FLOW OPTRODE

Abstract: There is disclosed a device capable of continuously measuring the presence and concentration of an analyte or analytes and a method for using said device in a liquid and/or a gas phase reaction volume. The inventive device comprises a sensor probe, a reservoir, and a detection means. The inventive device delivers reagent to the sensor probe in a flow method to directly and continuously renew reagent, thereby allowing the continuous measurement of the presence and the concentration of an analyte or analytes.

Description: Description FLOW OPTRODE Technical Field of the Invention The present invention relates to a device and a method to continuously measure the presence and concentration of an analyte or analytes in a liquid and/or a gas phase. The invention utilizes a flowing reagent that reacts with the analyte or analytes and the reaction product is detected in an optical sensor device. The reagent can use irreversible chemistry and reagent renewal is under direct and continuous control. Background of the Invention Fiberoptic-based chemical sensor devices are becoming established analytical tools for remote and in situ chemical sensing. The ideal optical sensor device must have the ability to measure the concentration of an analyte continuously through changes in the optical properties of the sensing reagent. For example, for oceanographic and environmental sampling, conventional methods for measuring nutrient distributions, species of geochemical interest, and toxic chemicals usually require cumbersome discrete sampling techniques. The required sample handling and processing techniques are often time- consuming and labor-intensive and are subject to contamination and storage problems. Because samples are often processed days or weeks after collection, these techniques are not well-suited for mapping distributions of chemical constituents in dynamic aquatic environments. Sensors are needed that can be deployed in situ for the rapid, remote measurement of chemical species in aquatic samples. Many colori etric or fluorometric techniques are irreversible because they form a tight binding complex or utilize reagents that generate an irreversibly colored adduct. Irreversible sensors can be used if they operate in an integrating mode; however, they must be replenished frequently with fresh sensing reagent. Accordingly, it is often difficult to reuse the same sensor probe for a large number of measurements or for the continuous measurement of an analyte. Many fiberoptic-based chemical sensors have been developed to attempt to continuously measure the presence and concentration of an analyte or analytes in a moving process stream. Many devices use an immobilized reagent to render them specific for an otherwise optically undetectable analyte of interest. These devices are often called "optrodes." Most optrodes have been limited to reversible reagent chemistries, because irreversible reagent reactions will severely limit the lifetime of the optrode probe and cause calibration problems. Moreover, the optrodes cannot use the vast number of irreversible reagent chemistries that form complex formations and colored products upon reaction with an analyte. Moreover, the optrodes with reversible reagent chemistries have had difficulties regarding probe-to-probe reproducibility, limited dynamic ranges, reagent photolability and leaching, and slow response times. One approach to this problem was developed by Luo et al., "Fiber-Optic Sensors Based On Reagent Delivery with Controlled-Release Polymers," Anal. Chem. 61:174-77 (1989) . Luo et al. describe a polymeric delivery system that attempts to deliver fresh sensing reagents for extended periods using a reversible sensing reagent. The Luo et al. probe releases the reversible reagent, entrapped in an ethylene-vinyl acetate polymer matrix, to the probe tip upon contact with an aqueous solution. Several problems noted by Luo et al. included the observation that when the polymer was below the fiber tip, the emerging light from the fiber was reflected by the polymer surface and part of this reflected light reentered the fiber, resulting in a systematic error in the signal. Further, the Luo et al. design resulted in the buildup of a large concentration gradient of dyes around the polymer. Another attempt by Inman et al., "Pressurized Membrane Indicator System for Fluorogenic-Based Fiber- Optic Chemical Sensors," Analvtica Chimica Acta. 217:249- 62 (1989) , describes a fluorogenic indicator that is forced through an ultrafiltration membrane into the analyte solution. The reaction product between the reagent and the analyte (in this case an indicator molecule and a target ion, respectively) occurs at the membrane/solution interface. Light from a bifurcated fiberoptic cable stimulates fluorescence and fluorescence emission from the membrane/solution interface is transmitted back up the cable to a photodiode detector. The Inman et al. sensor has an element of renewability by continually renewing the indicator (reagent) at the end of the fiberoptic probe. However, the buildup of the reaction product on the membrane/solution interface again limits the usefulness of this sensor probe. Accordingly, there is a need in the art to develop a completely renewable reagent sensor probe that can utilize the full spectrum of irreversible reagent chemistries as well as reversible reagent chemistries. Summary of the Invention The present invention is directed to a device capable of continuously measuring the presence and concentration of an analyte or analytes in a liquid or gas phase, such as in a process stream, comprising a sensor probe, a reservoir, and a detection means. The sensor probe comprises a reaction chamber defined by a permeable membrane. The sensor probe has an inflow opening, an outflow opening, a primary electromagnetic radiation source directed within the sensor probe, and a detection fiber. The primary electromagnetic radiation source directs electromagnetic radiation across a defined path within the reaction chamber toward the detection fiber. A reagent flows into the reaction chamber from the inflow opening, across the defined path, and out the outflow opening. Analyte or analytes diffuse across the permeable membrane into the reaction chamber, react with the reagent flowing within the reaction chamber of the sensor probe, and modulate electromagnetic radiation across the defined path. The reservoir communicates with the inflow opening of the sensor probe and contains the reagent in a liquid or gaseous state of matter. The reservoir feeds the inflow opening by a pumping means, or by gravity flow with a valve means to control flow rate. The reagent will chemically react with the analyte or analytes within the reaction chamber of the sensor probe to create a reaction product that modulates electromagnetic radiation differently from the unreacted reagent. The detection means communicates with the detection fiber in the sensor probe and measures the intensity of electromagnetic radiation that is directed across the defined path within the sensor probe, or that is generated by the reaction product, such as with chemiluminescence. Preferably, the electromagnetic radiation signal is modulated and the detection means demodulates the signal. In another embodiment, the reaction chamber is divided into two chambers. The first chamber is a reaction chamber defined by the permeable membrane and containing the inflow opening communicating with the reservoir. The second chamber, or detection chamber, is downstream from the first chamber and communicating with the first chamber. The second chamber comprises the electromagnetic radiation source, the detection fiber, and the outflow opening. The second chamber can be defined by any enclosed space communicating with the first chamber and a waste reservoir or waste for the ultimate destination of the reagent through the outflow opening. The permeable membrane can be either a semipermeable membrane or an ion exchange membrane. The semiper eable membrane allows the analyte or analytes to diffuse across the membrane by passive diffusion based upon molecular size and the ion exchange membrane transports the analyte or analytes by an ion exchange diffusion based upon charge and possibly size. Preferably, the device further comprises a secondary source of electromagnetic radiation within the sensor probe. The secondary source is a means for scattering and/or reflecting electromagnetic radiation from the primary electromagnetic radiation source. When a secondary source of electromagnetic radiation is present within the sensor probe, the defined path becomes the distance from the secondary source to the detection fiber. Light or electromagnetic radiation from the primary source is carried, via optical fiber, to the secondary source. The secondary source acts to redirect the electromagnetic radiation from the primary source so that it can be collected by the detection fiber after traversing the defined path. The reagent can use irreversible chemistry or reversible chemistry when binding to or reacting with the analyte. The reagent modulates or emits electromagnetic radiation upon reaction with or binding to the analyte. The electromagnetic radiation is preferably selected from the group consisting of UV light, visible light, or IR light. Examples of reagents utilizing irreversible chemistry are known to those of ordinary skill in the art and are described in, for example, J. Ruzicka and E. Hansen, "Flow Injection Analysis," Vol. 62 in Chemical Analysis. 