Patent Publication Number: US-2021181118-A1

Title: Systems, sensors and methods for determining a concentration of an analyte

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from Australian Provisional Patent Application No. 2018903323 filed 6 Sep. 2018, Australian Provisional Patent Application No. 2018904856 filed 20 Dec. 2018, and Australian Provisional Patent Application No. 2019902157 filed 20 Jun. 2019, the contents of each of which are incorporated herein in their entirety. 
     FIELD 
     The invention relates to systems, sensors and methods for determining a concentration of an analyte. 
     BACKGROUND 
     Sensors are used to sense various analytes for a variety of applications. These analytes may be biomarkers, contaminants, narcotics, or any other species where there is an interest in their detection and/or quantification. Such analytes may, for example, be biomarkers, where their presence, absence, or concentration in a sample may be indicative of a disease or condition. 
     For example, human chorionic gonadotropin (hCG) is a biomarker found in urine and blood after implantation during pregnancy. The presence of hCG in urine can indicate pregnancy in an individual, and this principle is used in pregnancy tests that use a sensor able to detect hCG in urine. 
     A sensor for sensing an analyte generally requires suitable sensitivity for the analyte in order to limit false negative results. For example, the EPA allowable limit for lead in drinking water is 15 ppb. Accordingly, a sensor for detecting lead in drinking water must be sensitive enough to reliably detect lead at least down to a concentration of 15 ppb. 
     Additionally, a sensor must have a suitable level of selectivity to the analyte in order to limit false positive results. For example, a sensor sensitive enough to detect lead in drinking water below 15 ppb will be of little use in that application if it has poor selectivity for lead. If the sensor responds similarly to sodium in drinking water, which is typically present in drinking water in much higher concentrations than lead, then such a sensor would yield many false positive results. 
     A sensor must also have suitable precision and accuracy for sensing the analyte depending upon the application area and sensing requirements. If a sensor is required only for detecting the presence or absence of an analyte, i.e. a qualitative sensing, the minimum precision and accuracy levels of the sensor may be less than if the sensor is to be used for quantifying the amount of the analyte. 
     Sensors in the prior art may have suitable sensitivity, selectivity, precision and/or accuracy for sensing an analyte, but only within a narrow analyte concentration range. For many analytes, however, it is necessary to detect and/or quantify their concentration over a wide variety of concentrations. Accordingly, there is a need to develop sensors and systems comprising sensors that have a wide dynamic range and/or a wide working range for sensing analytes at a variety of concentrations. Some sensors may meet these requirements, but may have a high equipment cost, for example they may require incorporation in a GC-MS. Further, it may be slow to determine an accurate concentration of an analyte using such sensors. 
     Low-cost sensors may suffer from one or more of difficult calibration yielding poor accuracy, poor repeatability yielding poor precision, poor selectivity, poor sensitivity, a narrow dynamic range and/or a narrow working range. Therefore, there is a need to develop low cost sensors and systems comprising low cost sensors having improvements in any one or more of these properties. There is also a need to develop sensors and systems comprising sensors suitable for rapid tests. 
     Sensors for certain biomarkers may only be effective for invasively collected samples, such as blood. For example, sensors for determining blood glucose levels for patients with diabetes are accurate and enable quantitative glucose concentration measurement, but only for invasively collected blood samples. Accordingly, there is also a need for sensors and systems comprising sensors with improved sensitivity, reliability and/or affordability that enable detection or quantitation of biomarkers in non-invasively collected samples, for example in urine, saliva, or breath. 
     For example, diabetes mellitus, commonly shortened to diabetes, is a serious health concern globally, and its prevalence and diabetes-related deaths have been increasing with rapid changes in lifestyle resulting in an increase in risk factors such as obesity. Recent estimates have shown the prevalence of diabetes is likely to continue to grow substantially in the coming decades. For instance, in 2000 approximately 1.0 million people in Australia aged over 25 years had diabetes. This figure is projected to reach 2.0-2.9 million by 2025, with the worldwide incidence of diabetes of 336 million in 2011 projected to grow to 552 million in 2030. The costs of diabetes are substantial to both individuals and governments, especially when complications occur. It is apparent that a cost-effective treatment of diabetes has tremendous potential to relieve pressure on healthcare systems. 
     Diabetes is caused by the body&#39;s inability to produce the hormone insulin (type 1), or the body&#39;s inability to utilise it (type 2). Diabetics can suffer from low insulin levels and the consequent elevated level of blood glucose results in an increase in the body&#39;s ketone body concentration. Ketone bodies are carbonyl containing compounds: acetoacetate, beta-hydroxybutyrate and acetone. Increased ketone body concentration in the body can cause diabetic ketoacidosis, a state of insulin deficiency that can progress rapidly and is considered a medical emergency. If left untreated it can result in coma or death due to the acidification of blood. Although this life-threatening condition primarily occurs in patients with newly diagnosed type 1 diabetes, it can also occur in patients with type 2 diabetes. Therefore, there is a need for new methods, sensors and systems suitable for monitoring levels of analytes, such as carbonyl-containing compounds, for example, ketone bodies. 
     A system, sensor, or method for determining the concentration of an analyte, such as a carbonyl containing compound (e.g. ketone bodies) would generally require suitable sensitivity for the carbonyl containing compound in order to limit false negative results. Additionally, such a system, sensor or method must have a suitable level of selectivity to the carbonyl containing compound in order to limit false positive results. A system, sensor or method should also have suitable precision and accuracy for determining the concentration of the carbonyl containing compound. 
     Devices and methods in the prior art for determining the concentration of carbonyl containing compounds (e.g. ketone bodies), have suitable sensitivity, selectivity, precision and/or accuracy, but only within a narrow analyte concentration range. For particular carbonyl containing compounds, however, it may be necessary to detect and/or quantify their concentration over a wide variety of concentrations. Accordingly, there is a need to develop systems, sensors and methods that have a wide dynamic range and/or a wide working range for determining analyte concentration, e.g., carbonyl containing compound concentration at a variety of concentrations. Some devices or methods meet these requirements, but have a high equipment cost, for example they may require incorporation in a GC-MS. Further, it may be slow to determine an accurate concentration of a carbonyl containing compound using such devices or methods. 
     Low-cost systems and sensors for determining various analyte concentrations, including carbonyl containing compound concentrations, commonly suffer from one or more of: difficult calibration yielding poor accuracy, poor repeatability yielding poor precision, poor selectivity, poor sensitivity, a narrow dynamic range and/or a narrow working range. Therefore, there is a need to develop low cost systems and sensors, and methods using low cost systems and sensors having improvements in one or more of these properties. There is also a need to develop sensors, systems and methods for determining analyte concentrations, such as carbonyl containing compound concentrations, that are suitable for rapid tests. 
     Many devices and methods for determining the concentration of diabetes biomarkers, such as ketone bodies, are only be effective for invasively collected samples, such as blood. For example, devices for determining blood glucose levels for patients with diabetes are accurate and enable quantitative glucose concentration measurement, but only for invasively collected blood samples. Accordingly, there is also a need for systems, devices, and methods with improved sensitivity, reliability and/or affordability that enable determination of the concentration of, e.g., diabetes biomarkers, such as ketone bodies, in non-invasively collected samples, for example in urine, saliva, or breath. There is also a need for rapid and accurate detection methods for analytes, such as carbonyl-containing compounds (e.g. ketone bodies), for various ketone body monitoring application, such fitness, dieting, and metabolic therapy treatments for a variety of diseases related to ketone body levels, such as diabetes and Alzheimer&#39;s disease. 
     It will be apparent to the skilled person that suitable detection methods for analytes such as carbonyl-containing compounds, may also be suitable for the detection of various other non-carbonyl-containing compound analytes, and further that suitable detection methods for carbonyl-containing compounds, such as ketone bodies for diabetes monitoring, may also be suitable for the detection of various other carbonyl-containing compound analytes, such as pentosidine and acetaldehyde. 
     It is an aim of the present invention to at least partially satisfy at least one of the above needs. 
     SUMMARY OF INVENTION 
     According to a first aspect there is provided a system for determining a concentration of an analyte in a fluid, said system comprising: 
     a sensor configured to generate a differential response to a constant concentration of the analyte; and 
     an acquisition device configured to determine the concentration of the analyte from the differential response. 
     The following options may be used in conjunction with the first aspect, either individually or in any suitable combination. 
     The differential response may be temporally differential, whereby the response varies over time. Alternatively, or additionally, the differential response may be spatially differential, whereby the sensor is configured to respond to the analyte at a plurality of detection positions on the sensor, said detection positions having different sensitivities to the analyte. 
     The sensitivity to the analyte may vary along a length of the sensor. It may vary monotonically along the length of the sensor. It may vary linearly or logarithmically along the length of the sensor. The plurality of detection positions may be spaced equidistantly along the length of the sensor. 
     The sensor may comprise a plurality of discrete regions each having a different sensitivity to the analyte. It may comprise a coating on a detection surface, said coating having spatially differential permeability to the analyte. It may comprise, for example, at least three detection positions having different sensitivities to the analyte. 
     The sensor may be configured to respond to the analyte at a plurality of detection positions on the sensor; said detection positions having different sensitivities to the analyte; and the acquisition device may be configured to calculate the concentration of the analyte utilising at least the following: 
     when the sensor reaches a saturated response level at all of the plurality of detection positions on the sensor:
         a response of the sensor to the analyte as a function of time at the detection position on the sensor having the least sensitivity to the analyte;
 
when the sensor does not reach a saturated response level at any of the plurality of detection positions on the sensor:
   a response of the sensor to the analyte at the detection position on the sensor having the greatest sensitivity to the analyte; and
 
if the sensor responds to the analyte to reach a saturated response level at some, but not all, of the plurality of detection positions on the sensor:
   one or more of:
           a ratio of the number of detection positions on the sensor that reach a saturated response level to the number of detection positions on the sensor that do not reach a saturated response level,   a time required to reach a saturated response level for at least one of the detection positions on the sensor that reaches a saturated response level, and   a response of the sensor to the analyte for at least one of the detection positions on the sensor that does not reach a saturated response level.   
               

     The system may further comprise a flow channel or an analysis chamber, wherein the sensor is disposed within said flow channel or analysis chamber. 
     The analyte may be a carbonyl-containing compound. The sensor may comprise an optionally O-substituted hydroxylammonium salt or an optionally N-substituted hydrazinium salt, whereby exposure of the fluid to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt in the presence of a liquid produces a change in a variable, said variable being dependent upon the pH of the liquid; and the acquisition device may comprise a detector for measuring the change in the variable, and a processor configured to calculate the concentration of the carbonyl-containing compound in the fluid from the change in the variable. The detector may be an ion sensitive field effect transistor (ISFET), chemiresistive sensor, potentiometric sensor, spectroscopic sensor, colorimetric sensor, fluorometric sensor, thermal sensor, or a combination thereof. 
     The sensor may comprise a sorbent material, whereby the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt is sorbed in and/or on the sorbent material. The system may further comprise a plurality of electrodes configured to apply an electric field across a surface of the sorbent material so as to concentrate one or more charged species on and/or in the sorbent material. The sorbent material may further comprise a weak base sorbed thereon and/or therein. The system may comprise an indicator which is sorbed in and/or on the sorbent material. The liquid volume may be about 1 mL or less. 
     The variable may be an absorbance, fluorescence, resistance, ion concentration, temperature, voltage, or current. The ion concentration may be, for example, a hydronium ion concentration. 
     In certain embodiments, the change in the variable is not measured using a titration. 
     The optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt may be selected from compounds of Formula (I): 
       [R m —Y—NH 3   + ] n X p   Formula (I)
 
     wherein Y=O or N;
 
m=1, 2, or 3;
 
n=1, 2, or 3;
 
p=1, 2, or 3;
 
R is independently selected from the group consisting of hydrogen, halogen, optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted heteroalkyl, optionally substituted cycloheteroalkyl, optionally substituted alkenyl, optionally substituted cycloalkenyl, optionally substituted alkylaryl, optionally substituted aryl, optionally substituted alkylheteroaryl, and optionally substituted heteroaryl; wherein said optionally substituted alkyl, cycloalkyl, heteroalkyl, cycloheteroalkyl, alkenyl, cycloalkenyl, alkylaryl, aryl, alkylheteroaryl, or heteroaryl is optionally substituted with one or more groups independently selected from the group consisting of halogen, hydroxyl, thiol, amino, aryl, heteroaryl, alkyl, alkenyl, alkynyl, heteroalkyl, alkylaryl, and alkylheteroaryl; and
 
X is an anion.
 
     The optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt may, for example, be selected from the group consisting of hydroxylammonium chloride, hydroxylammonium sulfate, hydroxylammonium azide, O-phenyl hydroxylammonium chloride, O-phenyl hydroxylammonium sulfate, O-phenyl hydroxylammonium azide, hydroxylamine picrate, hydroxylamine benzenesulfonate, hydroxylamine benzenesulfinate, hydroxylamine 1-sulfobutyl-3-methyl imidazole hydrosulphate salt, hydrazinium chloride, hydrazinium sulfate, hydrazinium hydrogensulfate, hydrazinium hydrogen sulfate, hydrazinium azide, hydrazinediium sulfate, hydrazinediium bis(6-carboxypyridazine-3-carboxylate), hydroxylammonium phosphate, hydroxylamine nitrate, and mixtures thereof. 
     The carbonyl-containing compound may, for example, be selected from the group consisting of acetoacetate, butyraldehyde, 4-hydroxy-2-hexenal, ethyl acetate, acetic acid, 4-hydroxy-2-nonenal, 2-pentanone, 2-butanone, 3-hydroxy-2-butanone, hydroxyacetaldehyde, acetaldehyde, pentanal, malondialdehyde, acrolein, beta-hydroxybutyrate, and acetone. 
     The fluid may be breath. 
     In a specific embodiment there is provided a system for determining a concentration of an acetone in breath, said system comprising a sensor configured to generate a spatially differential response to a constant concentration of acetone, whereby the sensor is configured to respond to acetone at a plurality of detection positions on the sensor, said detection positions having different sensitivities to the acetone, wherein the sensor comprises a hydroxylamine, hydroxylamine salt, or combination thereof, whereby exposure of the breath to the hydroxylamine, hydroxylamine salt, or combination thereof produces a change in a variable, said variable being dependent upon the pH of the liquid; and an acquisition device configured to determine the concentration of the acetone from the differential response, said acquisition device comprising a detector for measuring the change in the variable, and a processor configured to calculate the concentration of the acetone in the breath from the change in the variable, wherein the acquisition device is configured to calculate the concentration of the acetone utilising at least the following:
         if the sensor reaches a saturated response level at all of the plurality of detection positions on the sensor:
           a response of the sensor to the acetone as a function of time at the detection position on the sensor having the least sensitivity to the acetone;   
           if the sensor does not reach a saturated response level at any of the plurality of detection positions on the sensor:
           a response of the sensor to the acetone at the detection position on the sensor having the greatest sensitivity to the acetone; or   a response of the sensor to the acetone at one or more predetermined temporal points at a position on the sensor having the greatest sensitivity to the acetone; and   
           if the sensor responds to the acetone to reach a saturated response level at some, but not all, of the plurality of detection positions on the sensor:
           one or more of a ratio of the number of detection positions on the sensor that reach a saturated response level to the number of detection positions on the sensor that do not reach a saturated response level, a time required to reach a saturated response level for at least one of the detection positions on the sensor that reaches a saturated response level, and a response of the sensor to the acetone for at least one of the detection positions on the sensor that does not reach a saturated response level.   
               

     In a specific embodiment there is provided a system for determining a concentration of an acetone in breath, said system comprising a sensor configured to generate a spatially differential response to a constant concentration of acetone, whereby the sensor is configured to respond to acetone at a plurality of detection positions on the sensor, said detection positions having different sensitivities to the acetone, wherein the sensor comprises a hydroxylamine, hydroxylamine salt, or combination thereof, whereby exposure of the breath to the hydroxylamine, hydroxylamine salt, or combination thereof produces a change in a variable, said variable being dependent upon the pH of the liquid; and an acquisition device configured to determine the concentration of the acetone from the differential response, said acquisition device comprising a detector for measuring the change in the variable, and a processor configured to calculate the concentration of the acetone in the breath from the change in the variable, 
     wherein the acquisition device is configured to calculate the concentration of the acetone utilising at least the following:
         if the sensor reaches a saturated response level at all of the plurality of detection positions on the sensor:
           a response of the sensor to the acetone as a function of time at the detection position on the sensor having the least sensitivity to the acetone;   
           if the sensor does not reach a saturated response level at any of the plurality of detection positions on the sensor:
           a response of the sensor to the acetone at the detection position on the sensor having the greatest sensitivity to the acetone; or   a response of the sensor to the acetone at one or more predetermined temporal points at a position on the sensor having the greatest sensitivity to the acetone; and   
           if the sensor responds to the acetone to reach a saturated response level at some, but not all, of the plurality of detection positions on the sensor:
           one or more of a ratio of the number of detection positions on the sensor that reach a saturated response level to the number of detection positions on the sensor that do not reach a saturated response level, a time required to reach a saturated response level for at least one of the detection positions on the sensor that reaches a saturated response level, and a response of the sensor to the acetone for at least one of the detection positions on the sensor that does not reach a saturated response level;
 
wherein the sensor comprises a sorbent material, wherein the hydroxylamine, hydroxylamine salt, or combination thereof is sorbed in and/or on the sorbent material, wherein the sensor comprises a halochromic indicator which is sorbed in and/or on the sorbent material, wherein the variable is an absorbance, and the detector is a colorimetric sensor.
   
               

     In a specific embodiment there is provided a system for determining a concentration of an acetone in breath, said system comprising a sensor configured to generate a spatially differential response to a constant concentration of acetone, whereby the sensor is configured to respond to acetone at a plurality of detection positions on the sensor, said detection positions having different sensitivities to the acetone, wherein the sensor comprises a hydroxylamine, hydroxylamine salt, or combination thereof, whereby exposure of the breath to the hydroxylamine, hydroxylamine salt, or combination thereof produces a change in a variable, said variable being dependent upon the pH of the liquid; and an acquisition device configured to determine the concentration of the acetone from the differential response, said acquisition device comprising a detector for measuring the change in the variable, and a processor configured to calculate the concentration of the acetone in the breath from the change in the variable, wherein the acquisition device is configured to calculate the concentration of the acetone utilising at least the following:
         if the sensor reaches a saturated response level at all of the plurality of detection positions on the sensor:
           a response of the sensor to the acetone as a function of time at the detection position on the sensor having the least sensitivity to the acetone;   
           if the sensor does not reach a saturated response level at any of the plurality of detection positions on the sensor:
           a response of the sensor to the acetone at the detection position on the sensor having the greatest sensitivity to the acetone; or   a response of the sensor to the acetone at one or more predetermined temporal points at a position on the sensor having the greatest sensitivity to the acetone; and   
           if the sensor responds to the acetone to reach a saturated response level at some, but not all, of the plurality of detection positions on the sensor:
           one or more of a ratio of the number of detection positions on the sensor that reach a saturated response level to the number of detection positions on the sensor that do not reach a saturated response level, a time required to reach a saturated response level for at least one of the detection positions on the sensor that reaches a saturated response level, and a response of the sensor to the acetone for at least one of the detection positions on the sensor that does not reach a saturated response level;
 
wherein the sensor comprises a sorbent material, wherein the hydroxylamine, hydroxylamine salt, or combination thereof is sorbed in and/or on the sorbent material, wherein the sensor comprises a halochromic indicator which is sorbed in and/or on the sorbent material, wherein the variable is an absorbance, and the detector is a colorimetric sensor;
 
wherein the liquid volume is about 1 ml or less, and wherein the change in the absorbance is not measured using a titration.
   
               

     In a specific embodiment there is provided a system for determining a concentration of an acetone in breath, said system comprising a sensor configured to generate a spatially differential response to a constant concentration of acetone, whereby the sensor is configured to respond to acetone at a plurality of detection positions on the sensor, said detection positions having different sensitivities to the acetone, wherein the sensor comprises a hydroxylamine, hydroxylamine salt, or combination thereof, whereby exposure of the breath to the hydroxylamine, hydroxylamine salt, or combination thereof produces a change in a variable, said variable being dependent upon the pH of the liquid; and an acquisition device configured to determine the concentration of the acetone from the differential response, said acquisition device comprising a detector for measuring the change in the variable, and a processor configured to calculate the concentration of the acetone in the breath from the change in the variable, wherein the acquisition device is configured to calculate the concentration of the acetone utilising at least the following:
         if the sensor reaches a saturated response level at all of the plurality of detection positions on the sensor:
           a response of the sensor to the acetone as a function of time at the detection position on the sensor having the least sensitivity to the acetone;   
           if the sensor does not reach a saturated response level at any of the plurality of detection positions on the sensor:
           a response of the sensor to the acetone at the detection position on the sensor having the greatest sensitivity to the acetone; or   a response of the sensor to the acetone at one or more predetermined temporal points at a position on the sensor having the greatest sensitivity to the acetone; and   
           if the sensor responds to the acetone to reach a saturated response level at some, but not all, of the plurality of detection positions on the sensor:
           one or more of a ratio of the number of detection positions on the sensor that reach a saturated response level to the number of detection positions on the sensor that do not reach a saturated response level, a time required to reach a saturated response level for at least one of the detection positions on the sensor that reaches a saturated response level, and a response of the sensor to the acetone for at least one of the detection positions on the sensor that does not reach a saturated response level;
 
wherein the sensor comprises a sorbent material, wherein the hydroxylamine, hydroxylamine salt, or combination thereof is sorbed in and/or on the sorbent material, wherein the variable is a hydronium ion concentration, and the change in hydronium ion concentration is measured using an ion sensitive filed effect transistor, chemiresistive sensor, or potentiometric sensor;
 
wherein the liquid volume is about 1 ml or less, and wherein the change in the hydronium ion concentration is not measured using a titration.
   
               

     According to a second aspect there is provided a method of determining a concentration of an analyte in a fluid, the method comprising: 
     exposing the fluid to a sensor which is configured to generate a differential response to a constant concentration of the analyte; and 
     measuring a differential response of the sensor to the analyte. 
     The following options may be used in conjunction with the second aspect, either individually or in any suitable combination. 
     The method may comprise a step of calculating the concentration of the analyte utilising the differential response of the sensor to the analyte. The differential response may be spatially differential, whereby the sensor is configured to respond to the analyte at a plurality of detection positions on the sensor; said detection positions having different sensitivities to the analyte; and the calculating may utilise at least the following: 
     if the sensor reaches a saturated response level at all of the plurality of detection positions on the sensor:
         a response of the sensor to the analyte as a function of time at the detection position on the sensor having the least sensitivity to the analyte;
 
if the sensor does not reach a saturated response level at any of the plurality of detection positions on the sensor:
   a response of the sensor to the analyte at the detection position on the sensor having the greatest sensitivity to the analyte; or   a response of the sensor to the analyte at one or more predetermined temporal points to the acetone at a detection position on the sensor having the greatest sensitivity to the analyte; and
 
if the sensor responds to the analyte to reach a saturated response level at some, but not all, of the plurality of detection positions on the sensor:
   one or more of:
           a ratio of the number of detection positions on the sensor that reach a saturated response level to the number of detection positions on the sensor that do not reach a saturated response level,   a time required to reach a saturated response level for at least one of the detection positions on the sensor that reaches a saturated response level, and   a response of the sensor to the analyte for at least one of the detection positions on the sensor that does not reach a saturated response level.   
               

     Alternatively, or additionally, the differential response may be temporally differential, and the calculating may utilise at least one or more of the following:
         a time required to reach a predetermined response level for at least one position on the sensor after exposure of the fluid to the sensor; and   a response level for at least one position on the sensor at a predetermined time after exposure of the fluid to the sensor.       

     The analyte may be a carbonyl-containing compound. The method may comprise exposing the fluid to an optionally O-substituted hydroxylammonium salt or an optionally N-substituted hydrazinium salt in the presence of a liquid, so as to produce a change in a variable, said variable being dependent upon the pH of the liquid; measuring the change in the variable; and determining the concentration of the carbonyl-containing compound in the fluid from the change in the variable. 
     The optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt may be sorbed in and/or on a sorbent material. An indicator may be sorbed in and/or on the sorbent material. 
     The optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt and/or the indicator may be in contact with a weak base. 
     The change in the variable may be substantially independent of the volume of the fluid exposed to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt and/or the change in the variable is substantially independent of the flow rate of the fluid exposed to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt. The change in the variable may be measured using an ion sensitive field effect transistor (ISFET), chemiresistive sensor, potentiometric sensor, spectroscopic sensor, colorimetric sensor, fluorometric sensor, thermal sensor, or a combination thereof. 
     The variable may be an absorbance, fluorescence, resistance, ion concentration, temperature, voltage, or current. The ion concentration may, for example, be a hydronium ion concentration. 
     The liquid volume may be about 1 mL or less. 
     In certain embodiments, the change in the variable is not measured using a titration. 
     The optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt may be selected from compounds of Formula (I): 
       [R m —Y—NH 3   + ] n X p   Formula (I)
 
     wherein Y=O or N;
 
m=1, 2, or 3;
 
n=1, 2, or 3;
 
p=1, 2, or 3;
 
R is independently selected from the group consisting of hydrogen, halogen, optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted heteroalkyl, optionally substituted cycloheteroalkyl, optionally substituted alkenyl, optionally substituted cycloalkenyl, optionally substituted alkylaryl, optionally substituted aryl, optionally substituted alkylheteroaryl, and optionally substituted heteroaryl; wherein said optionally substituted alkyl, cycloalkyl, heteroalkyl, cycloheteroalkyl, alkenyl, cycloalkenyl, alkylaryl, aryl, alkylheteroaryl, or heteroaryl is optionally substituted with one or more groups independently selected from the group consisting of halogen, hydroxyl, thiol, amino, aryl, heteroaryl, alkyl, alkenyl, alkynyl, heteroalkyl, alkylaryl, and alkylheteroaryl; and
 
X is an anion.
 
     The optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt may, for example, be selected from the group consisting of hydroxylammonium chloride, hydroxylammonium sulfate, hydroxylammonium azide, O-phenyl hydroxylammonium chloride, O-phenyl hydroxylammonium sulfate, O-phenyl hydroxylammonium azide, hydroxylamine picrate, hydroxylamine benzenesulfonate, hydroxylamine benzenesulfinate, hydroxylamine 1-sulfobutyl-3-methyl imidazole hydrosulphate salt, hydrazinium chloride, hydrazinium sulfate, hydrazinium hydrogensulfate, hydrazinium hydrogen sulfate, hydrazinium azide, hydrazinediium sulfate, hydrazinediium bis(6-carboxypyridazine-3-carboxylate), hydroxylammonium phosphate, hydroxylamine nitrate, and mixtures thereof. 
     The carbonyl-containing compound may, for example, be selected from the group consisting of acetoacetate, butyraldehyde, 4-hydroxy-2-hexenal, ethyl acetate, acetic acid, 4-hydroxy-2-nonenal, 2-pentanone, 2-butanone, 3-hydroxy-2-butanone, hydroxyacetaldehyde, acetaldehyde, pentanal, malondialdehyde, acrolein, beta-hydroxybutyrate, and acetone. 
     The fluid may be breath. 
     In a specific embodiment, there is provided a method of determining a concentration of acetone in breath, the method comprising:
         exposing the breath to a sensor which is configured to generate a differential response to a constant concentration of the acetone; and   measuring a differential response of the sensor to the acetone;
 
whereby the method further comprises a step of calculating the concentration of the acetone utilising the differential response of the sensor to the acetone;
 
wherein the differential response is spatially differential, whereby the sensor is configured to respond to the acetone at a plurality of detection positions on the sensor; said detection positions having different sensitivities to the acetone; and
 
wherein said calculating utilises at least the following:
   if the sensor reaches a saturated response level at all of the plurality of detection positions on the sensor:
           a response of the sensor to the acetone as a function of time at the detection position on the sensor having the least sensitivity to the acetone;   
           if the sensor does not reach a saturated response level at any of the plurality of detection positions on the sensor:
           a response of the sensor to the acetone at the detection position on the sensor having the greatest sensitivity to the acetone; or   a response of the sensor to the acetone at one or more predetermined temporal points at a position on the sensor having the greatest sensitivity to the acetone; and   
           if the sensor responds to the acetone to reach a saturated response level at some, but not all, of the plurality of detection positions on the sensor:
           one or more of:
               a ratio of the number of detection positions on the sensor that reach a saturated response level to the number of detection positions on the sensor that do not reach a saturated response level,   a time required to reach a saturated response level for at least one of the detection positions on the sensor that reaches a saturated response level, and   a response of the sensor to the acetone for at least one of the detection positions on the sensor that does not reach a saturated response level;
 
wherein the method comprises:
   
               
           (a) exposing the breath to a hydroxylamine, hydroxylamine salt, or combination thereof in the presence of a liquid, so as to produce a change in a variable, said variable being dependent upon the pH of the liquid;   (b) measuring the change in the variable; and   (c) determining the concentration of the acetone in the breath from the change in the variable.       

     In a specific embodiment, there is provided a method of determining a concentration of acetone in breath, the method comprising:
         exposing the breath to a sensor which is configured to generate a differential response to a constant concentration of the acetone; and   measuring a differential response of the sensor to the acetone;
 
whereby the method further comprises a step of calculating the concentration of the acetone utilising the differential response of the sensor to the acetone;
 
wherein the differential response is spatially differential, whereby the sensor is configured to respond to the acetone at a plurality of detection positions on the sensor; said detection positions having different sensitivities to the acetone; and
 
wherein said calculating utilises at least the following:
   if the sensor reaches a saturated response level at all of the plurality of detection positions on the sensor:
           a response of the sensor to the acetone as a function of time at the detection position on the sensor having the least sensitivity to the acetone;   
           if the sensor does not reach a saturated response level at any of the plurality of detection positions on the sensor:
           a response of the sensor to the acetone at the detection position on the sensor having the greatest sensitivity to the acetone; or   a response of the sensor to the acetone at one or more predetermined temporal points at a position on the sensor having the greatest sensitivity to the acetone; and   
           if the sensor responds to the acetone to reach a saturated response level at some, but not all, of the plurality of detection positions on the sensor:
           one or more of:
               a ratio of the number of detection positions on the sensor that reach a saturated response level to the number of detection positions on the sensor that do not reach a saturated response level,   a time required to reach a saturated response level for at least one of the detection positions on the sensor that reaches a saturated response level, and   a response of the sensor to the acetone for at least one of the detection positions on the sensor that does not reach a saturated response level;
 
wherein the method comprises:
   
               
           (a) exposing the breath to a hydroxylamine, hydroxylamine salt, or combination thereof in the presence of a liquid, so as to produce a change in a variable, said variable being dependent upon the pH of the liquid;   (b) measuring the change in the variable; and   (c) determining the concentration of the acetone in the breath from the change in the variable
 
wherein the sensor comprises a sorbent material; the hydroxylamine, hydroxylamine salt, or combination thereof is sorbed in and/or on the sorbent material; the sensor comprises a halochromic indicator which is sorbed in and/or on the sorbent material; the variable is an absorbance; and the detector is a colorimetric sensor.
       

     In a specific embodiment, there is provided a method of determining a concentration of acetone in breath, the method comprising:
         exposing the breath to a sensor which is configured to generate a differential response to a constant concentration of the acetone; and   measuring a differential response of the sensor to the acetone;
 
whereby the method further comprises a step of calculating the concentration of the acetone utilising the differential response of the sensor to the acetone;
 
wherein the differential response is spatially differential, whereby the sensor is configured to respond to the acetone at a plurality of detection positions on the sensor; said detection positions having different sensitivities to the acetone; and
 
wherein said calculating utilises at least the following:
   if the sensor reaches a saturated response level at all of the plurality of detection positions on the sensor:
           a response of the sensor to the acetone as a function of time at the detection position on the sensor having the least sensitivity to the acetone;   
           if the sensor does not reach a saturated response level at any of the plurality of detection positions on the sensor:
           a response of the sensor to the acetone at the detection position on the sensor having the greatest sensitivity to the acetone; or   a response of the sensor to the acetone at one or more predetermined temporal points at a position on the sensor having the greatest sensitivity to the acetone; and   
           if the sensor responds to the acetone to reach a saturated response level at some, but not all, of the plurality of detection positions on the sensor:
           one or more of:
               a ratio of the number of detection positions on the sensor that reach a saturated response level to the number of detection positions on the sensor that do not reach a saturated response level,   a time required to reach a saturated response level for at least one of the detection positions on the sensor that reaches a saturated response level, and   a response of the sensor to the acetone for at least one of the detection positions on the sensor that does not reach a saturated response level;
 
wherein the method comprises:
   
               
           (a) exposing the breath to a hydroxylamine, hydroxylamine salt, or combination thereof in the presence of a liquid, so as to produce a change in a variable, said variable being dependent upon the pH of the liquid;   (b) measuring the change in the variable; and   (c) determining the concentration of the acetone in the breath from the change in the variable;
 
wherein the sensor comprises a sorbent material; the hydroxylamine, hydroxylamine salt, or combination thereof is sorbed in and/or on the sorbent material; the sensor comprises a halochromic indicator which is sorbed in and/or on the sorbent material; the variable is an absorbance, the detector is a colorimetric sensor; the liquid volume is about 1 ml or less; and the method does not comprise a titration.
       

