Method for detecting the presence or absence of a gas bubble by dynamic sensor response

A method for detecting the presence or absence of a gas bubble in an aqueous liquid is provided comprising providing a sensor positioned within a measuring chamber, wherein the sensor is configured to determine the concentration of a gaseous component dissolved in a liquid, the sensor comprising a sensitive region; setting a gas partial pressure at the sensor, wherein the gas partial pressure differs from an expected value of the gas partial pressure of the gaseous component of a liquid to be measured; exposing the sensor to the liquid to be measured; resting the liquid until standstill is attained; recording a signal from the sensor as a function of time until the signal becomes constant; and detecting the presence or absence of a gas bubble from the variation of the signal over time. The gas bubble, if present, is in at least partial contact with the sensitive region of the sensor.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Austrian patent application no. A 1468/2004 filed 2 Sep. 2004, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to analysis systems and methods thereof, and in particular to a method for detecting the presence or absence of a gas bubble in an aqueous liquid.

For the purposes of clinical diagnosis analysing systems are used for blood-gas analysis or for other measurements in liquid samples. Such systems are employed for instance for the determination of the partial pressure of oxygen or of carbon dioxide in blood, or of the hemoglobin parameters of whole blood, and for measurement of pH-value or ion concentration or special metabolites.

Complex analysis systems of this kind are usually provided with different sensor elements for determination of the parameters of interest, which elements are used for a multitude of measurements. Such sensor elements are for instance electrochemical or optical sensors for the determination of gas values, of the pH-value, of ion values or metabolite values, or optical measuring units for the determination of hemoglobin values.

In electrochemical gas sensors, also known as gas-selective or gas-sensitive electrodes, the gas molecules to be determined diffuse from an usually aqueous exterior solution or a gas phase into the interior electrolyte chamber of the sensor via a gas-permeable but essentially fluid- and ion-impermeable membrane. In addition to a liquid or solid interior electrolyte layer, the interior electrolyte chamber contains electrodes for electrochemical determination of the gas, especially measuring or working electrodes, counter-electrodes and reference electrodes. In the interior electrolyte chamber the electrochemical reactions for the determination of the gas by means of amperometric or potentiometric methods take place.

In the case of optical-chemical sensors a number of widely differing design variants and measurement principles have become known. In contrast to electrochemical sensors optical sensors do not require a reference electrode. An optical-chemical sensor typically consists of one or more layers of inorganic and/or organic, typically polymeric, substances applied on a transparent substrate, where at least one layer contains a dye whose optical properties (e.g., absorption, luminescence, etc.) are dependent on an analyte contained in the sample medium.

A frequently used gas sensor is the Clark oxygen sensor, for instance, where a gas-permeable membrane separates the interior electrolyte solution from the aqueous exterior medium, i.e., the medium to be measured. In the simplest case two electrodes dip into the interior electrolyte solution, one of which is placed immediately behind the membrane as a working electrode. After a polarization voltage of suitable strength has been applied, the oxygen which has diffused through the membrane from the measurement medium into the interior electrolyte chamber, is consumed by electrochemical reduction at the working electrode and an electric current corresponding to the substance consumed flows. This current is proportional to the partial pressure of oxygen in the medium to be measured and represents the primary measurement quantity.

Other frequently employed electrochemical gas sensors with gas-permeable membranes are for instance potentiometric sensors of the Severinghaus type for the measurement of carbon dioxide or electrochemical sensors for the determination of hydrogen by means of oxidation on platinum electrodes.

Such electrochemical gas sensors are often used in medical and diagnostic analyzers for the determination of partial gas pressures or gas concentrations in liquids. In particular, they are employed in blood gas analyzers, which play an important role in medical diagnostics. Blood gas analyzers often are provided with a plurality of sensors for diverse parameters, which are arranged in series. The sample fluid flows through these sensors, measurement often being taken by the “stop-flow-method”, i.e., with the sample at standstill during the measurement proper. Systems of this type are often used for routine measurements in clinics, laboratories and by medical practitioners, thus requiring the sensors used to have long service life, high accuracy and good reproducibility.

For the determination of oxygen in the OMNI® analyzer systems of Roche Diagnostics GmbH (Graz, Austria), for example, amperometric O2-sensors and potentiometric CO2-sensors are used. The oxygen sensors are miniaturized gas sensors of the Clark type. These gas sensor elements comprise, besides the actual sensor with its interior electrolytic chamber containing the electrodes, a sample passage for the transport and intermediate storage of the sample. Between the interior electrolytic chamber and the sample passage there is a gas-permeable and essentially ion- and fluid-impermeable plastic membrane separating the interior electrolytic chamber from the sample passage. The membrane is provided in a mechanically stretched state.

Frequently thin plastic membranes are employed in electro-chemical gas sensors, with layer thicknesses in the micrometer range, which are made from hydrophobic plastic materials, especially from polytetrafluoroethylene, polypropylene and polyethylene. Further details concerning typical membrane materials may be found in “Measurement of Oxygen by Membrane-covered Probes” (Ellis Horwood series in analytical chemistry, 1988, Ellis Horwood Limited, page 91f).