2nd ed. , J. Wiley and Sons, NY, NY (1988) and E. Jungreis, "Spot Test Analyses: Clinical, Environmental, Forensic and Geochemical Applications," Vol. 75 in Chemical Analysis. Further, the reaction product of the reagent and the analyte can create a fluorescent reaction product that absorbs light of one wavelength and emits light of a different wavelength. In another embodiment, the inventive device is capable of continuously measuring the presence and concentration of an analyte or analytes in a liquid or gas phase by chemiluminescence. The device does not contain a primary or secondary source of electromagnetic radiation because the chemiluminescent reaction product emits photons. The photons are detected by the detection fiber. The detection fiber communicates with a detection means that can quantitate the concentration of analyte or analytes according to the intensity of the light emission or the wavelength of the photons emitted. In yet another embodiment of the present invention is a device capable of continuously measuring the presence and concentration of an analyte or analytes in a reaction volume, such as a process stream. This embodiment is used when the analyte or analytes or reaction volume can or tend to foul the permeable membrane, thus preventing continued diffusion of the analyte or analytes across the permeable membrane. The device that measures the analyte or analytes in a reaction volume comprises a sensor probe, a reservoir, and a detection means. The sensor probe comprises a reactant inflow tube, a primary electromagnetic radiation source and a detection fiber. Therefore, the sensor probe does not contain a reaction chamber defined by a permeable membrane, because the reactant flows directly into the reaction volume or process stream containing the analyte or analytes to be detected, and the reaction product diffuses or is swept away by the process stream. Alternatively, the reaction chamber is defined by impermeable walls with an open end communicating with the analyte containing liquid or gas. The probe contains an outflow opening at a lower pressure than the inflow opening so as to be able to remove the reaction product and unreacted reagent from the reaction chamber. The primary electromagnetic radiation source is directed in a defined path within the reaction volume to the detection fiber. The inflow tube directs a flow of a reactant into the reaction volume, across the defined path and out the outflow tube. It is not necessary that all or even substantially all of the reactant (either unreacted or reacted with the analyte or analytes) gets removed by the outflow tube. Preferably, the outflow tube has a larger cross-sectional area than the inflow tube. The reservoir communicates with the inflow tube and contains the reagent in a liquid or a gaseous state of matter. The reagent chemically reacts with the analyte or analytes in the reaction volume or process stream. The flow of reagent and/or analyte or analytes in the process stream is directed across the defined path. Chemically reacted reagent can modulate electromagnetic radiation differently than unreacted reagent. This is detected by changes in the wavelength and/or intensity of the electromagnetic radiation in the detection fiber. The detection fiber communicates with a detection means that can quantitatively and qualitatively measure the intensity and/or wavelength of electromagnetic radiation reaching the detection fiber. This embodiment, without a permeable membrane, can further comprise a secondary source of electromagnetic radiation. The secondary source of electromagnetic radiation is a means for scattering and/or reflecting electromagnetic radiation from the primary electromagnetic radiation source. When a secondary electromagnetic radiation source is present, the defined path becomes the distance from the secondary source to the detection fiber. Preferably, the defined path is located across the process stream or across the flow path of reagent from the inflow tube to the outflow tube and within the reaction chamber. The present invention is further directed to a method for continuously measuring the presence and concentration of an analyte or analytes in a liquid or gas. Preferably, the method involves measuring the presence and concentration of an analyte or analytes in a liquid or gas process stream. The method comprises contacting the sensor probe with a liquid or gas, adjusting the flow rate of the reagent into the sensor probe which adjusts the sensitivity and useable measurement range of the sensor probe, and detecting the presence and concentration of the analyte or analytes by measuring the reaction product of the reagent and the analyte or analytes according to a modulation of electromagnetic radiation or by chemiluminescence. The utility of the present invention allows for the use of irreversible reagent chemistries which have been developed to a greater extent than the reversible reagent chemistries. Further, the sensor probe can be used continuously because the reagent is continuously renewed by flowing through the sensor probe. The device is compatible with numerous spectroscopic techniques including, but not limited to, absorbance, luminescence, chemiluminescence, fluorescence and light scattering for the analyte modulation of the optical signal. The control of the composition of delivered reagent and reagent flow rate allows for continuous adjustment of the dynamic sensitivity range of the probe for a given analyte while the probe is located in situ. Further, the probe has multiple analyte capability because the same probe can be used for a variety of different analytes simply by changing the nature and/or concentration of the reagent in the reservoir. Moreover, a small physical volume of the reaction chamber in the sensor probe (e.g., less than 5 βl) provides for a short diffusion path of the analyte across the permeable membrane and a rapid response time due to the rapid mixing of the analyte and reagent flowing out of the inflow opening. Further, when a reference fiber is employed in the probe, the dual beam geometry helps to correct nonanalyte-related modulation of the optical signal at a given wavelength of electromagnetic radiation. Preferably, the device modulates the electromagnetic radiation signal into probe and demodulates the signal at the detection means. Nonanalyte-related modulation of the signal may be caused by changes in the intensity of radiation emanating from the primary source, or losses, induced by bending or other means, by the optical fiber which transmits the primary source radiation to the probe head. The reference fiber allows correction for this modulation so that it is not falsely interpreted as a change in analyte concentration. Brief Description of the Drawings Figure 1 shows a schematic of one embodiment of a sensor probe showing the reaction chamber (9) , inflow tube (6) , outflow tube (7) , electromagnetic radiation source fiber (5) , detection fiber (1) , reference fiber (2) , an end cap which acts as a secondary source of electromagnetic radiation (4) and consists of a translucent material (3) and may further contain a reflecting overlayer, and permeable membrane (8) . Figure 2 is a schematic of the sensor probe of Figure 1 communicating with a series of other elements. The inflow tube (6) communicates with a pump and/or a reservoir that contains the reagent in a liquid or gaseous state of matter. The outflow tube (7) communicates with waste. The primary source of electromagnetic radiation (5) is a fiberoptic cable that communicates with a source of electromagnetic radiation and a modulator. The detection fiber (1) and the reference (2) communicate with a signal detector and a reference detector, respectively. Both the signal detector and the reference detector communicate with a demodulator and the signals are analyzed by computer. Figure 3 shows a sensor probe with a variable length to increase the surface area of the permeable membrane (8) , increase the volume of the reaction chamber to allow more time for diffusion of the analyte or analytes into the reaction volume and a defined path between the secondary electromagnetic radiation source (3, 4) and the detecting fiber (1) . The inflow tube (6) extends through the length of the reaction chamber allowing reagent to flow along the length of the reaction chamber through a tube (12) in the secondary source (3, 4) and into a detection chamber bounded by nonpermeable walls (10) and toward the outflow tube (7) . Thus, the sensitivity of this sensor probe is increased due to the increased reaction chamber length and increased surface area of the permeable membrane. Figure 4 shows an alternative embodiment of the sensor probe wherein the analyte or analytes enter a reaction chamber via a frit or filtering device (13) and a sampling capillary tube (12) . The reaction chamber is defined by nonpermeable barriers (11) and plugs which may be an epoxy filler material, which, at the distal end also acts as the secondary source of electromagnetic radiation (3). Figure 5 shows an alternative embodiment of the sensor probe wherein the semipermeable membrane (8) is located at the end of a glass capillary tube (20) . This figure further shows the reaction chamber (9) , inflow tube (6) , outflow tube (7) , primary electromagnetic radiation source fiber (5) , detection fiber (1) and reference fiber (2) . In this embodiment, the interior surface of the membrane (4) exposed to the reaction chamber serves as a secondary source of electromagnetic radiation by scattering or reflecting electromagnetic radiation from the primary electromagnetic radiation source fiber. Figure 6 shows an alternative embodiment of the sensor probe wherein the capillaries, optical fibers and membrane are fixed to an etched silicon structure, and then covered and sealed with another silicon structure which is a mirror image of the first. In this embodiment, the membrane is a hollow fiber membrane (18) and the reaction chamber is the interior volume of the hollow membrane. Opening (22) in the silicon structure permits contact of analyte with the membrane. Reagent flows into reaction chamber via the inflow tube (6) , and then passes to the detection chamber (19) as indicated by the arrows. The detection chamber contains a primary electromagnetic source fiber (5) , a secondary source of electromagnetic radiation (4) , a detection fiber (1) , a reference fiber (2) , and communicates with an outflow opening (7) . Figure 7 shows still a further embodiment of the sensor probe wherein the membrane is a planar membrane positioned upon the surface of an etched material (20) . Figure 7(a) is a top cross-sectional view of the sensor probe wherein the reagent flows into the inflow opening (6) and travels through the reaction chamber defined by the etched channel in the material and the membrane (18) . Analyte diffuses through the membrane and reacts with the reagent during the reagents transport through the reaction chamber. The reagent then passes to the detection chamber (19) and out the outflow tube (7) . The detection chamber contains a primary electromagnetic source fiber (5) , a secondary