     In a specific embodiment, there is provided a method of determining a concentration of acetone in breath, the method comprising:
         exposing the breath to a sensor which is configured to generate a differential response to a constant concentration of the acetone; and   measuring a differential response of the sensor to the acetone;
 
whereby the method further comprises a step of calculating the concentration of the acetone utilising the differential response of the sensor to the acetone;
 
wherein the differential response is spatially differential, whereby the sensor is configured to respond to the acetone at a plurality of detection positions on the sensor; said detection positions having different sensitivities to the acetone; and
 
wherein said calculating utilises at least the following:
   if the sensor reaches a saturated response level at all of the plurality of detection positions on the sensor:
           a response of the sensor to the acetone as a function of time at the detection position on the sensor having the least sensitivity to the acetone;   
           if the sensor does not reach a saturated response level at any of the plurality of detection positions on the sensor:
           a response of the sensor to the acetone at the detection position on the sensor having the greatest sensitivity to the acetone; or   a response of the sensor to the acetone at one or more predetermined temporal points at a position on the sensor having the greatest sensitivity to the acetone; and   
           if the sensor responds to the acetone to reach a saturated response level at some, but not all, of the plurality of detection positions on the sensor:
           one or more of:
               a ratio of the number of detection positions on the sensor that reach a saturated response level to the number of detection positions on the sensor that do not reach a saturated response level,   a time required to reach a saturated response level for at least one of the detection positions on the sensor that reaches a saturated response level, and   a response of the sensor to the acetone for at least one of the detection positions on the sensor that does not reach a saturated response level;
 
wherein the method comprises:
   
               
           (a) exposing the breath to a hydroxylamine, hydroxylamine salt, or combination thereof in the presence of a liquid, so as to produce a change in a variable, said variable being dependent upon the pH of the liquid;   (b) measuring the change in the variable; and   (c) determining the concentration of the acetone in the breath from the change in the variable
 
wherein the sensor comprises a sorbent material; the hydroxylamine, hydroxylamine salt, or combination thereof is sorbed in and/or on the sorbent material; the variable is a hydronium ion concentration, and the change in hydronium ion concentration is measured using an ion sensitive filed effect transistor, chemiresistive sensor, or potentiometric sensor; the liquid volume is about 1 ml or less; and the method does not comprise a titration.
       

     In a specific embodiment, the method is for managing diabetes in a patient on a diabetes treatment plan, wherein the carbonyl-containing compound is acetone, the fluid is breath, and the method comprises determining an acetone concentration of a breath sample of the patient by the method described earlier; comparing the acetone concentration to a reference acetone concentration range; and if the acetone concentration is outside the acetone concentration reference range, adjusting the diabetes treatment plan. 
     In another specific embodiment, the method is to assist in diagnosing diabetes in a patient, wherein the carbonyl-containing compound is acetone, the fluid is breath, and the method comprises determining an acetone concentration of a breath sample of the patient by the method described earlier; and comparing the acetone concentration to a reference acetone concentration range to determine if the patient has diabetes. 
     The method of the second aspect may use the system of the first aspect. The system of the first aspect may be used in the method of the second aspect. 
     According to a third aspect there is provided a sensor for sensing a concentration of an analyte in a fluid, the sensor being as defined according to the first or second aspect. 
     In a specific embodiment there is provided a sensor configured to generate a spatially differential response to a constant concentration of acetone, whereby the sensor is configured to respond to acetone at a plurality of detection positions on the sensor, said detection positions having different sensitivities to the analyte; wherein the sensor comprises a hydroxylamine, hydroxylamine salt, or combination thereof; and the sensor comprises a sorbent material, wherein the hydroxylamine, hydroxylamine salt, or combination thereof is sorbed in and/or on the sorbent material. 
     In a specific embodiment there is provided a sensor configured to generate a spatially differential response to a constant concentration of acetone, whereby the sensor is configured to respond to acetone at a plurality of detection positions on the sensor, said detection positions having different sensitivities to the analyte; wherein the sensor comprises a hydroxylamine, hydroxylamine salt, or combination thereof; the sensor comprises a sorbent material, wherein the hydroxylamine, hydroxylamine salt, or combination thereof is sorbed in and/or on the sorbent material; and wherein the sensor comprises a halochromic indicator which is sorbed in and/or on the sorbent material. 
     The sensor of the third aspect may be used in the method of the second aspect. The method of the second aspect may use the sensor of the third aspect. 
     The system of the first aspect may use the sensor of the third aspect. The sensor of the third aspect may be a component of the system of the first aspect. 
     According to a fourth aspect there is provided the use of a system for diagnosing and/or managing diabetes in a patient, wherein the system is capable of determining a concentration of acetone in a breath sample of the patient, and the system comprises: 
     an optionally O-substituted hydroxylammonium salt or an optionally N-substituted hydrazinium salt, whereby exposure of the breath to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt in the presence of a liquid produces a change in a variable, said variable being dependent upon the pH of the liquid; 
     a detector for measuring the change in the variable; and 
     a processor configured to calculate the concentration of the acetone in the breath from the change in the variable. 
     The use of the fourth aspect may be a use of the system of the first aspect. The system of the first aspect may be used for the use of the fourth aspect. The use of the fourth aspect may use the method of the second aspect. The method of the second aspect may be used in the use of the fourth aspect. 
     According to a fifth aspect there is provided a method of managing diabetes in a patient on a diabetes treatment plan, said method comprising: 
     (a) determining an acetone concentration of a breath sample of the patient using the method according to the second aspect; 
     (b) comparing the acetone concentration to a reference acetone concentration range; 
     and 
     (c) if the acetone concentration is outside the acetone concentration reference range, adjusting the diabetes treatment plan. 
     The method of the fifth aspect may use the system of the first aspect. The system of the first aspect may be used for the method of the fifth aspect. The method of the fifth aspect may use the sensor of the third aspect. The sensor of the third aspect may be used for the method of the fifth aspect. 
     According to a sixth aspect there is provided a method to assist in diagnosing diabetes in a patient, said method comprising: 
     (a) determining an acetone concentration of a breath sample of the patient using the method according to the second aspect; and
 
(b) comparing the acetone concentration to a reference acetone concentration range to determine if the patient has diabetes.
 
     The method of the sixth aspect may use the system of the first aspect. The system of the first aspect may be used for the method of the sixth aspect. The method of the sixth aspect may use the sensor of the third aspect. The sensor of the third aspect may be used for the method of the sixth aspect. 
     According to a first embodiment there is provided a system for determining a concentration of an analyte in a fluid, said system comprising a sensor configured to generate a differential response to a constant concentration of the analyte; and an acquisition device configured to determine the concentration of the analyte from the differential response. 
     The following options may be used in conjunction with the first embodiment, either individually or in any suitable combination. 
     The differential response may be temporally differential, whereby the response varies over time. Alternatively, or additionally, it may be spatially differential, whereby the sensor is configured to respond to the analyte at a plurality of detection positions on the sensor; said detection positions having different sensitivities to the analyte. 
     The plurality of detection positions may be spaced equidistantly along the length of the sensor. Sensitivity to the analyte may vary along a length of the sensor. It may vary monotonically along the length of the sensor. It may vary linearly along the length of the sensor. Alternatively, it may vary nonlinearly, e.g. logarithmically, along the length of the sensor. It may vary linearly or logarithmically along the length of the sensor. The sensor may comprise a plurality of discrete regions each having a different sensitivity to the analyte. It may comprise a coating on a detection surface, said coating having spatially differential permeability to the analyte. It may comprise at least three detection positions having different sensitivities to the analyte. 
     The acquisition device may comprise a processor configured to calculate the concentration of the analyte utilising the differential response of the sensor to the analyte. 
     In an embodiment the sensor is configured to respond to the analyte at a plurality of detection positions on the sensor; said detection positions having different sensitivities to the analyte. In this embodiment the acquisition device is configured to calculate the concentration of the analyte utilising at least the following:
         if the sensor reaches a saturated response level at all of the plurality of detection positions on the sensor:
           a response of the sensor to the analyte as a function of time at the detection position on the sensor having the least sensitivity to the analyte;   
           if the sensor does not reach a saturated response level at any of the plurality of detection positions on the sensor:
           a response of the sensor to the analyte at the detection position on the sensor having the greatest sensitivity to the analyte; or   a response of the sensor to the acetone at one or more predetermined temporal points at a position on the sensor having the greatest sensitivity to the acetone; and   
           if the sensor responds to the analyte to reach a saturated response level at some, but not all, of the plurality of detection positions on the sensor:
           one or more of a ratio of the number of detection positions on the sensor that reach a saturated response level to the number of detection positions on the sensor that do not reach a saturated response level, a time required to reach a saturated response level for at least one of the detection positions on the sensor that reaches a saturated response level, and a response of the sensor to the analyte for at least one of the detection positions on the sensor that does not reach a saturated response level.   
               

     The system may further comprise a flow channel or an analysis chamber. The sensor may be disposed within said flow channel or analysis chamber. In some instances, the sensor surface may form a portion of a wall of the flow channel or analysis chamber. The system may further comprise a flow controller configured to control and/or monitor a flow rate of the fluid into said analysis chamber or through said flow channel. It may further comprise a volume controller configured to control and/or monitor the volume of the fluid passing into said analysis chamber or through said flow channel. 
     The analyte may be a biomarker for diet, a disease, or a disorder. It may be a metal, protein, antibody, or small organic molecule. It may, for example, be acetone (which is a biomarker for diabetes). The sensor may comprise a compound capable of reacting with acetone. The compound may be selected from the group consisting of hydroxylamines, hydroxylamine salts, amines, alcohols, and combinations thereof. 
     In a specific embodiment there is provided a system for determining a concentration of an acetone in a fluid, said system comprising a sensor configured to generate a spatially differential response to a constant concentration of acetone, whereby the sensor is configured to respond to acetone at a plurality of detection positions on the sensor, said detection positions having different sensitivities to the analyte, wherein the sensor comprises a compound capable of reacting with acetone, said compound being selected from the group consisting of hydroxylamines, hydroxylamine salts, and combinations thereof; and an acquisition device configured to determine the concentration of the acetone from the differential response, the acquisition device configured to calculate the concentration of the analyte utilising at least the following:
         if the sensor reaches a saturated response level at all of the plurality of detection positions on the sensor:
           a response of the sensor to the analyte as a function of time at the detection position on the sensor having the least sensitivity to the analyte;   
           if the sensor does not reach a saturated response level at any of the plurality of detection positions on the sensor:
           a response of the sensor to the analyte at the detection position on the sensor having the greatest sensitivity to the analyte; or   a response of the sensor to the acetone at one or more predetermined temporal points at a position on the sensor having the greatest sensitivity to the acetone; and   
           if the sensor responds to the analyte to reach a saturated response level at some, but not all, of the plurality of detection positions on the sensor:
           one or more of a ratio of the number of detection positions on the sensor that reach a saturated response level to the number of detection positions on the sensor that do not reach a saturated response level, a time required to reach a saturated response level for at least one of the detection positions on the sensor that reaches a saturated response level, and a response of the sensor to the analyte for at least one of the detection positions on the sensor that does not reach a saturated response level.   
               

     According to a second embodiment there is provided a method of determining a concentration of an analyte in a fluid, the method comprising exposing the fluid to a sensor which is configured to generate a differential response to a constant concentration of the analyte; and measuring a differential response of the sensor to the analyte. 
     The following options may be used in conjunction with the second embodiment, either individually or in any suitable combination. 
     The method may further comprise the step of calculating the concentration of the analyte utilising a differential response of the sensor to the analyte. The differential response may be spatially differential, and the sensor may be configured to respond to the analyte at a plurality of detection positions on the sensor; said detection positions having different sensitivities to the analyte. In this case, the calculating may utilise at least the following:
         if the sensor reaches a saturated response level at all of the plurality of detection positions on the sensor:
           a response of the sensor to the analyte as a function of time at a detection position on the sensor having the least sensitivity to the analyte;   
           if the sensor does not reach a saturated response level at any of the plurality of detection positions on the sensor:
           a response of the sensor to the analyte at a detection position on the sensor having the greatest sensitivity to the analyte; or   a response of the sensor to the analyte at one or more predetermined temporal points at a detection position on the sensor having the greatest sensitivity to the analyte;   or   
           if the sensor responds to the analyte to reach a saturated response level at some, but not all, of the plurality of detection positions on the sensor:
           one or more of a ratio of the number of detection positions on the sensor that reach a saturated response level to the number of detection positions on the sensor that do not reach a saturated response level, a time required to reach a saturated response level for at least one of the detection positions on the sensor that reaches a saturated response level, and a response of the sensor to the analyte for at least one of the detection positions on the sensor that does not reach a saturated response level.   
               

     The differential response may be temporally differential, whereby the response varies over time. In this instance the calculating may utilise one or both of: a time required to reach a predetermined response level for at least one position on the sensor after exposure of the fluid to the sensor; and a response level for at least one position on the sensor at a predetermined time after exposure of the fluid to the sensor. 
     The method of the second embodiment may use the system of the first embodiment. The system of the first embodiment may be used in the method of the second embodiment. 
     According to a third embodiment there is provided a sensor for sensing a concentration of an analyte in a fluid, said sensor configured to generate a differential response to a constant concentration of the analyte. 
     The following options may be used in conjunction with the third embodiment, either individually or in any suitable combination. 
     The differential response may be temporally differential, whereby the response varies over time. 
     The differential response may be spatially differential. Thus, the sensor may be configured to respond to the analyte at a plurality of detection positions on the sensor; said detection positions having different sensitivities to the analyte. The plurality of detection positions may be spaced equidistantly along the length of the sensor. Sensitivity to the analyte may vary along a length of the sensor. It may vary monotonically along a length of the sensor. It may vary linearly along a length of the sensor. Alternatively, it may vary non-linearly, e.g. logarithmically, along a length of the sensor. It may vary linearly of logarithmically along a length of the sensor. The sensor may comprise a plurality of discrete regions each having a different sensitivity to the analyte. It may comprise a coating on a detection surface, said coating having spatially differential permeability to the analyte. It may comprise at least three detection positions having different sensitivities to the analyte. 
     The analyte may be a biomarker for diet, a disease, or a disorder. It may be a metal, protein, antibody, or small organic molecule. It may be acetone, and the sensor may comprise a compound capable of reacting with acetone. The compound may be selected from the group consisting of hydroxylamines, hydroxylamine salts, amines, alcohols, and combinations thereof. 
     In a specific embodiment there is provided a sensor configured to generate a spatially differential response to a constant concentration of acetone, whereby the sensor is configured to respond to acetone at a plurality of detection positions on the sensor, said detection positions having different sensitivities to the analyte; wherein the sensor comprises a compound capable of reacting with acetone, said compound being selected from the group consisting of hydroxylamines, hydroxylamine salts, and combinations thereof. 
     The sensor of the third embodiment may be used in the system of the first embodiment. The system of the first embodiment may incorporate the sensor of the third embodiment. 
     The sensor of the third embodiment may be used in the method of the second embodiment. The method of the second embodiment may use the sensor of the third embodiment. 
     According to a fourth embodiment there is provided a device for determining a concentration of a carbonyl containing compound in a fluid, said device comprising:
         a sorbent material having an optionally substituted hydroxylamine salt and an indicator sorbed therein and/or thereon, whereby exposure of the fluid to the sorbent material produces a change in absorption spectrum of the indicator;   a detector for measuring the change in the absorption spectrum of the indicator; and   a processor configured to calculate the concentration of the carbonyl containing compound in the fluid from the change in the absorption spectrum of the indicator.       

     The following options may be used in conjunction with the fourth embodiment, either individually or in any suitable combination. 
     The device may further comprise a plurality of electrodes configured to apply an electric field across a surface of the sorbent material so as to concentrate one or more charged species on and/or in the sorbent material. It may further comprise a detection region which is in fluid connection with the sorbent material and is optically coupled to the detector. In some instances, the device may further comprise a neutralisation chamber, said neutralisation chamber comprising a neutralisation reagent capable of neutralising a compound sorbed on and/or in the sorbent material. 
     The sorbent material may comprise a plurality of exposure regions. The plurality of exposure regions may have different sensitivities to the carbonyl containing compound. The sorbent material may have an exposure surface for exposure to the fluid, said exposure surface comprising a coating having spatially differential permeability to the carbonyl containing compound. 
     The detector may be a CCD, an active pixel sensor, or a hybrid thereof. 
     The fluid may be a gas and/or vapour. It may, for example, be breath. 
     The carbonyl containing compound may be selected from the group consisting of ketones, aldehydes, esters, thioesters, amides, and carboxylic acids. It may, for example, be selected from the group consisting of acetoacetate, butyraldehyde, 4-hydroxy-2-hexenal, ethyl acetate, acetic acid, 4-hydroxy-2-nonenal, 2-pentanone, 2-butanone, 3-hydroxy-2-butanone, hydroxyacetaldehyde, acetaldehyde, pentanal, malondialdehyde, acrolein, beta-hydroxybutyrate, and acetone. 
     The sorbent material may further comprise a weak base sorbed thereon and/or therein. The weak base may be selected from the group consisting of bicarbonate salts, ammonia, aniline, phenolate salts, and mixtures thereof. It may be, for example, a bicarbonate salt. 
     The optionally substituted hydroxylamine salt may be selected from the group consisting of hydroxylamine hydrochloride, hydroxylammonium sulfate, O-benzylhydroxylamine hydrochloride, hydroxylammonium phosphate, hydroxylamine nitrate, and mixtures thereof. 
     The indicator may be a halochromic indicator. The halochromic indicator may be selected from the group consisting of methyl orange, methyl red, methyl yellow chlorophenol red, bromocresol green, congo red, thymol blue, bromophenol blue, cresol red, metacresol purple, malachite green, ethyl violet, crystal violet, 2,4-dinitrophenol, orange IV, erythrosin B, p-(phenylazo)diphenylamine, p-phenylazoaniline and mixtures thereof. It may be, for example, selected from the group consisting of thymol blue, bromophenol blue and mixtures thereof. 
     The change in the absorption spectrum of the indicator may be a change of absorption maximum of the indicator. In this case, the change in the absorption maximum of the indicator may comprise a change in wavelength. In some instances, the change in the absorption maximum of the indicator may comprise a change in intensity. It may be both a change in wavelength and a change in intensity. 
     In a specific embodiment there is provided a device for determining a concentration of a carbonyl containing compound in breath, said device comprising:
         a sorbent material having hydroxylamine hydrochloride, sodium bicarbonate and a halochromic indicator sorbed therein and/or thereon, whereby exposure of the breath to the sorbent material produces a change in absorption spectrum e.g. in the absorption maximum of the indicator;   a detector for measuring the change in the absorption spectrum of the indicator; and   a processor configured to calculate the concentration of the carbonyl containing compound in the breath from the change in the absorption spectrum of the indicator;   wherein the carbonyl containing compound is acetone, and the halochromic indicator is selected from the group consisting of thymol blue, bromophenol blue, and mixtures thereof.       

     According to a fifth embodiment there is provided a method of determining a concentration of a carbonyl containing compound in a fluid, said method comprising:
         (a) exposing the fluid to an optionally substituted hydroxylamine salt and an indicator so as to produce a change in absorption spectrum of the indicator, wherein the optionally substituted hydroxylamine salt and indicator are sorbed in and/or on a sorbent material;   (b) measuring the change in the absorption spectrum of the indicator; and   (c) determining the concentration of the carbonyl containing compound in the fluid from the change in the absorption spectrum of the indicator.       

     The following options may be used in conjunction with the fifth embodiment, either individually or in any suitable combination. 
     The optionally substituted hydroxylamine salt and indicator may be sorbed in and/or on a sorbent material. The optionally substituted hydroxylamine salt may be selected from the group consisting of hydroxylamine hydrochloride, hydroxylammonium sulfate, O-benzylhydroxylamine hydrochloride, hydroxylammonium phosphate, hydroxylamine nitrate, and mixtures thereof. 
     The optionally substituted hydroxylamine salt and/or indicator may be in contact with a weak base. The weak base may be selected from the group consisting of bicarbonate salts, ammonia, aniline, phenolate salts, and mixtures thereof. The fluid may be a gas and/or vapour. It may be breath. The carbonyl containing compound may be selected from the group consisting of ketones, aldehydes, esters, thioesters, amides, and carboxylic acids. It may, for example, be selected from the group consisting of acetoacetate, butyraldehyde, 4-hydroxy-2-hexenal, ethyl acetate, acetic acid, 4-hydroxy-2-nonenal, 2-pentanone, 2-butanone, 3-hydroxy-2-butanone, hydroxyacetaldehyde, acetaldehyde, pentanal, malondialdehyde, acrolein, beta-hydroxybutyrate, and acetone. 
     The change in the absorption spectrum of the indicator may be a change of absorption maximum of the indicator. In this case, the change in the absorption maximum of the indicator may comprise a change in wavelength. It may comprise a change in intensity. The change in the absorption spectrum may be substantially independent of the volume of the fluid exposed to the optionally substituted hydroxylamine salt and halochromic indicator. It may be substantially independent of the flow rate of the fluid exposed to the optionally substituted hydroxylamine salt and halochromic indicator. 
     The indicator may be a halochromic indicator. The absorption spectrum of the halochromic indicator may vary monotonically over an operational pH range. The halochromic indicator may be selected from the group consisting of methyl orange, methyl red, methyl yellow chlorophenol red, bromocresol green, congo red, thymol blue, bromophenol blue, cresol red, metacresol purple, malachite green, ethyl violet, crystal violet, 2,4-dinitrophenol, orange IV, erythrosin B, p-(phenylazo)diphenylamine, p-phenylazoaniline and mixtures thereof. It may, for example, be selected from the group consisting of thymol blue, bromophenol blue, and mixtures thereof. 
     In a specific embodiment, the method is for managing diabetes in a patient on a diabetes treatment plan, wherein the carbonyl containing compound is acetone, the fluid is breath, and the method comprises determining an acetone concentration of a breath sample of the patient by the method described earlier; comparing the acetone concentration to a reference acetone concentration range; and if the acetone concentration is outside the acetone concentration reference range, adjusting the diabetes treatment plan. 
     In another specific embodiment, the method is for diagnosing diabetes in a patient, wherein the carbonyl containing compound is acetone, the fluid is breath, and the method comprises determining an acetone concentration of a breath sample of the patient by the method described earlier; and comparing the acetone concentration to a reference acetone concentration range to determine if the patient has diabetes. 
     The method of the fifth embodiment may use the device of the fourth embodiment. The device of the fourth embodiment may be used in the method of the fifth embodiment. 
     According to a sixth embodiment there is provided the use of a device for diagnosing and/or managing diabetes in a patient, wherein the device is capable of determining a concentration of acetone in a breath sample of the patient, and the device comprises:
         a sorbent material having an optionally substituted hydroxylamine salt and an indicator sorbed therein and/or thereon, whereby exposure of the breath sample to the sorbent material produces a change in absorption spectrum of the indicator;   a detector for measuring the change in the absorption spectrum of the indicator; and   a processor configured to calculate the concentration of the acetone in the breath sample from the change in the absorption spectrum of the indicator.       

     The use of the sixth embodiment may be a use of the device of the fourth embodiment. The device of the fourth embodiment may be used for the use of the sixth embodiment. The use of the sixth embodiment may use the method of the fifth embodiment. The method of the fifth embodiment may be used in the use of the sixth embodiment. 
     According to a seventh embodiment there is provided a method of managing diabetes in a patient on a diabetes treatment plan, said method comprising:
         (a) determining an acetone concentration of a breath sample of the patient using the method of the fifth embodiment;   (b) comparing the acetone concentration to a reference acetone concentration range; and   (c) if the acetone concentration is outside the acetone concentration reference range, adjusting the diabetes treatment plan.       

     The method of the seventh embodiment may use the device of the fourth embodiment. The device of the fourth embodiment may be used for the method of the seventh embodiment. 
     According to an eighth embodiment there is provided a method of diagnosing diabetes in a patient, said method comprising:
         (a) determining an acetone concentration of a breath sample of the patient using the method of the fifth embodiment; and   (b) comparing the acetone concentration to a reference acetone concentration range to determine if the patient has diabetes.       

     The method of the eighth embodiment may use the device of the fourth embodiment. The device of the fourth embodiment may be used for the method of the eighth embodiment. 
     According to a ninth embodiment there is provided a device for determining a concentration of a carbonyl-containing compound in a fluid, said device comprising:
         an optionally O-substituted hydroxylammonium salt or an optionally N-substituted hydrazinium salt, whereby exposure of the fluid to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt in the presence of a liquid produces a change in a variable, said variable being dependent upon the pH of the liquid;   a detector for measuring the change in the variable; and   a processor configured to calculate the concentration of the carbonyl-containing compound in the fluid from the change in the variable.       

     The following options may be used in conjunction with the ninth embodiment, either individually or in any suitable combination. 
     The variable may be an absorbance, fluorescence, resistance, ion concentration, temperature, voltage, or current. In the case where the variable is an ion concentration, the ion concentration may be a hydronium ion concentration. 
     In particular embodiments, the change in the variable is not measured using a titration. 
     The liquid volume may be about 1 mL or less. 
     In some instances, the device may further comprise a sorbent material, whereby the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt is sorbed in and/or on the sorbent material. 
     The detector may be an ion sensitive field effect transistor (ISFET), chemiresistive sensor, potentiometric sensor, spectroscopic sensor, colorimetric sensor, fluorometric sensor, thermal sensor, or a combination thereof. 
     The device may further comprise a flow controller configured to control and/or monitor the volume of the fluid which is exposed to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt. 
     The optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt may be selected from compounds of Formula (I): 
       [R m —Y—NH 3   + ] n X p   Formula (I)
 
     wherein Y=O or N;
 
m=1, 2, or 3;
 
n=1, 2, or 3;
 
p=1, 2, or 3;
 
R is independently selected from the group consisting of hydrogen, halogen, optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted heteroalkyl, optionally substituted cycloheteroalkyl, optionally substituted alkenyl, optionally substituted cycloalkenyl, optionally substituted alkylaryl, optionally substituted aryl, optionally substituted alkylheteroaryl, and optionally substituted heteroaryl; wherein said optionally substituted alkyl, cycloalkyl, heteroalkyl, cycloheteroalkyl, alkenyl, cycloalkenyl, alkylaryl, aryl, alkylheteroaryl, or heteroaryl is optionally substituted with one or more groups independently selected from the group consisting of halogen, hydroxyl, thiol, amino, aryl, heteroaryl, alkyl, alkenyl, alkynyl, heteroalkyl, alkylaryl, and alkylheteroaryl; and
 
X is an anion.
 
     The optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt may be selected from the group consisting of hydroxylammonium chloride, hydroxylammonium sulfate, hydroxylammonium azide, O-phenyl hydroxylammonium chloride, O-phenyl hydroxylammonium sulfate, O-phenyl hydroxylammonium azide, hydroxylamine picrate, hydroxylamine benzenesulfonate, hydroxylamine benzenesulfinate, hydroxylamine 1-sulfobutyl-3-methyl imidazole hydrosulfate salt, hydrazinium chloride, hydrazinium sulfate, hydrazinium hydrogensulfate, hydrazinium hydrogen sulfate, hydrazinium azide, hydrazinediium sulfate, hydrazinediium bis(6-carboxypyridazine-3-carboxylate), hydroxylammonium phosphate, hydroxylamine nitrate, and mixtures thereof. 
     The fluid may be a gas and/or vapour. It may, for example, be breath. 
     The carbonyl-containing compound may be selected from the group consisting of ketones, aldehydes, esters, thioesters, amides, and carboxylic acids. It may, for example, be selected from the group consisting of acetoacetate, butyraldehyde, 4-hydroxy-2-hexenal, ethyl acetate, acetic acid, 4-hydroxy-2-nonenal, 2-pentanone, 2-butanone, 3-hydroxy-2-butanone, hydroxyacetaldehyde, acetaldehyde, pentanal, malondialdehyde, acrolein, beta-hydroxybutyrate, and acetone. 
     In a specific embodiment there is provided a device for determining a concentration of acetone in breath, said device comprising:
         a sorbent material having hydroxylamine hydrochloride sorbed in and/or on the sorbent material, whereby exposure of the breath to the hydroxylamine hydrochloride in the presence of a liquid produces a change in hydronium ion concentration of the liquid;   an ion-sensitive field effect transistor for measuring the change in the hydronium ion concentration of the liquid; and   a processor configured to calculate the concentration of the acetone in the breath from the change in the hydronium ion concentration of the liquid.       

     In a further specific embodiment there is provided a device for determining a concentration of acetone in breath, said device comprising:
         a sorbent material having hydroxylamine hydrochloride sorbed in and/or on the sorbent material, whereby exposure of the breath to the hydroxylamine hydrochloride in the presence of a liquid produces a change in hydronium ion concentration of the liquid;   an ion-sensitive field effect transistor for measuring the change in the hydronium ion concentration of the liquid; and   a processor configured to calculate the concentration of the acetone in the breath from the change in the hydronium ion concentration of the liquid,   wherein the liquid volume is about 1 mL or less, and wherein the change in hydronium concentration is not measured using a titration.       

     According to a tenth embodiment there is provided a method for determining a concentration of a carbonyl-containing compound in a fluid, said method comprising:
         (a) exposing the fluid to an optionally O-substituted hydroxylammonium salt or an optionally N-substituted hydrazinium salt in the presence of a liquid, so as to produce a change in a variable, said variable being dependent upon the pH of the liquid;   (b) measuring the change in the variable; and   (c) determining the concentration of the carbonyl-containing compound in the fluid from the change in the variable.       

     The following options may be used in conjunction with the tenth embodiment, either individually or in any suitable combination. 
     The variable may be an absorbance, fluorescence, resistance, ion concentration, temperature, voltage, or current. In this case, the ion concentration may be a hydronium ion concentration. 
     The liquid volume may be about 1 mL or less. 
     In some instances, the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt may be sorbed in and/or on a sorbent material. 
     The change in the variable may be measured using an ion sensitive field effect transistor (ISFET), chemiresistive sensor, potentiometric sensor, spectroscopic sensor, colorimetric sensor, fluorometric sensor, thermal sensor, or a combination thereof. In particular embodiments, the change in the variable is not measured using a titration. 
     The method may further comprise controlling and/or monitoring the volume of the fluid which is exposed to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt. 
     The optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt may be selected from compounds of Formula (I): 
       [R m —Y—NH 3   + ] n X p   Formula (I)
 
     wherein Y=O or N;
 
m=1, 2, or 3;
 
n=1, 2, or 3;
 
p=1, 2, or 3;
 
R is independently selected from the group consisting of hydrogen, halogen, optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted heteroalkyl, optionally substituted cycloheteroalkyl, optionally substituted alkenyl, optionally substituted cycloalkenyl, optionally substituted alkylaryl, optionally substituted aryl, optionally substituted alkylheteroaryl, and optionally substituted heteroaryl; wherein said optionally substituted alkyl, cycloalkyl, heteroalkyl, cycloheteroalkyl, alkenyl, cycloalkenyl, alkylaryl, aryl, alkylheteroaryl, or heteroaryl is optionally substituted with one or more groups independently selected from the group consisting of halogen, hydroxyl, thiol, amino, aryl, heteroaryl, alkyl, alkenyl, alkynyl, heteroalkyl, alkylaryl, and alkylheteroaryl; and
 
X is an anion.
 