When gaseous analytes are determined in aqueous solutions by means of electrochemical gas sensors, especially in physiological fluids such as whole blood, serum or urine, problems may occur in certain rare cases during sample measurement or during calibration or quality control, if the sample or the calibrating or control medium does not completely fill the sample passage or if the solution contains gas bubbles, e.g., air bubbles, in the region of the sensors. Especially in blood-gas analyzer systems with sensor elements for small sample volumes gas bubbles may cause measurement errors, which will necessitate efficient checking for the presence or absence of gas bubbles in this case. Gas bubbles mostly adhere to the membrane surface. This phenomenon is observed when during the filling process of the sample passage the aqueous fluid avoids the hydrophobic surface of the membrane on one or both sides. If it is possible for the front of the fluid to laterally bypass the membrane before it is completely covered by the fluid, a gas bubble will form in the area of the membrane. Already existing as well as newly formed gas bubbles will mostly adhere to the membrane and will often not be removed by the fluid flow. A gas bubble adhering to the membrane or remaining in the immediate vicinity of the membrane will result in a measurement error, which will not be recognized as such without additional efforts to detect such bubbles.

The problem of enclosed air bubbles, which cause measurement errors by impeding sufficient wetting of the surface of the sensors used, is pointed out in the prior art. Measures for recognition of such errors will be necessary above all in automated analyzers, where the filling process of the measuring capillary or the absence of bubbles in the sample must be controlled automatically.

Certain known methods cannot efficiently detect air bubbles, however, which do not extend over the whole cross-section of the measuring passage or the measuring capillary. Resistance measurement would in such cases show slight variations in the measurement signal, which however could not be discerned from variations in the signal caused by different conductivity of the individual samples due to differing hematocrit values, for instance.

SUMMARY OF THE INVENTION

It is against the above background that the present invention provides certain unobvious advantages and advancements over the prior art. In particular, the inventors have recognized a need for improvements in methods for detecting the presence or absence of a gas bubble in liquids.

Although the present invention is not limited to specific advantages or functionality, it is noted that the present invention provides a method for the detection of a gas bubble, which typically is in at least partial contact with the sensitive region of a sensor located in a measuring chamber, in such a way that even very small gas bubbles may be definitely detected without additions or changes to the measuring chamber, thus making possible suitable counter measures.

In accordance with one embodiment of the present invention, a method for detecting the presence or absence of a gas bubble in an aqueous liquid is provided comprising providing a sensor positioned within a measuring chamber, wherein the sensor is configured to determine the concentration of a gaseous component dissolved in a liquid, the sensor comprising a sensitive region; setting a gas partial pressure at the sensor, wherein the gas partial pressure differs from an expected value of the gas partial pressure of the gaseous component of a liquid to be measured; exposing the sensor to the liquid to be measured; resting the liquid until standstill is attained; recording a signal from the sensor as a function of time until the signal becomes constant; and detecting the presence or absence of a gas bubble from the variation of the signal over time. The gas bubble, if present, is in at least partial contact with the sensitive region of the sensor.

The method of the invention, in accordance with at least one embodiment, utilizes the fact that gas sensors based on optical-chemical or electrochemical measurement principles have a certain response kinetics of the sensor signal. It is assumed that the gas component to be determined by the sensor is also present in the gas bubble.

Thus amperometric O2-sensors require a certain time for the signal to change from a first signal value corresponding to a preset pO2, to a second signal value corresponding to the pO2of the sample. The response kinetics is slower if the O2-reservoir of the sensor is larger. The O2-reservoir depends on the size of the sensor and the kind of sensor materials in the immediate vicinity of the O2-sensitive elements.

If amperometric O2-sensors are used, in accordance with one embodiment, the method of the invention also utilizes the fact that these sensors consume the analyte (O2) and that O2-depletion in the sample depends on the supply over time of gas molecules in the immediate vicinity of the O2-sensitive elements of the sensor.

According to another embodiment of the present invention, a signal curve value is measured for at least two points in time during the response time of the sensor, and these values are compared with known values obtained from bubble-free samples. In accordance with still another embodiment of the present invention, it is of advantage if, preceding measurement, a gas partial pressure is set at the sensor, which is greater than the gas partial pressure of the gaseous component in the liquid to be measured.

According to yet another embodiment of the present invention, the gas partial pressure at the sensor, preceding the introduction of the liquid to be measured, is set by using an aqueous liquid which contains the dissolved gaseous component to be determined in suitable concentration, for instance a suitably tonometered calibrating, control or rinsing fluid.

In accordance with yet still another embodiment of the present invention, it is also possible to set the gas partial pressure at the sensor, preceding the introduction of the liquid to be measured, by using a gas or mixture of gases which contains the gaseous component to be determined in suitable concentration.