source of electromagnetic radiation (4) , a detection fiber (1) and a reference fiber (2) . Depending upon the specific application, an optional secondary source of reagent (11) may be added to the reagent stream prior to detection. The arrows in the figure represent the direction of reagent travel through the sensor probe. Figure 7(b) is a side view of the sensor probe taken along the dashed line of Figure 7(a). Figure 8 is a schematic of the sensor probe constructed in Example 1. The arrows show the movement of reagent through the reaction chamber and the movement of analyte across the permeable membrane. Figure 9 shows a plot of the response of the probe constructed in Example 1 to varying concentrations of ammonia in aqueous solution. The response is shown at several reagent flow rates showing differential sensitivity and dynamic range as a function of reagent delivery rate. Figure 10 illustrates the full wafer mask patterns utilized in the manufacture of the sensor probe depicted in Figure 6. Figure 11 illustrates a number of the steps involved in the fabrication of the sensor probe depicted in Figure 6. Detailed Description of the Invention The present invention relates to a device and a method for continuously measuring the presence and the concentration of an analyte or analytes in a liquid or gas phase. Preferably, the analyte or analytes are located in a process stream, wherein the inventive device is capable of continuously monitoring the concentration of a critical analyte so as to provide data to monitor the process and adjust various process parameters, as necessary. The invention is a flow optrode. Optrodes are optical transducers used at the distal end of an optical fiber, which provide for chemical analytical specificity in a remote fiber spectroscopy measurement. Chemical optrodes respond to changes in chemical concentrations of an analyte or analytes. Classic chemical optrodes use entrapped or immobilized reagents at the fiber tip that react with an analyte in such a way that an optical signal is produced, or an existing optical signal is modulated in response to changes in the analyte concentration. The classical optrode usually consists of a reagent indicator with unique absorption or fluorescence properties that change as a function of sample concentration. The optrode is read by sending a probe beam down the optical fiber and monitoring the light that returns back up the fiber or up a separate return fiber. For an absorption optrode, the attenuation of the probe beam is related to the sample concentration, while in a fluorescent optrode, the intensity, frequency, or some other characteristic of the fluorescence emission is used to monitor sample concentration. Optrode measurements are usually made in either one of two ways, by monitoring attenuation in the probe beam at one or more wavelengths, or by monitoring fluorescence generated by the probe beam. The probe beam comprising a single wavelength or a wavelength region of electromagnetic radiation can be attenuated by a number of different mechanisms, including absorption, scattering, and reflection at the fiber surface. The measurements are made by a detection means. Preferred detection means include, for example, a photomultiplier tube and a solid state photodetector. The type of fiber used will depend upon the application. Optical fibers should be selected according to usable ranges of wavelengths, inherent fiber fluorescence, length of fiber, and cost. For remote measurements of distances greater than 100 meters, the selection of the fiber is important. Fibers are available that cover most of the spectral region of the electromagnetic radiation spectrum from the ultraviolet (220 nm) to the near-infrared. A typical fiber optic has a diameter of approximately 200 μm. Fibers ranging from 50 i to 500 μm. in diameter are preferred for use with the flow optrode in order to keep the flow optrode probe volume small. The materials used in optical fibers must be both flexible and transparent. The fiber must have a core that is surrounded by a physically compatible cladding with a lower refractive index. Both glasses and plastics can be used. Low-melting silicate glasses are appropriate for fiber fabrication but are limited to transmission in the visible range of the electromagnetic spectrum. Further, silicate glasses are not suitable for longer range transmission of light. Silicate glasses typically have core diameters of from 100 to 200 μM. Silica fiber can also be coated with a transparent plastic such as a silicone polymer or FEP Teflon ® . Plastic clad silica, often referred to as PCS, has a higher numerical aperture (NA) and is transparent in the ultraviolet (UV) range. Plastic fibers do not transmit light as efficiently as glass or silica fibers but are inexpensive and can be fabricated with larger core diameters. The transmission characteristics depend upon the particular plastics or glasses used in the fiber. The problem with the fixed reagent optrodes has been the short life span of the usefulness of the sensor probe when irreversible reagent chemistries are used. The first attempt to solve this problem was to develop reversible reagent chemistries. However, reversible reagent chemistries limit the number of analytes that can be determined and the chemical specificity of the probe. The present invention solves this problem by directly controlling the renewal of the reagent in the sensor probe. This is done by flowing reagent in a liquid or a gas medium from a reservoir into the reaction chamber of the sensor probe and out again to a waste collection. The analyte or analytes from the process stream or sample diffuse across the permeable membrane which defines, in part, the reaction chamber. The permeable membrane can allow diffusion of the analyte or analytes into the reaction chamber by a passive diffusion process with a semipermeable membrane creating some degree of specificity for the analyte or analytes by only allowing molecules of a specific size or chemical type to pass through. Alternatively, the permeable membrane can be an ion exchange barrier that will only allow an analyte or analytes to pass into the reaction chamber based upon their charge and size. Examples of semipermeable membranes for passive diffusion based on size or type include, but are not limited to silicone rubber, porous polypropylene, such as Celgard X-20 or X-10, and porous teflon. Each of the semipermeable membranes have pore sizes that control the movement of molecules based on the size of the molecules. For example, the pore size may range from about 0.05 lm to about 10 μm. The permeable membrane may also be an ion exchange membrane to separate analytes by size and charge. More specifically, anion exchange membranes include aminated polystyrene, divinyl benzene, aminated polypropylene, aminated polyethylene, other aminated polymers and other polymers with functional groups, such as trimethyl amine, ethyl dimethyl amine, and dimethyl ethanol amine. Cation exchange membranes include Nafion ® , and sulfonated polystyrene, polyacrylates and polypropylene. The ion exchange membrane can also comprise radiation grafted polymers such as polypropylene, polyethylene, and polystyrene, with various charged functional groups. The choice of the semipermeable or ion exchange membrane depends upon the molecular size and charge characteristics of the analyte or analytes to be detected. It is within the scope of the present invention to have a hybrid membrane comprising a section of semipermeable membrane and a section of ion exchange membrane such that two or more different analytes may pass into the reaction chamber for detection by one or a plurality of reversible or irreversible reagents. In addition, the membrane can be made from a rigid support matrix (such as an aluminum oxide membrane) with an organic deposit thereon (such as polyethylene, naphthalene or Teflon ® . The sensor probe comprises a reaction chamber defined by the permeable membrane, and inflow opening, an outflow opening, a primary electromagnetic radiation source and a detection fiber. The primary electromagnetic radiation source directs electromagnetic radiation across a defined path within the reaction chamber toward the detection fiber. For example, the primary electromagnetic radiation source can be ultraviolet, visible, or infrared light. Alternatively, the electromagnetic radiation can be an excitation wavelength of light that will allow a fluorescent molecule to emit light at a different wavelength. The inflow opening communicates with a reservoir containing the reagent in a liquid or a gas state. The reservoir can be located within the sensor probe or external to the sensor probe but communicating with the sensor probe by the inflow opening. The outflow opening communicates with waste or an outflow reservoir that then stores the used reagent for disposal. The greater the path length, the greater the sensitivity of the sensor probe. Therefore, for measuring low concentrations of analytes, it may be desirable to increase the path length for electromagnetic radiation in the sensor probe and reaction or detection chambers to create a longer defined path length. The reagent delivery flow rate has an effect upon the sensitivity of the sensor probe and the dynamic range of analyte concentration under which the probe operates. For a given external concentration of an analyte, such as ammonia , the amount of time that the analyte has to diffuse across the permeable membrane and interact with the internal reagent solution is important. The interaction time is a function of the volumetric flow rate of the reagent solution through the reaction volume. A slower flow rate will result in a larger absorbance signal than a faster flow rate. This is illustrated in Figure 9, which shows the probe of Example 1 response to various ammonia concentrations at five different reagent delivery flow rates (0.5, 2, 5, 10, and 15 μL/min.). The slower flow rates offer the advantage of increased sensitivity, while at faster flow rates the probe is useful over a larger dynamic range. By continuously renewing the reagent in the probe, the response and recovery times are improved over static reagent sensors. The reagent flow in mixing within the probe, creates a steep gradient in the chemical potential of the analyte across the permeable membrane