     The optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt may be selected from the group consisting of hydroxylammonium chloride, hydroxylammonium sulfate, hydroxylammonium azide, O-phenyl hydroxylammonium chloride, O-phenyl hydroxylammonium sulfate, O-phenyl hydroxylammonium azide, hydroxylamine picrate, hydroxylamine benzenesulfonate, hydroxylamine benzenesulfinate, hydroxylamine 1-sulfobutyl-3-methyl imidazole hydrosulfate salt, hydrazinium chloride, hydrazinium sulfate, hydrazinium hydrogensulfate, hydrazinium hydrogen sulfate, hydrazinium azide, hydrazinediium sulfate, hydrazinediium bis(6-carboxypyridazine-3-carboxylate), hydroxylammonium phosphate, hydroxylamine nitrate, and mixtures thereof. 
     The fluid may be a gas and/or vapour. It may, for example, be breath. 
     The carbonyl-containing compound may be selected from the group consisting of ketones, aldehydes, esters, thioesters, amides, and carboxylic acids. It may, for example, be selected from the group consisting of acetoacetate, butyraldehyde, 4-hydroxy-2-hexenal, ethyl acetate, acetic acid, 4-hydroxy-2-nonenal, 2-pentanone, 2-butanone, 3-hydroxy-2-butanone, hydroxyacetaldehyde, acetaldehyde, pentanal, malondialdehyde, acrolein, beta-hydroxybutyrate, and acetone. 
     In a specific embodiment, the method is for managing diabetes in a patient on a diabetes treatment plan, wherein the carbonyl-containing compound is acetone, the fluid is breath, and the method comprises determining an acetone concentration of a breath sample of the patient by the method described earlier; comparing the acetone concentration to a reference acetone concentration range; and if the acetone concentration is outside the acetone concentration reference range, adjusting the diabetes treatment plan. 
     In another specific embodiment, the method is to assist in diagnosing diabetes in a patient, wherein the carbonyl-containing compound is acetone, the fluid is breath, and the method comprises determining an acetone concentration of a breath sample of the patient by the method described earlier; and comparing the acetone concentration to a reference acetone concentration range to determine if the patient has diabetes. 
     The method of the tenth embodiment may use the device of the ninth embodiment. The device of the ninth embodiment may be used in the method of the tenth embodiment. 
     According to an eleventh embodiment there is provided the use of a device for diagnosing and/or managing diabetes in a patient, wherein the device is capable of determining a concentration of acetone in a breath sample of the patient, and the device comprises:
         an optionally O-substituted hydroxylammonium salt or an optionally N-substituted hydrazinium salt, whereby exposure of the breath to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt in the presence of a liquid produces a change in a variable, said variable being dependent upon the pH of the liquid;   a detector for measuring the change in the variable; and   a processor configured to calculate the concentration of the acetone in the breath from the change in the variable.       

     The use of the eleventh embodiment may be a use of the device of the ninth embodiment. The device of the ninth embodiment may be used for the use of the eleventh embodiment. The use of the eleventh embodiment may use the method of the tenth embodiment. The method of the tenth embodiment may be used in the use of the eleventh embodiment. 
     According to a twelfth embodiment there is provided a method of managing diabetes in a patient on a diabetes treatment plan, said method comprising:
         (a) determining an acetone concentration of a breath sample of the patient using the method according to the tenth embodiment;   (b) comparing the acetone concentration to a reference acetone concentration range; and   (c) if the acetone concentration is outside the acetone concentration reference range, adjusting the diabetes treatment plan.       

     The method of the twelfth embodiment may use the device of the ninth embodiment. The device of the ninth embodiment may be used for the method of the twelfth embodiment. 
     According to a thirteenth embodiment there is provided a method to assist in diagnosing diabetes in a patient, said method comprising:
         (a) determining an acetone concentration of a breath sample of the patient using the method according to the tenth embodiment; and   (b) comparing the acetone concentration to a reference acetone concentration range to determine if the patient has diabetes.       

     The method of the thirteenth embodiment may use the device of the ninth embodiment. The device of the ninth embodiment may be used for the method of the thirteenth embodiment. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1 : Schematic diagrams of example sensors according to specific embodiments of the present invention. One sensor is depicted as having a continuous spatial arrangement of sensitivities to an analyte, and another is depicted as having a discrete spatial arrangement of sensitivities to an analyte. 
         FIG. 2 : Depicts an example sensor according to a specific embodiment of the present invention having an internal treatment that affects the local concentration of an analyte at different sensor positions. 
         FIG. 3 : Depicts an example sensor according to a specific embodiment of the present invention having an internal treatment that affects the local exposure of an analyte at different sensor positions. 
         FIG. 4 : Example sensor responses as a function of time at a position of the sensor after exposure of the sensor to the analyte.  FIG. 4( a )  depicts a reversible non-saturated response,  FIG. 4( b )  depicts a reversible saturated response,  FIGS. 4( c ) and 4( d )  depict irreversible non-saturated responses, and  FIG. 4( e )  depicts an irreversible saturated response. 
         FIG. 5( a ) : An example sensor according to a specific embodiment of the present invention having increased sensitivity to an analyte from left to right along its length, displaying a sensor response divided into segment A, where the sensor response does not reach saturation; and segment B, where the sensor response reaches saturation. 
         FIG. 5( b ) : A graph showing sensor response (signal) as a function of sensor sensitivity and distance across the sensor depicted in  FIG. 5( a ) . 
         FIG. 5( c ) : A graph showing the time for the sensor response to reach saturation (100%) as a function of sensor sensitivity and distance across the sensor depicted in  FIG. 5( a ) . 
         FIG. 6 : Graphs showing sensor response (signal) as a function of sensor sensitivity and distance across the sensor depicted in  FIG. 5( a ) , and the time for the sensor response to reach saturation as a function of sensor sensitivity and distance across the sensor depicted in  FIG. 5( a ) . These graphs are for each of three cases of sensor response: where the sensor response does not reach saturation at any position (Case  1 ); where the sensor response reaches saturation at some, but not all positions (Case  2 ); and where the sensor response reaches saturation at all positions (Case  3 ). 
         FIG. 7 : A schematic depiction of a device according to a specific embodiment of the present invention. The device is a component of a system which analyses and stores data from the device. The device has a sorbent material strip on/in which an optionally O-substituted hydroxylammonium salt or an optionally N-substituted hydrazinium salt is sorbed. 
         FIG. 8 : A schematic depiction of a device according to a specific embodiment of the present invention. The device is a component of a system which analyses and stores data from the device. The device has a sorbent material strip on/in which an optionally O-substituted hydroxylammonium salt or an optionally N-substituted hydrazinium salt is sorbed, and also comprises an ISFET detector. 
         FIG. 9 : A schematic depiction of a device according to a specific embodiment of the present invention. The device is a component of a system which analyses and stores data from the device. The device has a sorbent material strip and a neutralisation chamber comprising a neutralisation agent which is capable of neutralising a compound sorbed on and/or in the sorbent material. 
         FIG. 10( a ) : A diagram of a sorbent material component of a device according to a specific embodiment of the present invention. 
         FIG. 10( b ) : A diagram of a sorbent material component of a device according to a specific embodiment of the present invention. The sorbent material has a plurality of exposure regions. 
         FIG. 11 : Depicts a sorbent material according to a specific embodiment of the present invention, the sorbent material having an internal treatment that affects the local concentration of a carbonyl containing compound at different sorbent material positions. 
         FIG. 12 : Depicts a sorbent material according to a specific embodiment of the present invention, the sorbent material having an internal treatment that affects the local exposure of a carbonyl containing compound at different sorbent material positions. 
         FIG. 13( a ) : Depicts example sensor responses as a function of length across the sensor for a continuous spatially differential sensor such as depicted in  FIG. 5( a ) , at a high, medium and low analyte concentration. 
         FIG. 13( b ) : Depicts example sensor responses as a function of exposure time for a sensor such as depicted in  FIG. 3 , at a high, medium and low analyte concentration. 
         FIG. 14 : Depicts example sensor responses as a function of time exhibiting delays in their initial response. 
         FIG. 15( a ) : A schematic depiction of a device according to a specific embodiment of the present invention having an ISFET detector system. The ISFET detector system comprises an ISFET and a traditional reference electrode. 
         FIG. 15( b ) : A schematic depiction of a device according to a specific embodiment of the present invention having an ISFET detector system. The ISFET detector system comprises an ISFET and a reference field effect transistor (REFET). 
         FIG. 15( c ) : A schematic depiction of a device according to a specific embodiment of the present invention having an ISFET detector system. The ISFET detector system comprises an ISFET and a disposable pseudo reference electrode. 
         FIG. 16( a ) : A schematic depiction of a device according to a specific embodiment of the present invention having a chemiresistive detector system. The chemiresistive detector system utilises a direct resistivity measurement. 
         FIG. 16( b ) : A schematic depiction of a device according to a specific embodiment of the present invention having a chemiresistive detector system. The chemiresistive detector system utilises an indirect resistivity measurement. 
         FIG. 16( c ) : A schematic depiction of a device according to a specific embodiment of the present invention having a chemiresistive detector system. The chemiresistive detector comprises a Capacitively-Coupled Contactless Conductivity Detector (C 4 D). 
         FIG. 17 : A device according to a specific embodiment of the present invention comprising a smartphone. 
         FIG. 18 : Example sensors according to a specific embodiment of the present invention, each with two discrete triangular shaped regions with higher (top right triangle) and lower (bottom left triangle) sensitivity to acetone, shown after exposure to: (a) 10 ppm acetone, and (b) 50 ppm acetone. 
         FIG. 19 : The sensor response plotted as a function of time for a position of an example sensor after exposure to 50 ppm acetone. 
         FIG. 20 : An exploded view of a device according to a specific embodiment of the present invention. 
         FIG. 21 : An exploded view of a device according to a specific embodiment of the present invention. The sorbent material has a plurality of exposure regions. 
         FIG. 22 : Depicts sensor responses (i.e. change in absorption spectrum of an indicator) at two different wavelengths for different acetone concentrations in gas samples using a method according to a specific embodiment of the present invention. The hydroxylamine salt and halochromic indicator are in solution. 
         FIG. 23 : Depicts sensor responses (i.e. change in absorption spectrum of an indicator) at two different wavelengths for different acetone concentrations in gas samples using a method according to a specific embodiment of the present invention. The hydroxylamine salt and halochromic indicator are sorbed in and/or on a sorbent material test strip. 
         FIG. 24 : Depicts sensor responses (i.e. change in absorption spectrum of an indicator) for different acetone concentrations in gas samples of two different gas volumes (0.5 L or 0.75 L) using a method according to a specific embodiment of the present invention. 
         FIG. 25 : Depicts sensor responses (i.e. change in absorption spectrum of an indicator) at two different wavelengths for different acetone concentrations in gas samples using a method according to a specific embodiment of the present invention. The method comprises exposing the gas samples to sodium bicarbonate. 
         FIG. 26 : The change in absorption spectrum of an indicator (i.e. response) plotted as a function of time for a position of an example sensor after exposure to 50 ppm acetone. 
         FIG. 27 : Depicts sensor response (i.e. change in voltage) or different acetone concentrations in gas samples using a method according to a specific embodiment of the present invention. 
     
    
    