DETAILED DESCRIPTION OF THE INVENTION

The measurement set-up schematically shown inFIG. 1has a measuring chamber1with a through-going sample passage2whose inlet opening is indicated by3. At the outlet opening4a pumping or suction device6is provided, for instance a peristaltic pump, for transporting the calibrating, control, rinsing or sample fluid. The sample passage2may contain a plurality of different electrochemical and/or optical-chemical gas sensors; shown in the figure is an amperometric sensor7, for instance an O2-sensor, whose sensitive region8protrudes into the sample passage2and into the measurement liquid9(e.g., a calibrating, control, rinsing or sample fluid) contained therein. The measurement liquid9can be separated at the inlet side by a separating gas bubble10from the subsequent liquid sample. A noxious gas bubble11is in at least partial contact with the sensitive region8of the amperometric sensor7.

The method of the invention is particularly suited for measurements in biological fluids, e.g., for in-vitro determination of the pO2of blood by means of an amperometric O2-sensor (seeFIG. 1). In order that the invention may be more readily understood, reference is made to the following example, which is intended to illustrate the invention, but not limit the scope thereof.

Immediately before the sensor7is exposed to the measurement fluid9(for instance a blood sample) a known pO2value is set at the sensor. Setting of the pO2value is performed using a liquid or gaseous medium containing oxygen (e.g., air of the separating air bubble10or a suitably tonometered rinsing solution). Typically, an O2gas partial pressure is set which differs from the expected value of the gas partial pressure of the measurement fluid. In an especially typical way an O2gas partial pressure is set which is greater than the expected value of the gas partial pressure of the measurement fluid.

The sensor7is exposed to the measurement fluid9by filling the sample passage2with the measurement fluid9, rinsing the passage with the measurement fluid if necessary, and then movement of the measurement fluid is stopped at time tS, with the sensor now in contact with the unmoving measurement fluid. The oxygen partial pressure in the sensitive region8of the sensor7and in the sample will then be equalized through diffusion processes. The signal curve due to the equalizing process is shown inFIG. 2, where the raw data of the signal curve without air bubble (solid line) and with air bubble (dash-dotted line) are shown. Measurement may be continuous, but it is also possible to record only individual pointwise measurement values Sp1to Sp5(seeFIG. 3) during the response time of the sensor.

FIG. 3shows the standardized signal curves of the data fromFIG. 2, where the measured curves have been standardized in such a way that the curves coincide at time t>150 s. InFIGS. 2 and 3a voltage U (in mV) proportional to the O2-concentration of the sample, e.g., a blood sample, is plotted on the ordinate versus time t (in s) on the abscissa. The amperometric signal is obtained from a current/voltage converter and measured in the form of a voltage proportional to the current. Evaluation of the signal curves to infer the presence of a gas bubble may be carried out directly from the measured voltage values, converting the signal curves to pressure units (pO2[mmHg]) and evaluation on the basis of pressure units could be done using calibration data, but is not necessary.

If an amperometric O2-sensor, for instance, is set to a pO2of ca. 160 Torr with a suitable calibrating medium and is then brought into contact with a blood sample, which has a lower pO2(e.g., 140 Torr), an exponentially decaying signal curve is observed over the time t. The signal change (dS/dt) tends to zero. Subsequently, t99will designate that period of time during which 99% of the signal change has occurred. The profile of the signal curve mirrors the diffusion processes of oxygen from sensor to sample and vice versa.

The pO2-value of the sensor7is to be understood as the pO2which the materials of the sensor have in the immediate vicinity of the sensitive elements.

When the pO2-value of a blood sample was determined with an amperometric sensor7, it was unexpectedly found in the measurement situation described above, that in the presence of a gas bubble11in direct contact with the sensitive region8of the sensor7the response behavior of the sensor was faster (see curve B inFIG. 3) than in the absence of the gas bubble (see curve A inFIG. 3). FromFIG. 3it may further be seen that t99is reached sooner if a gas bubble is present. From the difference in the signal curves the presence of a gas bubble may definitely be inferred and suitable counter measures may be taken.

The different signal curves may be explained by differences in diffusion properties, diffusion paths and gas volumes in the presence or absence of a gas bubble. If a gas bubble is present in the immediate vicinity of the sensor, for instance, resulting diffusion paths and gas reservoirs will be different from those in a bubble-free measurement liquid. Inside a gas bubble diffusion kinetics are generally faster than in an aqueous liquid. A gas bubble will generally contain more gas than an equal volume of aqueous liquid.

FIG. 4shows signal curves for different measurement values, with the voltages being converted to partial pressures (pO2mmHg) by means of calibration data. From top to bottom the individual curves are as follows:

P1′: sample 100 mmHg distorted to 140 by air bubble

P2′: sample 70 mmHg distorted to 120 by air bubble

P3′: sample 40 mmHg distorted to 100 by air bubble

P4′: sample 10 mmHg distorted to 80 by air bubble