interface, which results in a rapid steady state concentration. Using an ammonia probe, such as the one described in Example 1, and the reagent bromothymol blue (BTB) , response times (to 95% of the steady state signal) ranged from 15 to 40 seconds. The longest response times were observed at lower analyte concentrations and slower flow rates. As the flow rate increased, the response time decreased. Recovery time was also reduced. There was no need to wait for the analyte to diffuse out of the membrane, since the products of the interaction were simply washed out of the sensing volume with fresh reagents. Using the ammonia sensor of Example 1 with BTB as the reagent, recovery times to base line ranged from 10 to 140 seconds, with the slowest recovery observed at the lower flow rates and at higher concentrations of ammonia. As the flow rate increased, a point was reached where the reagent within the probe, was replaced so rapidly that the analyte concentration made little difference in the rate of recovery. It is also possible to configure the flow optrode to have a stop flow mode for the flow rate of the reagent. The stop flow mode will attain maximum sensitivity. If the flow rate is sufficiently fast, there will be no response to external analyte concentrations. When the flow is stopped, the analyte diffuses through the permeable membrane at a rate determined by the external analyte concentration. The slope of the increase in absorbance is a function of the external analyte concentration. Alternatively, the reagent pump can be stopped for a specified period of time, and the signal level at that time is related to analyte concentration. Once a slope or a point has been determined, the reagent pump can be turned on again and the signal will return to base line. In this mode, the flow optrode is self- referencing. Using a pH of 5.3, and a reagent concentration of 50 μM BTB, ammonia as the analyte, and a renewal rate of 15 μL/min. , several aqueous ammonia (analyte) samples were determined with this method. For each determination, the pump was stopped for exactly three minutes, a value taken, and the pump restarted to return the signal to the base line. The concentration of reagent also affects the sensitivity of the flow optrode. When using BTB as a reagent and ammonia as the analyte and a flow rate of 5 μL/min. in the flow optrode described in Example 1, the sensitivity of the probe increased as the BTB reagent concentration decreased. One explanation why the sensitivity decreased as the reagent concentration increased is that the reagent also acted as a buffer. Therefore, as the concentration of the reagent increased, so did buffer capacity. As a result, the flow optrode became less sensitive to small changes in analyte concentrations. This effect should only hold when the reagent also acts as a buffer. The flow rate is adjusted by a valve or by the pump speed within the fluidics system of the sensor probe. The fluidics system comprises the reservoir, the communication means with the inflow opening of the sensor probe, the outflow opening of the sensor probe, and a waste reservoir communicating with the outflow opening. The pump and/or valve system that controls the flow rate and the ability to stop the flow can be located between the reagent reservoir and the inflow opening or between the outflow opening and the waste reservoir. For example, it is possible to deliver reagent in a liquid state with a high precision microinjection pump such as the model CMA/100 from Carnegie-Medicin (Solna, Sweden) when the reagent flow rate is relatively low (e.g., from about 0.1 μl/min. to about 20 μl/min.). The reagent reservoir can communicate with the inflow opening with tubing such as Teflon ® tubing or a fused silica capillary (e.g., Model TSP100/170, Polymicro Technologies) . The fused silica capillary can have dimensions, for example, of 100 μm internal diameter and 165 μm outer diameter. The diameter of the tubing is further used to control the flow rate, with larger inner-diameter tubing being capable of having a faster flow rate. In summary, the inventive flow optrode can adjust the sensitivity and dynamic range to the concentration of the analyte or changes in concentration of the analyte by varying the flow rate, the reagent composition, or by operating in a stop flow mode. Therefore, the inventive device and the inventive method for using the inventive device offer significant advantages over previously developed optrodes. For example, reagent photodecomposition and leaching result in limited lifetimes and calibration problems when an immobilized reagent is used within a conventional sensor probe. The inventive device, however, avoids this problem by renewing the reagent within the reaction chamber with fresh reagents as required. While it is possible to adjust the sensitivity and dynamic range of conventional optrodes by varying the reagent composition, a new probe must be constructed in each case. On the other hand, the present inventive flow optrode is able to change reagent reservoirs to change the reagent chemistry without having to construct a new sensor probe. Response and return times greater than five minutes are common for conventional optrodes. The response and return times can be significantly improved with the flow optrode. Response times are rapid as a result of the steep gradient in analyte concentration created by the reagent flow. Recovery times are relatively fast because the reacted products of reagent and analyte are simply washed out of the reaction chamber with fresh reagents. Irreproducible construction of conventional optrodes is often the result of physical differences of the substrate on which the reagent is immobilized, and/or different reagent loadings from probe to probe. However, the flow optrode has a standard physical design and can be constructed reproducibly without the variable nature of the reagent loading being a factor. The reagent composition will be reproducible so long as the composition of the reservoir, from which the reagent is pumped, is prepared reproducibly. Another significant advantage of the flow optrode is its ability to be used with both reversible and irreversible reagent chemistry. In previous optrodes, based upon irreversible reaction, a large reagent reservoir was needed to supply reagent over the desired lifetime of the probe. This has several disadvantages, including a limited lifetime at high analyte concentration. The flow optrode, however, allows an irreversible reagent/analyte reaction to occur and the device can be simply reset with fresh reagent as the reaction products are removed from the reaction chamber through the outflow opening and carried off to waste. The extent of irreversible reagent chemistries is greater than known reversible reagent chemistries. Further, irreversible reagent chemistries exhibit higher binding constants in their detection reactions. Therefore, the proper choice of the permeable membrane for analyte selectivity, the proper choice of the specific reagent chemistry to further analyte selectivity, the proper choice of flow rate or stopped flow mode to further analyte selectivity, and the proper choice of spectral analysis can allow flow optrodes to be prepared for a wide variety of analytes and analyte concentrations. The reagent is designed to interact with the analyte so as to render it detectable through the detection fiber. The reactant directly or indirectly reacts with the analyte to form an optically detectable product. Alternatively, the reactant can be a catalyst to accelerate the rate at which analyte is converted to an optically detectable product or the reagent can be a substrate for the detection of catalysts. Further, the reagent can be an adsorbent or extractant to preconcentrate an optically detectable analyte within the defined path to modulate the electromagnetic radiation. Many reagents are indicators that change color upon interacting with the analyte. The measurement is the change in color rather than a change in the intensity of a single color. Thus, the primary source of information is the spectral shift accompanying a reaction between an indicator reagent and an analyte. Examples of indicator type reagent reactions are an acid-base reaction to analyze pH, carbon dioxide, and ammonia. A complexation type of indicator reaction can be used to measure the concentration of aluminum (III) . A ligand exchange can measure the concentration of water vapor and oxygen. Luminescence can measure oxygen as an analyte, and a quenching reaction can measure halide-type analytes. Other reagents include phenol red and the trisodium salt of 8-hydroxy-l,3,6-pyrenetrisulfonic acid (HPTS) as pH indicators. HPTS is a fluorescent reagent. Other pH reagents include 7-hydroxycoumarin-3-carboxylie acid, fluoresceina ine, fluorescein and derivatives. A carbon dioxide sensor can use both phenol red and HPTS as reagents plus 4-methylumbelliferone. Oxygen sensors are often based on fluorescence quenching with pyrenebutyric acid as the preferred indicator. Glucose may be analyzed by a combination reagent that includes glucose oxidase and a fluorophor sensitive to oxygen quenching. The glucose oxidase-catalyzed oxidation of glucose consumes oxygen to reduce steady-state oxygen concentrations in the reagent phase, which leads to increases in observed fluorescence intensity. In one embodiment of the inventive device, as illustrated in Figure 1, and based upon absorbance type of reagent chemistries, the sensor probe comprises a reaction chamber formed from a semipermeable membrane (8) in tubular form of a small diameter (typically less than 0.5 mm) and sealed at both ends by epoxy or some other means (3) . A reagent delivery system causes reagent to flow into the reaction chamber, or lumen of the tubing, via a small diameter capillary made of fused silica or a chemically resistant polymer (6) , and drains through a suitable tubing (7) to a waste collection. As reagent flows from the distal to proximal ends of the reaction chamber, analyte can pass through the membrane (8) , mix and react with the reagents, forming a colored species that may optically detected. Detection is accomplished via a fiber optic (5) connected to a modulated source as shown in Figure 2, a source reference fiber (2) and a detection fiber (1) connected to their respective detectors