     DEFINITIONS 
     As used herein, the term “about”, is relative to the actual value stated, as will be appreciated by those of skill in the art, and allows for approximations, inaccuracies and limits of measurement under the relevant circumstances. Depending on context, it may allow a variation from the stated value of ±10%, ±5%, ±2%, ±1%, ±0.5%, ±0.2%, ±0.1%, ±0.05%, ±0.02%, or ±0.01%. 
     As used herein, the term “comprising” indicates the presence of the specified integer(s), but allows for the possibility of other integers, unspecified. This term does not imply any particular proportion of the specified integers. Variations of the word “comprising”, such as “comprise” and “comprises”, have correspondingly similar meanings. 
     As used herein, the term “small organic molecule” indicates an organic molecule having a molecular weight below about 900 Daltons. 
     As used herein, the term “accuracy” in reference to a sensor is a measure of how closely the analyte concentration quantified from the sensor response agrees with the “true” or expected analyte concentration. Accuracy may be expressed as an absolute error (absolute error=|obtained result-expected result|). It may be expressed as a percentage relative error (percentage relative error=[(|obtained result-expected result|)×100]/expected result). For example, the term “accuracy” in reference to a device, system, sensor or method for determining the concentration of a carbonyl containing compound may be a measure of how closely the carbonyl containing compound concentration quantified using the device, system, sensor, or method respectively agrees with the “true” or expected carbonyl containing compound concentration. Accuracy may be expressed as an absolute error (absolute error=|obtained result-expected result|). It may be expressed as a percentage relative error (percentage relative error=[(|obtained result-expected result|)×100]/expected result). 
     As used herein, the term “precision” in reference to the sensor is a measure of the reproducibility and repeatability of the sensor response at a given analyte concentration. This may be expressed as a percentage relative standard deviation. For example, the term “precision” in reference to a device, sensor, system or method for determining the concentration of a carbonyl-containing compound may be a measure of the reproducibility and repeatability of the change in the variable at a given carbonyl-containing compound concentration. This may be expressed as a percentage relative standard deviation. In certain embodiments the term “precision” in reference to a device, sensor, system, or method for determining the concentration of a carbonyl containing compound may be a measure of the reproducibility and repeatability of the change in the absorption spectrum of the indicator at a given carbonyl containing compound concentration. This may be expressed as a percentage relative standard deviation. 
     As used herein, the term “selectivity” in reference to the sensor for an analyte in a fluid, is a measure of the sensor&#39;s response to the analyte compared with the sensor&#39;s response to a non-analyte component of the fluid. For example, as used herein, the term “selectivity” in reference to a device, sensor, system, or method for determining the concentration of a carbonyl-containing compound, may be a measure of the change in the variable for a carbonyl-containing compound concentration in the fluid compared with another component of the fluid. In certain embodiments the term “selectivity” in reference to a device, system, sensor or method for determining the concentration of a carbonyl containing compound, may be a measure of the change in the absorption spectrum of the indicator for a carbonyl containing compound concentration in the fluid compared with another component of the fluid. 
     As used herein, the term “sensitivity” in reference to a position of the sensor is a measure of the sensor&#39;s response to a given concentration of an analyte at the position of the sensor. For example, a low sensitivity position of the sensor may produce a low response level or no response to a low concentration of an analyte, whereas a high sensitivity position of the sensor may produce a higher response level to the same analyte concentration. 
     As used herein, the term “dynamic range” in reference to the sensor is a measure of the concentration range of an analyte in which the sensor is able to sense or detect the analyte and produce a response that is distinguishable from any background response. For example, the term “dynamic range” in reference to a device, system, sensor or method for determining the concentration of a carbonyl-containing compound may be a measure of the concentration range of a carbonyl-containing compound in which the device, system, sensor or method is able to produce a change in the variable that is distinguishable from any background change in the variable. In certain embodiments the term “dynamic range” in reference to a device, system, sensor or method for determining the concentration of a carbonyl containing compound may be a measure of the concentration range of a carbonyl containing compound in which the device, system, sensor or method is able to produce a change in the absorption spectrum of the indicator that is distinguishable from any background change in the absorption spectrum of the indicator. 
     As used herein, the term “working range” in reference to the sensor is a measure of the concentration range of an analyte in which the analyte concentration can be quantified from the sensor response. For example, the term “working range” in reference to a device, system, sensor or method for determining the concentration of a carbonyl-containing compound may be a measure of the concentration range of a carbonyl-containing compound in which the carbonyl-containing compound concentration can be quantified from the change in the variable. For example, the term “working range” in reference to a device, system, sensor or method for determining the concentration of a carbonyl-containing compound may be a measure of the concentration range of a carbonyl-containing compound in which the carbonyl-containing compound concentration can be quantified from the change in the variable. In certain embodiments, the term “working range” in reference to the device, system, sensor, or method for determining the concentration of a carbonyl containing compound may be a measure of the concentration range of a carbonyl containing compound in which the carbonyl containing compound concentration can be quantified from the change in the absorption spectrum of the indicator. 
     As used herein, the term “differential response” in reference to the sensor means that the sensor can generate more than one response level when exposed to a constant concentration of the analyte. It may mean, for example, that the sensor can generate different responses at different times after exposure to a constant concentration of the analyte (i.e. temporally differential). It may mean that the sensor can generate different responses at different positions on the sensor after exposure to a constant concentration of the analyte (i.e. spatially differential). It may mean that the sensor can generate different responses at different positions on the sensor and at different times after exposure to a constant concentration of the analyte (i.e. both temporally and spatially differential). Thus, a differential response is a response which varies with a variation in one or more parameters other than analyte concentration, e.g. a position on the sensor, time from initial exposure to the analyte, or both of these. 
     As used herein, the term “detection position” or “detection positions” in reference to the sensor mean a position or positions of the sensor where the response may be measured. 
     As used herein, the term “configured” means to set up for operation, especially in a particular way. 
     As used herein, the term “exposure region” or “exposure regions” in reference to the device or system mean a region or regions of the sorbent material or sensor which is exposed to the fluid. 
     As used herein, the term “small organic molecule” indicates an organic molecule having a molecular weight below about 900 Daltons. 
     As used herein, the term “sorbent” means a material capable of adsorbing and/or absorbing a liquid, solid, and/or gaseous compound therein and/or thereon. 
     As used herein, the term “halochromic” means a material which changes colour when pH changes occur. 
     As used herein, the term “absorption maximum” means a maximum wavelength and/or intensity of electromagnetic absorption. 
     As used herein, the term “peak response level” means the highest response level at a position of the sensor reached upon exposure of the sensor to the analyte-containing fluid. For example, the term “peak response level” in relation to a device, system, sensor, or method for determining the concentration of a carbonyl-containing compound may mean the highest response level (i.e. greatest change in the variable) reached upon exposure of the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt to a concentration of the analyte (i.e. the carbonyl-containing compound) in a fluid. In certain embodiments, the term “peak response level” in relation to a device, system, sensor, or method for determining the concentration of a carbonyl-containing compound may mean the highest response level (i.e. greatest change in absorption spectrum of the indicator) reached at a position of the sorbent material upon exposure to a concentration of the analyte (i.e. the carbonyl containing compound) in a fluid. 
     As used herein, the term “steady state response level” means the response level upon exposure of a position of the sensor to a concentration of the analyte in a fluid, when the response has reached a steady state, or local equilibrium, i.e. no longer changes over time. For example, the term “steady state response level” in relation to a device, system, sensor, or method for determining the concentration of a carbonyl-containing compound may mean the response level (i.e. change in variable) upon exposure of the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt to a concentration of the analyte (i.e. the carbonyl-containing compound) in a fluid, when the response has reached a steady state, or local equilibrium, i.e. no longer changes over time. In certain embodiments, the term “steady state response level” in relation to a device, system, sensor, or method for determining the concentration of a carbonyl-containing compound may mean the response level (i.e. change in absorption spectrum of the indicator) reached at a position of the sorbent material upon exposure to a concentration of the analyte (i.e. the carbonyl containing compound) in a fluid, when the response has reached a steady state, or local equilibrium, i.e. no longer changes over time. 
     As used herein, the term “baseline response level” means the sensor response level at a position of the sensor prior to exposure of the sensor to a concentration of the analyte in a fluid. For example, the term “baseline response level” in relation to a device, system, sensor, or method for determining the concentration of a carbonyl-containing compound may mean the response level (i.e. change in variable) prior to exposure of the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt to a concentration of the analyte (i.e. the carbonyl-containing compound) in a fluid. In certain embodiments, the term “baseline response level” in relation to a device, system, sensor, or method for determining the concentration of a carbonyl-containing compound may mean the response level (i.e. change in absorption spectrum of the indicator) at a position of the sorbent material prior to exposure of the sorbent material to a concentration of the analyte (i.e. the carbonyl containing compound) in a fluid. 
     A “saturated response level” may mean that the sensor response level reaches about 100% of the maximum possible response level to the analyte at a position of the sensor, or it may mean, for example, that the sensor response level reaches from about 80% to about 100%, from about 85% to about 100%, from about 90% to about 100%, from about 95% to about 100%, from about 97% to about 100%, from about 98% to about 100%, from about 99% to about 100%, from about 99.5% to about 100%, from about 99.7% to about 100%, or from about 99.9% to about 100% of the maximum possible response level to the analyte at a position of the sensor. For example, the term “saturated response level” in relation to a device, system, sensor, or method for determining the concentration of a carbonyl-containing compound may mean that the response level (i.e. change in variable) reaches about 100%, from about 80% to about 100%, from about 85% to about 100%, from about 90% to about 100%, from about 95% to about 100%, from about 97% to about 100%, from about 98% to about 100%, from about 99% to about 100%, from about 99.5% to about 100%, from about 99.7% to about 100%, or from about 99.9% to about 100% of the maximum possible response level to the analyte (i.e. the carbonyl-containing compound) at a position of the sensor. In certain embodiments, the term “saturated response level” in relation to a device, system, sensor, or method for determining the concentration of a carbonyl-containing compound may mean that the response level (i.e. change in absorption spectrum of the indicator) reaches about 100%, from about 80% to about 100%, from about 85% to about 100%, from about 90% to about 100%, from about 95% to about 100%, from about 97% to about 100%, from about 98% to about 100%, from about 99% to about 100%, from about 99.5% to about 100%, from about 99.7% to about 100%, or from about 99.9% to about 100% of the maximum possible response level to the analyte (i.e. the carbonyl containing compound) at a position of the sorbent material. 
     As used herein, the term “non-saturated response level” means that the response level reaches less than the saturated response level to the analyte at a position of the sensor. For example, the term “non-saturated response level” in relation to a device, system, sensor, or method for determining the concentration of a carbonyl-containing compound may mean that the response level (i.e. change in variable) reaches less than the saturated response level to the analyte (i.e. the carbonyl-containing compound) at a position of the sorbent material. In certain embodiments, the term “non-saturated response level” in relation to a device, system, sensor, or method for determining the concentration of a carbonyl-containing compound may mean that the response level (i.e. change in absorption spectrum of the indicator) reaches less than the saturated response level to the analyte (i.e. the carbonyl containing compound) at a position of the sorbent material. 
     As used herein, the term “maximum possible response level” means the highest possible response level to the analyte at a position of the sensor. It is the response level to the analyte if a position of the sensor was exposed to pure undiluted analyte for sufficient time for the response to reach a steady state. For example, the term “maximum possible response level” in relation to a device, system, sensor, or method for determining the concentration of a carbonyl-containing compound may mean the highest possible response level (i.e. change in variable) to the analyte (i.e. the carbonyl-containing compound). It may be the response level to the analyte if the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt were exposed to pure undiluted analyte for sufficient time for the response to reach a steady state. In certain embodiments, the term “maximum possible response level” in relation to a device, system, sensor, or method for determining the concentration of a carbonyl-containing compound may mean the highest possible response level (i.e. change in absorption spectrum of the indicator) to the analyte (i.e. the carbonyl containing compound) at a position of the sorbent material. It may mean the response level to the analyte if the position of the sorbent material were exposed to pure undiluted analyte for sufficient time for the response to reach a steady state. 
     DESCRIPTION OF EMBODIMENTS 
     Disclosed herein is a system for determining a concentration of an analyte in a fluid. The system comprises a sensor configured to generate a differential response to a constant concentration of the analyte, and an acquisition device configured to determine the concentration of the analyte from the differential response. 
     In certain embodiments there is provided a device for determining a concentration of a carbonyl-containing compound in a fluid. The device comprises an optionally O-substituted hydroxylammonium salt or an optionally N-substituted hydrazinium salt, a detector and a processor. Exposure of the fluid to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt in the presence of a liquid produces a change in a variable. The variable is dependent upon the pH of the liquid. The detector is able to measure the change in the variable. The processor is configured to calculate the concentration of the carbonyl-containing compound in the fluid from the change in the variable. 
     In certain embodiments there is provided a device for determining a concentration of a carbonyl containing compound in a fluid. The device comprises a sorbent material, a detector and a processor. The sorbent material has an optionally substituted hydroxylamine salt and an indicator sorbed therein and/or thereon, such that exposure of the fluid to the sorbent material produces a change in absorption spectrum of the indicator. The detector is able to measure the change in the absorption spectrum of the indicator. The processor is configured to calculate the concentration of the carbonyl containing compound in the fluid from the change in the absorption spectrum of the indicator. 
     With respect to certain embodiments of the present invention the inventors have surprisingly discovered that a sorbent material having an optionally substituted hydroxylamine salt and an indicator sorbed therein and/or thereon in the presence of a fluid comprising a carbonyl containing molecule analyte produces a change in the absorption spectrum of the indicator that is substantially dependent upon the concentration of the analyte in the fluid, but is substantially independent of the fluid volume exposed to the sorbent material. This enables the concentration of the analyte to be determined without the need for strict control or accurate measurement of the fluid volume. 
     System 
     The system may comprise the sensor and an acquisition device configured to measure a response of the sensor to the analyte. The system may be portable. It may be non-portable. It may be a detector. It may be a handheld system, for example a handheld detector. The sensor may be removably disposed within the system. 
     The system may further comprise a flow channel or an analysis chamber. The sensor may be disposed within the flow channel or analysis chamber. It may be located so as to be exposed to fluid in the flow channel or analysis chamber. It may, for example, form part of an inner surface or wall of the flow channel or analysis chamber. The system may further comprise a flow controller configured to control and/or monitor a flow rate of the fluid into the analysis chamber or through the flow channel. It may comprise a volume controller configured to control and/or monitor the volume of the fluid passing into the analysis chamber or through the flow channel. 
     Sensor 
     The sensor may be for sensing a concentration of an analyte in a fluid. It may be a chemical sensor. It may be a biosensor. It may be a single-use sensor. It may be reusable. It may be suitable for sensing a concentration of an analyte in a fluid, for example, a liquid, vapour, gas or mixture thereof. The sensor may comprise a strip, or it may be a part of a strip. The skilled person will understand that the sensor may be any suitable shape or size. It may be flat, or essentially two-dimensional, or it may be three-dimensional in shape. The sensor may comprise a sorbent material. 
     The sensor is configured to generate a differential response to a constant concentration of the analyte. The differential response may be temporally differential, whereby the response varies over time. In other words, the sensor may generate a response to a constant concentration of the analyte that varies over time. The differential response may be spatially differential, whereby the sensor is configured to respond to the analyte at a plurality of detection positions on the sensor, the detection positions having different sensitivities to the analyte. The differential response may be a combination of temporally differential and spatially differential. In other words, the response may be different at different positions, for example due to a difference in sensitivity at different positions of the sensor, and the response at a position may vary over time, after exposure of the sensor to the analyte. 
     The sensor may respond to the analyte by producing a sensor response at a position on the sensor. This response may vary over time. It may vary in intensity or response level over time. For example, the response level at a position on the sensor may be at 0% of the maximum possible response level, or it may produce no response, or a baseline response level prior to exposure of the sensor to the analyte. Upon exposure of the sensor to the analyte the response level at a position of the sensor may increase over a period of time. The response level at a position of the sensor upon exposure to the analyte may increase to a peak response level, a steady state response level, or a saturated response level over a period of time. If the concentration of the analyte is below a threshold concentration for a position of the sensor, the response level at the position of the sensor may not increase above its baseline response level. The sensor response may be measured at a defined, or predetermined time after exposure, or commencement of exposure of the sensor to the analyte. 
     The sensor response may be a change in a variable. It may, for example, be a change in an absorbance, fluorescence, resistance, ion concentration, temperature, voltage, or current. The sensor response may be a change in colour, or a change in absorbance of electromagnetic radiation. The electromagnetic radiation may be, for example, infrared, far infrared, near infrared, visible, ultraviolet, or some other electromagnetic radiation. The sensor response may be a change in resistance. It may be a change in temperature. It may be a change in voltage. It may be a change in electric current. It may be production of electromagnetic radiation, for example x-ray, fluorescent, phosphorescent, or chemiluminescent radiation. 
     The sensor may comprise a compound which selectively interacts or reacts with the analyte. The compound may be adsorbed, absorbed, and/or bound in the sensor. It may be adsorbed, absorbed, or bound in and/or on an absorbent, adsorbent, and/or functionalised media in the sensor. The skilled person will understand that the choice of compound will depend upon the analyte to be sensed by the sensor. For example, in the case where the analyte is a carbonyl-containing compound, such as a ketone (e.g. acetone), the compound may be an optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt. It may be, for example, a hydroxylamine, hydroxylamine salt, amine, alcohol, or a combination thereof. Specific examples include hydroxylamine hydrochloride, hydroxylamine sulfate, and combinations thereof. The compound may react with or bind to the analyte. The interaction of the compound with the analyte may produce a change, for example it may produce a colour change, a change in resistance, or a change in temperature. In the case where the sensor has spatially differential sensitivity to the analyte, the spatially differential sensitivity may result from a spatially differential local concentration or local amount of the compound at different points of the sensor. In the case where the sensor has spatially differential sensitivity to the analyte, the sensitivity to the analyte may increase at a position of the sensor having increased local concentration or amount of the compound when compared to a position of the sensor having decreased local concentration or amount of the compound. 
     In the case where the sensor has spatially differential sensitivity to the analyte, the spatially differential sensitivity may result in a differential sensor response at different positions of the sensor when the sensor is exposed to a concentration of the analyte in a fluid. 
     The response of the sensor upon exposure to a concentration of the analyte in a fluid may vary from about 0% (no response) to a saturated response. It may vary from about 0% (no response) to a saturated response at different positions of the sensor. It may vary from about 0% (no response) to a saturated response at different times after exposure to the fluid. The sensor response to the analyte may be spatially dependent. In other words, the sensor response may be dependent upon the position on the sensor. The sensor response to the analyte may be time dependent. In other words, the sensor response may be dependent upon the time after exposure to the fluid. The sensor response to the analyte may be a combination of time dependent and spatially dependent. In some instances, the time dependence may vary at different positions on the sensor. 
     The sensitivity to the analyte may be uniform across a length of the sensor. It may be uniform across all detection positions on the sensor. In the case where the sensor has spatially differential sensitivity to the analyte, the sensitivity to the analyte at any particular position of the sensor may be from about 0% to about 100% of the sensitivity of a sensor position having the greatest sensitivity to the analyte, or it may be from about 0.1% to about 100%, about 0.5% to about 100%, about 1% to about 100%, about 5% to about 100%, about 10% to about 100%, about 20% to about 100%, about 30% to about 100%, about 40% to about 100%, about 50% to about 100%, about 60% to about 100%, about 70% to about 100%, about 80% to about 100%, about 90% to about 100%, or about 95% to about 100% of the sensitivity of a sensor position having the greatest sensitivity to the analyte. 
     The sensor may comprise one or more positions having different sensitivity to the analyte. Alternatively, the sensor may have uniform sensitivity to the analyte. In the case where the sensor has spatially differential sensitivity to the analyte, the sensor may comprise at least two positions having different sensitivities to the analyte, or it may comprise at least 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, 30, 40, or 50, or more than 50 positions having different sensitivities to the analyte. It may comprise from about 2 to about 200 positions having different sensitivities to the analyte, or it may comprise from about 2 to about 100, about 2 to about 50, about 2 to about 20, about 3 to about 10, about 5 to about 50, or about 10 to about 50 positions having different sensitivities to the analyte. 
     The skilled person will understand that the detection positions of the sensor may be in any arrangement. They may, for example, be spaced equidistantly along a length of the sensor. They may be non-equidistantly spaced along a length of the sensor. They may be arranged in a regular pattern on the sensor. They may be arranged in a non-regular pattern on the sensor. They may be arranged on a single face of the sensor, or on more than one face of the sensor. They may be arranged in a linear or radial arrangement on the sensor. They may be arranged in an array. They may be arranged through, or within the sensor. They may be in an orthogonal arrangement to each other. In the case where the sensor has spatially differential sensitivity to the analyte they may be arranged, for example, to have a large region comprising a larger amount of positions with suitable sensitivity for detecting concentrations of the analyte within a specified concentration range, and smaller regions comprising a smaller amount of positions with suitable sensitivity for detecting concentrations of the analyte outside of the specified concentration range. 
     In the case where the sensor has spatially differential sensitivity to the analyte, the sensitivity to the analyte may vary along a length of the sensor. It may vary monotonically along a length of the sensor. It may increase continuously along a length of the sensor. It may vary regularly along a length of the sensor. It may increase regularly along a length of the sensor. It may increase non-regularly along a length of the sensor. It may vary linearly or logarithmically along a length of the sensor. It may increase linearly or logarithmically along a length of the sensor. It may vary continuously along a length of the sensor. It may vary radially outward from a position of the sensor. It may increase radially outward from a position of the sensor. It may decrease radially outwards from a position of the sensor. It may vary in a pattern arrangement on the sensor. The difference in sensitivity across the sensor may be discreet or continuous as depicted in  FIG. 1 , or it may be arranged in a different configuration. The skilled person will understand that in the case where the sensor has spatially differential sensitivity to the analyte, the detection positions having different sensitivity to the analyte may be arranged in any manner. Their arrangement may depend, for example, on the kinetics of the interaction between the sensor and the analyte. It may depend, for example, on the kinetics of a reaction between the analyte and a compound in the sensor that reacts with the analyte to produce a sensor response at a position of the sensor. 
     In certain embodiments where the sensitivity to the analyte varies continuously along a length of the sensor, the sensor may comprise a sensitivity gradient (ds/dl), that is the rate of change of the sensitivity to the analyte (in response level per analyte concentration (in mol·L −1 ), for example ΔΩ·L·mol −1  in the case where the response is a change in resistance) as a function of the distance along the length of the sensor (in mm). The sensitivity gradient along the sensor may be adjusted to suit a variety of analyte concentration ranges. The sensitivity gradient may be greater than about 0, or it may be greater than about 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1, 1.5, 2, 2.5, 5, 7.5, or 10. The sensitivity gradient may be constant along a length of the sensor, or it may vary along a length of the sensor. 
     In certain embodiments the sensor may comprise a plurality of discrete regions each having a different sensitivity to the analyte. They may be adjacent to each other, or they may be separated by areas having no sensitivity to the analyte. The discrete regions may be equally sized, or they may have different sizes. The sensor may comprise at least two discrete regions each having different sensitivity to the analyte, or it may comprise at least 3, 4, 5, 6, 7, 8, 9, or 10, or more than 10 discrete regions each having different sensitivity to the analyte. It may comprise from about 2 to about 50 discrete regions each having different sensitivity to the analyte, or it may comprise from about 2 to about 20, about 2 to about 10, about 5 to about 20, or about 10 to about 30 discrete regions each having different sensitivity to the analyte. 
     In certain embodiments, the sensor may comprise a coating on a detection surface. The coating may have spatially differential permeability to the analyte so as to provide spatially differential sensitivity to the sensor. The coating may increase or decrease the concentration of the analyte concentration at the detecting surface when compared with the analyte concentration exposed to the sensor. The coating may have variable thickness, such that, for example, the sensor has higher sensitivity to the analyte at a position where the coating is thinner, and lower sensitivity to the analyte as a position where the coating is thicker. The coating may comprise apertures, e.g. pores, allowing a fraction of analyte to pass therethrough. It may comprise channels, for example microfluidic channels allowing a fraction of analyte to pass therethrough. The channels may, for example, have varying flow rates and/or permeability to the analyte. In the case where the sensor has spatially differential sensitivity to the analyte, the spatially differential sensitivity may result from the varying flow rate of the channels. For example, the sensor portion below the coating may have uniform sensitivity, but the varying flow rates of the channels of the coating may provide the sensor spatially differential sensitivity. In the case where the sensor has spatially differential sensitivity to the analyte, the spatially differential sensitivity may result from the spatially differential permeability of the coating. For example, the sensor portion below the coating may have uniform sensitivity, but the spatially differential permeability of the coating may provide the sensor spatially differential sensitivity. In the case where the sensor has spatially differential sensitivity to the analyte, the spatially differential sensitivity may result from a differential in time of exposure at a sensor portion below the coating because of the differential permeability of the coating. For example, the sensor portion below the coating may have uniform sensitivity, but the varying exposure time at a sensor portion below the coating as a result of the coating may provide the sensor spatially differential sensitivity. The sensitivity to the analyte may increase at a position of the sensor having increased permeability of the coating when compared to a position of the sensor having decreased permeability of the coating. The coating may comprise a blocking agent that interacts with the analyte. The blocking agent may react with, bind to, and/or adsorb the analyte. 
     The blocking agent may affect the dynamic range of the sensor by adjusting the local concentration of the analyte at a position of the sensor. The blocking agent may be used in a different concentration or amount at a plurality of positions of the sensor. In the case where the sensor has spatially differential sensitivity to the analyte, the spatially differential sensitivity may result from the different concentration or amount of the blocking agent at different positions of the sensor. For example, the sensor portion below the coating may have uniform sensitivity, but the differential concentration or amount of the blocking agent may provide the sensor spatially differential sensitivity as depicted in  FIG. 2 . The sensitivity to the analyte may increase at a position of the sensor having decreased local concentration or amount of the blocking agent when compared to a position of the sensor having increased local concentration or amount of the blocking agent. 
     The sensor may comprise channels, for example microfluidic channels allowing a fraction of analyte to pass therethrough. The channels may, for example, have varying flow rates and/or permeability to the analyte. The fluid may pass through a microfluidic channel prior to contacting a detection position. In the case where the sensor has spatially differential sensitivity to the analyte, the spatially differential sensitivity may result from the varying flow rate of the channels, providing a variable amount or volume of the fluid to the plurality of detection positions. For example, the detection positions of the sensor may have uniform sensitivity, but the varying flow rates of the channels of the coating may provide the sensor spatially differential sensitivity. In the case where the sensor has spatially differential sensitivity to the analyte, the spatially differential sensitivity may result from varying exposure time at the plurality of detection positions. For example, the sensor positions may have uniform sensitivity, but the varying exposure time at different detection positions may provide the sensor spatially differential sensitivity. This example is depicted in  FIG. 3 , where a concentration of the analyte is incident upon the sensor, where it is split into three regions which provide low (t 1 ), medium (t 2 ), and high (t 3 ) exposure times at different detection positions. 
     The sensor may be disposed within a flow channel, or an analysis chamber. It may be disposed on or in a test strip. It may be a component of a detector. It may a component of a sensor array. It may be a component of a system. It may be a component of a device. 
     The response of the sensor upon exposure to a concentration of the analyte in a fluid may vary from about 0% (no response) to a saturated response. The response of the sensor upon exposure to a concentration of the analyte in a fluid may vary from about 0% (no response) to a saturated response at different positions of the sensor. The response of the sensor upon exposure to a concentration of the analyte in a fluid may change as a function of time at a position of the sensor. The response at a position of the sensor may initially be 0% prior to exposure to the analyte. The response may increase at the position of the sensor upon exposure to the analyte until a peak, or steady state response level is reached, which may be from about 0% (no response) to a saturated response. After all positions of the sensor have reached their peak, or steady state response level, there may be between about 0% and about 100% of positions of the sensor at a saturated response level, and between about 0% and about 100% of positions of the sensor at a non-saturated response level. 
     The response of the sensor upon exposure to a concentration of the analyte in a fluid may be reversible, that is it may return to zero over time after cessation of exposure to the analyte. In this case the sensor may be reusable.  FIGS. 4( a ) and 4( b )  depict example reversible sensor responses as a function of time at a position of the sensor.  FIG. 4( a )  depicts a non-saturated response with a peak response level R p .  FIG. 4( b )  depicts a saturated response. 
     Alternatively, the sensor response may be irreversible, that is it may not return to zero over time after exposure to the analyte. In this case the sensor may be non-reusable.  FIGS. 4 ( c - e ) depict example irreversible sensor responses as a function of time at a position of the sensor.  FIGS. 4 ( c - d ) depict a non-saturated response with a peak response level R p  and a steady state response level R ss .  FIG. 4( e )  depicts a saturated response. In  FIG. 4( c )  the sensor response increases over time after exposure to the analyte, until it reaches a steady state response level. In this case the peak response level and steady state response levels are equivalent. In  FIG. 4( d )  the sensor response increases over time after exposure to the analyte, until it reaches a peak response level. The response level subsequently varies until it reaches a steady state response level as the sensor and analyte reach a local equilibrium. 
     The time for a position of the sensor to reach its steady state response level upon exposure to a concentration of the analyte in the fluid may be from about 0.1 seconds to about 120 minutes, or it may be from about 0.1 seconds to about 60 minutes, about 0.1 seconds to about 30 minutes, about 0.1 seconds to about 20 minutes, about 0.1 seconds to about 10 minutes, about 0.1 seconds to about 5 minutes, about 0.1 seconds to about 2 minutes, about 0.1 seconds to about 1 minute, about 0.1 seconds to about 30 seconds, about 1 second to about 10 minutes, about 30 seconds to about 60 minutes, about 1 minute to about 10 minutes, about 1 minute to about 60 minutes, about 5 minutes to about 60 minutes, about 10 minutes to about 60 minutes, about 5 minutes to about 30 minutes, or about 10 minutes to about 30 minutes. It may be less that about 60 minutes, or less than about 30, 20, 10, 5, 2, 1, or 0.5 minutes. 
     The time for a position of the sensor to reach its peak response level upon exposure to a concentration of the analyte in the fluid may be from about 0.01 seconds to about 120 minutes, or it may be from about 0.01 seconds to about 60 minutes, about 0.01 seconds to about 30 minutes, about 0.01 seconds to about 20 minutes, about 0.01 seconds to about 10 minutes, about 0.01 seconds to about 5 minutes, about 0.01 seconds to about 2 minutes, about 0.01 seconds to about 1 minute, about 0.01 seconds to about 30 seconds, about 1 second to about 10 minutes, about 30 seconds to about 60 minutes, about 1 minute to about 60 minutes, about 5 minutes to about 60 minutes, about 10 minutes to about 60 minutes, about 5 minutes to about 30 minutes, or about 10 minutes to about 30 minutes. It may be less that about 60 minutes, or less than about 30, 20, 10, 5, 2, 1, 0.5, or 0.1 minutes. 
     The sensor may be selective for the analyte. It may respond to the analyte but not substantially respond to any other component of the fluid at a position of the sensor. It may produce a higher response to the analyte when compared with its response to a non-analyte component of the fluid at the same concentration at a position of the sensor. It may produce a faster response to the analyte when compared with its response to a non-analyte component of the fluid at the same concentration at a position of the sensor. The ratio of the sensor response to the analyte, to the sensor response to any particular non-analyte component of the fluid at the same concentration at a position of the sensor may be from about 1.5 to about 1×10 6 , or it may be from about 2 to about 1×10 6 , about 5 to about 1×10 6 , about 10 to about 1×10 6 , about 50 to about 1×10 6 , about 100 to about 1×10 6 , or about 1×10 3  to about 1×10 6 . It may be greater than about 1.5, 2, 5, 10, 20, 50, 100, 200, 500, 1000, or 1×10 4 . 
     The sensor may be sensitive to the analyte. It may have a limit of detection (LOD) for the analyte of less than about 1000 ppm, or less than about 500 ppm, 200 ppm, 100 ppm, 50 ppm, 20 ppm, 10 ppm, 5 ppm, 2 ppm, 1 ppm, 500 ppb, 200 ppb, 100 ppb, 50 ppb, 10 ppb, 5 ppb, 2 ppb, or 1 ppb. It may have a LOD for the analyte of from about 0.1 ppb to about 1000 ppm, or from about 5 ppb to about 500 ppm, about 10 ppb to about 500 ppm, about 10 ppb to about 1000 ppm, about 100 ppb to about 500 ppm, about 500 ppb to about 500 ppm, or about 1 ppm to about 500 ppm. It may have a limit of quantitation (LOQ) for the analyte of less than about 1000 ppm, or less than about 500 ppm, 200 ppm, 100 ppm, 50 ppm, 20 ppm, 10 ppm, 5 ppm, 2 ppm, 1 ppm, 500 ppb, 200 ppb, 100 ppb, 50 ppb, 10 ppb, 5 ppb, 2 ppb, or 1 ppb. It may have a LOQ for the analyte of from about 0.1 ppb to about 1000 ppm, or from about 5 ppb to about 500 ppm, about 10 ppb to about 500 ppm, about 100 ppb to about 500 ppm, about 500 ppb to about 500 ppm, or about 1 ppm to about 500 ppm. 
     The dynamic range of the sensor may be from about 0.1 ppb to about 1000 g/L, or from about 5 ppb to about 1000 g/L, about 10 ppb to about 1000 g/L, about 10 ppb to about 1000 ppm, about 100 ppb to about 1000 g/L, about 500 ppb to about 1000 g/L, about 1 ppm to about 1000 g/L, about 10 ppm to about 1000 g/L, about 100 ppm to about 1000 g/L, or about 1000 ppm to about 1000 g/L. 
     The working range of the sensor may be from about 0.1 ppb to about 1000 g/L, or from about 5 ppb to about 1000 g/L, about 10 ppb to about 1000 g/L, about 100 ppb to about 1000 g/L, about 500 ppb to about 1000 g/L, about 1 ppm to about 1000 g/L, about 10 ppm to about 1000 g/L, about 100 ppm to about 1000 g/L, or about 1000 ppm to about 1000 g/L. 
     The accuracy of the sensor may be such that its percentage relative error for the analyte may be less than about 20%, or less than about 10%, 5%, 2%, 1%, 0.5%, 0.2%, 0.1%, 0.05%, 0.02%, or 0.01%. 
     The precision of the sensor may be such that its percentage relative standard deviation for an analyte concentration may be less than about 20%, or less than about 10%, 5%, 2%, 1%, 0.5%, 0.2%, 0.1%, 0.05%, 0.02%, or 0.01%. 
     The sensor may be a component of a system for determining a concentration of an analyte in a fluid. It may be a component of a device for determining a concentration of an analyte in a fluid. When the sensor is exposed to a concentration of the analyte in the fluid, the response of the sensor may vary from about 0% (no response) to a saturated response at different detection positions of the sensor. After all detection positions of the sensor have reached their peak, or steady state response level, there may be between about 0% and about 100% of the positions of the sensor at a saturated response level, and between about 0% and about 100% of the positions of the sensor at a non-saturated response level. In the example spatially differential response sensor shown in  FIG. 5( a ) , Segment A depicts positions of the sensor at a non-saturated response level, and Segment B depicts positions of the sensor at a saturated response level. 
       FIG. 5( b )  exhibits results that may be indicative of segments A and B as shown on the sensor after reaction of the sensing material with the analyte. For segment A and B, these are the gradient of response, and the time taken to reach 100% threshold respectively. As shown in  FIGS. 5( b ) and ( c ) , the response of the sensor may provide three key data points: the slope of segment A (m 1 ), the time response gradient of segment B (m 2 ), and the length of segment A (d) or B (L−d) where L is the length of the sensor. High concentrations of the analyte may result in a steeper slope of m 1 , a shorter length of d, and a shallower slope of m 2 . In contrast, low concentrations of the analyte may result in a shallower slope of m 1 , a longer length of d and a steeper slope of m 2 . Depending on the concentration of analyte, the sensor response may be composed entirely of segment A (Case  1 ), B (Case  3 ), or a combination of both (Case  2 ). The three different cases of sensor response for an example spatially differential response sensor are depicted in  FIG. 6 . 
     Device 
     The device may be the system as hereinbefore described. It may be a component of the system as hereinbefore described. The device may be a single-use device. It may be reusable. It may be portable. It may be non-portable. It may be a handheld device. It may be integrated within a portable health monitoring device. It may be suitable for determining a concentration of an analyte, e.g., a carbonyl-containing compound in a fluid, for example, a liquid, vapour, gas or mixture thereof. The device may comprise a strip. It may be a lab-on chip. The skilled person will understand that the device may be any suitable shape or size. It may comprise an inlet for passing the fluid through. In the case where the fluid is breath, the device may comprise a mouthpiece for breathing through. The device may comprise a sorbent material removably disposed therein. It may comprise a sensor removably disposed therein. The device may be connected to a larger system that utilises analytical methods such as machine learning and neural networking to provide both historical and predictive analysis of a user&#39;s carbonyl-containing compound levels. The device may be attachable, or connectable to a smartphone. It may comprise a smartphone. The device may be suitable for an unskilled operator to use. That is, it may be simple to use, such that a person without any formal training may be able to use the device by, for example, following a set of oral, written, and/or pictorial instructions. 
     The device may comprise a flow channel or an analysis chamber. The optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt may be disposed within the flow channel or analysis chamber. It may be located so as to be exposed to fluid in the flow channel or analysis chamber. It may, for example, form part of an inner surface or wall of the flow channel or analysis chamber. The device may further comprise a flow controller configured to control and/or monitor a flow rate of the fluid into the analysis chamber or through the flow channel. It may comprise a flow controller configured to control and/or monitor the volume of the fluid which is exposed to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt. It may comprise a volume controller configured to control and/or monitor the volume of the fluid passing into the analysis chamber or through the flow channel. In some instances, the device does not comprise a flow channel or analysis chamber. The device may comprise a flow monitor system, comprising a regulator valve, flow meter, timer, or any combination of the three. In this instance the regulator valve may be capable of limiting the minimum and/or maximum flow rate of the fluid when the device is in use. The timer may be used to monitor or control the length of time that the fluid is exposed to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt. The flow meter may be able to measure the flow rate of the fluid during use of the device. The flow meter system may be, for example, an ultrasonic flow meter, turbine flow meter, or reflective light flowmeter. Alternatively, an intermediate storage device such as an air bag may be utilised to contain a fixed volume of the fluid. In some instances, the device does not comprise a flow controller and/or a volume controller. 
     The change in the variable (i.e. the response) upon exposure of the fluid to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt may vary from about 0% (no response) to a saturated response. In the case where the device comprises a sorbent material, the change in the variable upon exposure of the fluid to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt may vary from about 0% (no response) to a saturated response at different positions of the sorbent material. The change in the variable upon exposure to a concentration of the analyte (i.e. the carbonyl-containing compound) in a fluid may change as a function of time at a position of the sorbent material. The response at a position of the sorbent material may initially be 0% prior to exposure to the analyte. The response may increase at the position of the sorbent material upon exposure to the analyte until a peak, or steady state response level is reached, which may be from about 0% (no response) to a saturated response. After all positions of the sorbent material have reached their peak, or steady state response level, there may be between about 0% and about 100% of positions of the sorbent material at a saturated response level, and between about 0% and about 100% of positions of the sorbent material at a non-saturated response level. 
     In the case where the device comprises a sorbent material, the time for a position of the sorbent material to reach its steady state response level upon exposure to a concentration of the carbonyl-containing compound in the fluid may be from about 0.1 seconds to about 120 minutes, or it may be from about 0.1 seconds to about 60 minutes, about 0.1 seconds to about 30 minutes, about 0.1 seconds to about 20 minutes, about 0.1 seconds to about 10 minutes, about 0.1 seconds to about 5 minutes, about 0.1 seconds to about 2 minutes, about 0.1 seconds to about 1 minute, about 0.1 seconds to about 30 seconds, about 1 second to about 10 minutes, about 30 seconds to about 60 minutes, about 1 minute to about 10 minutes, about 1 minute to about 60 minutes, about 5 minutes to about 60 minutes, about 10 minutes to about 60 minutes, about 5 minutes to about 30 minutes, or about 10 minutes to about 30 minutes. It may be less that about 60 minutes, or less than about 30, 20, 10, 5, 2, 1, or 0.5 minutes. 
     In the case where the device comprises a sorbent material, the time for a position of the sorbent material to reach its peak response level upon exposure to a concentration of the carbonyl-containing compound in the fluid may be from about 0.01 seconds to about 120 minutes, or it may be from about 0.01 seconds to about 60 minutes, about 0.01 seconds to about 30 minutes, about 0.01 seconds to about 20 minutes, about 0.01 seconds to about 15 minutes, about 0.01 seconds to about 10 minutes, about 0.01 seconds to about 5 minutes, about 0.01 seconds to about 2 minutes, about 0.01 seconds to about 1 minute, about 0.01 seconds to about 30 seconds, about 1 second to about 10 minutes, about 1 second to about 15 minutes, about 30 seconds to about 15 minutes, about 1 minute to about 15 minutes, about 5 minutes to about 15 minutes, about 10 minutes to about 15 minutes, about 5 minutes to about 30 minutes, or about 10 minutes to about 30 minutes. It may be less that about 60 minutes, or less than about 30, 20, 15, 10, 5, 2, 1, 0.5, or 0.1 minutes. 
     The device may be selective for the carbonyl-containing compound. That is, it may respond to the carbonyl-containing compound (i.e. produce a change in the variable), but not substantially respond (i.e. may not produce a substantial change in the variable) to any other component of the fluid. It may respond to the carbonyl-containing compound (i.e. may produce a change in the variable), but not substantially respond (i.e. may not produce a substantial change in the variable) to any other non-carbonyl component of the fluid. It may produce a higher response to the carbonyl-containing compound when compared with its response to any other component of the fluid at the same concentration. It may produce a higher response to the carbonyl-containing compound when compared with its response to any other non-carbonyl component of the fluid at the same concentration. It may produce a faster response to the carbonyl-containing compound when compared with its response to any other component of the fluid. It may produce a faster response to the carbonyl-containing compound when compared with its response to any other non-carbonyl component of the fluid. The ratio of the device response to the carbonyl-containing compound, to the device response to any other component of the fluid at the same concentration, optionally to any other non-carbonyl component of the fluid at the same concentration, may be from about 1.5 to about 1×10 6 , or it may be from about 2 to about 1×10 6 , about 5 to about 1×10 6 , about 10 to about 1×10 6 , about 50 to about 1×10 6 , about 100 to about 1×10 6 , or about 1×10 3  to about 1×10 6 . It may be greater than about 1.5, 2, 5, 10, 20, 50, 100, 200, 500, 1000, or 1×10 4 . 
     The device may be sensitive to the carbonyl-containing compound. It may have a limit of detection (LOD) for the carbonyl-containing compound of less than about 1000 ppm (v/v), or less than about 500 ppm (v/v), 200 ppm (v/v), 100 ppm (v/v), 50 ppm (v/v), 20 ppm (v/v), 10 ppm (v/v), 5 ppm (v/v), 2 ppm (v/v), 1 ppm (v/v), 500 ppb (v/v), 200 ppb (v/v), 100 ppb (v/v), 50 ppb (v/v), 10 ppb (v/v), 5 ppb (v/v), 2 ppb (v/v), or 1 ppb (v/v). It may have a LOD for the carbonyl-containing compound of from about 0.1 ppb (v/v) to about 1000 ppm (v/v), or from about 5 ppb (v/v) to about 500 ppm (v/v), about 10 ppb (v/v) to about 500 ppm (v/v), about 10 ppb (v/v) to about 1000 ppm (v/v), about 100 ppb (v/v) to about 500 ppm (v/v), about 500 ppb (v/v) to about 500 ppm (v/v), or about 1 ppm (v/v) to about 500 ppm (v/v). It may have a limit of quantitation (LOQ) for the carbonyl-containing compound of less than about 1000 ppm (v/v), or less than about 500 ppm (v/v), 200 ppm (v/v), 100 ppm (v/v), 50 ppm (v/v), 20 ppm (v/v), 10 ppm (v/v), 5 ppm (v/v), 2 ppm (v/v), 1 ppm (v/v), 500 ppb (v/v), 200 ppb (v/v), 100 ppb (v/v), 50 ppb (v/v), 10 ppb (v/v), 5 ppb (v/v), 2 ppb (v/v), or 1 ppb (v/v). It may have a LOQ for the carbonyl-containing compound of from about 0.1 ppb (v/v) to about 1000 ppm (v/v), or from about 5 ppb (v/v) to about 500 ppm (v/v), about 10 ppb (v/v) to about 500 ppm (v/v), about 100 ppb (v/v) to about 500 ppm (v/v), about 500 ppb (v/v) to about 500 ppm (v/v), or about 1 ppm (v/v) to about 500 ppm (v/v). 
     The dynamic range of the device may be from about 0.1 ppb (v/v) to about 100,000 ppm (v/v), or from about 5 ppb (v/v) to about 10,000 ppm (v/v), about 500 ppb (v/v) to about 100 ppm (v/v), about 100 ppb (v/v) to about 10,000 ppm (v/v), about 10 ppb (v/v) to about 10,000 ppm (v/v), about 10 ppb (v/v) to about 1000 ppm (v/v), about 100 ppb (v/v) to about 10,000 ppm (v/v), about 500 ppb (v/v) to about 10,000 ppm, about 1 ppm (v/v) to about 10,000 ppm (v/v), about 10 ppm (v/v) to about 10,000 ppm (v/v), about 100 ppm (v/v) to about 10,000 ppm (v/v), or about 1000 ppm (v/v) to about 10,000 ppm (v/v). 