and demodulators in Figure 2. In the embodiment shown in Figure 1, fibers 5 and 2 are embedded in a secondary source of electromagnetic radiation which is a transparent epoxy end-cap (3) . A silver-filled epoxy (or other suitable material) is placed over or mixed in (3) to increase the intensity of back-reflected light and to eliminate effects of external refractive index changes. The epoxy end-cap (3) becomes a secondary source produced by light scattered and back-reflected from the primary electromagnetic radiation source, fiber (5) . Fiber 2 is used as a reference for the intensity of the secondary source of electromagnetic radiation at the same modulated signal wavelength. Light or electromagnetic radiation emanating from the end-cap is collected by fiber (1) after traversing the reaction chamber volume and conducts the analyte modulated light signal back to a suitable detector. A computer is used for processing the optical signals as shown in Figure 2. An example of how a secondary source of electromagnetic radiation can be created is to embed the distal end of an optical fiber carrying a light from the primary radiation source in translucent epoxy. The epoxy will be illuminated to form the secondary source. Light may then be more effectively directed downward the detection fiber by covering part of the secondary source with a reflective coating, such as a silver-filled epoxy. When using chemiluminescent reagent chemistries, only the detection fiber for signal collection is needed since no source or reference fiber would be required. The chemiluminescent reaction has the energy needed to cause excitation of a species derived from a chemical reaction. Light is emitted when the excited species decays back to the ground state. In sensor probe geometries containing two or more fibers, their placement in the sensor probe relative to one another is variable. An example is the detection of a fluorescence modulation in which excitation, emission signal, and reference fibers (a total of three fibers) could all be placed at the same distance from the secondary source within the reaction chamber. The relative positions of reagent delivery capillaries and fibers are also variable and dependent upon reaction conditions, flow rate, and the membrane transport rate of the analyte or analytes. Figure 3 shows a sensor probe which employs a long length of permeable membrane, which could be coiled into a compact probe to allow for a longer time for signal development before detection. Further, in some process systems, such as those operating above ambient pressure, it is possible to eliminate the permeable membrane from the probe. Figure 4 illustrates a device in which an open capillary continuously samples the process stream (13) . Reagent is added and a signal is developed in the defined path of the optical fibers. A frit may be employed to prevent plugging of the sampling capillary. As in other embodiments of the inventive device, the placement of the optical fibers may be altered, depending upon the spectroscopic technique being employed. When using an embodiment that directly sample analyte from a process stream, the membrane (11) should be impermeable to the diffusion of analyte or analytes. Figure 5 illustrates an alternative embodiment of the sensor probe wherein the membrane (8) is located at the end of a glass capillary (20) . The reagent passes into the reaction chamber (9) via the inflow tube (6) , and the analyte or analytes enter the reaction chamber by diffusion across the membrane. The reaction chamber contains a primary electromagnetic radiation source fiber (5) , a reference fiber (2) , and a detection fiber (1) . The interior surface of the membrane (4) serves as a secondary source of electromagnetic radiation by scattering and/or reflecting electromagnetic radiation from the primary electromagnetic radiation source fiber for subsequent detection by the detection fiber. An outflow tube (7) is also provided. Figure 6 shows an embodiment of the sensor probe in a planar form. The membrane is in the shape of a hollow fiber (18) wherein the reagent passes through the interior of the membrane. The analyte or analytes are present within an opening in the sensor (22) , across which the hollow membrane fiber passes. Transport of the reagent within the sensor probe is confined to channels etched or notched in the surface of the material used to construct the probe. Mirror images of the etched or notched material are joined to form the desired channels. In this embodiment, the reaction chamber (i.e., the interior volume of the hollow fiber membrane) is separate form the detection chamber (19) . After passing through the reaction chamber, reagent is delivered to the detection chamber which contains a primary electromagnetic radiation source fiber (5) , a detection fiber (1) , a reference fiber (2) , and a secondary source of electromagnetic radiation (4) . The detection chamber also contains an outflow opening (7) . Figure 7 illustrates yet a further embodiment of a planar sensor probe. Figure 7(a) is a top cross- sectional view of a sensor probe utilizing a planar membrane (18) laid over an channel etched upon the surface of a silicon sheet (20) . The reagent enters the reaction chamber through an inlet opening (6) and travels through the reaction chamber defined by the length of channel in contact with the membrane surface. The reagent then passes to a detection chamber which contains a primary electromagnetic radiation source fiber (5) , a detection fiber (1) , a reference fiber (2) , and a secondary source of electromagnetic radiation (4) , and communicates with an outflow opening (7). Figure 7(b) is a cross sectional view of the sensor probe taken along the dashed line of Figure 7(a). Two sheets of silicon, a bottom etched sheet (20) and a top sheet (22) containing the planar membrane (18) , are joined along their surfaces to form the sensor probe. For some types of chemical reactions, introduction of a second reagent may be necessary. For example, a specific reaction may require an initial analyte-reagent reaction followed by subsequent reagent additions to produce a color change which may be detected in the detection chamber. In such circumstances, an optional secondary source of reagent (11) may be provided at a point between the reaction chamber and detection chamber. In addition, mixing of the reaction product may be facilitated by passage through a tortuous path (not shown) prior to entering the detection chamber. The following examples are designed to illustrate several aspects of the present invention and are not to be construed to limit the scope of the invention. EXAMPLE 1 This example illustrates the construction of a device capable of continuously measuring pH in a process stream. The sensor probe was constructed with a visible light source (tungsten filament lamp) as the source of electromagnetic radiation. The light source was modulated at 200 Hz with a mechanical chopper (Stanford Research Systems, Inc., Palo Alto, Calif., Model SR540) . The electromagnetic radiation was input into the sensor probe with a source input optical fiber (ensign-Bickford, Avon, Conn., Model HCR-H0110T-14) with a core diameter of 110 μm, a cladding diameter of 125 μm, a buffer diameter of 250 μm and a numerical aperture of 0.48-0.44. The sensor probe was constructed with a detection fiber and a reference fiber from Polymicro Technologies, Phoenix, Ariz., Model FHP 100/110/125 with core diameters of 100 μm, cladding diameters of 110 μm, buffer diameters of 125 μm, and numerical apertures of 0.22. The electromagnetic radiation entering the reference and detection fibers was taken to respective photomultiplier tubes or detection means (Pacific Instruments, Concord, Calif., Model 3150S) . Each photomultiplier tube was fitted with 620 nm (maximum absorbance wavelength of the non-protonated form of the reagent, Bromothymol blue) band-pass filters (Microcoatings, Inc., Westford, Mass.). The signals entered respective lock-in amplifiers (Evans Electronics, Berkeley, Calif., Model 4110). All three of the output signals were connected to an AD board of a lock-in amplifier (Stanford Research, Model SR575) . This lock-in amplifier was used to acquire, store, and display data on a Leading Edge PC with software written by Stanford Research (SR575 Lock-in Data Acquisition Software) . The bromothymol blue (BTB) reagent was delivered with a high precision microinjection pump (Carnegie- Medicin, Solna, Sweden, Model CMA/100) . A 5 mL gas-tight syringe (Hamilton Co., Reno, Nev.) was connected to a 10 cm length of 0.007 inch internal diameter, 0.063 inch outer diameter. Teflon ® (Alltech Associates, Inc., Deerfield, 111., Flexon HP). The tubing was, in turn, connected to a 1 meter length of 100 μm internal diameter, 165 μm outer diameter fused silica capillary (Polymicro Technologies, Model TSP 100/170) which delivered the reagent to the sensor probe. Microporous hollow fiber membranes were made of polypropylene and had 400 μm internal diameters, 0.03 μm average pore size, 40% porosity and 25 μm wall thickness (Hoechst Celanese, Charlotte, N.C., Model Celgard X-20) . Bromothymol blue (BTB) (Aldrich, Milwaukee, Wis.) was used. All other reagents were analytical grade. A 50 μM BTB solution in the basic form (molar absortivity of 37,000 Lmol -1 cm "1 at 620 nm) will give an absorbance of 1 unit for a 5 mm defined path length. All solutions were prepared in distilled and deionized water. The sensor probe is shown in a schematic diagram in Figure 8. A 12 mm length of polypropylene hollow fiber was placed in a 7 mm length of 1 mm internal diameter glass capillary. The membrane was pushed out of the glass capillary so that 8 mm of the membrane was sticking out. The membrane was then epoxied in this position with a rugged 2-part fluoropolymer epoxy (Master Bond, Hackensack, N.J. , Model EP21HTAR) . After curing, a bifurcated fiberoptic pair consisting of the electromagnetic source input fiber and the reference fiber (glued together with a Duco Cement-acetone mixture) , was pushed through the glass capillary and the hollow fiber membrane until just shy of the end of the membrane. The membrane tip was then filled with fluoro-polymer epoxy for a distance of 1 mm. This buries the end faces of the optical fibers, making them impervious to the internal reagent-analyte