     The working range of the device may be from about 0.1 ppb (v/v) to about 100,000 ppm (v/v), or from about 5 ppb (v/v) to about 10,000 ppm (v/v), about 500 ppb (v/v) to about 100 ppm (v/v), about 100 ppb (v/v) to about 10,000 ppm (v/v), about 10 ppb (v/v) to about 10,000 ppm (v/v), about 100 ppb (v/v) to about 10,000 ppm (v/v), about 500 ppb (v/v) to about 10,000 ppm (v/v), about 1 ppm (v/v) to about 10,000 ppm (v/v), about 10 ppm (v/v) to about 10,000 ppm (v/v), about 100 ppm (v/v) to about 10,000 ppm (v/v), or about 1000 ppm (v/v) to about 10,000 ppm (v/v). 
     The accuracy of the device may be such that its percentage relative error for a concentration of the carbonyl-containing compound may be less than about 20%, or less than about 10%, 5%, 2%, 1%, 0.5%, 0.2%, 0.1%, 0.05%, 0.02%, or 0.01%. 
     The precision of the device may be such that its percentage relative standard deviation for a concentration of the carbonyl-containing compound may be less than about 20%, or less than about 10%, 5%, 2%, 1%, 0.5%, 0.2%, 0.1%, 0.05%, 0.02%, or 0.01%. 
     The device may further comprise a neutralisation chamber, said neutralisation chamber comprising a neutralisation reagent capable of neutralising a compound, such as the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt, or a product of the reaction between the carbonyl-containing compound and the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt. In this context, “neutralisation” indicates that the compound is converted to a less hazardous and/or more environmentally friendly product or products. The neutralisation reagent may comprise a biological and/or a chemical material. It may comprise a bacterium. It may, for example, comprise a bacterium selected from the  Nitrosomonas  genus. It may, for example, comprise  Nitrosomonas europaea . It may comprise an enzyme. It may comprise an oxidant. It may comprise, for example, one or more compound selected from the group consisting of hydrogen peroxide, chlorite salts, chlorate salts, perchlorate salts, hypochlorite salts, and mixtures thereof. The neutralisation reagent may be able to react with the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt. It may be able, for example, to oxidise the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt. This may serve to convert the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt into a less hazardous and/or more environmentally friendly product or products, such as ammonia, water and dilute nitric acid. 
     The device may be a component of a system. The system may be for personal carbonyl-containing compound (e.g. ketone body) measurement and monitoring over a period.  FIG. 7  displays an example system. In this system, the device  700  comprises a flow monitor  720 , optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt sorbed on a sorbent material  750 , a detector  760 , and a smart device (i.e. processor)  740 . The user breathes into the device, thereby supplying breath  710 , which passes through the flow monitor  720 . The sample flow rate and/or volume information for the sample are sent via path  730  from flow monitor  720  to smart device  740 . The breath sample is exposed to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt sorbed on the sorbent material  750 , producing a change in a variable which is measured by the detector  760 . The change in the variable measurement is sent from detector  760  to smart device  740  via path  780 . The smart device calculates the concentration of the carbonyl-containing compound from the sample flow rate and/or volume information and the change in the variable. 
     Further, in the example system depicted in  FIG. 7 , the sensor data can be transmitted to the smart device such as a phone via wired or wireless connections at paths  730  and  780 . This allows various methods of local or cloud hosted computation and analytics to be performed to provide an accurate measurement of breath ketone bodies. Over time the device software can be upgraded which will enable more reliable results, and auxiliary data, such as activity and location data, obtained from the user&#39;s smartphone or paired devices such as a smart watch can be recorded at the time of ketone analysis. Data from the total user population can then be aggregated within a cloud service  795 . This data may be for used for machine learning, enabling predictions in user ketone body breath concentration trends based on regularly reported auxiliary data. 
       FIG. 8  depicts another example system. In this system, the device  800  comprises disposable sorbent material  820  in a tape format. The tape is impregnated with the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt reagent in multiple discrete regions, and is coiled within a storage cartridge  830 . The device comprises a mouth piece for introducing a breath sample  805 , and a flow regulator  810  capable of measuring and/or regulating the flow of the breath sample. The liquid storage and delivery may be integrated into the device as shown at vessel  835  or may be supplied by external means (not shown). The device has a detector comprising an ISFET  825 . The ISFET  825  is capable of measuring a change in voltage when the carbonyl-containing compound reacts with the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt. The device also comprises a processor  815  configured to calculate the concentration of the carbonyl-containing compound in the fluid from the change in the voltage. The system comprises a connection component  850  which is capable of sending information from processor  815  to smartphone  860 . 
     Before testing, the user mechanically aligns a new section of the sorbent material  820  into the region above the ISFET  825  by moving a section of the sorbent material out of the storage cartridge  830 . Liquid is absorbed into the sorbent material through capillary channels connected to a reservoir  835 , that are positioned beneath the sorbent material  820  and adjacent to the ISFET  825 . Liquid absorption is limited by the saturation level of the sorbent material  820  and the liquid dissolves the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt reagent into a solution that is contained within the reaction 
     The dried alkoxyamine/hydrazine reagents are impregnated within the absorbent reaction media that is segmented into multiple discreet regions and aligned in an array along a flexible tape. The tape stores numerous discreet regions that are suitable for a single use and is coiled within a storage container. 
     The user breathes through a mouthpiece and into the device. Within the mouthpiece inlet there is a flow regulator  810 , comprising a flow meter and timer, that is used to control, measure and provide flow information on the user&#39;s breath to the processor  815 . The breath contacts the wet sorbent material and imine formation takes place, leading to the associated formation of H 3 O +  ions and a reduction in pH. 
     The ISFET detector  825  measures the resultant change in voltage at the sorbent material and transmits this information to the processor  815 . The processor  815  uses the ISFET data and flow information to calculate the concentration of the user&#39;s breath ketone level. The device can be connected to a smart phone  860  as shown, either through a wired or wireless connection  850 , where the user&#39;s breath ketone level is displayed. Data from the total user population can then be aggregated within a cloud service  870  via wireless connection  865 . 
     The used sorbent material  820  portion is then moved away from the ISFET and into waste cartridge  840 , moving a new portion of the sorbent material  820  into contact with the ISFET  825  ready for the next breath sample analysis. 
     The device may further comprise a sorbent material, whereby the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt may be sorbed in and/or on the sorbent material. 
     In certain embodiments the device may be a single-use device. It may be reusable. It may be portable. It may be non-portable. It may be a handheld device. It may be integrated within a portable health monitoring device. It may be suitable for determining a concentration of a carbonyl containing compound in a fluid, for example, a liquid, vapour, gas or mixture thereof. The device may comprise a strip. It may be a lab-on chip. The skilled person will understand that the device may be any suitable shape or size. It may comprise an inlet for passing the fluid through. In the case where the fluid is breath, the device may comprise a mouthpiece for breathing through. The sorbent material may be removably disposed within the device. This device may be connected to a larger system that utilises analytical methods such as machine learning and neural networking to provide both historical and predictive analysis of a user&#39;s carbonyl containing compound levels. The device may be attachable, or connectable to a smartphone. It may comprise a smartphone. 
     The device may comprise a flow channel or an analysis chamber. The sorbent material may be disposed within the flow channel or analysis chamber. It may be located so as to be exposed to fluid in the flow channel or analysis chamber. It may, for example, form part of an inner surface or wall of the flow channel or analysis chamber. The device may further comprise a flow controller configured to control and/or monitor a flow rate of the fluid into the analysis chamber or through the flow channel. It may comprise a volume controller configured to control and/or monitor the volume of the fluid passing into the analysis chamber or through the flow channel. In some instances, the device does not comprise a flow channel or analysis chamber. In some instances, the device does not comprise a flow controller and/or a volume controller. 
     The device may comprise a plurality of electrodes configured to apply an electric field across a surface of the sorbent material so as to concentrate one or more charged species on and/or in the sorbent material. The electrodes may be in pairs, each electrode pair capable of applying a potential difference of from about 0.01 volts to about 20 volts, or about 0.01 volts to about 10 volts, about 0.01 volts to about 5 volts, about 0.01 volts to about 2 volts, about 0.01 volts to about 1 volt, about 0.01 volts to about 0.5 volts, about 0.01 volts to about 0.2 volts, about 0.01 volts to about 0.1 volts, about 0.1 volts to about 20 volts, about 0.1 volts to about 10 volts, about 1 volt to about 20 volts, about 1 volt to about 15 volts, about 1 volt to about 10 volts, or about 1 volt to about 5 volts. Each electrode pair may be capable of applying a potential difference of about 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 15 or 20 volts. When a potential difference is applied between a pair of electrodes, a positively charged species may migrate to, and be concentrated at, a position proximate to a negatively charged electrode, whereas a negatively charged species may migrate to, and be concentrated at, a position proximate to a positively charged electrode. This concentrating effect may be utilised to improve the sensitivity of the device by, for example, concentrating charged indicator species so that their local concentration is increased and produces a greater change in absorption spectrum at a position of increased local concentration. 
     The change in absorption spectrum of the indicator (i.e. the response) upon exposure of the fluid to the sorbent material may vary from about 0% (no response) to a saturated response. The change in absorption spectrum of the indicator upon exposure of the fluid to the sorbent material may vary from about 0% (no response) to a saturated response at different positions of the sorbent material. The change in absorption spectrum of the indicator upon exposure to a concentration of the analyte (i.e. the carbonyl containing compound) in a fluid may change as a function of time at a position of the sorbent material. The response at a position of the sorbent material may initially be 0% prior to exposure to the analyte. The response may increase at the position of the sorbent material upon exposure to the analyte until a peak, or steady state response level is reached, which may be from about 0% (no response) to a saturated response. After all positions of the sorbent material have reached their peak, or steady state response level, there may be between about 0% and about 100% of positions of the sorbent material at a saturated response level, and between about 0% and about 100% of positions of the sorbent material at a non-saturated response level. 
     The time for a position of the sorbent material to reach its steady state response level upon exposure to a concentration of the carbonyl containing compound in the fluid may be from about 0.1 seconds to about 120 minutes, or it may be from about 0.1 seconds to about 60 minutes, about 0.1 seconds to about 30 minutes, about 0.1 seconds to about 20 minutes, about 0.1 seconds to about 10 minutes, about 0.1 seconds to about 5 minutes, about 0.1 seconds to about 2 minutes, about 0.1 seconds to about 1 minute, about 0.1 seconds to about 30 seconds, about 1 second to about 10 minutes, about 30 seconds to about 60 minutes, about 1 minute to about 10 minutes, about 1 minute to about 60 minutes, about 5 minutes to about 60 minutes, about 10 minutes to about 60 minutes, about 5 minutes to about 30 minutes, or about 10 minutes to about 30 minutes. It may be less that about 60 minutes, or less than about 30, 20, 10, 5, 2, 1, or 0.5 minutes. 
     The time for a position of the sorbent material to reach its peak response level upon exposure to a concentration of the carbonyl containing compound in the fluid may be from about 0.01 seconds to about 120 minutes, or it may be from about 0.01 seconds to about 60 minutes, about 0.01 seconds to about 30 minutes, about 0.01 seconds to about 20 minutes, about 0.01 seconds to about 15 minutes, about 0.01 seconds to about 10 minutes, about 0.01 seconds to about 5 minutes, about 0.01 seconds to about 2 minutes, about 0.01 seconds to about 1 minute, about 0.01 seconds to about 30 seconds, about 1 second to about 10 minutes, about 1 second to about 15 minutes, about 30 seconds to about 15 minutes, about 1 minute to about 15 minutes, about 5 minutes to about 15 minutes, about 10 minutes to about 15 minutes, about 5 minutes to about 30 minutes, or about 10 minutes to about 30 minutes. It may be less that about 60 minutes, or less than about 30, 20, 15, 10, 5, 2, 1, 0.5, or 0.1 minutes. 
     The device may be selective for the carbonyl containing compound. That is, it may respond to the carbonyl containing compound (i.e. produce a change in absorption spectrum of the indicator), but not substantially respond (i.e. may not produce a substantial change in absorption spectrum of the indicator) to any other component of the fluid. It may respond to the carbonyl containing compound (i.e. may produce a change in absorption spectrum of the indicator), but not substantially respond (i.e. may not produce a substantial change in absorption spectrum of the indicator) to any other non-carbonyl component of the fluid. It may produce a higher response to the carbonyl containing compound when compared with its response to any other component of the fluid at the same concentration. It may produce a higher response to the carbonyl containing compound when compared with its response to any other non-carbonyl component of the fluid at the same concentration. It may produce a faster response to the carbonyl containing compound when compared with its response to any other component of the fluid. It may produce a faster response to the carbonyl containing compound when compared with its response to any other non-carbonyl component of the fluid. The ratio of the device response to the carbonyl containing compound, to the device response to any other component of the fluid at the same concentration, optionally to any other non-carbonyl component of the fluid at the same concentration, may be from about 1.5 to about 1×10 6 , or it may be from about 2 to about 1×10 6 , about 5 to about 1×10 6 , about 10 to about 1×10 6 , about 50 to about 1×10 6 , about 100 to about 1×10 6 , or about 1×10 3  to about 1×10 6 . It may be greater than about 1.5, 2, 5, 10, 20, 50, 100, 200, 500, 1000, or 1×10 4 . 
     The device may be sensitive to the carbonyl containing compound. It may have a limit of detection (LOD) for the carbonyl containing compound of less than about 1000 ppm (v/v), or less than about 500 ppm (v/v), 200 ppm (v/v), 100 ppm (v/v), 50 ppm (v/v), 20 ppm (v/v), 10 ppm (v/v), 5 ppm (v/v), 2 ppm (v/v), 1 ppm (v/v), 500 ppb (v/v), 200 ppb (v/v), 100 ppb (v/v), 50 ppb (v/v), 10 ppb (v/v), 5 ppb (v/v), 2 ppb (v/v), or 1 ppb (v/v). It may have a LOD for the carbonyl containing compound of from about 0.1 ppb (v/v) to about 1000 ppm (v/v), or from about 5 ppb (v/v) to about 500 ppm (v/v), about 10 ppb (v/v) to about 500 ppm (v/v), about 10 ppb (v/v) to about 1000 ppm (v/v), about 100 ppb (v/v) to about 500 ppm (v/v), about 500 ppb (v/v) to about 500 ppm (v/v), or about 1 ppm (v/v) to about 500 ppm (v/v). It may have a limit of quantitation (LOQ) for the carbonyl containing compound of less than about 1000 ppm (v/v), or less than about 500 ppm (v/v), 200 ppm (v/v), 100 ppm (v/v), 50 ppm (v/v), 20 ppm (v/v), 10 ppm (v/v), 5 ppm (v/v), 2 ppm (v/v), 1 ppm (v/v), 500 ppb (v/v), 200 ppb (v/v), 100 ppb (v/v), 50 ppb (v/v), 10 ppb (v/v), 5 ppb (v/v), 2 ppb (v/v), or 1 ppb (v/v). It may have a LOQ for the carbonyl containing compound of from about 0.1 ppb (v/v) to about 1000 ppm (v/v), or from about 5 ppb (v/v) to about 500 ppm (v/v), about 10 ppb (v/v) to about 500 ppm (v/v), about 100 ppb (v/v) to about 500 ppm (v/v), about 500 ppb (v/v) to about 500 ppm (v/v), or about 1 ppm (v/v) to about 500 ppm (v/v). 
     The dynamic range of the device may be from about 0.1 ppb (v/v) to about 100,000 ppm (v/v), or from about 5 ppb (v/v) to about 10,000 ppm (v/v), about 500 ppb (v/v) to about 100 ppm (v/v), about 100 ppb (v/v) to about 10,000 ppm (v/v), about 10 ppb (v/v) to about 10,000 ppm (v/v), about 10 ppb (v/v) to about 1000 ppm (v/v), about 100 ppb (v/v) to about 10,000 ppm (v/v), about 500 ppb (v/v) to about 10,000 ppm, about 1 ppm (v/v) to about 10,000 ppm (v/v), about 10 ppm (v/v) to about 10,000 ppm (v/v), about 100 ppm (v/v) to about 10,000 ppm (v/v), or about 1000 ppm (v/v) to about 10,000 ppm (v/v). 
     The working range of the device may be from about 0.1 ppb (v/v) to about 100,000 ppm (v/v), or from about 5 ppb (v/v) to about 10,000 ppm (v/v), about 500 ppb (v/v) to about 100 ppm (v/v), about 100 ppb (v/v) to about 10,000 ppm (v/v), about 10 ppb (v/v) to about 10,000 ppm (v/v), about 100 ppb (v/v) to about 10,000 ppm (v/v), about 500 ppb (v/v) to about 10,000 ppm (v/v), about 1 ppm (v/v) to about 10,000 ppm (v/v), about 10 ppm (v/v) to about 10,000 ppm (v/v), about 100 ppm (v/v) to about 10,000 ppm (v/v), or about 1000 ppm (v/v) to about 10,000 ppm (v/v). 
     The accuracy of the device may be such that its percentage relative error for a concentration of the carbonyl containing compound may be less than about 20%, or less than about 10%, 5%, 2%, 1%, 0.5%, 0.2%, 0.1%, 0.05%, 0.02%, or 0.01%. 
     The precision of the device may be such that its percentage relative standard deviation for a concentration of the carbonyl containing compound may be less than about 20%, or less than about 10%, 5%, 2%, 1%, 0.5%, 0.2%, 0.1%, 0.05%, 0.02%, or 0.01%. 
     The device may further comprise a detection region which is in fluid connection with the sorbent material and is optically coupled to the detector. The detection region may be distal to the sorbent material. In the case where the fluid comprises a liquid, or if a liquid is added to the fluid, the detection region may be configured to reduce meniscus formation at the edges of the gas-liquid interface within the detection region. This may reduce interference in the detector measurement of the change in absorption spectrum of the indicator. 
     The device may further comprise a neutralisation chamber, said neutralisation chamber comprising a neutralisation reagent capable of neutralising a compound sorbed on and/or in the sorbent material. In this context, “neutralisation” indicates that the compound is converted to a less hazardous and/or more environmentally friendly product or products. The neutralisation reagent may comprise a biological and/or a chemical material. It may comprise a bacterium. It may, for example, comprise a bacterium selected from the  Nitrosomonas  genus. It may, for example, comprise  Nitrosomonas europaea . It may comprise an enzyme. It may comprise an oxidant. It may comprise, for example, one or more compound selected from the group consisting of hydrogen peroxide, chlorite salts, chlorate salts, perchlorate salts, hypochlorite salts, and mixtures thereof. The neutralisation reagent may be able to react with the optionally substituted hydroxylamine. It may be able, for example, to oxidise the optionally substituted hydroxylamine. This may serve to convert the optionally substituted hydroxylamine into a less hazardous and/or more environmentally friendly product or products, such as ammonia, water and dilute nitric acid. 
     The device may be a component of a system. The system may be for personal carbonyl containing compound (e.g. ketone body) measurement and monitoring over a period.  FIG. 9  displays an example system. In this system, the device  900  comprises disposable sorbent material  909  in a tape format. The tape is impregnated with the halochromic indicator and hydroxylamine salt reagents, and is coiled within a storage cartridge  910 . The device comprises a mouth piece  901  for introducing a breath sample, and reaction chamber  903  where a breath sample can be exposed to the sorbent material. Solvent storage and delivery may be integrated into the device as shown at the pre-treatment chamber  902  or may be supplied by external means (not shown). The device has a detector comprising a spectrometric sensor  913  with a lens  912 , and a light source  911 . The sensor  913  is capable of measuring a change in absorption at sensing region  908 . The detector comprises a connection component  914  which is capable of sending information from detector to the smartphone. The device also comprises a waste collection cartridge  906 , where used sorbent material can be treated with neutralisation reagent  904  which can be dispensed by opening valve  905 . Any waste gas generated may be released through the gas outlet  915 . 
     Before testing, the user mechanically aligns a new section of the sorbent material  909  into the sensing region  908  above the detector by moving a section of the sorbent material out of the storage cartridge  910 . The light source  911  irradiates the sensing region with electromagnetic radiation, in this case visible light. Any light which is not absorbed by the indicator sorbed in and/or on the sorbent material is reflected by the sorbent material to produce an absorption spectrum which passes through the lens  912  where it is concentrated and measured at the sensor  913 . After the user breathes into the device through the mouth piece  901 , the fluid passes through the pretreatment area  902  and enters the reaction chamber  903  where it is exposed to the sorbent material at the sensing region  908 . The sensor  913  measures the absorption spectrum of the indicator and sends this information via the connection component  914  to a smart device (i.e. the processor). The smart device calculates the concentration of the analyte, which is recorded and analysed. In this example, the used sorbent material is passed into the waste collection cartridge  906 , where it is treated with neutralisation reagent  904  which can be dispensed by opening a valve  905 . Any waste gas generated may be released through the gas outlet  915 . This system may enable environmentally friendly storage, safe handling and disposal of multiple ketone sensing chips. 
     Further, in the example system depicted in  FIG. 9 , the sensor data can be transmitted to the smart device such as a phone via wired or wireless connections. This allows various methods of local or cloud hosted computation and analytics to be performed to provide an accurate measurement of breath ketone bodies. Over time the device software can be upgraded which will enable more reliable results, and auxiliary data, such as activity and location data, obtained from the user&#39;s smartphone or paired devices such as a smart watch can be recorded at the time of ketone analysis. Data from the total user population can then be aggregated within a cloud service. This data may be for used for machine learning, enabling predictions in user ketone body breath concentration trends based on regularly reported auxiliary data. 
     Sorbent Material 
     The sorbent material may be a component of the sensor as hereinbefore described. The sensor as hereinbefore described may comprise the sorbent material. The sorbent material may be transparent or at least partially transparent. It may be opaque. The sorbent material may be disposed within a flow channel, or an analysis chamber. It may be disposed on or in a test strip. 
     The sorbent material may be a surface. It may be a glass surface. It may be a polymer surface. It may be a surface-modified material, for example a surface modified glass. It may be a liquid vessel, sponge, hydrogel, paper, or an absorbent polymer. The sorbent material may comprise a polymer fibre. It may be a porous material, such as paper. It may comprise a wettable polymer. It may comprise cellulose, nitrocellulose, polypropylene or a combination thereof. It may comprise a hydrophilic region and/or a hydrophobic region. The hydrophilic region may comprise a wettable material. It may be a porous material. The hydrophobic region may comprise a hydrophobic polymer, a wax, or a combination thereof. The sorbent material may be disposed on an at least partially optically transparent substrate. It may be disposed on an optically opaque substrate. The sorbent material may be positioned in direct contact with the detector. In some instances, the sorbent material is not in direct contact with the detector. 
     The variable may be an absorbance, fluorescence, resistance, ion concentration, temperature, voltage, or current. In the case where the variable is an ion concentration, it may be a hydronium ion concentration. In particular embodiments, the change in the variable is not measured using a titration. In the case where the change in variable is not measured using a titration, the device may require only a small liquid volume, such as less than 1 mL. This may enable the device to have a simplified device design when compared with devices requiring a titration step, and/or may enable the device to be portable. In such a case, the device may be simple to use. That is, it may not require a highly skilled operator. It may, for example, be suitable for an unskilled operator to use. 
     In the case where the variable is an absorbance, it may be the absorbance of an indicator. In this instance, a change in the absorption spectrum of the indicator may comprise a change in wavelength. It may comprise a change in intensity. The change in the absorption spectrum may be substantially independent of the volume of the fluid exposed to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt and halochromic indicator. It may be substantially independent of the flow rate of the fluid exposed to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt and halochromic indicator. 
     The change in the absorption spectrum of the indicator may be a change of absorption maximum of the indicator. In this case, the change in the absorption maximum of the indicator may comprise a change in wavelength of the absorption maximum of the indicator. It may comprise a change in intensity of the absorption maximum of the indicator. It may comprise a change in wavelength and intensity. The change in the absorption spectrum of the indicator may comprise a change in intensity of the absorption of the indicator at a particular wavelength or at more than one wavelengths. It may be a change in colour, or a change in absorption e.g. absorbance of electromagnetic radiation. The electromagnetic radiation may be, for example, infrared, far infrared, near infrared, visible, ultraviolet, or some other electromagnetic radiation. 
     In the case where the indicator is a halochromic indicator, the absorption spectrum of the halochromic indicator may vary monotonically over an operational pH range. The halochromic indicator may be selected from the group consisting of methyl orange, methyl red, methyl yellow chlorophenol red, bromocresol green, congo red, thymol blue, bromophenol blue, cresol red, metacresol purple, malachite green, ethyl violet, crystal violet, 2,4-dinitrophenol, orange IV, erythrosin B, p-(phenylazo)diphenylamine, p-phenylazoaniline, phenol blue, and mixtures thereof. It may, for example, be selected from the group consisting of thymol blue, bromophenol blue, and mixtures thereof. 
     In the case where the change in absorption spectrum of the indicator is a change in intensity at a particular wavelength or wavelengths, each particular wavelength or wavelengths may be within the range of from about 10 nm to about 300 μm, or from about 390 nm to about 700 nm, about 10 nm to about 400 nm, about 700 nm to about 300 μm, about 390 nm to about 500 nm, about 500 nm to about 600 nm, or about 600 nm to about 700 nm. It may be, for example, at about 10 nm, 50 nm, 100 nm, 200 nm, 300 nm, 400 nm, 425 nm, 450 nm, 475 nm, 500 nm, 525 nm, 550 nm, 575 nm, 600 nm, 625 nm, 650 nm, 675 nm, 700 nm, 800 nm, 900 nm, 1 μm, 2 μm, 5 μm, 10 μm, 20 μm, 50 μm, 100 μm, 200 μm, or 300 μm. 
     In the case where the change in the absorption spectrum of the indicator is a change in wavelength of the absorption maximum of the indicator, the absorption maximum may mean the maximum absorption in a particular region of the electromagnetic spectrum. It may mean, for example, the maximum absorption in a region of from about 10 nm to about 300 μm, or from about 390 nm to about 700 nm, about 10 nm to about 400 nm, about 700 nm to about 300 μm, about 390 nm to about 500 nm, about 500 nm to about 600 nm, or about 600 nm to about 700 nm. 
     The indicator may be a material that changes due to the concentration of optionally O-substituted hydroxylammonium salts and/or optionally N-substituted hydrazinium salts. It may be a halochromic indicator. In the case where the indicator is a halochromic indicator, it may be an indicator which changes absorption spectrum as a result of a change in pH. The indicator may comprise more than one such halochromic indicator. It may comprise a plurality of indicators, each indicator having a transition pH range, whereby the transition pH ranges of the plurality of indicators together span the operational pH range. It may, for example, be selected from the group consisting of methyl orange, methyl red, methyl yellow chlorophenol red, bromocresol green, congo red, thymol blue, bromophenol blue, cresol red, metacresol purple, malachite green, ethyl violet, crystal violet, 2,4-dinitrophenol, orange IV, erythrosin B, p-(phenylazo)diphenylamine, p-phenylazoaniline, phenol blue, and mixtures thereof. It may, for example, be selected from the group consisting of thymol blue, bromophenol blue, and mixtures thereof. The absorption spectrum of the halochromic indicator may vary monotonically over an operational pH range. The term “operational pH range” may mean the pH range at a position of the sorbent material from the pH if the position was not exposed to any analyte, to the pH if the position was exposed to pure undiluted analyte for sufficient time for the pH to reach a steady state. The term “transition pH range” means the pH range in which small changes in pH produce a change in the indicator maximum absorption. The operational pH range may be from about 1 to about 9, about 1 to about 8, about 1 to about 7, about 1 to about 6, about 1 to about 5, about 2 to about 7, about 3 to about 7, about 4 to about 7, about 2 to about 5, or about 2 to about 4. The halochromic indicator may be selected to have an operational pH range that includes a pH range below the pH at a position of the sorbent material prior to exposure of the sorbent material to a concentration of the analyte (i.e. the carbonyl-containing compound) in a fluid. It may be selected to have an operational pH range that includes a pH range below the pH of the fluid prior to exposure of the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt to a concentration of the analyte (i.e. the carbonyl-containing compound) in a fluid. The halochromic indicator may change from a neutral species at a higher pH to a charged species at a lower pH. 
     In the case where the variable is a fluorescence, the device may comprise one or more fluorophores that are sensitive to one or more products from the reaction of a carbonyl-containing compound with an optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt. In this case, the detector may be, for example, a fluorometer. 
     In the case where the variable is a fluorescence, the change in fluorescence may be a change in intensity of fluorescence at a particular wavelength or wavelengths, each particular wavelength or wavelengths may be within the range of from about 10 nm to about 300 μm, or from about 390 nm to about 700 nm, about 10 nm to about 400 nm, about 700 nm to about 300 μm, about 390 nm to about 500 nm, about 500 nm to about 600 nm, or about 600 nm to about 700 nm. It may be, for example, at about 10 nm, 50 nm, 100 nm, 200 nm, 300 nm, 400 nm, 425 nm, 450 nm, 475 nm, 500 nm, 525 nm, 550 nm, 575 nm, 600 nm, 625 nm, 650 nm, 675 nm, 700 nm, 800 nm, 900 nm, 1 μm, 2 μm, 5 μm, 10 μm, 20 μm, 50 μm, 100 μm, 200 μm, or 300 μm. 
     In the case where the variable is an ion concentration, it may be, for example, a hydronium concentration. It may be an optionally O-substituted hydroxylammonium or optionally N-substituted hydrazinium concentration. It may be a concentration of the counter ion (i.e. anion) of an optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt. It may be a concentration of a product ion from the reaction of a carbonyl-containing compound and an optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt. 
     In the case where the variable is an ion concentration, the change in ion concentration may be a change in ion concentration in the liquid. The change in ion concentration may be within the range of from about 0.001 mM to about 100 mM, or from about 0.01 mM to about 50 mM, about 0.01 mM to about 20 mM, about 0.01 mM to about 10 mM, about 0.01 mM to about 1 mM, about 0.01 mM to about 0.5 mM, or about 0.01 mM to about 0.1 mM. 
     In the case where the variable is a voltage and/or current, the detector may be, for example, a potentiostat, or an ISFET. In the case where the detector is a potentiostat, it may comprise three electrodes in contact with the liquid and/or sorbent material, where the voltage and/or current flow between a reference electrode and working electrode varies as a function of ion concentration (e.g. pH) of the liquid. 
     In the case where the variable is a voltage, the change in voltage may be a change in voltage between two electrodes at different positions of the liquid and/or sorbent material. The change in voltage may be within the range of from about 0.01 V to about 10 V, or from about 0.01 V to about 5 V, about 0.01 V to about 2 V, about 0.01 V to about 1 V, about 0.01 V to about 0.5 V, about 0.01 V to about 0.2 V, or about 0.01 V to about 0.1 V. 
     In the case where the variable is a current, the change in current may be a change in current between two electrodes at different positions of the liquid and/or sorbent material. The change in current may be within the range of from about 0.001 A to about 1 A, or from about 0.001 A to about 0.5 A, about 0.001 A to about 0.2 A, about 0.001 A to about 0.1 A, about 0.001 A to about 0.05 A, about 0.001 A to about 0.02 A, or about 0.001 A to about 0.01 A. 
     In the case where the variable is a resistance, the detector may be, for example, a chemiresistor sensor. In this case the liquid and/or sorbent material may be placed in direct contact with the chemiresistor sensor. The chemiresistor may comprise two opposing electrodes separated by the liquid and/or sorbent material, through which a current is passed at a stable voltage. The chemiresistor may measure the resistance across the liquid and/or sorbent material. A higher number of ions (e.g. hydronium ions) in the liquid and/or sorbent material may result in a lower resistance measurement. 
     In the case where the variable is a resistance, the change in resistance may be a change in resistance between two electrodes at different positions of the liquid and/or sorbent material. The change in resistance may be within the range of from about 0.1Ω to about 10,000Ω, or from about 0.1Ω to about 5000Ω, about 0.1Ω to about 2000Ω, about 0.1Ω to about 1000Ω, about 0.1Ω to about 500Ω, about 0.1Ω to about 200Ω, about 0.1Ω to about 100Ω, about 0.1Ω to about 50Ω, about 0.1Ω to about 20Ω, about 0.1Ω to about 10Ω, about 0.1Ω to about 5Ω, about 0. 1Ω to about 2Ω, or about 0.1Ω to about 1Ω. 
     In the case where the variable is a temperature, the detector may be, for example, a thermal sensor, such as a thermometer. In this case it may measure the temperature change caused by the enthalpy of reaction of a carbonyl-containing compound with an optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt. 
     In the case where the variable is a temperature, the change in temperature may be a change in temperature at a position of the liquid and/or sorbent material. The change in temperature may be within the range of from about 0.001° C. to about 10° C., or from about 0.001° C. to about 5° C., about 0.001° C. to about 2° C., about 0.001° C. to about 1° C., about 0.001° C. to about 0.5° C., about 0.001° C. to about 0.2° C., or about 0.001° C. to about 0.1° C. 
     In certain embodiments the sorbent material has an optionally substituted hydroxylamine salt and an indicator sorbed therein and/or thereon, such that exposure of the fluid to the sorbent material produces a change in absorption spectrum of the indicator. The sorbent material may be transparent or at least partially transparent. It may be opaque. The sorbent material may be disposed within a flow channel, or an analysis chamber. It may be disposed on or in a test strip. 
     The sorbent material may be a surface. It may be a glass surface. It may be a polymer surface. It may be a surface-modified material, for example a surface modified glass. The sorbent material may comprise a polymer fibre. It may be a porous material, such as paper. It may comprise a wettable polymer. It may comprise cellulose, nitrocellulose, polypropylene or a combination thereof. It may comprise a hydrophilic region and/or a hydrophobic region. The hydrophilic region may comprise a wettable material. It may be a porous material. The hydrophobic region may comprise a hydrophobic polymer, a wax, or a combination thereof. The sorbent material may be disposed on an at least partially optically transparent substrate. 
     As shown in the example sorbent material depicted in  FIG. 10( a ) , sorbent material  1000  has a hydrophilic sensing region  1020 , bordered by a hydrophobic containment region  1010 . The sorbent material is disposed on an at least partially optically transparent substrate  1030 . In this case hydroxylamine hydrochloride and an indicator may be sorbed in and/or on the hydrophilic region. Upon exposure of the sorbent material to the fluid, the hydrophobic containment region restricts the fluid and/or any reagents from spreading. In this example, the at least partially transparent substrate enables spectroscopic measurement therethrough. The example sorbent material depicted in  FIG. 10( b )  has three exposure areas, and is described elsewhere herein. 
     The change in the absorption spectrum of the indicator may be a change of absorption maximum of the indicator. In this case, the change in the absorption maximum of the indicator may comprise a change in wavelength of the absorption maximum of the indicator. It may comprise a change in intensity of the absorption maximum of the indicator. It may comprise a change in wavelength and intensity. The change in the absorption spectrum of the indicator may comprise a change in intensity of the absorption of the indicator at a particular wavelength or at more than one wavelengths. It may be a change in colour, or a change in absorption e.g. absorbance of electromagnetic radiation. The electromagnetic radiation may be, for example, infrared, far infrared, near infrared, visible, ultraviolet, or some other electromagnetic radiation. 
     In the case where the change in absorption spectrum of the indicator is a change in intensity at a particular wavelength or wavelengths, each particular wavelength or wavelengths may be within the range of from about 10 nm to about 300 μm, or from about 390 nm to about 700 nm, about 10 nm to about 400 nm, about 700 nm to about 300 μm, about 390 nm to about 500 nm, about 500 nm to about 600 nm, or about 600 nm to about 700 nm. It may be, for example, at about 10 nm, 50 nm, 100 nm, 200 nm, 300 nm, 400 nm, 425 nm, 450 nm, 475 nm, 500 nm, 525 nm, 550 nm, 575 nm, 600 nm, 625 nm, 650 nm, 675 nm, 700 nm, 800 nm, 900 nm, 1 μm, 2 μm, 5 μm, 10 μm, 20 μm, 50 μm, 100 μm, 200 μm, or 300 μm. 
     In the case where the change in the absorption spectrum of the indicator is a change in wavelength of the absorption maximum of the indicator, the absorption maximum may mean the maximum absorption in a particular region of the electromagnetic spectrum. It may mean, for example, the maximum absorption in a region of from about 10 nm to about 300 μm, or from about 390 nm to about 700 nm, about 10 nm to about 400 nm, about 700 nm to about 300 μm, about 390 nm to about 500 nm, about 500 nm to about 600 nm, or about 600 nm to about 700 nm. 
     The optionally substituted hydroxylamine salt may be O-substituted with a C 1 -C 12  alkyl group, C 1 -C 12  alkenyl group, C 1 -C 12  alkynyl group, C 1 -C 12  alkyloxy group, or C 1 -C 12 -alkylaryl group; wherein the C 1 -C 12  alkyl group, C 1 -C 12  alkenyl group, C 1 -C 12  alkynyl group, C 1 -C 12  alkyloxy group, or C 1 -C 12 -alkylaryl group is optionally substituted with one or more groups independently selected from the group consisting of halo, hydroxyl, thiol, aryl, C 1 -C 12  alkyl, C 1 -C 12  alkenyl, C 1 -C 12  alkynyl, C 1 -C 12  alkyloxy, or C 1 -C 12 -alkylaryl. The optionally substituted hydroxylamine salt may be, for example, selected from the group consisting of hydroxylamine hydrochloride, hydroxylammonium sulfate, O-benzylhydroxylamine hydrochloride, hydroxylammonium phosphate, hydroxylamine nitrate, and mixtures thereof. It may be hydroxylamine hydrochloride. The optionally substituted hydroxylamine salt may be used in a molar excess when compared with the indicator. It may be used in a molar excess when compared with the weak base. 
     The indicator may be a material that changes due to the concentration of optionally substituted hydroxylamine salts. It may be a halochromic indicator. In the case where the indicator is a halochromic indicator, it may be an indicator which changes absorption spectrum as a result of a change in pH. The indicator may comprise more than one such halochromic indicator. It may comprise a plurality of indicators, each indicator having a transition pH range, whereby the transition pH ranges of the plurality of indicators together span the operational pH range. It may, for example, be selected from the group consisting of methyl orange, methyl red, methyl yellow chlorophenol red, bromocresol green, congo red, thymol blue, bromophenol blue, cresol red, metacresol purple, malachite green, ethyl violet, crystal violet, 2,4-dinitrophenol, orange IV, erythrosin B, p-(phenylazo)diphenylamine, p-phenylazoaniline, and mixtures thereof. It may, for example, be selected from the group consisting of thymol blue, bromophenol blue, and mixtures thereof. The absorption spectrum of the halochromic indicator may vary monotonically over an operational pH range. The term “operational pH range” may mean the pH range at a position of the sorbent material from the pH if the position was not exposed to any analyte, to the pH if the position was exposed to pure undiluted analyte for sufficient time for the pH to reach a steady state. The term “transition pH range” means the pH range in which small changes in pH produce a change in the indicator maximum absorption. The operational pH range may be from about 1 to about 9, about 1 to about 8, about 1 to about 7, about 1 to about 6, about 1 to about 5, about 2 to about 7, about 3 to about 7, about 4 to about 7, about 2 to about 5, or about 2 to about 4. The halochromic indicator may be selected to have an operational pH range that includes a pH range below the pH at a position of the sorbent material prior to exposure of the sorbent material to a concentration of the analyte (i.e. the carbonyl containing compound) in a fluid. The halochromic indicator may change from a neutral species at a higher pH to a charged species at a lower pH. 
     The device may have uniform sensitivity to the carbonyl containing compound. Alternatively, the sorbent material may comprise a plurality of exposure regions, said plurality of exposure regions having different sensitivities to the carbonyl containing compound. In this case, the sensitivity to the carbonyl containing compound at any particular exposure region of the sorbent material may be from about 0% to about 100% of the sensitivity of an exposure region having the greatest sensitivity to the carbonyl containing compound, or it may be from about 0.1% to about 100%, about 0.5% to about 100%, about 1% to about 100%, about 5% to about 100%, about 10% to about 100%, about 20% to about 100%, about 30% to about 100%, about 40% to about 100%, about 50% to about 100%, about 60% to about 100%, about 70% to about 100%, about 80% to about 100%, about 90% to about 100%, or about 95% to about 100% of the sensitivity of an exposure region having the greatest sensitivity to the carbonyl containing compound. 
     In the case where the sorbent material comprises a plurality of exposure regions, the sorbent material may comprise at least two exposure regions having different sensitivities to the carbonyl containing compound, or it may comprise at least 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, 30, 40, or 50, or more than 50 exposure regions having different sensitivities to the carbonyl containing compound. It may comprise from about 2 to about 200 exposure regions having different sensitivities to the carbonyl containing compound, or it may comprise from about 2 to about 100, about 2 to about 50, about 2 to about 20, about 3 to about 10, about 5 to about 50, or about 10 to about 50 exposure regions having different sensitivities to the carbonyl containing compound. 
     The skilled person will understand that the exposure regions of the sorbent material may be in any arrangement. They may, for example, be spaced equidistantly along a length of the sorbent material. They may be non-equidistantly spaced along a length of the sorbent material. They may be arranged in a regular pattern on the sorbent material. They may be arranged in a non-regular pattern on the sorbent material. They may be arranged on a single face of the sorbent material, or on more than one face of the sorbent material. They may be arranged in a linear or radial arrangement on the sorbent material. They may be arranged in an array. They may be arranged through, or within the sorbent material. They may be in an orthogonal arrangement to each other. They may be arranged, for example, to have a large exposure region comprising a larger amount of positions with suitable sensitivity for detecting concentrations of the carbonyl containing compound within a specified concentration range, and smaller regions comprising a smaller amount of positions with suitable sensitivity for detecting concentrations of the carbonyl containing compound outside of the specified concentration range. 
       FIG. 10( b )  shows an example sorbent material  1001  having three exposure regions  1050   a ,  1050   b ,  1050   c  each having different sensitivity to the carbonyl containing compound. In this example each exposure region comprises a hydrophilic material and is surrounded by a hydrophobic containment border  1040  which separates the exposure regions and prevents the fluid to pass from one exposure region to another. In this example, the sorbent material is disposed on an at least partially transparent optical substrate  1060 . 
     In the case where the sorbent material comprises a plurality of exposure regions, said plurality of exposure regions having different sensitivities to the carbonyl containing compound, the different sensitivities may result from a spatially differential local concentration or local amount of the optionally substituted hydroxylamine salt and/or the indicator at different exposure regions of the sorbent material. In this case, the sensitivity to the carbonyl containing compound may increase at an exposure region of the sorbent material having increased local concentration or amount of the optionally substituted hydroxylamine salt and/or the indicator when compared to an exposure region of the sorbent material having decreased local concentration or amount of the optionally substituted hydroxylamine salt and/or the indicator. 
     In certain embodiments, the sorbent material may have an exposure surface for exposure to the fluid. The exposure surface may comprise a coating having spatially differential permeability to the analyte (e.g. carbonyl containing compound), so as to provide spatially differential sensitivity to the sorbent material. The coating may increase or decrease the concentration of the carbonyl containing compound concentration at the detecting surface when compared with the carbonyl containing compound concentration exposed to the sorbent material. The coating may have variable thickness, such that, for example, the sorbent material has higher sensitivity to the carbonyl containing compound at a position where the coating is thinner, and lower sensitivity to the carbonyl containing compound at a position where the coating is thicker. The coating may comprise apertures, e.g. pores, allowing a fraction of the carbonyl containing compound to pass therethrough. It may comprise channels, for example microfluidic channels allowing a fraction of the carbonyl containing compound to pass therethrough. The channels may, for example, have varying flow rates and/or permeability to the carbonyl containing compound. In the case where the sorbent material has spatially differential sensitivity to the carbonyl containing compound, the spatially differential sensitivity may result from the varying flow rate of the channels. For example, the sorbent material portion below the coating may have uniform sensitivity, but the varying flow rates of the channels of the coating may provide the sorbent material spatially differential sensitivity. In the case where the sorbent material has spatially differential sensitivity to the carbonyl containing compound, the spatially differential sensitivity may result from the spatially differential permeability of the coating. For example, the sorbent material portion below the coating may have uniform sensitivity, but the spatially differential permeability of the coating may provide the sorbent material spatially differential sensitivity. In the case where the sorbent material has spatially differential sensitivity to the carbonyl containing compound, the spatially differential sensitivity may result from a differential in time of exposure at a sorbent material portion below the coating because of the differential permeability of the coating. For example, the sorbent material portion below the coating may have uniform sensitivity, but the varying exposure time at a sorbent material portion below the coating as a result of the coating may provide the sorbent material spatially differential sensitivity. The sensitivity to the carbonyl containing compound may increase at a position of the sorbent material having increased permeability of the coating when compared to a position of the sorbent material having decreased permeability of the coating. The coating may comprise a blocking agent that interacts with the carbonyl containing compound. The blocking agent may react with, bind to, and/or adsorb the carbonyl containing compound. 
     The blocking agent may affect the dynamic range of the device by adjusting the local concentration of the analyte (e.g. carbonyl containing compound) at a position of the sorbent material. The blocking agent may be used in a different concentration or amount at a plurality of positions of the sorbent material. In the case where the sorbent has spatially differential sensitivity to the carbonyl containing compound, the spatially differential sensitivity may result from the different concentration or amount of the blocking agent at different positions of the sorbent material. For example, the sorbent material portion below the coating may have uniform sensitivity, but the differential concentration or amount of the blocking agent may provide the sorbent material spatially differential sensitivity as depicted in  FIG. 11 . In this example, a fluid incident upon the device is subjected to a differential treatment such that different positions of the sorbent material have different local analyte concentrations, shown as low, medium and high local concentrations in  FIG. 11 . The sensitivity to the carbonyl containing compound may increase at a position of the sorbent material having decreased local concentration or amount of the blocking agent when compared to a position of the sorbent material having increased local concentration or amount of the blocking agent. 
     The sorbent material may comprise channels, for example microfluidic channels allowing a fraction of the analyte (e.g. carbonyl containing compound) to pass therethrough. The channels may, for example, have varying flow rates and/or permeability to the carbonyl containing compound. The fluid may pass through a microfluidic channel prior to contacting an exposure region. In the case where the sorbent material has spatially differential sensitivity to the carbonyl containing compound, the spatially differential sensitivity may result from varying exposure time at the plurality of exposure regions. For example, the exposure regions may have inherent uniform sensitivity, but the varying exposure time at different exposure regions may provide the sorbent material spatially differential sensitivity. This example is depicted in  FIG. 12 , where a concentration of the carbonyl containing compound is incident upon the sorbent layer, where it is split into three sections which provide low (t 1 ), medium (t 2 ), and high (t 3 ) exposure times at different exposure regions. 
     The sorbent material may further comprise a weak base sorbed thereon and/or therein. The weak base may be, for example, selected from the group consisting of bicarbonate salts, ammonia, aniline, phenolate salts, and mixtures thereof. It may be, for example, a bicarbonate salt. It may be sodium bicarbonate. The weak base may be used to adjust the working range of the device by increasing the pH at a surface of the sorbent material. The weak base may have a pKb of greater than about 3, or greater than about 4, 5, 6, 7, 8, 9, or 10. The weak base may have a pKb of from about 3 to about 12, or from about 3 to about 10, about 3 to about 8, about 5 to about 12, about 5 to about 10, about 6 to about 9, or about 7 to about 8. It may be, for example, about 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.2, 7.4, 7.6, 7.8, 8, 8.2, 8.4, 8.6, 8.8, 9, 9.5, 10, 10.5, 11, 11.5, or 12. 
     Optionally O-Substituted Hydroxylammonium Salt or Optionally N-Substituted Hydrazinium Salt 
     The optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt may be selected from compounds of Formula (I): 
       [R m —Y—NH 3   + ] n X p   Formula (I)
         wherein Y=O or N; m=1, 2, or 3; n=1, 2, or 3; p=1, 2, or 3; R is independently selected from the group consisting of hydrogen, halogen, optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted heteroalkyl, optionally substituted cycloheteroalkyl, optionally substituted alkenyl, optionally substituted cycloalkenyl, optionally substituted alkylaryl, optionally substituted aryl, optionally substituted alkylheteroaryl, and optionally substituted heteroaryl; wherein said optionally substituted alkyl, cycloalkyl, heteroalkyl, cycloheteroalkyl, alkenyl, cycloalkenyl, alkylaryl, aryl, alkylheteroaryl, or heteroaryl is optionally substituted with one or more groups independently selected from the group consisting of halogen, hydroxyl, thiol, amino, aryl, heteroaryl, alkyl, alkenyl, alkynyl, heteroalkyl, alkylaryl, and alkylheteroaryl; and X is an anion.       

     The optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt may be, for example, selected from the group consisting of hydroxylammonium chloride, hydroxylammonium sulfate, hydroxylammonium azide, O-phenyl hydroxylammonium chloride, O-phenyl hydroxylammonium sulfate, O-phenyl hydroxylammonium azide, hydroxylamine picrate, hydroxylamine benzenesulfonate, hydroxylamine benzenesulfinate, hydroxylamine 1-sulfobutyl-3-methyl imidazole hydrosulfate salt, hydrazinium chloride, hydrazinium sulfate, hydrazinium hydrogensulfate, hydrazinium hydrogen sulfate, hydrazinium azide, hydrazinediium sulfate, hydrazinediium bis(6-carboxypyridazine-3-carboxylate), hydroxylammonium phosphate, hydroxylamine nitrate, and mixtures thereof. It may be hydroxylamine hydrochloride. It may be hydrazinium chloride. 
     The device may have uniform sensitivity to the carbonyl-containing compound. Alternatively, in the case where the device comprises a sorbent material, the sorbent material may comprise a plurality of exposure regions, said plurality of exposure regions having different sensitivities to the carbonyl-containing compound. In this case, the sensitivity to the carbonyl-containing compound at any particular exposure region of the sorbent material may be from about 0% to about 100% of the sensitivity of an exposure region having the greatest sensitivity to the carbonyl-containing compound, or it may be from about 0.1% to about 100%, about 0.5% to about 100%, about 1% to about 100%, about 5% to about 100%, about 10% to about 100%, about 20% to about 100%, about 30% to about 100%, about 40% to about 100%, about 50% to about 100%, about 60% to about 100%, about 70% to about 100%, about 80% to about 100%, about 90% to about 100%, or about 95% to about 100% of the sensitivity of an exposure region having the greatest sensitivity to the carbonyl-containing compound. 
     In the case where the sorbent material comprises a plurality of exposure regions, the sorbent material may comprise at least two exposure regions having different sensitivities to the carbonyl-containing compound, or it may comprise at least 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, 30, 40, or 50, or more than 50 exposure regions having different sensitivities to the carbonyl-containing compound. It may comprise from about 2 to about 200 exposure regions having different sensitivities to the carbonyl-containing compound, or it may comprise from about 2 to about 100, about 2 to about 50, about 2 to about 20, about 3 to about 10, about 5 to about 50, or about 10 to about 50 exposure regions having different sensitivities to the carbonyl-containing compound. 
     The skilled person will understand that the exposure regions of the sorbent material may be in any arrangement. They may, for example, be spaced equidistantly along a length of the sorbent material. They may be non-equidistantly spaced along a length of the sorbent material. They may be arranged in a regular pattern on the sorbent material. They may be arranged in a non-regular pattern on the sorbent material. They may be arranged on a single face of the sorbent material, or on more than one face of the sorbent material. They may be arranged in a linear or radial arrangement on the sorbent material. They may be arranged in an array. They may be arranged through, or within the sorbent material. They may be in an orthogonal arrangement to each other. They may be arranged, for example, to have a large exposure region comprising a larger amount of positions with suitable sensitivity for detecting concentrations of the carbonyl-containing compound within a specified concentration range, and smaller regions comprising a smaller amount of positions with suitable sensitivity for detecting concentrations of the carbonyl-containing compound outside of the specified concentration range. 
     In the case where the sorbent material comprises a plurality of exposure regions, said plurality of exposure regions having different sensitivities to the carbonyl-containing compound, the different sensitivities may result from a spatially differential local concentration or local amount of the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt at different exposure regions of the sorbent material. In this case, the sensitivity to the carbonyl-containing compound may increase at an exposure region of the sorbent material having increased local concentration or amount of the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt when compared to an exposure region of the sorbent material having decreased local concentration or amount of the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt. 
     In certain embodiments, the sorbent material may have an exposure surface for exposure to the fluid. The exposure surface may comprise a coating having spatially differential permeability to the carbonyl-containing compound, so as to provide spatially differential sensitivity to the sorbent material. The coating may increase or decrease the concentration of the carbonyl-containing compound concentration at the detecting surface when compared with the carbonyl-containing compound concentration exposed to the sorbent material. The coating may have variable thickness, such that, for example, the sorbent material has higher sensitivity to the carbonyl-containing compound at a position where the coating is thinner, and lower sensitivity to the carbonyl-containing compound at a position where the coating is thicker. The coating may comprise apertures, e.g. pores, allowing a fraction of the carbonyl-containing compound to pass therethrough. It may comprise channels, for example microfluidic channels allowing a fraction of the carbonyl-containing compound to pass therethrough. The channels may, for example, have varying flow rates and/or permeability to the carbonyl-containing compound. In the case where the sorbent material has spatially differential sensitivity to the carbonyl-containing compound, the spatially differential sensitivity may result from the varying flow rate of the channels. For example, the sorbent material portion below the coating may have uniform sensitivity, but the varying flow rates of the channels of the coating may provide the sorbent material spatially differential sensitivity. In the case where the sorbent material has spatially differential sensitivity to the carbonyl-containing compound, the spatially differential sensitivity may result from the spatially differential permeability of the coating. For example, the sorbent material portion below the coating may have uniform sensitivity, but the spatially differential permeability of the coating may provide the sorbent material spatially differential sensitivity. In the case where the sorbent material has spatially differential sensitivity to the carbonyl-containing compound, the spatially differential sensitivity may result from a differential in time of exposure at a sorbent material portion below the coating because of the differential permeability of the coating. For example, the sorbent material portion below the coating may have uniform sensitivity, but the varying exposure time at a sorbent material portion below the coating as a result of the coating may provide the sorbent material spatially differential sensitivity. The sensitivity to the carbonyl-containing compound may increase at a position of the sorbent material having increased permeability of the coating when compared to a position of the sorbent material having decreased permeability of the coating. The coating may comprise a blocking agent that interacts with the carbonyl-containing compound. The blocking agent may react with, bind to, and/or adsorb the carbonyl-containing compound. 
     The blocking agent may affect the dynamic range of the device by adjusting the local concentration of the carbonyl-containing compound at a position of the sorbent material. The blocking agent may be used in a different concentration or amount at a plurality of positions of the sorbent material. In the case where the sorbent has spatially differential sensitivity to the carbonyl-containing compound, the spatially differential sensitivity may result from the different concentration or amount of the blocking agent at different positions of the sorbent material. The sensitivity to the carbonyl-containing compound may increase at a position of the sorbent material having decreased local concentration or amount of the blocking agent when compared to a position of the sorbent material having increased local concentration or amount of the blocking agent. 
     The sorbent material may comprise channels, for example microfluidic channels allowing a fraction of the carbonyl-containing compound to pass therethrough. The channels may, for example, have varying flow rates and/or permeability to the carbonyl-containing compound. The fluid may pass through a microfluidic channel prior to contacting an exposure region. In the case where the sorbent material has spatially differential sensitivity to the carbonyl-containing compound, the spatially differential sensitivity may result from varying exposure time at the plurality of exposure regions. For example, the exposure regions may have inherent uniform sensitivity, but the varying exposure time at different exposure regions may provide the sorbent material spatially differential sensitivity. 
     The sorbent material may further comprise a weak base sorbed thereon and/or therein. The weak base may be, for example, selected from the group consisting of bicarbonate salts, ammonia, aniline, phenolate salts, and mixtures thereof. It may be, for example, a bicarbonate salt. It may be sodium bicarbonate. The weak base may be used to adjust the working range of the device by increasing the pH at a surface of the sorbent material. The weak base may have a pKb of greater than about 3, or greater than about 4, 5, 6, 7, 8, 9, or 10. The weak base may have a pKb of from about 3 to about 12, or from about 3 to about 10, about 3 to about 8, about 5 to about 12, about 5 to about 10, about 6 to about 9, or about 7 to about 8. It may be, for example, about 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.2, 7.4, 7.6, 7.8, 8, 8.2, 8.4, 8.6, 8.8, 9, 9.5, 10, 10.5, 11, 11.5, or 12. 
     Analyte 
     The analyte may be a metal, non-metal, cation, anion, radioisotope, organic or non-organic chemical. It may be a biomarker for diet, a disease, or a disorder. It may be a metal, protein, antibody, or small organic molecule. The analyte may be, for example, a ketone, such as acetone. The analyte may be a carbonyl-containing compound. It may, for example, be acetone. 
     The analyte may be a carbonyl-containing compound. The carbonyl-containing compound may be selected from the group consisting of ketones, aldehydes, esters, thioesters, amides, and carboxylic acids. It may be a biomarker for diet, a disease, or a disorder. It may be a protein, antibody, or small organic molecule. It may, for example, be selected from the group consisting of acetoacetate, butyraldehyde, 4-hydroxy-2-hexenal, ethyl acetate, acetic acid, 4-hydroxy-2-nonenal, 2-pentanone, 2-butanone, 3-hydroxy-2-butanone, hydroxyacetaldehyde, acetaldehyde, pentanal, malondialdehyde, acrolein, beta-hydroxybutyrate, and acetone. It may, for example, be acetone. 
     Fluid 
     The fluid may be a gas, liquid, vapour or combination thereof. It may be a dispersion. It may be an emulsion. It may be a microemulsion. It may be a sol. It may be a solution, for example an aqueous or organic solution. It may be a suspension. It may be a biological fluid. It may be, for example, whole blood, breath, sweat, sputum, mucus, urine, serum, semen, plasma, or saliva. The fluid may be derived from a solid, for example by dissolution of the solid in a solvent. 
     Liquid 
     The liquid may be water, or some other suitable solvent. It may be an organic solvent, an aqueous solution, or a combination thereof. It may be suitable for dissolving the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt. It may be suitable for dissolving the analyte (e.g. carbonyl-containing compound). The liquid, or a portion thereof, may be a component of the fluid. Alternatively, or additionally, the liquid, or a portion thereof, may be separate to the fluid. In the case where the liquid is separate to the fluid, the liquid may be added to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt prior to or during exposure of the fluid to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt. The liquid may, for example, be pre-sorbed onto the sorbent material. The optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt may be in a solution with the liquid prior to exposure to the fluid. 
     The liquid volume may be about 1 mL or less. It may be less than about 0.5 mL, 0.2 mL, 0.1 mL, 0.05 mL, 0.02 mL, or 0.01 mL. The liquid volume may be from about 0.001 mL to about 1 mL, or from about 0.001 mL to about 0.5 mL, about 0.001 mL to about 0.2 mL, about 0.005 mL to about 0.5 mL, about 0.005 mL to about 0.05 mL, or about 0.005 mL to about 1 mL. 
     Acquisition Device 
     The acquisition device may be configured to determine the concentration of the analyte from the differential response. It may be configured to measure a response of the sensor to the analyte as a function of time. It may be configured to determine the concentration of the analyte utilising at least one detection position of the sensor. It may be configured to measure the sensor response at a defined or predetermined time after exposure of the sensor to the analyte. The time may be, for example, from about 0.01 seconds to about 120 minutes, or it may be from about 0.01 seconds to about 60 minutes, about 0.01 seconds to about 30 minutes, about 0.01 seconds to about 20 minutes, about 0.01 seconds to about 10 minutes, about 0.01 seconds to about 5 minutes, about 0.01 seconds to about 2 minutes, about 0.01 seconds to about 1 minute, about 0.01 seconds to about 30 seconds, about 30 seconds to about 60 minutes, about 1 minute to about 60 minutes, about 5 minutes to about 60 minutes, about 10 minutes to about 60 minutes, about 5 minutes to about 30 minutes, or about 10 minutes to about 30 minutes. It may be less than about 60 minutes, or less than about 30, 20, 10, 5, 2, 1, 0.5, or 0.1 minutes. It may be, for example, about 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, or 55 seconds. It may be, for example, about 1, 2, 5, 10, 15, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, or 120 minutes. 
     The acquisition device may be configured to measure the time taken for at least one of the sensor detection positions to reach a defined response level after exposure of the sensor to the analyte. The defined response level may be, for example, from about 0% to about 100% of the maximum possible response level, or it may be from about 0% to about 90%, about 0% to about 80%, about 0% to about 70%, about 0% to about 60%, about 0% to about 50%, about 0% to about 40%, about 0% to about 30%, about 0% to about 20%, about 0% to about 10%, about 0% to about 5%, about 0.5% to about 100%, about 1% to about 100%, about 5% to about 100%, about 10% to about 100%, about 20% to about 100%, about 20% to about 60%, about 40% to about 60%, or about 20% to about 50% of the maximum possible response level. It may be, for example, about 0, 0.01, 0.02, 0.05, 0.1, 0.2. 0.5, 1, 2, 5, 10, 20, 25, 30, 40, 50, 60, 70, 75, 80, 90, or 100% of the maximum possible response level. 
     The acquisition device may be configured to measure the time taken for at least one of the sensor detection positions to reach a saturated response level after exposure of the sensor to the analyte. It may be configured to measure the time taken for at least one of the sensor detection positions to reach a peak response level after exposure of the sensor to the analyte. It may be configured to measure the time taken for at least one of the sensor positions to reach a steady state response level after exposure of the sensor to the analyte 
     The acquisition device may comprise a processor configured to calculate the concentration of the analyte utilising the differential response of the sensor to the analyte. The processor may be configured to calculate the concentration of the analyte based on a response of the sensor to the analyte as a function of time. The processor may be configured to calculate the concentration of the analyte based on a response of the sensor to the analyte at a defined period after exposure of the sensor to the analyte. The processor may be configured to calculate the concentration of the analyte based on the time taken for at least one of the sensor detection positions to reach a defined response level after exposure of the sensor to the analyte. The processor may be configured to calculate the concentration of the analyte based on the time taken for at least one of the sensor detection positions to reach a saturated response level after exposure of the sensor to the analyte. The processor may be configured to calculate the concentration of the analyte based on the time taken for at least one of the sensor detection positions to reach a peak response level after exposure of the sensor to the analyte. The processor may be configured to calculate the concentration of the analyte based on the time taken for at least one of the sensor detection positions to reach a steady state response level after exposure of the sensor to the analyte. The processor may be configured to calculate the concentration of the analyte based on the area under the curve of a plot of the sensor response as a function of time for at least one of the sensor detection positions. It may be configured to calculate the concentration of the analyte based on the integral the sensor response as a function of time for at least one of the sensor detection positions. It may be configured to calculate the concentration of the analyte based on the gradient or slope of the sensor response as a function of time for at least one of the sensor detection positions.  FIG. 13( a )  shows sensor responses as a function of length across the sensor for a continuous spatially differential sensor such as depicted in  FIG. 5( a ) , at a high, medium and low analyte concentration.  FIG. 13( b )  shows sensor responses as a function of exposure time for a sensor such as depicted in  FIG. 3 , at a high, medium and low analyte concentration. The differences in curve shape, integration and gradient at each concentration can be utilised by the processor to determine the analyte concentration.  FIGS. 14( a ) and 14( b )  depict example sensor responses as a function of time, where the responses exhibiting initial delays. In this example case, the slope or gradient of the sensor response as a function of time may be used to determine the concentration of the analyte. 
     As discussed earlier, the sensor may be configured to respond to the analyte at a plurality of detection positions on the sensor having different sensitivities to the analyte. In this case the acquisition device may be configured to calculate the concentration of the analyte utilising at least the following: 
     if the sensor reaches a saturated response level at all of the plurality of detection positions on the sensor:
         a response of the sensor to the analyte as a function of time at the detection position on the sensor having the least sensitivity to the analyte;
 