interaction. After curing, the tip of the membrane was coated with a silver-filled epoxy (Epoxy Technology, Inc., Billerica, Mass., Model EPO-TEK) which serves as a reflective surface of the source electromagnetic radiation. Finally, the membrane tip was dipped in melted black Apeazon wax in order to optically isolate the source and reference fibers from the external environment. Next, the inflow opening or reagent delivery capillary and the analytical signal fiber were glued together so that the end face of the capillary was 5 mm beyond the end face of the analytical signal fiber. This pair was pushed through the glass capillary and through the open end of the membrane, until the reagent delivery capillary just made contact with the epoxy plug at the tip of the membrane (sensor probe) . This step was repeated until the maximum analytical signal level was obtained. The epoxy plug at the tip of the membrane serves as a diffuse secondary source of electromagnetic radiation. Thus, the optical path length or defined path was approximately 5 mm, making the sensing volume of the reaction chamber approximately 400 nL. Finally, a Teflon ® drain tube (0.30 mm internal diameter, 0.76 mm outer diameter, 1 m length) was put into the open end of the glass capillary and that end of the glass capillary was sealed with fluoropolymer epoxy. This device was capable of continuously measuring the presence and concentration of an analyte, depending upon the reagent used. In the example illustrated herein, the reagent was BTB and the analyte was ammonia. EXAMPLE 2 This example illustrates the use of the device of Example 1 measuring various ammonia concentrations. Calibration samples were prepared by adding appropriate microliter volumes of ammonium chloride standard to 20 mL aliquots of 0.01 M sodium hydroxide. The sensor probe constructed in Example 1 was threaded through a hole in a piece of aluminum stock, which was padded on the bottom. For each sample, before measurement, the vial cap was removed, a stir bar added, and a cap with a hole was put on. The vial was placed in a stir plate which was on a jack. The vial was raised so that the probe passed through the hole in the cap and into the aqueous solution, until the vial cap and padding formed a tight seal. In this way, the integrity of the ammonia concentrations were preserved during measurements. All samples were stirred during measurement. Using the ratio signal output, absorbance values were calculated as log (I/I Q ) where I and I Q were the sample and incident radiation intensities, respectively. The incident radiation intensity was measured as the signal of a sample solution containing no ammonia. Since the BTB reagent pumped through the sensor probe is a pH indicator, the response to ammonia becomes a titration curve. The pH transition interval of BTB is between pH 6.0-7.6 for a color change from yellow to blue. In order to obtain ' maximum sensitivity, it is desirable to have the indicator at an initial pH just below the steepest portion of the titration curve. For a BTB concentration of 50 μM and a continuous flow rate of 5 μL/min, the sensitivity of the probe increased as the pH of the indicator solution increased. However, as the pH increased beyond 5.3, there began to be a decrease in the dynamic range. This is because the probe was operating in the upper portion of the titration curve where the sensitivity to ammonia drops off. The concentration of the reagent also affected sensitivity. Four concentrations of BTB, each at pH 5.0, were pumped through the sensor probe at a flow rate of 5 μL/min. The results show that sensitivity increased as BTB concentration decreased. The BTB reagent or dye may have been acting as a buffer which is why the sensitivity decreased as the BTB concentration increased. Therefore, as the concentration of the reagent increased, so did the buffer capacity, and as a result, the sensor probe became less sensitive to small changes in ammonia concentration. Thus, the BTB concentration of 50 μM appeared to be optimal choice. Alternatively, calculation of the optimal BTB concentration which gave absorbances ranging from 0 to 1 absorbance units, based on a 5 mm optical path length and a molar absorptivity at 620 nm of approximately 37,000 Lmol -1 cm -1 , also resulted in a BTB concentration of approximately 50 μM. Based on the results of the reagent pH and concentration studies, the optimal reagent was found to be pH 5.3 and 50 μM BTB. EXAMPLE 3 This example illustrates the construction of the planar sensor probe depicted in Figure 6. The sensor probe (size = 15 mm x 15 mm x 1 mm) was prepared by fixing the capillaries (i.e., the inflow tube and the outflow tube), the optical fibers (i.e., the primary electromagnetic radiation source fiber, the detection fiber, and the reference fiber) and the membrane in an anisotropically etched silicon structure (channel depth — 300 μm) , and covering and sealing it with another silicon structure which is a mirror image to the first structure. All silicon microfabrication steps were conducted in a Class 100 clean room. Three-inch diameter single crystal silicon wafers were employed (Silica-Source Technology Corp., Tempe, Ariz.) and were p-type (doped with boron, 10-20 ohm/cm resistivity) with a (100) crystal orientation. Since the components to be inserted in the silicon channels were fairly large (maximum diameter = 450 μm) , it was necessary to use relatively thick wafers (19- 21 mil, «500 μm) . Because the final sensor probe size (15 x 15 mm x 1 mm) was much larger than usually can be accommodated with the clean room microfabrication systems, the mask making procedure was modified. Normally, a single integrated circuit photomask is cut in rubylith 100X the actual size. The film is placed over a green- lighted backdrop and a 10X reduced imaged is developed on a glass emulsion plate with a step and repeat camera which, in the process, results in another 10X reduction. The maximum size pattern that can be used with this camera is 1.5 inch x 1.5 inch, which results in a maximum device size of 0.15 inch x 0.15 inch (i.e., 3.8 mm x 3.8 mm). Since this size was too small to practically fit the fiber optics, capillaries and membrane employed in this example, an alternative photomask making procedure was utilized. The alternative procedure utilized full wafer mask patterns (six mirror image pairs per 3" wafer) were prepared actual size on a computer (either a CAD or DRAW program). See Figure 10(a). The images were printed electronically as opaque on transparent film with a high resolution (2540 dpi) Linotronic L-300 Laser Imagesetter. The photomasks were then contact printed to high resolution emulsion coated glass plates (Precision Photoglass, Inc., Palo Alto, Cal., Item 13122-AA209, Ultraflat, 4" x 5" x 0.06") by the following procedure. Developer and fixer for black and white film (Kodak HC- 110) were prepared in deionized water and placed in separate baths. With the room lights off, the mask transparency was sandwiched between an emulsion plate (emulsion side against mask) , and a clear glass plate. This was placed under a contact print camera lamp (Durst Nevoneg, Model M301) , and the lamp turned on for 5 minutes. Immediately thereafter, the emulsion plate was placed in the developer for 5 minutes with agitation. Next, the emulsion plate was rinsed in flowing deionized water for 1 minute to dilute the developer and then it was placed in the fixer solution for 3 minutes. The photomask plates appear as black with transparent images defining the areas to be etched. Since there are two depths needed in the final device structure (the opening or window and the channel), two photomasks were used. See Figures 10(b) and 10(c). The first photomask etches the window (4 mm x 5 mm) , which will eventually be etched all the way through, to a depth of 260 μm. The second photomask contains the entire structure and defines the flow channels (500 μra wide) , reaction and detection chambers (1800 μm wide) and window, and is etched 250 μm deep. The fabrication procedure is illustrated in Figure 11. Wearing complete safety clothing (goggles, shield, rubber gloves and plastic apron) , the wafer was cleaned in three stages. First, it was placed in a piranha wash (degrease) , prepared by adding 300 mL of 30% H 0 2 to 800 mL of 96% H2SO4, for 15 minutes. After rinsing in a quick dump (rinses wafer in deionized water, then empties, then rinses again, for 3 cycles) , the wafer was placed in a buffered HF solution (10:1) for 1 minute. It was then dipped in and out of a water bath to rinse off HF and then washed in the quick dump. Finally, the wafer was placed in a heated HCl bath for 10 minutes, rinsed in the quick dump and spin dried with hot N 2 . Next, the wafer (1) (see Figure 11(a)) was thermally oxidized in an initial oxidation furnace (1100 β C) to grow a Si0 2 surface masking layer (2) (see Figure 11(b)). The wafer was loaded on a quartz boat, polished surface facing towards the furnace heat, which was placed in a quartz elephant. The furnace cap was removed and replaced with the quartz elephant and the boat containing the wafer was pushed with a glass rod, over a three minute period, until it sat in the center of the furnace. The door to the furnace was closed and N 2 continued to flow for 5 minutes. Then 0 2 flow was turned on and N 2 off, and 0 2 alone was flowed for 5 minutes. Next, with 0 2 still on, H was turned on and the wafer was oxidized for 2 hour and 50 minutes to grow an approximately 7000 A oxide layer (confirmed by laser ellipsometry) . After this amount of time, the H 2 was turned off, then after 5 minutes N 2 was turned on and 0 off. The glass rod was pushed in until it grabbed the boat which was pulled out slowly over 3 minutes. Next, the dehydrated wafer was placed on a photoresist spinner (Solitec Wafer Spinner) polished side down, adhesion promoter (American Hoechst Corp. AZ Adhesion Promoter) was pipetted onto the wafer surface and the wafer spun to spread the chemical out thinly and evenly (5 sec spread, 30 sec spin dry, 2000 rpm) . Then the photoresist (American Hoechst Corp. , AZ Photoresist) was pipetted on the wafer surface and spread as was the adhesion promoter. The wafer was placed in a soft bake oven (90°C) for 15 minutes for solvent removal and then the polished surface of the wafer was coated with photoresist (3) (see