if the sensor does not reach a saturated response level at any of the plurality of detection positions on the sensor:
   a response of the sensor to the analyte at the detection position on the sensor having the greatest sensitivity to the analyte; and
 
if the sensor responds to the analyte to reach a saturated response level at some, but not all, of the plurality of detection positions on the sensor:
   one or more of:
           the ratio of the number of detection positions on the sensor that reach a saturated response level to the number of detection positions on the sensor that do not reach a saturated response level,   the time required to reach a saturated response level for at least one of the detection positions on the sensor that reaches a saturated response level, and   a response of the sensor to the analyte for at least one of the detection positions on the sensor that does not reach a saturated response level.   
               

     In the case where the differential response of the sensor is temporally differential, the acquisition device may be configured to calculate the concentration of the analyte utilising at least one or more of a time required to reach a predetermined response level for at least one position on the sensor after exposure of the fluid to the sensor; and a response level for at least one position on the sensor at a predetermined time after exposure of the fluid to the sensor. 
     In the case where the differential response of the sensor is temporally differential and spatially differential, the acquisition device may be configured to calculate the concentration of the analyte using a sensor response at two or more temporal points at one or more positions of the sensor. 
     The processor may be configured to calculate the concentration of the analyte using the sensor response at one or more of the detection positions of the sensor, optionally 2, 3, 4, 5, 10, 50, 100 or more than 100 detection positions thereof. At a detection position of the sensor where the sensor reaches a saturated response level upon exposure to the concentration of the analyte, the processor may be configured to calculate the concentration of the analyte based on the time taken for the sensor to reach saturation. At a detection position of the sensor where the sensor reaches a non-saturated response level upon exposure to the concentration of the analyte, the processor may be configured to calculate the concentration of the analyte based on any one or more of: the peak response level, the steady state response level, the time taken for the sensor to reach the peak response level, and the time taken for the sensor to reach the peak response level. 
     In the case where the sensor response to the analyte produces a colour change, the acquisition device may comprise a camera and a light source. The acquisition device may measure data from the sensor over a period of time, such that the final colour (which may be, for example, the steady state response, or peak response) of a detection position of the sensor is measured. It may measure the time taken to reach the steady state response at a detection position of the sensor. It may measure the time taken to reach the peak response at a detection position of the sensor. It may measure the time taken to reach a saturated response for at least one detection position of the sensor that reaches a saturation level. 
     In the case where the sensor response to the analyte produces a change in resistance, the acquisition device may measure changes in local conductivity across the sensor. The change in resistance may be caused by changes in the ionic concentration of the sensor following the interaction with the analyte. The acquisition device may for example comprise multiple electrode pairs that may be connected at either side of the sensor, perpendicular to a sensitivity gradient, and the conductivity of the sensor material may be measured by the acquisition device for at least one detection position of the sensor. 
     The acquisition device may comprise a detector. It may comprise a processor. It may comprise a processor and a detector. 
     Detector 
     The detector may be an ion sensitive field effect transistor (ISFET), chemiresistive sensor, potentiometric sensor, spectroscopic sensor, colorimetric sensor, fluorometric sensor, thermal sensor, or a combination thereof. It may be a combined sensor, for example a combined ISFET and colorimetric sensor. In the case where the detector is a spectroscopic or colorimetric sensor, it may be sensitive to a defined range of electromagnetic radiation, for example, ultraviolet-A (UVA), ultraviolet-B (UVB), ultraviolet-C (UVC), visible light, near-infrared (NIR), far-infrared (FIR), or some combination thereof. It may be a change-coupled device (CCD), an active pixel sensor, or a hybrid thereof. It may, for example, be a complementary metal-oxide-semiconductor (CMOS) sensor, or a scientific complementary metal-oxide-semiconductor (sCMOS) sensor. The skilled person will understand that various sensing apparatuses can be used as the detector such as, for example, a red-green-blue (RGB) colour sensor, a photodiode, a photoresistor, a phototransistor, a light emitting diode (LED), a photovoltaic cell, or an image sensor used, for example, in a smart phone. The detector may comprise a camera and a light source. It may comprise a smart phone. The device may comprise one or more filters, optionally optical filters, which selectively allow particular wavelengths or combinations of wavelengths of electromagnetic radiation to reach the spectroscopic sensor. 
     In the case where the detector is an ISFET, the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt sorbent material may be utilised as the gate electrode for the transistor and an oxide surface may be used as a gate material to separate the reaction media from the transistor channel. Typical gate materials may include SiO 2 , Si 3 N 4 , Al 2 O 3  or Ta 2 O 5 . Hydrolysis of the gate material may occur when it is exposed to ions in the sorbent material, causing the threshold voltage of the ISFET to vary as a function of ion concentration.  FIG. 15 ( a - c ) show different example layouts of the ISFET detector within the device. In certain embodiments where the detector is an ISFET, the sorbent material may be disposable, that is suitable for single use, and the ISFET may be reusable. In particular embodiments, a reference electrode may be positioned adjacent to the ISFET and may be reusable, i.e. non-disposable. The ISFET and the reference electrode may both be in contact with the analyte (i.e. the carbonyl containing compound in the liquid). In another embodiment, the reference electrode may be an Ag/AgCl ‘pseudo reference electrode’, which may be disposable and may be positioned within, or on the surface of the sorbent material. The disposable pseudo reference electrode may be brought into conductive contact with conductive pads when the disposable sorbent material is inserted into the device, thereby forming a sensing circuit. 
     Example device  1500  shown in  FIG. 15( a )  comprises ISFET  1510 , sorbent material  1505  (having optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt sorbed therein and/or thereon), a standard reference electrode  1515  (consisting of electrode wire  1516 , electrolyte  1518 , and membrane  1517 ) and processor  1525 . When a user supplies a breath sample  1520  to the sorbent material  1505 , reaction of a carbonyl-containing compound in the breath sample  1520  with the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt sorbed in and/or sorbed on sorbent material  1505 , produces an increase in hydronium ion concentration  1522 , resulting in a change in the threshold voltage of the ISFET  1510  relative to the standard reference electrode  1515 . The processor  1525  calculates the concentration of the carbonyl-containing compound in the breath sample from the change in voltage. 
     Example device  1530  shown in  FIG. 15( b )  comprises ISFET  1540 , sorbent material  1535  (having optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt sorbed therein and/or thereon), a Reference Electrode Field Effect Transistor (REFET)  1545  and processor  1555 . When a user supplies a breath sample  1550  to the sorbent material  1535 , reaction of a carbonyl-containing compound in the breath sample  1550  with the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt sorbed in and/or sorbed on sorbent material  1535 , produces an increase in hydronium ion concentration  1552 , resulting in a change in the threshold voltage of the ISFET  1540  relative to the REFET  1545 . The processor  1555  calculates the concentration of the carbonyl-containing compound in the breath sample from the change in voltage. 
     Example device  1560  shown in  FIG. 15( c )  comprises ISFET  1570 , sorbent material  1585  (having optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt sorbed therein and/or thereon), a disposable pseudo reference electrode  1575  and processor  1590 . When a user supplies a breath sample  1580  to the sorbent material  1585 , reaction of a carbonyl-containing compound in the breath sample  1580  with the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt sorbed in and/or sorbed on sorbent material  1565 , produces an increase in hydronium ion concentration  1585 , resulting in a change in the threshold voltage of the ISFET  1570  relative to the disposable pseudo reference electrode  1575 . The processor  1590  calculates the concentration of the carbonyl-containing compound in the breath sample from the change in voltage. 
     In the case where the detector is a potentiometric sensor, it may be a potentiostat. In this case, it may comprise three electrodes in contact with the liquid and/or sorbent material, where the potential between a reference electrode and working electrode varies as a function of ion concentration (e.g. pH) of the liquid. 
     In the case where the detector is a fluorometric sensor, it may be, for example, a fluorometer. In this case, the device may comprise one or more fluorophores that are sensitive to one or more products from the reaction of a carbonyl-containing compound with an optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt. 
     In the case where the detector is a thermal sensor, it may be, for example, a thermometer. In this case the detector may measure the temperature change caused by the enthalpy of reaction of a carbonyl-containing compound with an optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt. 
     In the case where the detector is a chemiresistor sensor, the liquid and/or sorbent material may be placed in direct contact with the chemiresistor sensor. The chemiresistor may comprise two opposing electrodes separated by the liquid and/or sorbent material, through which a current is passed at a stable voltage. The chemiresistor measures the resistance across the liquid and/or sorbent material. A higher number of ions (e.g. hydronium ions) in the liquid and/or sorbent material may result in a lower resistance measurement. 
     Alternatively, ion sensitive intermediate substances may be utilised, such as conductive polymers or carbon materials that alter their resistivity depending on the concentration of available ions in their surrounding environment. Furthermore, another method of chemiresistive sensing, a C 4 D sensor (Capacitively-Coupled Contactless Conductivity Detector) may be utilised to determine the resistivity of the liquid and/or sorbent material. An example of device arrangements for a direct resistivity measurement, an intermediate resistivity measurement and C 4 D sensor measurement are shown in  FIGS. 16( a ), 16( b ) and 16( c )  respectively. 
     The example device  1600  depicted in  FIG. 16( a )  comprises sorbent material  1605  (having optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt sorbed therein and/or thereon) between a cathode  1610  and anode  1615 . When a breath sample is exposed to the device, the reaction of a carbonyl-containing compound in the breath sample with the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt sorbed in and/or sorbed on sorbent material  1605 , produces an increase in hydronium ion concentration, resulting in a change in the resistance between cathode  1610  and anode  1615 . A processor (not shown) calculates the concentration of the carbonyl-containing compound in the breath sample from the change in resistance. 
     The example device  1630  depicted in  FIG. 16( b )  comprises sorbent material  1635  (having optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt sorbed therein and/or thereon) in close contact with an ion-sensitive material  1640  which is located between cathode  1645  and anode  1650 . The resistivity of the ion-sensitive material changes depending on the concentration of available ions in the surrounding environment. When a breath sample is exposed to the device, the reaction of a carbonyl-containing compound in the breath sample with the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt sorbed in and/or sorbed on sorbent material  1635 , produces an increase in hydronium ion concentration in the sorbent material, resulting in a change in the resistance in the ion-sensitive material  1640  between cathode  1610  and anode  1615 . A processor (not shown) calculates the concentration of the carbonyl-containing compound in the breath sample from the change in resistance. 
     The example device  1660  depicted in  FIG. 16( c )  comprises liquid  1665  (comprising optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt) circumferentially surrounded by channel wall  1670 . Device  1660  also comprises actuator electrode  1675  and pickup electrode  1680 . When a breath sample is exposed to the liquid, the reaction of a carbonyl-containing compound in the breath sample with the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt in liquid  1665 , produces an increase in hydronium ion concentration in the liquid, resulting in a change in the resistance between actuator electrode  1675  and pickup electrode  1680 . A processor (not shown) calculates the concentration of the carbonyl-containing compound in the breath sample from the change in resistance. 
     The detector may measure the change in the variable over a period of time, such that the final change in the variable (which may be, for example, the steady state response, or peak response) is measured. It may measure the time taken to reach the steady state change in the variable. It may measure the time taken to reach the peak response (i.e. change in variable maximum). It may measure the time taken to reach a saturated response (i.e. change in variable). 
     The detector may be configured to measure a response (i.e. the change in the variable) of the device as a function of time. It may be configured to measure the change in the variable at a defined or predetermined time after exposure of the optionally O-substituted hydroxylammonium salt or an optionally N-substituted hydrazinium salt to the carbonyl-containing compound. The time may be, for example, from about 0.01 seconds to about 120 minutes, or it may be from about 0.01 seconds to about 60 minutes, about 0.01 seconds to about 30 minutes, about 0.01 seconds to about 20 minutes, about 0.01 seconds to about 10 minutes, about 0.01 seconds to about 5 minutes, about 0.01 seconds to about 2 minutes, about 0.01 seconds to about 1 minute, about 0.01 seconds to about 30 seconds, about 30 seconds to about 60 minutes, about 1 minute to about 60 minutes, about 5 minutes to about 60 minutes, about 10 minutes to about 60 minutes, about 5 minutes to about 30 minutes, or about 10 minutes to about 30 minutes. It may be less than about 60 minutes, or less than about 30, 20, 10, 5, 2, 1, 0.5, or 0.1 minutes. It may be, for example, about 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, or 55 seconds. It may be, for example, about 1, 2, 5, 10, 15, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, or 120 minutes. 
     The detector may be configured to measure the time taken for the device to reach a defined response level (i.e. change in variable) after exposure of the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt to the carbonyl-containing compound. The defined response level may be, for example, from about 0% to about 100% of the maximum possible response level, or it may be from about 0% to about 90%, about 0% to about 80%, about 0% to about 70%, about 0% to about 60%, about 0% to about 50%, about 0% to about 40%, about 0% to about 30%, about 0% to about 20%, about 0% to about 10%, about 0% to about 5%, about 0.5% to about 100%, about 1% to about 100%, about 5% to about 100%, about 10% to about 100%, about 20% to about 100%, about 20% to about 60%, about 40% to about 60%, or about 20% to about 50% of the maximum possible response level. It may be, for example, about 0, 0.01, 0.02, 0.05, 0.1, 0.2. 0.5, 1, 2, 5, 10, 20, 25, 30, 40, 50, 60, 70, 75, 80, 90, or 100% of the maximum possible response level. 
     The detector may be configured to measure the time taken for the device to reach a saturated response level (i.e. change in variable) after exposure of the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt to the carbonyl-containing compound. It may be configured to measure the time taken for the device to reach a peak response level (i.e. change in variable) after exposure of the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt to the carbonyl-containing compound. It may be configured to measure the time taken for the device to reach a steady state response level (i.e. change in variable) after exposure of the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt to the carbonyl-containing compound. 
     In certain embodiments the detector may be sensitive of a defined range of electromagnetic radiation, for example, UVA, UVB, UVC, visible light, NIR, FIR, or some combination thereof. It may be a CCD, an active pixel sensor, or a hybrid thereof. It may, for example, be a CMOS sensor, or a sCMOS sensor. The skilled person will understand that various sensing apparatuses can be used as the detector such as, for example, an RGB colour sensor, a photodiode, a photoresistor, a phototransistor, an LED, a photovoltaic cell, or an image sensor used, for example, in a smart phone. 
     The detector may comprise a camera and a light source. It may comprise a smart phone. For example, as depicted in  FIG. 17 , the device  1700  comprises a smart phone  1730  which performs both the detector and processor functions. That is, it is able to both measure the change in the absorption spectrum of the indicator and calculate the concentration of the carbonyl containing compound in the fluid from the change in the absorption spectrum of the indicator. In this example, the device comprises a chip  1710  that comprises the sorbent material (not shown), and a breath input  1720  for a user to breath into, thereby exposing the sorbent material to the fluid (e.g. the user&#39;s breath). In this case, the smart phone is a removable component of the device. The digital nature of the device data captured in, for example, a smart phone component of the device may allow easy transferal to external sources for data analysis. In this example a smartphone could be utilised to perform the spectrometric measurement of the change in absorption spectrum, in addition to analysis and computation. Here, a spectrometric assessment of the chip may be performed both before and after contact with user&#39;s breath, a device housing is utilised such that the camera of the smartphone is positioned beneath the carbonyl containing compound sensing chip, and the change in absorption spectrum is measured via the phone camera to quantify breath carbonyl containing compound levels. 
     External sources include portable computing units, such as those found in smart phones and cloud-based computing technologies. This enables the analysis of data and improvement of the detection of the carbonyl containing compound in the breath through methods such as machine learning. The data processing and analytics of the device response can be processed via hardware and/or software approaches via various platforms such as microcontroller, cloud computing and a smart phone. 
     The detector may measure the change in the absorption maximum over a period of time, such that the final change in absorption maximum (which may be, for example, the steady state response, or peak response) is measured. It may measure the time taken to reach the steady state change in absorption maximum. It may measure the time taken to reach the peak response (i.e. change in absorption maximum). It may measure the time taken to reach a saturated response (i.e. change in absorption maximum). 
     The detector may be configured to measure a response (e.g. the change in the absorption spectrum of the indicator) of the device as a function of time. It may be configured to measure the change in the absorption spectrum of the indicator at a defined or predetermined time after exposure of the sorbent material to the carbonyl containing compound. The time may be, for example, from about 0.01 seconds to about 120 minutes, or it may be from about 0.01 seconds to about 60 minutes, about 0.01 seconds to about 30 minutes, about 0.01 seconds to about 20 minutes, about 0.01 seconds to about 10 minutes, about 0.01 seconds to about 5 minutes, about 0.01 seconds to about 2 minutes, about 0.01 seconds to about 1 minute, about 0.01 seconds to about 30 seconds, about 30 seconds to about 60 minutes, about 1 minute to about 60 minutes, about 5 minutes to about 60 minutes, about 10 minutes to about 60 minutes, about 5 minutes to about 30 minutes, or about 10 minutes to about 30 minutes. It may be less than about 60 minutes, or less than about 30, 20, 10, 5, 2, 1, 0.5, or 0.1 minutes. It may be, for example, about 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, or 55 seconds. It may be, for example, about 1, 2, 5, 10, 15, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, or 120 minutes. 
     The detector may be configured to measure the time taken for the device to reach a defined response level (i.e. change in absorption spectrum of the indicator) after exposure of the sorbent material to the carbonyl containing compound. The defined response level may be, for example, from about 0% to about 100% of the maximum possible response level, or it may be from about 0% to about 90%, about 0% to about 80%, about 0% to about 70%, about 0% to about 60%, about 0% to about 50%, about 0% to about 40%, about 0% to about 30%, about 0% to about 20%, about 0% to about 10%, about 0% to about 5%, about 0.5% to about 100%, about 1% to about 100%, about 5% to about 100%, about 10% to about 100%, about 20% to about 100%, about 20% to about 60%, about 40% to about 60%, or about 20% to about 50% of the maximum possible response level. It may be, for example, about 0, 0.01, 0.02, 0.05, 0.1, 0.2. 0.5, 1, 2, 5, 10, 20, 25, 30, 40, 50, 60, 70, 75, 80, 90, or 100% of the maximum possible response level. 
     The detector may be configured to measure the time taken for the device to reach a saturated response level (i.e. change in absorption spectrum of the indicator) after exposure of the sorbent material to the carbonyl containing compound. It may be configured to measure the time taken for the device to reach a peak response level (i.e. change in absorption spectrum of the indicator) after exposure of the sorbent material to the carbonyl containing compound. It may be configured to measure the time taken for the device to reach a steady state response level (i.e. change in absorption spectrum of the indicator) after exposure of the sorbent material to the carbonyl containing compound. 
     Processor 
     The processor may be configured to calculate the concentration of the carbonyl-containing compound in the fluid from the change in the variable. It may be configured to determine the concentration of the carbonyl-containing compound utilising at least one exposure region of the sensor. It may, for example, determine the concentration of the carbonyl-containing compound by comparing the change in variable to a standard curve. In this example, the standard curve may be a plot of the change in variable for a variety of known carbonyl-containing compound concentrations. The standard curve may, for example, be expressed as a mathematical function. It may, for example, express the concentration of the carbonyl-containing compound as a function of the change in variable. In this case the processor may use the function to calculate the concentration of the carbonyl-containing compound from the change in variable measured by the detector. 
     The processor may be configured to calculate the concentration of the analyte using the response at one or more of the exposure regions of the sorbent layer, optionally 2, 3, 4, 5, 10, 50, 100 or more than 100 exposure regions thereof. 
     In certain embodiments the processor may be configured to calculate the concentration of the carbonyl containing compound in the fluid from the change in the absorption spectrum of the indicator. It may be configured to determine the concentration of the carbonyl containing compound utilising at least one exposure region of the sensor. It may, for example, determine the concentration of the carbonyl containing compound by comparing the change in absorption spectrum of the indicator to a standard curve. In this example, the standard curve may be a plot of the change in absorption spectrum of the indicator for a variety of known carbonyl containing compound concentrations. The standard curve may, for example, be expressed as a mathematical function. It may, for example, express the concentration of the carbonyl containing compound as a function of the change in absorption spectrum of the indicator. In this case the processor may use the function to calculate the concentration of the carbonyl containing compound from the change in absorption spectrum measured by the detector. 
     The processor may be configured to calculate the concentration of the analyte using the response at one or more of the exposure regions of the sorbent layer, optionally 2, 3, 4, 5, 10, 50, 100 or more than 100 exposure regions thereof. 
     Method of Determining a Concentration of an Analyte 
     Disclosed herein is a method of determining a concentration of an analyte in a fluid, the method comprising exposing the fluid to a sensor which is configured to generate a differential response to a constant concentration of the analyte; and measuring a differential response of the sensor to the analyte. 
     The method may further comprise the step of calculating the concentration of the analyte utilising the differential response of the sensor to the analyte. The differential response may be spatially differential, and the sensor may be configured to respond to the analyte at a plurality of detection positions on the sensor having different sensitivities to the analyte. In this case the calculating may utilise at least the following:
         if the sensor reaches a saturated response level at all of the plurality of detection positions on the sensor:
           a response of the sensor to the analyte as a function of time at the detection position on the sensor having the least sensitivity to the analyte;   
           if the sensor does not reach a saturated response level at any of the plurality of detection positions on the sensor:
           a response of the sensor to the analyte at the detection position on the sensor having the greatest sensitivity to the analyte; or   
           if the sensor responds to the analyte to reach a saturated response level at some, but not all, of the plurality of detection positions on the sensor:
           one or more of a ratio of the number of detection positions on the sensor that reach a saturated response level to the number of detection positions on the sensor that do not reach a saturated response level, a time required to reach a saturated response level for at least one of the detection positions on the sensor that reaches a saturated response level, and a response of the sensor to the analyte for at least one of the detection positions on the sensor that does not reach a saturated response level.   
               

     The differential response may be temporally differential, and said calculating may utilise at least one or more of a time required to reach a predetermined response level for at least one position on the sensor after exposure of the fluid to the sensor; and a response level for at least one position on the sensor at a predetermined time after exposure of the fluid to the sensor. 
     The method may further comprise passing the fluid through a flow channel, or into an analysis chamber, in which the sensor is disposed. 
     The method may use the system and/or the sensor disclosed herein. The system of the disclosed herein may be used in the method. 
     The method may further comprise the step of calculating the concentration of the analyte based on a response of the sensor to the analyte as a function of time. The calculating may be performed by the processor. 
     Method of Determining a Concentration of a Carbonyl-Containing Compound 
     In certain embodiments the method as hereinbefore described may be a method of determining a concentration of a carbonyl-containing compound in a fluid. The method may comprise exposing the fluid to an optionally O-substituted hydroxylammonium salt or an optionally N-substituted hydrazinium salt in the presence of a liquid, so as to produce a change in a variable, the variable being dependent upon the pH of the liquid; measuring the change in the variable; and determining the concentration of the carbonyl-containing compound in the fluid from the change in the variable. 
     The carbonyl-containing compound, fluid, and optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt may be as described hereinbefore in relation to the device. The optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt may be sorbed in and/or on a sorbent material. The sorbent material may be as described herein in relation to the device. The liquid may be as described herein in relation to the device. 
     The variable may be an absorbance, fluorescence, resistance, ion concentration, temperature, voltage, or current. In the case where the variable is an ion concentration, it may be a hydronium ion concentration. 
     In the case where the variable is an absorbance, it may be the absorbance of an indicator. In this instance, a change in the absorption spectrum of the indicator may comprise a change in wavelength. It may comprise a change in intensity. The change in the absorption spectrum may be substantially independent of the volume of the fluid exposed to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt and halochromic indicator. It may be substantially independent of the flow rate of the fluid exposed to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt and halochromic indicator. In the case where the indicator is a halochromic indicator, the absorption spectrum of the halochromic indicator may vary monotonically over an operational pH range. The halochromic indicator may be selected from the group consisting of methyl orange, methyl red, methyl yellow chlorophenol red, bromocresol green, congo red, thymol blue, bromophenol blue, cresol red, metacresol purple, malachite green, ethyl violet, crystal violet, 2,4-dinitrophenol, orange IV, erythrosin B, p-(phenylazo)diphenylamine, p-phenylazoaniline, phenol blue, and mixtures thereof. It may, for example, be selected from the group consisting of thymol blue, bromophenol blue, and mixtures thereof. 
     The method may further comprise adding the liquid to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt, prior to or during exposure of the fluid to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt. 
     The method may be capable of rapidly determining the concentration of the carbonyl-containing compound in the fluid. It may, for example, be capable of determining the concentration of the carbonyl-containing compound in the fluid in less than about 10 minutes, or less than about 5 minutes, 2 minutes, 1 minute, 45 seconds or 30 seconds after exposure of the fluid to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt. 
     In some instances, the method does not comprise a titration step. In the case where the method does not comprise a titration step, the method may require only a small liquid volume, such as less than 1 mL. This may enable the method to be integrated into a simplified device design when compared with devices requiring a titration step, and/or may enable the device to be portable. In such a case, the method may be simple to use. That is, it may not require a highly skilled operator. It may, for example, be suitable for an unskilled operator to use. 
     The method may use the device described herein. The device described herein may be used in the method. 
     In certain embodiments there is provided a method of determining a concentration of a carbonyl containing compound in a fluid. The method may comprise exposing the fluid to an optionally substituted hydroxylamine salt and an indicator so as to produce a change in absorption spectrum of the indicator, wherein the optionally substituted hydroxylamine salt and indicator are sorbed in and/or on a sorbent material; measuring the change in the absorption spectrum of the indicator; and determining the concentration of the carbonyl containing compound in the fluid from the change in the absorption spectrum of the indicator. 
     The carbonyl containing compound, fluid, indicator, and optionally substituted hydroxylamine salt may be as described hereinbefore in relation to the device. The optionally substituted hydroxylamine salt and/or indicator may be in contact with a weak base. The weak base may be as described herein in relation to the device. 
     The change in the absorption spectrum of the indicator may comprise a change in wavelength. It may comprise a change in intensity. The change in the absorption spectrum may be substantially independent of the volume of the fluid exposed to the optionally substituted hydroxylamine salt and halochromic indicator. It may be substantially independent of the flow rate of the fluid exposed to the optionally substituted hydroxylamine salt and halochromic indicator. In the case where the indicator is a halochromic indicator, the absorption spectrum of the halochromic indicator may vary monotonically over an operational pH range. The halochromic indicator may be selected from the group consisting of methyl orange, methyl red, methyl yellow chlorophenol red, bromocresol green, congo red, thymol blue, bromophenol blue, cresol red, metacresol purple, malachite green, ethyl violet, crystal violet, 2,4-dinitrophenol, orange IV, erythrosin B, p-(phenylazo)diphenylamine, p-phenylazoaniline and mixtures thereof. It may, for example, be selected from the group consisting of thymol blue, bromophenol blue, and mixtures thereof. 
     The sorbent material may be as described hereinabove with respect to the device. The method may further comprise adding a liquid carrier, optionally to the sorbent material, prior to or during exposure of the fluid to the optionally substituted hydroxylamine salt and indicator. The liquid carrier may be water, or some other suitable solvent. 
     The method may be capable of rapidly determining the concentration of the carbonyl containing compound in the fluid. It may, for example, be capable of determining the concentration of the carbonyl containing compound in the fluid in less than about 10 minutes, or less than about 5 minutes, 2 minutes, 1 minute, 45 seconds or 30 seconds after exposure of the fluid to the optionally substituted hydroxylamine salt and indicator. 
     In some instances, the method does not comprise a titration step. 
     The method may use the device described herein. The device described herein may be used in the method. 
     Use of a Device for Diagnosing and/or Managing Diabetes in a Patient 
     Disclosed herein is the use of a device for diagnosing and/or managing diabetes in a patient, wherein the device is capable of determining a concentration of acetone in a breath sample of the patient, and the device comprises a sorbent material having an optionally O-substituted hydroxylammonium salt or an optionally N-substituted hydrazinium salt, whereby exposure of the breath to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt in the presence of a liquid produces a change in a variable, said variable being dependent upon the pH of the liquid; a detector for measuring the change in the variable; and a processor configured to calculate the concentration of acetone in the breath sample from the change in the variable. 
     The optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt, sorbent material, detector, and processor may be as hereinbefore described. 
     The use may be a use of the device as hereinbefore described. The device as hereinbefore described may be used for the use. The use may use the method as hereinbefore described. The method as hereinbefore described may be used in the use. 
     Disclosed herein is the use of a device for diagnosing and/or managing diabetes in a patient, wherein the device is capable of determining a concentration of acetone in a breath sample of the patient, and the device comprises a sorbent material having an optionally substituted hydroxylamine salt and an indicator sorbed therein and/or thereon, whereby exposure of the breath sample to the sorbent material produces a change in absorption spectrum of the indicator; a detector for measuring the change in the absorption spectrum of the indicator; and a processor configured to calculate the concentration of acetone in the breath sample from the change in the absorption spectrum of the indicator. 
     The sorbent material, optionally substituted hydroxylamine salt, and indicator may be as hereinbefore described. 
     The use may be a use of the device as hereinbefore described. The device as hereinbefore described may be used for the use. The use may use the method as hereinbefore described. The method as hereinbefore described may be used in the use. 
     Methods of Managing and Diagnosing Diabetes 
     Disclosed herein is a method of managing diabetes in a patient on a diabetes treatment plan. The method comprises determining an acetone concentration of a breath sample of the patient using the method of determining a concentration of acetone in a fluid as described above; comparing the acetone concentration to a reference acetone concentration range; and if the acetone concentration is outside the acetone concentration reference range, adjusting the diabetes treatment plan. 
     The diabetes treatment plan may be adjusted, for example, by modifying the administration timing or dosage of a blood glucose management medication. It may be adjusted, for example, by an administration of insulin, or modification of the patient&#39;s total daily dose of insulin. It may be adjusted by modifying the fluid and/or nutrient intake of the patient. It may be adjusted by an administration of glucagon. It may be adjusted, for example, by initiating immediate medical attention, in particular in the case of ketoacidosis. 
     The method may use the device disclosed herein. The device disclosed herein may be used in the method. 
     Disclosed herein is a method to assist in diagnosing diabetes in a patient. The method comprises determining an acetone concentration of a breath sample of the patient using the method of determining a concentration of acetone in a fluid as described above; and comparing the acetone concentration to a reference acetone concentration range to determine if the patient has diabetes. 
     The method may be a component of a diabetes diagnosis method. For example, it may be possible to diagnose diabetes in a patient by using the method and additionally considering other factors which may affect the concentration of ketones in the breath of the patient, such as the diet of the patient. 
     The method may use the device disclosed herein. The device disclosed herein may be used in the method. 
     Disclosed herein is a method of managing diabetes in a patient on a diabetes treatment plan. The method comprises determining an acetone concentration of a breath sample of the patient using the method of determining a concentration of acetone in a fluid as described above; comparing the acetone concentration to a reference acetone concentration range; and if the acetone concentration is outside the acetone concentration reference range, adjusting the diabetes treatment plan. 
     The diabetes treatment plan may be adjusted, for example, by modifying the administration timing or dosage of a blood glucose management medication. It may be adjusted, for example, by an administration of insulin, or modification of the patient&#39;s total daily dose of insulin. It may be adjusted by modifying the fluid and/or nutrient intake of the patient. It may be adjusted by an administration of glucagon. 
     The method may use the device disclosed herein. The device disclosed herein may be used in the method. 
     Disclosed herein is a method of diagnosing diabetes in a patient. The method comprises determining an acetone concentration of a breath sample of the patient using the method of determining a concentration of acetone in a fluid as described above; and comparing the acetone concentration to a reference acetone concentration range to determine if the patient has diabetes. 
     The method may use the device disclosed herein. The device disclosed herein may be used in the method. 
     EXAMPLES 
     The present invention will now be further described in greater detail with reference to the following specific examples, which should not be construed as in any way limiting the scope of the invention. 
     Example 1: A Sensor for the Measurement of Breath Acetone 
     Diabetes mellitus, commonly shortened to diabetes, is a serious health concern globally, and its prevalence and diabetes-related deaths have been increasing with rapid transitions of lifestyle resulting in factors such as obesity. Furthermore, recent estimates have shown the prevalence of diabetes is likely to continue to grow substantially in coming decades. 
     Diabetes is caused by the lack of the hormone insulin, or the body&#39;s inability to use insulin. Diabetics can suffer from low insulin levels, and the elevated levels of blood glucose results in an increase in the body&#39;s ketone concentration. This can cause diabetic ketoacidosis, a state of insulin deficiency that can progress rapidly and which if left untreated can result in coma or death. Therefore, the ability to monitor ketone levels is beneficial as it allows self-monitoring which can help prevent diabetic ketoacidosis. 
     Blood testing currently provides accurate and quantitative measurements, using blood glucose meters. However, the invasive nature of blood testing can be painful for patients, is expensive, and presents risk of infection as well as injury if performed incorrectly. Urine testing is also available but is inaccurate. Acetone levels in human breath have been correlated with ketone bodies in plasma, and therefore in this embodiment, as sensor is provided that is able to sense a concentration of acetone in human breath. 
     In this example, the differential sensitivity to acetone on the sensor is produced by a varying concentration or amount of a selectively reactive chemical, such as hydroxylamine hydrochloride, hydroxylammonium sulfate, or a combination thereof at positions of the sensor. 
     The sensor produces a colour change response upon exposure of the sensor to acetone in a breath sample. The colour change at different sensor positions depends upon the reaction between acetone and the selectively reactive chemical. The colour-metric detection method is utilised to detect the degree of reaction between the breath acetone and the reagent. This reaction causes a colour change across the sensor, to varying degrees, divided into segment A and B as depicted in  FIG. 5( a ) . Segment A consists of the area with a noticeable colour gradient, while segment B consists of the area of the sensor with a homogenous colour. 
     The change in sensitivity across the sensor can be achieved through multiple methods. In this case, the concentration of hydroxylamine hydrochloride is adjusted across the sensor, such that there is a gradient from low concentration to high concentration. This change in concentration causes a change in sensitivity along the length of the sensor, both through changes in the degree of reaction due to the equilibrium between acetone and hydroxylamine, and the initial concentration of H +  due to partial dissociation of hydroxylamine hydrochloride leading to a difference in the signal produced by the colour change of the secondary reaction. 
     In this case, the 0% response for a given local section of the sensor, is defined as the colour of the media before any reaction with acetone. The steady state response level is the point at which there is no further colour change, despite further increases in H +  caused by the reaction. The colour change may be assessed visually. Alternatively, spectroscopic methods may be used to accurately quantify the change. 
     Example 2: Spatially Differential Response Acetone Sensor 
     The sensors depicted in  FIG. 18  each have two discrete triangular shaped regions with higher (top right triangle) and lower (bottom left triangle) sensitivity to acetone. A concentration of 0.633 molL −1  and 1.27 molL −1  of hydroxylamine hydrochloride solution was used for the higher and lower sensitivity regions respectively.  FIG. 18( a )  shows the colour change in the sensor after exposure to 10 ppm acetone. At this acetone concentration the sensor clearly produces distinct colours at each region.  FIG. 18( b )  shows the colour change in the sensor after exposure to 50 ppm acetone. At this acetone concentration the two regions of the sensor produce the same colour, due to a saturated detector response at both regions. 
       FIG. 19  shows a sensor response plotted as a function of time for a position of an example sensor having a rectangular shape and using a 1.08 molL −1  hydroxylamine hydrochloride solution. A TCS230 colour sensor was used to detect the red colour change as a function of time for the first 40 seconds after exposure of the sensor to a fluid with a 50 ppm concentration of acetone. 
     Prototype structures to achieve breath ketone measurement based on the invention may vary. In general, they can be divided into two categories, with air storage and without air storage i.e. a pass-through approach. The former requires the user to breath into an intermediate storage bag, to contain a specified volume of breath. After this bag is filled, the breath is passed over the sensor at a controlled flow rate, to produce a signal response as previously described. The latter requires the user to breathe directly over the sensor, without the use of intermediate storage, the volume of breath that the patient supplies can either be controlled, or measured, for calibration with the signal response produced by the sensor. 
     Example 3: Lab on Chip Device Design 
     An example of the device in a lab on chip design is shown in an exploded view in  FIG. 20 . The device  2000  comprises five layers: liquid layer  2010 , exposure layer  2020 , electrode layer  2030 , reaction layer  2040  and sensing layer  2050 . The reaction layer  2040  consists of a hydrophilic sorbent material, impregnated with hydroxylamine hydrochloride and a halochromic indicator. The sorbent material is selected to allow for fast diffusivity of solution and ion migration. The sensing layer is opaque and contains optically transparent windows positioned beneath the electrode areas, such that spectrometric measurement of these two critical areas can be performed. A spectrophotometric sensor  2060  is positioned below the sensing layer such that it can analyse the optical change at position  2055  through the transparent windows of the sensing layer during the reaction. A processor (not shown) is used to calculate the concentration of the carbonyl containing compound in the fluid from the optical change measured by the spectrophotometric sensor  2060 . 
     The liquid layer  2010  contains on-chip storage of suitable reaction liquids  2075  that can be dispensed on demand into the reaction layer via mechanical actuation (not shown). The device has an opening  2070  in it such that the gas-liquid interface area is exposed for reaction with breath analyte, and has small openings  2095   a ,  2095   b  for dispensing liquids to the sensor regions  2090   a ,  2090   b . By dispensing the solvent through these distinct openings separated from the exposure area, meniscus formation is reduced at the edges of the gas-liquid interface within the sensing region. This reduces interference in the detector measurement of the change in absorption spectrum of the indicator. 
     The electrode layer has both a cathode  2080  and an anode  2085 , positioned above opposite ends of the reaction layer for applying an electric field. When the electric field is applied to the reaction layer, the charged reaction products formed after exposure of the fluid to the sorbent material of the reaction layer migrate and concentrate to a localised area of the sensor based on the charge of the species. This causes all the charged species to concentrate to a smaller sensing area, relative to the total reaction area, and consequently the localised sensor response is increased. 
     Immediately prior to exposing the fluid to the device, a suitable solvent, such as water, may be dispensed and distributed into the sensing region of the chip. Moreover, liquid neutralising reagents may be utilised to react with the chemical formulation present in the device after use, reducing the environmental impact associated with waste disposal of used devices. An additional benefit of this structure shown in  FIG. 20 , is an on-chip method of storing and dispensing these fluids, whereby two containments  2075   a ,  2075   b  for each respective fluid are positioned on the liquid layer. The fluids stored in these containers are dispensed on demand by proper actuation such as mechanical actuation, and are directed into the reaction layer where they diffuse across the hydrophilic sensing region. 
     Prior to breathing into the device  2000 , a liquid such as water or a solution comprising a weak base is dispensed from liquid storage  2075   a  on the liquid storage layer  2010 , thereby pre-wetting and/or adjusting the pH at the reaction layer  2040 . When a user breathes into the device  2000 , the breath sample passes through opening  2070  in exposure layer  2020 , and moves to reaction layer  2040 , where it is exposed to the hydroxylamine salt and halochromic indicator sorbed therein and/or thereon. The hydroxylamine salt reacts with a carbonyl containing compound in the breath sample, reducing the local pH and resulting in a change in absorption spectrum of the indicator. A potential is applied across the electrodes  2080 ,  2085  on the electrode layer  2030 , which causes charged species formed as the indicator is exposed to the reduced local pH to migrate through small openings  2095   a ,  2095   b  to sensor areas  2090   a ,  2090   b , thereby concentrating the charged species in these areas. The spectrophotometric sensor  2060  measures the absorption spectrum change at position  2055  compared to a baseline level, and the processor (not shown) determines the concentration of carbonyl containing compound from the absorption spectrum change. After use, a neutralising agent is dispensed from liquid storage  2075   b  that neutralises the chemical formulation present in the device after use, thereby reducing the environmental impact associated with waste disposal of the used device and/or layers thereof. 
     Example 4: Lab on Chip Device Design with Multiple Exposure Regions Having Different Sensitivities to the Carbonyl Containing Compound 
       FIG. 21  shows an example of a lab on chip device design depicted in an exploded view, where the sorbent material has multiple exposure regions having different sensitivities to the carbonyl compound. Each layer of device  2100  has a similar function to that described in Example 3 (i.e. exposure layer  2110 , electrode layer  2120 , reaction layer  2130  and sensing layer  2140 ). However, in this case the reaction layer contains multiple distinct exposure regions which provide for an increase in working range of the device, and the electrode layer  2120  contains additional electrodes (two of which are labelled as  2170   a ,  2170   b ) such that an individual electric field can be applied to each distinct sensing region, enabling an increase in sensitivity through concentrating charged species in the respective sensor regions. The example device is also depicted with a detector  2150  and having liquid dispenser containers  2160 , both similar to those described in Example 3. The device also comprises a processor (not shown) configured to calculate the concentration of the carbonyl containing compound in the fluid from the change in the absorption spectrum of the indicator. 
     In use, a breath sample is passed through opening  2165  and is directed through the device to three sorbent material comprising reaction areas  2135   a ,  2135   b , and  2135   c  where the breath sample is exposed to the hydroxylamine salt and halochromic indicator. An electric field applied across each reaction zone by the electrodes in the electrode layer  2120  concentrates the charged species in sensing zones (two of which are labelled as  2190   a ,  2190   b ), where the change in absorption spectrum is measured by the detector  2150 . The processor (not shown) calculates the carbonyl containing compound concentration in the breath sample from the change in absorption spectrum at one or more of the sensing zones (two of which are labelled as  2190   a ,  2190   b ). Such a device configuration allows determination of a wide concentration of analyte due to the different sensitivity exposure areas of the sorbent material. For example, a high concentration of analyte may be determined more accurately from the change in absorption spectrum at a position of lower sensitivity to the analyte, whereas a lower concentration of analyte may be determined more accurately from the change in absorption spectrum at a position of higher sensitivity to the analyte. 
     Example 5: Acetone Detection System with Hydroxylamine Hydrochloride and a Halochromic Indicator in Solution 
     A reagent solution comprising hydroxylamine hydrochloride and a halochromic indicator was added to an absorbent material. Gas samples comprising various concentrations of acetone were exposed to the absorbent material for a period of 30 seconds, which approximates the duration of a standard human breath. The sensor response was measured at two different wavelengths (response  1  and  2 ) using a spectrophotometer. The results in  FIG. 22  show that the response changed as a function of the concentration of acetone in the gas. 
     It is clear that the ketone detection system could be used to provide a different response with different concentration of acetone. With higher concentration of acetone, the sensor could provide greater negative response. The resolution of this system is defined as 
       Resolution=Acetone concentration (ppm)/Sensor response 
     The resolution of the system is around 2 ppm for both responses (i.e. wavelength measurements) in a low concentration range, and 5 ppm for high concentration gas. It is thought that the negative value at concentration of 0 ppm is related to the drying effect that concentrates the reagent on the testing strip and provides a greater response for both channels. 
     Example 6: Acetone Detection System with Hydroxylamine Hydrochloride and a Halochromic Indicator Pre-Impregnated in the Testing Strip 
     Hydroxylamine hydrochloride and a halochromic indicator were pre-impregnated in a testing strip. The strip was wetted with water to form a solution before the test. Gas samples comprising various concentrations of acetone were exposed to the absorbent material for a period of 30 seconds, which approximates the duration of a standard human breath. The sensor response was measured at two different wavelengths (response  1  and  2 ) using a spectrophotometer. The results in  FIG. 23  show that the response changed as a function of the concentration of acetone in the gas. 
       FIG. 23  shows that the sensor response was greater for this device when comparing the results for the same acetone concentration exposure for the device described in Example 5 above. Although the resolution was higher in the low concentration range of the device results shown in  FIG. 23 , there was some cross over between the responses at the higher concentration range, presumably because the chemical sensor was operating close to its saturation limit. 
     Example 7: Acetone Detection System with Varying Gas Volume/Flow Rate 
     Hydroxylamine hydrochloride and a halochromic indicator were added to a testing strip. The strip was prewetted with water. Gas samples comprising various concentrations of acetone were exposed to the absorbent material for a period of 30 seconds, at a flow rate of 0.5 L/min or 0.75 L/min. The sensor response was measured using a spectrophotometer. The results in  FIG. 24  show that the response changed as a function of the concentration of acetone in the gas, but the response did not substantially change as a function of the flow rate (or volume) of gas. 
     The chart illustrates that increasing the volume of gas by 50% will not significantly change the sensor response, and the response at the different concentrations does not overlap (for a particular flow rate). Although different gas volumes did not produce identical results, a higher volume sample with a lower acetone concentration will clearly not produce the same response as a lower volume sample with a higher ketone concentration. In other words, the ketone detection system is substantially fluid volume independent. 
     Example 8: Acetone Detection System with a Weak Base 
     The acetone detection system described at Example 6 can be adjusted with the addition of a weak base to the testing strip. Base adjustment using sodium bicarbonate increased the resolution and accuracy for samples in the higher concentration range.  FIG. 25  shows the sensor response with base adjustment on the testing strip. The accuracy of the test could be estimated by the extent or presence of error bars crossing each other with samples having different gas concentrations, in which case the system cannot distinguish the concentration based on the sensor reading. The results in  FIG. 25  show that the addition of the weak base allows a higher accuracy at a high concentration range could be achieved when compared with the results of  FIG. 23  which show some cross over at 40 and 50 ppm. The resolution in the example system with the base adjustment ( FIG. 25 ) is 2 ppm—in the low concentration range and 5 ppm—in the higher concentration range tested. 
     Example 9: Acetone Sensor 
       FIG. 26  shows a response (i.e. a change in absorption spectrum of the indicator) plotted as a function of time for a position of an example sensor having a rectangular shape and using a 1.08 molL −1  hydroxylamine hydrochloride solution. A TCS230 colour sensor was used to detect the red colour change as a function of time for the first 40 seconds after exposure of the sensor to a fluid with a 50 ppm—concentration of acetone. 
     Prototype structures to achieve breath ketone measurement based on the invention may vary. In general, it can be divided into two categories, with air storage and without air storage i.e. a pass-through approach. The former requires the user to breath into an intermediate storage bag, to contain a specified volume of breath. After this bag is filled, the breath is passed over the sorbent material at a controlled flow rate, to produce a signal response (i.e. a change in absorption spectrum of the indicator) as previously described. The latter requires the user to breathe directly over the sensor, without the use of intermediate storage, the volume of breath that the patient supplies can either be controlled, or measured, for calibration with the signal response produced by the device. 
     Example 10: Acetone Detection System with Hydroxylamine Hydrochloride and an ISFET pH Sensor 
     An experimental study was carried out based on the usage of an ISFEST pH sensor. The hydrogen ion sensing layer was made of silicon nitride, and the reference electrode was made of Ag/AgCl contained in potassium chloride solution (similar to the device design depicted in  FIG. 15( a ) ). The test was carried out at 26° C. at 65% humidity. In the experiment, hydroxylamine hydrochloride solution was added on a sorbent material strip before being exposed to a gas consisting of a mixture of air and acetone. Three concentrations of acetone gas (75, 150 and 300 ppm) were used in the testing at a gas flowrate of 1.2 L/min for an exposure period of 30 seconds. After the reaction was finished, distilled water was added onto the strip. Both the ISFET sensor and the reference electrode were attached to the strip so as to produce a voltage reading. For each gas concentration, six measurements were performed and the average value was taken so as to minimize any experimental errors. The test results at different acetone concentrations are shown in  FIG. 27 . 
     As can be seen from the results at  FIG. 27 , the ISFET sensor shows a voltage drop with an increasing amount of acetone. This is due to an increasing amount of H 3 O +  through the reaction of acetone with the hydroxylamine hydrochloride to produce H 3 O + . The sensor exhibits a voltage difference which is dependent upon the concentration of acetone in the gas sample. The resolution of the device was defined as follows: 
       Resolution=Acetone concentration (ppm)/Sensor response 
     Yielding a resolution for the device of around 2.65 ppm/mV. 
     The skilled person will appreciate that the examples disclosed herewith with respect to sensors, systems and methods for determining the concentration of carbonyl-containing compound (e.g. acetone) samples could be adapted without undue burden to sensors, systems and methods for determining the concentration of a variety of other analytes. 
     Disclosed herein are the following forms: 
     A1. A system for determining a concentration of an analyte in a fluid, said system comprising: 
     a sensor configured to generate a differential response to a constant concentration of the analyte; and 
     an acquisition device configured to determine the concentration of the analyte from the differential response. 
     A2. The system of form A1, wherein the differential response is temporally differential, whereby the response varies over time. 
     A3. The system of form A1 or A2, wherein the differential response is spatially differential, whereby the sensor is configured to respond to the analyte at a plurality of detection positions on the sensor, said detection positions having different sensitivities to the analyte. 
     A4. The system of form A3, wherein sensitivity to the analyte varies along a length of the sensor. 
     A5. The system of form A4, wherein the plurality of detection positions are spaced equidistantly along the length of the sensor. 
     A6. The system of form A4 or A5, wherein sensitivity to the analyte varies monotonically along the length of the sensor. 
     A7. The system of any one of forms A4 to A6, wherein sensitivity to the analyte varies linearly or logarithmically along the length of the sensor. 
     A8. The system of any one of forms A3 to A7, wherein the sensor comprises a plurality of discrete regions each having a different sensitivity to the analyte. 
     A9. The system of any one of forms A3 to A8, wherein the sensor comprises a coating on a detection surface, said coating having spatially differential permeability to the analyte. 
     A10. The system of any one of forms A3 to A9, wherein the sensor comprises at least three detection positions having different sensitivities to the analyte. 
     A11. The system of any one of forms A1 to A10, wherein the acquisition device comprises a processor configured to calculate the concentration of the analyte utilising the differential response of the sensor to the analyte. 
     A12. The system of any one of forms A1 to A11, wherein the sensor is configured to respond to the analyte at a plurality of detection positions on the sensor; said detection positions having different sensitivities to the analyte; and 
     the acquisition device is configured to calculate the concentration of the analyte utilising at least the following:
         when the sensor reaches a saturated response level at all of the plurality of detection positions on the sensor:
           a response of the sensor to the analyte as a function of time at the detection position on the sensor having the least sensitivity to the analyte;   
           when the sensor does not reach a saturated response level at any of the plurality of detection positions on the sensor:
           a response of the sensor to the analyte at the detection position on the sensor having the greatest sensitivity to the analyte; and   
           if the sensor responds to the analyte to reach a saturated response level at some, but not all, of the plurality of detection positions on the sensor:
           one or more of:
               a ratio of the number of detection positions on the sensor that reach a saturated response level to the number of detection positions on the sensor that do not reach a saturated response level,   a time required to reach a saturated response level for at least one of the detection positions on the sensor that reaches a saturated response level, and   a response of the sensor to the analyte for at least one of the detection positions on the sensor that does not reach a saturated response level.   
               