Figure 11(c) as described above. After another 30 minutes in the soft bake oven, the wafer was placed in an aligner (Quintel 2001 Aligner) and exposed to UV radiation for 15 seconds (see Figure 11(d)) through the chosen photomask (4). Immediately thereafter, the wafer was immersed in photoresist developer (American Hoechst Corp., AZ-351 Developer) which was diluted 5X with water, rinsed in a deionized water bath, and dried with N . The wafer was hard baked (110"C) for 30 minutes to remove any remaining solvents. Next, the exposed oxide layer (5) (see Figure 11(e)) was removed in a buffered HF (10:1) etch by immersion for 17 minutes (Si0 2 etch rate = 500 A/min) . The disappearance of the oxide layer is evident (see Figure 11(f)) because the mask pattern areas are clearly hydrophobic (Si) when in contact with water. After the photoresist was removed by spraying the wafer with acetone while spinning on the wafer spinner (see Figure 11(g)), the wafer is ready for silicon etching. An anisotropic chemical etchant containing ethylenediamine, pyrocatechol and water (EPW) , was prepared by adding 900 mL ethylenediamine (Aldrich Chemical Co.), 288 g of catechol (Aldrich Chemical Co.), 288 mL of deionized water, and 5.4 g of pyrazine (Aldrich Chemical Co.). The exact concentration of the EPW solution used is dependent on the amount of silicon to be etched and the solution described above will etch approximately 9 g of silicon, which is far more than was etched in this example (1-2 g) . When refluxed at 115 β C, the pink-purple solution etches (100) oriented silicon preferentially at a rate of 1.6 μm/min (confirmed by Profilimeter measurement) , while it etches the Si0 mask at a much slower rate of 5 A/min. Therefore, if the entire 510 μm thick wafer is to be etched through at 1.6 μm/min, 319 minutes are required during which approximately 1600 A of Si0 2 will be etched, which is far less than the total oxide thickness. Anisotropic etching of (100) oriented silicon results in angled walls (54.74°) because the (111) surfaces are attacked at a rate which is 35X slower. With the first mask image on the wafer, it was placed in a quartz carrier and lowered into the refluxing EPW solution. After etching for 162 minutes (260 μm deep) , the wafer was removed, washed with deionized water and quick dumped (see Figure 11(h)). The wafer was dehydrated by baking at 110 β C for 1 hour, and then the second mask pattern was aligned and imaged on the wafer according to the steps described above. Once all the channels have been etched (second etch step requires 156 minutes) , the oxide layer is removed in buffered HF (10:1), and the devices cut out with an automatic dicing saw (1 mil blade, 28,000 rpm, 0.5 inch/second) . The source and reference fibers (Ensign-Bickfort Optics Co., HCN-HO300T-12, core diameter 300 μm and HCN- H0200T-12, core diameter 200 μm, respectively) were fixed together with a Duco cement/ cetone glue so that their endfaces were even with each other. The analytical signal fiber (Ensign-Bickfort Optics Co., HCN-H0200T-12, core diameter 200 μm) was attached to the source fiber with its endface 5 mm back. A drain capillary (Polymicro Technologies, TSP 320/450) was fixed 2 mm behind the signal fiber. This bundle of four components was placed in the detection chamber of the silicon wafer and the source and reference were sealed in place with fluoro- polymer epoxy. The optical configuration is similar to that sensor probe of Example 1, except source radiation reflects off the silicon walls and is diffusely reflected back through the optical cell to the signal fiber. The hollow fiber polypropylene membrane (Celgard X-20, 400 μm i.d.) was placed across the window and fixed at both ends with epoxy. Also, the reagent inlet capillary (Polymicro Technologies, TSP 100/170) was inserted and fixed in place with epoxy. Epoxy was thinly spread over the top surface of the silicon wafer and its mirror image was bonded to it. After curing, the points where the fibers, membrane and capillaries enter the device were sealed with epoxy, the fibers and capillaries were put in bundles and protected with heat shrink tubing, and the entire sensor was coated with apeazon for further protection. At a flow rate of 10 μL/min, this sensor probe responded reproducibly to analyte chlorine concentrations of 5, 10 and 15 milligrams/liter, utilizing a reagent which included a mixture of N,N,-diethyl-p- phenylenediamine sulphate and phosphate buffer (pH 6) . Response times and recovery times were approximately 2 and 5 minutes, respectively. From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and the scope of the invention.

Claims: Claims 1. A device capable of continuously measuring the presence and concentration of an analyte or analytes in a liquid and/or a gas phase, comprising: a sensor probe wherein the sensor probe comprises a reaction chamber defined by a permeable membrane, an inflow opening, an outflow opening, a primary electromagnetic radiation source and a detection fiber, wherein the primary electromagnetic radiation source directs electromagnetic radiation across a defined path within the reaction chamber toward the detection fiber, and wherein a reagent flows into the reaction chamber from the inflow opening, across the defined path and out the outflow opening, and wherein the analyte or analytes are capable of diffusion across the permeable membrane into the reaction chamber; a reservoir communicating with the inflow opening of the sensor probe, wherein the reservoir contains the reagent, and wherein the reagent chemically reacts with the analyte or analytes to modulate electromagnetic radiation differently than unreacted reagent; and a detection means communicating with the detection fiber in the sensor probe. 2. The device of claim 1 wherein the reaction chamber is divided into a first chamber defined by the permeable membrane and the inflow opening and a second chamber downstream from and communicating with the first chamber, wherein the second chamber comprises the electromagnetic radiation source, the detection fiber and the outflow opening, wherein the second chamber is defined by a closed space communicating with the first chamber and with a waste reservoir communicating with the outflow opening. 3. The device of claim 1, further comprising a secondary source of electromagnetic radiation within the sensor probe, wherein the secondary source is a means for scattering and/or reflecting electromagnetic radiation from the primary electromagnetic radiation source, and wherein the defined path is the distance from the secondary source to the detection fiber. 4. The device of claim 1 wherein the permeable membrane is selected from the group consisting of a permeable membrane, an ion exchange membrane and a combination thereof. 5. The device of claim 4 wherein the semipermeable membrane is made from porous teflon, porous polypropylene or silicon rubber. 6. The device of claim 4 wherein the ion exchange membrane is selected from the group consisting of aminated polystyrene, aminated polypropylene, aminated polyethylene, Nafion ® , sulfonated polystyrene polyacrylates and polypropylene, radiation grafted polymers having trimethyl amine, ethyl dimethyl amine and dimethylethanolamine as functional groups, and combinations thereof. 7. The device of claim 1 wherein the primary electromagnetic radiation source is selected from the group consisting of ultraviolet light, visible light, and infrared light. 8. The device of claim 3 wherein the secondary source of electromagnetic radiation is made from an optically transparent or translucent material that scatters or reflects electromagnetic radiation. 9. The device of claim 3, further comprising a source reference detector fiber communicating with the detection means, wherein the source reference detector monitors the intensity of the primary or secondary source of electromagnetic radiation. 10. The device of claim 1, further comprising a means for regulating the flow rate of reagent into and out of the reaction chamber. 11. The device of claim 10 wherein the means for regulating flow rate is a valve. 12. The device of claim 1 wherein the detection means is selected from the group consisting of a photomultiplier, a solid state photodetector, and combinations thereof. 13. A method for continuously measuring the presence and concentration of an analyte or analytes in a liquid and/or a gas phase, comprising: contacting a sensor probe with the liquid and/or a gas phase material, wherein the sensor probe comprises a reaction chamber defined by a permeable membrane, an inflow opening, an outflow opening, a primary electromagnetic radiation source, and a detection fiber, wherein the primary electromagnetic radiation source directs electromagnetic radiation across a defined path within the reaction chamber toward the detection fiber, and wherein a reagent flows into the reaction chamber from the inflow opening, across the defined path, and out the outflow opening, and wherein the analyte or analytes are capable of diffusion across the permeable membrane into the reaction chamber and are further capable of reacting with the reagent, and wherein the detection fiber communicates with a detection means; adjusting the flow rate and the concentration of the reagent into the reaction chamber to adjust the sensitivity and the dynamic range of the sensor probe; and detecting the presence and concentration of the analyte or analytes by having the analyte or analytes diffuse across the permeable membrane, react with the reagent flowing through the reaction chamber and across the defined path, and modulate the electromagnetic radiation directed toward the detection fiber. 14. The method of claim 13 wherein the reaction chamber is divided into a first chamber defined by the permeable membrane and the inflow opening, and a second chamber downstream from and communicating with the first chamber, wherein the second chamber comprises the electromagnetic radiation source, the detection fiber and the outflow opening, wherein the second chamber is defined by a closed space communicating with the first chamber and a waste reservoir communicating with the outflow opening, and wherein the second chamber is not defined by a permeable membrane. 15. The method of claim 13 wherein the sensor probe further comprises a secondary source of electromagnetic radiation within the sensor probe chamber, wherein the secondary source is a means for scattering and/or reflecting electromagnetic radiation from the primary electromagnetic radiation source, and wherein the path is the distance from the secondary source to the detection fiber. 16. The method of claim 15 wherein the secondary source of electromagnetic radiation is made from optically transparent or translucent material. 17. The method of claim 15 wherein the sensor probe further comprises a source reference detector fiber communicating with the detection means, wherein the source reference detector monitors the intensity of the primary or secondary source of electromagnetic radiation. 18. The method of claim 13 wherein the electromagnetic radiation is selected from the group consisting of ultraviolet light, visible light, and infrared light. 19. The method of claim 13 wherein the permeable membrane allows for diffusion of the analyte or analytes into the reaction chamber by passive diffusion through a semipermeable membrane, wherein the composition of the semipermeable membrane is selected from the group consisting of porous Teflon ® , porous polypropylene, and silicone rubber. 20. The method of claim 13 wherein the flow rate of the reagent into the reaction chamber is adjusted by means of a valve communicating with either the inflow opening or the outflow opening. 21. The method of claim 13 wherein the specificity of the dynamic range of the sensor probe for the analyte of analytes is conferred by permeable membrane specificity, the choice and concentration of reagent, the particular wavelength or wavelength range of electromagnetic radiation, the flow rate of reagent, and the geometry of the sensor probe. 22. The method of claim 13 wherein the detection means detects the reaction product of the analyte or analytes and the reagent by absorbance, fluorescence or chemiluminescence. 23. A device capable of continuously measuring the presence and concentration of an analyte or analytes in a liquid or gas phase by chemiluminescence, comprising: a sensor probe, wherein the sensor probe comprises a reaction chamber defined by a permeable membrane, an inflow opening, an outflow opening, and a detection fiber, wherein a reagent or reagents flows into the reaction volume and reacts with the analyte or analytes to produce photons from a chemiluminescent reaction, wherein the photons are detected by the detection fiber, and wherein the analyte or analytes are capable of diffusion across the permeable membrane into the reaction chamber; a reservoir communicating with the inflow opening of the sensor probe wherein the reservoir contains the reagent or reagents ; and a detection means communicating with the detection fiber in the sensor probe. 24. A method for continuously measuring the presence and concentration of an analyte or analytes in a liquid and/or a gas phase using photons emitted from a chemiluminescent reaction, comprising: contacting a sensor probe with the liquid and/or gas phase material, wherein the sensor probe comprises a reaction chamber defined by a permeable membrane, an inflow opening, an outflow opening, and a detection fiber, wherein a chemiluminescent reagent or reagents flow into the reaction chamber from the inflow opening and out the outflow opening, wherein the chemiluminescent reagent is capable of reacting with the analyte or analytes to produce photons, wherein the photons are detected by the detection fiber; adjusting the flow rate of the reagent or reagents into the reaction chamber to adjust the sensitivity of the sensor probe; and detecting the presence and concentration of the analyte or analytes with a detection means communicating with the detection fiber. 25. The method of claim 24 wherein the permeable membrane allows for diffusion of the analyte or analytes into the reaction chamber by passive diffusion through a semipermeable membrane, wherein the composition of the semipermeable membrane is selected from the group consisting of porous Teflon, porous polypropylene, and silicone rubber. 26. The method of claim 24 wherein the detection means is a photomultiplier tube. 27. A device capable of continuously measuring the presence and concentration of an analyte or analytes in a reaction volume, comprising: a sensor probe, wherein the sensor probe comprises a reagent inflow tube, a reagent outflow tube, a primary electromagnetic radiation source and a detection fiber, wherein the primary electromagnetic radiation source is directed in a defined path within the reaction volume to the detection fiber and wherein the inflow tube directs a flow of a reagent into the reaction volume, across the defined path, and out the outflow tube; a reservoir communicating with the inflow tube of the sensor probe, wherein the reservoir contains the reagent and wherein the reagent chemically reacts with the analyte or analytes to modulate electromagnetic radiation differently than unreacted reagent; and a detection means communicating with the detection fiber in the sensor probe. 28. The device of claim 27 wherein the cross- sectional area of the outflow tube is larger than the cross- sectional area of the inflow tube. 29. The device of claim 27, further comprising a secondary source of electromagnetic radiation wherein the secondary source is a means for scattering and/or reflecting electromagnetic radiation from the primary electromagnetic radiation source, and wherein the defined path is the distance from the secondary source to the detection fiber. 30. The device of claim 29 wherein the secondary source of electromagnetic radiation is made from an optically transparent or translucent material. 31. The device of claim 29 further comprising a source reference detector fiber communicating with the detection means, wherein the source reference detector monitors the intensity of the primary or secondary source of electromagnetic radiation. 32. The device of claim 27 wherein the primary electromagnetic radiation source is selected from the group consisting of ultraviolet light, visible light, and infrared light. 33. The device of claim 27, further comprising a sampling capillary communicating with the reaction volume. 34. A method for continuously measuring the presence and concentration of an analyte or analytes in a liquid or gas process stream when the analyte or analytes or process stream would foul a permeable membrane, comprising: contacting a sensor probe with the liquid or gas process stream, wherein the sensor probe comprises a reagent inflow tube, a reagent outflow tube, a primary electromagnetic radiation source, and a detection fiber, wherein the primary electromagnetic radiation source is directed in a defined path within the process stream to the detection fiber, and wherein the inflow tube directs a flow of a reagent into the process stream, across the defined path, and out the outflow tube; adjusting the flow rate of the reagent into the process stream to adjust the sensitivity of the sensor probe; and detecting the presence and concentration of the analyte or analytes by reacting with the reagent in the process stream, across the defined path, and modulating the electromagnetic radiation directed toward the detection fiber. 35. The method of claim 34 wherein the sensor probe further comprises a secondary source of electromagnetic radiation, wherein the secondary source is a means for scattering and/or reflecting electromagnetic radiation from the primary electromagnetic radiation source, and wherein the defined path is the distance from the secondary source to the detection fiber. 36. The method of claim 35 wherein the secondary source of electromagnetic radiation is made from optically transparent or translucent material. 37. The method of claim 36 wherein the sensor probe further comprises a source reference detector fiber communicating with the detection means, wherein the source reference detector monitors the intensity of the primary or secondary source of electromagnetic radiation. 38. The method of claim 34 wherein the electromagnetic radiation is selected from the group consisting of ultraviolet light, visible light, and infrared light. 39. The method of claim 34 wherein the cross- sectional area of the outflow tube is larger than the cross- sectional area of the inflow tube. 40. A device capable of continuously measuring the presence and concentration of an analyte or analytes in a reaction volume, comprising: a sensor probe, wherein the sensor probe comprises a reaction chamber defined by a closed volume with an opening communicating with the reaction volume, a reagent inflow tube, a reagent outflow tube, a primary electromagnetic radiation source and a detection fiber, wherein the primary electromagnetic radiation source is directed in a defined path within the reaction chamber to the detection fiber, and wherein the inflow tube directs a flow of reagent into the reaction chamber, across the defined path, and out the outflow tube; a reservoir communicating with the inflow tube of the sensor probe, wherein the reservoir contains the reagent, and wherein the reagent chemically reacts with the analyte or analytes to modulate electromagnetic radiation differently than unreacted reagent; and a detection means communicating with the detection fiber in the sensor probe. 41. The device of claim 40 wherein the cross- sectional area of the outflow tube is larger than the cross- sectional area of the inflow tube. 42. The device of claim 40 wherein the reaction chamber opening contains a frit to filter debris from the reaction volume. 43. The device of claim 40 wherein the secondary source is a means for scattering and/or reflecting electromagnetic radiation from the primary electromagnetic radiation source, and wherein the defined path is the distance from the secondary source to the detection fiber. 44. The device of claim 43 wherein the secondary source of electromagnetic radiation is made from an optically transparent or translucent material. 45. The device of claim 40 wherein the primary electromagnetic radiation source is selected from the group consisting of ultraviolet light, visible light, and infrared light.

Inventors: BURGESS LLOYD W JR, BURGESS, LLOYD, W., JR.
Applicants: WASHINGTON RES FOUND, THE WASHINGTON RESEARCH FOUNDATION
Application year: 1990
Main classification: G01N21
IPC codes: G01N21