               

     A13. The system of any one of forms A1 to A12, further comprising a flow channel or an analysis chamber, wherein the sensor is disposed within said flow channel or analysis chamber. 
     A14. The system of form A13, further comprising a flow controller configured to control and/or monitor a flow rate of the fluid into said analysis chamber or through said flow channel. 
     A15. The system of form A13 or A14, further comprising a volume controller configured to control and/or monitor the volume of the fluid passing into said analysis chamber or through said flow channel. 
     A16. The system of any one of forms A1 to A15, wherein the analyte is a biomarker for diet, a disease, or a disorder. 
     A17. The system of any one of forms A1 to A16, wherein the analyte is a metal, protein, antibody, or small organic molecule. 
     A18. The system of any one of forms A1 to A17, wherein the analyte is acetone, and the sensor comprises a compound capable of reacting with acetone. 
     A19. The system of form A18, wherein the compound is selected from the group consisting of hydroxylamines, hydroxylamine salts, amines, alcohols, and combinations thereof. 
     A20. A method of determining a concentration of an analyte in a fluid, the method comprising:
         exposing the fluid to a sensor which is configured to generate a differential response to a constant concentration of the analyte; and   measuring a differential response of the sensor to the analyte.       

     A21. The method of form A20, further comprising the step of calculating the concentration of the analyte utilising the differential response of the sensor to the analyte. 
     A22. The method of form A21, wherein the differential response is spatially differential, whereby the sensor is configured to respond to the analyte at a plurality of detection positions on the sensor; said detection positions having different sensitivities to the analyte; and 
     wherein said calculating utilises at least the following:
         if the sensor reaches a saturated response level at all of the plurality of detection positions on the sensor:
           a response of the sensor to the analyte as a function of time at the detection position on the sensor having the least sensitivity to the analyte;   
           if the sensor does not reach a saturated response level at any of the plurality of detection positions on the sensor:
           a response of the sensor to the analyte at the detection position on the sensor having the greatest sensitivity to the analyte; or   
           if the sensor responds to the analyte to reach a saturated response level at some, but not all, of the plurality of detection positions on the sensor:
           one or more of:
               a ratio of the number of detection positions on the sensor that reach a saturated response level to the number of detection positions on the sensor that do not reach a saturated response level,   a time required to reach a saturated response level for at least one of the detection positions on the sensor that reaches a saturated response level, and   a response of the sensor to the analyte for at least one of the detection positions on the sensor that does not reach a saturated response level.   
               
               

     A23. The method of form A21, wherein the differential response is temporally differential, and wherein said calculating utilises at least one or more of the following:
         a time required to reach a predetermined response level for at least one position on the sensor after exposure of the fluid to the sensor; and   a response level for at least one position on the sensor at a predetermined time after exposure of the fluid to the sensor.       

     A24. The method of any one of forms A20 to A23 further comprising passing the fluid through a flow channel, or into an analysis chamber, in which the sensor is disposed. 
     A25. A sensor for sensing a concentration of an analyte in a fluid, said sensor configured to generate a differential response to a constant concentration of the analyte. 
     A26. The sensor of form A25, wherein the differential response is temporally differential, whereby the response varies over time. 
     A27. The sensor of form A25 or A26, wherein the differential response is spatially differential, whereby the sensor is configured to respond to the analyte at a plurality of detection positions on the sensor, said detection positions having different sensitivities to the analyte. 
     A28. The sensor of form A27, wherein sensitivity to the analyte varies along a length of the sensor. 
     A29. The sensor of form A28 wherein the plurality of detection positions are spaced equidistantly along the length of the sensor. 
     A30. The sensor of forms A28 or A29, wherein sensitivity to the analyte varies monotonically along the length of the sensor. 
     A31. The sensor of any one of forms A28 to A30, wherein sensitivity to the analyte varies linearly or logarithmically along the length of the sensor. 
     A32. The sensor of any one of forms A27 to A31, comprising a plurality of discrete regions each having a different sensitivity to the analyte. 
     A33. The sensor of any one of forms A27 to A32, comprising a coating on a detection surface, said coating having spatially differential permeability to the analyte. 
     A34. The sensor of any one of forms A27 to A33, comprising at least three detection positions having different sensitivities to the analyte. 
     A35. The sensor of any one of forms A25 to A34, wherein the analyte is a biomarker for diet, a disease, or a disorder. 
     A36. The sensor of any one of forms A25 to A35, wherein the analyte is a metal, protein, antibody, or small organic molecule. 
     A37. The sensor of any one of forms A25 to A36, wherein the analyte is acetone, and the sensor comprises a compound capable of reacting with acetone. 
     A38. The sensor of form A37, wherein the compound is selected from the group consisting of hydroxylamines, hydroxylamine salts, amines, alcohols, and combinations thereof. 
     B1. A device for determining a concentration of a carbonyl containing compound in a fluid, said device comprising:
         a sorbent material having an optionally substituted hydroxylamine salt and an indicator sorbed therein and/or thereon, whereby exposure of the fluid to the sorbent material produces a change in absorption spectrum of the indicator;   a detector for measuring the change in the absorption spectrum of the indicator; and   a processor configured to calculate the concentration of the carbonyl containing compound in the fluid from the change in the absorption spectrum of the indicator.       

     B2. The device of form B1, further comprising a plurality of electrodes configured to apply an electric field across a surface of the sorbent material so as to concentrate one or more charged species on and/or in the sorbent material. 
     B3. The device of form B1 or B2, further comprising a detection region which is in fluid connection with the sorbent material and is optically coupled to the detector. 
     B4. The device of any one of forms B1 to B3, wherein the sorbent material comprises a plurality of exposure regions, said plurality of exposure regions having different sensitivities to the carbonyl containing compound. 
     B5. The device of any one of forms B1 to B4, wherein the sorbent material has an exposure surface for exposure to the fluid, said exposure surface comprising a coating having spatially differential permeability to the carbonyl containing compound. 
     B6. The device of any one of forms B1 to B5, wherein the detector is a CCD, an active-pixel sensor, or a hybrid thereof. 
     B7. The device of any one of forms B1 to B6, wherein the sorbent material further comprises a weak base sorbed thereon and/or therein. 
     B8. The device of form B7, wherein the base is selected from the group consisting of bicarbonate salts, ammonia, aniline, phenolate salts, and mixtures thereof. 
     B9. The device of any one of forms B1 to B8, wherein the optionally substituted hydroxylamine salt is selected from the group consisting of hydroxylamine hydrochloride, hydroxylammonium sulfate, O-benzylhydroxylamine hydrochloride, hydroxylammonium phosphate, hydroxylamine nitrate, and mixtures thereof. 
     B10. The device of any one of forms B1 to B9, wherein the indicator is a halochromic indicator. 
     B11. The device of form B10, wherein the halochromic indicator is selected from the group consisting of methyl orange, methyl red, methyl yellow, methyl green, methyl violet, chlorophenol red, bromocresol green, congo red, thymol blue, bromophenol blue, cresol red, metacresol purple, malachite green, ethyl violet, crystal violet, 2,4-dinitrophenol, orange IV, erythrosin B, p-(phenylazo)diphenylamine, p-phenylazoaniline and mixtures thereof. 
     B12. The device of any one of forms B1 to B11, wherein the change in absorption spectrum of the indicator is a change of absorption maximum of the indicator. 
     B13. The device of form B12, wherein the change in the absorption maximum of the indicator comprises a change in wavelength. 
     B14. The device of form B12 or B13, wherein the change in the absorption maximum of the indicator comprises a change in intensity. 
     B15. The device of any one of forms B1 to B14, wherein the fluid is a gas and/or vapour. 
     B16. The device of any one of forms B1 to B15, wherein the fluid is breath. 
     B17. The device of any one of forms B1 to B16, wherein the carbonyl containing compound is selected from the group consisting of ketones, aldehydes, esters, thioesters, amides, and carboxylic acids. 
     B18. The device of any one of forms B1 to B17, wherein the carbonyl containing compound is selected from the group consisting of acetoacetate, butyraldehyde, 4-hydroxy-2-hexenal, ethyl acetate, acetic acid, 4-hydroxy-2-nonenal, 2-pentanone, 2-butanone, 3-hydroxy-2-butanone, hydroxyacetaldehyde, acetaldehyde, pentanal, malondialdehyde, acrolein, beta-hydroxybutyrate, and acetone. 
     B19. The device of any one of forms B1 to B18, further comprising a neutralisation chamber, said neutralisation chamber comprising a neutralisation reagent capable of neutralising a compound sorbed on and/or in the sorbent material. 
     B20. A method of determining a concentration of a carbonyl containing compound in a fluid, said method comprising:
         (a) exposing the fluid to an optionally substituted hydroxylamine salt and an indicator so as to produce a change in absorption spectrum of the indicator, wherein the optionally substituted hydroxylamine salt and indicator are sorbed in and/or on a sorbent material;   (b) measuring the change in the absorption spectrum of the indicator; and   (c) determining the concentration of the carbonyl containing compound in the fluid from the change in the absorption spectrum of the indicator.       

     B21. The method of form B20, wherein the optionally substituted hydroxylamine salt and/or the indicator are in contact with a weak base. 
     B22. The method of form B21, wherein the weak base is selected from the group consisting of bicarbonate salts, ammonia, aniline, phenolate salts, and mixtures thereof. 
     B23. The method of any one of forms B20 to B22, wherein the change in the absorption spectrum is substantially independent of the volume of the fluid exposed to the optionally substituted hydroxylamine salt and an indicator. 
     B24. The method of any one of forms B20 to B23, wherein the optionally substituted hydroxylamine salt is selected from the group consisting of hydroxylamine hydrochloride, hydroxylammonium sulfate, O-benzylhydroxylamine hydrochloride, hydroxylammonium phosphate, hydroxylamine nitrate, and mixtures thereof. 
     B25. The method of any one of forms B20 to B24, wherein the indicator is a halochromic indicator. 
     B26. The method of form B25, wherein the absorption spectrum of the halochromic indicator varies monotonically over an operational pH range. 
     B27. The method of form B25 or B26, wherein the halochromic indicator is selected from the group consisting of methyl orange, methyl red, methyl yellow chlorophenol red, bromocresol green, congo red, thymol blue, bromophenol blue, cresol red, metacresol purple, malachite green, ethyl violet, crystal violet, 2,4-dinitrophenol, orange IV, erythrosin B, p-(phenylazo)diphenylamine, p-phenylazoaniline and mixtures thereof. 
     B28. The method of any one of forms B20 to B27, wherein the change in absorption spectrum of the indicator is a change of absorption maximum of the indicator. 
     B29. The method of form B28, wherein the change in the absorption maximum of the indicator comprises a change in wavelength. 
     B30. The method of form B28 or B29, wherein the change in the absorption maximum of the indicator comprises a change in intensity. 
     B31. The method of any one of forms B20 to B30, wherein the fluid is a gas and/or vapour. 
     B32. The method of any one of forms B20 to B31, wherein the fluid is breath. 
     B33. The method of any one of forms B20 to B32, wherein the carbonyl containing compound is selected from the group consisting of ketones, aldehydes, esters, thioesters, amides, and carboxylic acids. 
     B34. The method of any one of forms B20 to B33, wherein the carbonyl containing compound is selected from the group consisting of acetoacetate, butyraldehyde, 4-hydroxy-2-hexenal, ethyl acetate, acetic acid, 4-hydroxy-2-nonenal, 2-pentanone, 2-butanone, 3-hydroxy-2-butanone, hydroxyacetaldehyde, acetaldehyde, pentanal, malondialdehyde, acrolein, beta-hydroxybutyrate, and acetone. 
     B35. Use of a device for diagnosing and/or managing diabetes in a patient, 
     wherein the device is capable of determining a concentration of acetone in a breath sample of the patient, and the device comprises:
         a sorbent material having an optionally substituted hydroxylamine salt and an indicator sorbed therein and/or thereon, whereby exposure of the breath sample to the sorbent material produces a change in absorption spectrum of the indicator;   a detector for measuring the change in the absorption spectrum of the indicator; and   a processor configured to calculate the concentration of the acetone in the breath sample from the change in the absorption spectrum of the indicator.       

     B36. A method of managing diabetes in a patient on a diabetes treatment plan, said method comprising:
         (a) determining an acetone concentration of a breath sample of the patient using the method of form B32;   (b) comparing the acetone concentration to a reference acetone concentration range; and   (c) if the acetone concentration is outside the acetone concentration reference range, adjusting the diabetes treatment plan.       

     B37. A method of diagnosing diabetes in a patient, said method comprising:
         (a) determining an acetone concentration of a breath sample of the patient using the method of form B32; and   (b) comparing the acetone concentration to a reference acetone concentration range to determine if the patient has diabetes.       

     C1. A device for determining a concentration of a carbonyl-containing compound in a fluid, said device comprising:
         an optionally O-substituted hydroxylammonium salt or an optionally N-substituted hydrazinium salt, whereby exposure of the fluid to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt in the presence of a liquid produces a change in a variable, said variable being dependent upon the pH of the liquid;   a detector for measuring the change in the variable; and   a processor configured to calculate the concentration of the carbonyl-containing compound in the fluid from the change in the variable.       

     C2. The device of form C1, wherein the variable is an absorbance, fluorescence, resistance, ion concentration, temperature, voltage, or current. 
     C3. The device of form C2, wherein the ion concentration is a hydronium ion concentration. 
     C4. The device of any one of forms C1 to C3, wherein the liquid volume is about 1 mL or less. 
     C5. The device of any one of forms C1 to C4, wherein the device further comprises a sorbent material, whereby the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt is sorbed in and/or on the sorbent material. 
     C6. The device of any one of forms C1 to C5, wherein the change in the variable is not measured using a titration. 
     C7. The device of any one of forms C1 to C6, wherein the detector is an ion sensitive field effect transistor (ISFET), chemiresistive sensor, potentiometric sensor, spectroscopic sensor, colorimetric sensor, fluorometric sensor, thermal sensor, or a combination thereof. 
     C8. The device of any one of forms C1 to C7, further comprising a flow controller configured to control and/or monitor the volume of the fluid which is exposed to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt. 
     C9. The device of any one of forms C1 to C8, wherein the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt is selected from compounds of Formula (I): 
       [R m —Y—NH 3   + ] n X p   Formula (I)
         wherein Y=O or N;   m=1, 2, or 3;   n=1, 2, or 3;   p=1, 2, or 3;   R is independently selected from the group consisting of hydrogen, halogen, optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted heteroalkyl, optionally substituted cycloheteroalkyl, optionally substituted alkenyl, optionally substituted cycloalkenyl, optionally substituted alkylaryl, optionally substituted aryl, optionally substituted alkylheteroaryl, and optionally substituted heteroaryl; wherein said optionally substituted alkyl, cycloalkyl, heteroalkyl, cycloheteroalkyl, alkenyl, cycloalkenyl, alkylaryl, aryl, alkylheteroaryl, or heteroaryl is optionally substituted with one or more groups independently selected from the group consisting of halogen, hydroxyl, thiol, amino, aryl, heteroaryl, alkyl, alkenyl, alkynyl, heteroalkyl, alkylaryl, and alkylheteroaryl; and   X is an anion.       

     C10. The device of any one of forms C1 to C9, wherein the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt is selected from the group consisting of hydroxylammonium chloride, hydroxylammonium sulfate, hydroxylammonium azide, O-phenyl hydroxylammonium chloride, O-phenyl hydroxylammonium sulfate, O-phenyl hydroxylammonium azide, hydroxylamine picrate, hydroxylamine benzenesulfonate, hydroxylamine benzenesulfinate, hydroxylamine 1-sulfobutyl-3-methyl imidazole hydrosulphate salt, hydrazinium chloride, hydrazinium sulfate, hydrazinium hydrogensulfate, hydrazinium hydrogen sulfate, hydrazinium azide, hydrazinediium sulfate, hydrazinediium bis(6-carboxypyridazine-3-carboxylate), hydroxylammonium phosphate, hydroxylamine nitrate, and mixtures thereof. 
     C11. The device of any one of forms C1 to C10, wherein the fluid is a gas and/or vapour. 
     C12. The device of any one of forms C1 to C11, wherein the fluid is breath. 
     C13. The device of any one of forms C1 to C12, wherein the carbonyl-containing compound is selected from the group consisting of ketones, aldehydes, esters, thioesters, amides and carboxylic acids. 
     C14. The device of any one of forms C1 to C13, wherein the carbonyl-containing compound is selected from the group consisting of acetoacetate, butyraldehyde, 4-hydroxy-2-hexenal, ethyl acetate, acetic acid, 4-hydroxy-2-nonenal, 2-pentanone, 2-butanone, 3-hydroxy-2-butanone, hydroxyacetaldehyde, acetaldehyde, pentanal, malondialdehyde, acrolein, beta-hydroxybutyrate, and acetone. 
     C15. A method for determining a concentration of a carbonyl-containing compound in a fluid, said method comprising:
         (a) exposing the fluid to an optionally O-substituted hydroxylammonium salt or an optionally N-substituted hydrazinium salt in the presence of a liquid, so as to produce a change in a variable, said variable being dependent upon the pH of the liquid;   (b) measuring the change in the variable; and   (c) determining the concentration of the carbonyl-containing compound in the fluid from the change in the variable.       

     C16. The method of form C15, wherein the variable is an absorbance, fluorescence, resistance, ion concentration, temperature, voltage, or current. 
     C17. The method of form C16, wherein the ion concentration is a hydronium ion concentration. 
     C18. The method of any one of forms C15 to C17, wherein the liquid volume is about 1 mL or less. 
     C19. The method of any one of forms C15 to C18, wherein the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt is sorbed in and/or on a sorbent material. 
     C20. The method of any one of forms C15 to C19, wherein the change in the variable is not measured using a titration. 
     C21. The method of any one of forms C15 to C20, wherein the change in the variable is measured using an ion sensitive field effect transistor (ISFET), chemiresistive sensor, potentiometric sensor, spectroscopic sensor, colorimetric sensor, fluorometric sensor, thermal sensor, or a combination thereof. 
     C22. The method of any one of forms C15 to C21, further comprising controlling and/or monitoring the volume of the fluid which is exposed to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt. 
     C23. The method of any one of forms C15 to C22, wherein the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt is selected from compounds of Formula (I): 
       [R m —Y—NH 3   + ] n X p   Formula (I)
         wherein Y=O or N;   m=1, 2, or 3;   n=1, 2, or 3;   p=1, 2, or 3;   R is independently selected from the group consisting of hydrogen, halogen, optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted heteroalkyl, optionally substituted cycloheteroalkyl, optionally substituted alkenyl, optionally substituted cycloalkenyl, optionally substituted alkylaryl, optionally substituted aryl, optionally substituted alkylheteroaryl, and optionally substituted heteroaryl; wherein said optionally substituted alkyl, cycloalkyl, heteroalkyl, cycloheteroalkyl, alkenyl, cycloalkenyl, alkylaryl, aryl, alkylheteroaryl, or heteroaryl is optionally substituted with one or more groups independently selected from the group consisting of halogen, hydroxyl, thiol, amino, aryl, heteroaryl, alkyl, alkenyl, alkynyl, heteroalkyl, alkylaryl, and alkylheteroaryl; and   X is an anion.       

     C24. The method of any one of forms C15 to C23, wherein the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt is selected from the group consisting of hydroxylammonium chloride, hydroxylammonium sulfate, hydroxylammonium azide, O-phenyl hydroxylammonium chloride, O-phenyl hydroxylammonium sulfate, O-phenyl hydroxylammonium azide, hydroxylamine picrate, hydroxylamine benzenesulfonate, hydroxylamine benzenesulfinate, hydroxylamine 1-sulfobutyl-3-methyl imidazole hydrosulphate salt, hydrazinium chloride, hydrazinium sulfate, hydrazinium hydrogensulfate, hydrazinium hydrogen sulfate, hydrazinium azide, hydrazinediium sulfate, hydrazinediium bis(6-carboxypyridazine-3-carboxylate), hydroxylammonium phosphate, hydroxylamine nitrate, and mixtures thereof. 
     C25. The method of any one of forms C15 to C24, wherein the fluid is a gas and/or vapour. 
     C26. The method of any one of forms C15 to C25, wherein the fluid is breath. 
     C27. The method of any one of forms C15 to C26, wherein the carbonyl-containing compound is selected from the group consisting of ketones, aldehydes, esters, thioesters, amides and carboxylic acids. 
     C28. The method of any one of forms C15 to C27, wherein the carbonyl-containing compound is selected from the group consisting of acetoacetate, butyraldehyde, 4-hydroxy-2-hexenal, ethyl acetate, acetic acid, 4-hydroxy-2-nonenal, 2-pentanone, 2-butanone, 3-hydroxy-2-butanone, hydroxyacetaldehyde, acetaldehyde, pentanal, malondialdehyde, acrolein, beta-hydroxybutyrate, and acetone. 
     C29. Use of a device for diagnosing and/or managing diabetes in a patient, wherein the device is capable of determining a concentration of acetone in a breath sample of the patient, and the device comprises:
         an optionally O-substituted hydroxylammonium salt or an optionally N-substituted hydrazinium salt, whereby exposure of the breath to the optionally O-substituted hydroxylammonium salt or optionally N-substituted hydrazinium salt in the presence of a liquid produces a change in a variable, said variable being dependent upon the pH of the liquid;   a detector for measuring the change in the variable; and   a processor configured to calculate the concentration of the acetone in the breath from the change in the variable.       

     C30. A method of managing diabetes in a patient on a diabetes treatment plan, said method comprising:
         (a) determining an acetone concentration of a breath sample of the patient using the method of form C26;   (b) comparing the acetone concentration to a reference acetone concentration range; and   (c) if the acetone concentration is outside the acetone concentration reference range, adjusting the diabetes treatment plan.       

     C31. A method to assist in diagnosing diabetes in a patient, said method comprising:
         (a) determining an acetone concentration of a breath sample of the patient using the method of form C26; and   (b) comparing the acetone concentration to a reference acetone concentration range to determine if the patient has diabetes.       

     It will be appreciated that, although specific embodiments of the invention have been described herein for the purpose of illustration, various modifications may be made without deviating from the spirit and scope of the invention as defined in the following claims.