Optode for the determination of gases

Details Optode for the determination of gases Optode for the determination of gases

1998-08-28

2001-05-15

Williams, Hezron (Department: 2856)

Measuring and testing

Gas analysis

Detector detail

C073S023200, C422S082060, C422S082090, C436S167000, C356S437000

Reexamination Certificate

active

06230545

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns a sensor membrane of an optode, a method for the determination of gases in gas mixtures, and an apparatus for the determination of gases in gas mixtures.
2. Description of Related Art
The increasing demand for direct, reliable, and rapid determination of gas concentrations in gas mixtures, for example in medicine, environmental engineering, smoke and fire detection, the monitoring of chemical processes, or also for measuring the exhaust gases of internal combustion engines, is leading to greater interest in the development of novel gas sensors. Only at great cost and in relatively unreliable fashion, for example, has it hitherto been possible to use electrochemical sensors based on mixed oxide materials for controlling climate control systems in automobiles (A. Zeppenfeld et al., Sensor 97 Conference Report 1, p. 113 ff.). Also known, for example, is the measurement of oxygen partial pressure in the exhaust gases of internal combustion engines by means of “lambda probes,” which are also based on ceramic materials. An overview of methods for determining gases using solid electrolytes is provided by the article by I. Lundström, Sensor and Actuators B 1996, 35-36, p. 11 to p. 19, which also, in particular, discusses production, miniaturization, and cross-sensitivity problems. An attempt to eliminate, in particular, cross-sensitivities in gas determination is described in the article by E. Hermanns in Sensors and Actuators 1984, 5, pp. 181 to 186. In this context, gases are determined via the change in the electrical conductivity of polymers caused by gas absorption. The change in the layer thickness of polymer films which can absorb gases is measured by means of interference reflection (EP 0 737 768 A2). U.S. Pat. No. 5,030,420 discloses an apparatus for determining oxygen in gaseous mixtures by means of an optical sensor, also referred to as an optode, in which the quenching luminescence of ruthenium(II) complexes is measured. Another route to the determination, in particular the optical measurement, of chemical compounds and ions is offered by “ionophores.” These are lipophilic ligands which have the ability selectively to complex specific ions and to transport them through membranes by means of a carrier mechanism (EP 0 578 630 B1), so that various ions are selectively determined in solution and can be detected calorimetrically. Among these ionophores, the electroneutral ionically active substances have gained particularly wide applicability as components in ion-selective liquid membrane electrodes, for example as polymer liquid membrane electrodes in clinical analysis, as microelectrodes in electrophysiology, and in high-selectivity ion transport through artificial membranes. Recently, they have also been used in optode membranes for reversible optical detection of charged and electrically neutral species in solution (U. E. Spichiger et al., Proceedings SPIE—Int. Soc. Opt. Eng. 1991, 1510, 118). Pronounced selectivit-es were obtained, in particular, for the direct determination of H
+
, Li
+
, Na
+
, K
+
, Cs
+
, Mg
2+
, Ca
2+
, Sr
2+
, Ba
2+
, Tl
+
, UO
2
2+
, Cl
−
, and CO
3
2−
. Two possible principles were used in this context: on the one hand, only one ion-selective chromophoric or chromogenic ligand is used, which in contact with the species to be detected changes, in particular, its color. On the other hand, the combination of electroneutral ion carriers with “chromoionophores” in very thin polymer liquid membranes allows a determination of the optical activity of those cations and anions. Both principles are usable only in pH-buffered solutions. With none of these proposed solutions, however, was it possible to accomplish a simple and rapid accurate determination of gaseous species in gas mixtures, reliably and with no great need for complex equipment, while also avoiding cross-sensitivities.
SUMMARY OF THE INVENTION
The use of a sensor membrane of an optode for determination of a physical and/or chemical parameter of a sample whose absorption properties change, on the basis of an indicator substance contained in it, due to at least indirect contact with a gas and/or gas mixture makes it possible, in extremely simple fashion, to produce miniaturized gas sensors, called “optodes,” using said sensor membrane. Since only the tiniest quantities of substance are required for detection, miniaturization is particularly simple. The cost factor is also decisively reduced thereby.
Because of the fact that in the method for the determination of gases and gas mixtures, the absorption properties of the indicator substance exposed at least directly to the gas mixture are measured, it is possible in very simple fashion, by means of simple optical apparatuses, to determine gases in concentrations of a few ppb up to a few % without cross-sensitivities. This was difficult to implement with the existing measurement methods, and moreover required much more complex equipment. The indicator substance used can be, for example, a chromogenic ligand which, in a physical and/or chemical interaction with the gas species to be determined, changes its absorption spectrum for electromagnetic radiation. It is also possible to use a selective ligand in conjunction with a chromoionophore.
The detection limit can be adjusted as required, in particularly advantageous and simple fashion, by varying the layer thickness and layer composition, in particular the concentration and quantities of the compounds used, since they are correlated with one another.
In an advantageous apparatus for the determination of gases in gas mixtures, in which a gas-permeable membrane is provided whose optical absorption properties change upon at least indirect contact with a gas and/or gas mixture, at least one source of electromagnetic radiation, in particular an LED, having a discrete and selectable wavelength is provided. This makes it possible to dispense with a complex device which cannot be miniaturized, for example a UV/visible or IR device. The sensor membrane having the indicator substance contained therein can thus be irradiated with a discrete, selectable wavelength which can be determined for each gas by prior calibration, so that a transmission can be measured when an absorption is present. It is moreover also possible, in very simple fashion, to combine multiple sources of electromagnetic radiation with multiple sensor membranes which are sensitive to various gases, so that multiple gases can be measured simultaneously by means of a single apparatus. It is also possible to use one source of electromagnetic radiation with multiple discrete or integrated detectors. It is also possible to use one source of electromagnetic radiation which covers the entire spectral region, in conjunction with wavelength selectors.
Further advantageous embodiments and developments of the invention are described in the dependent claims.
In a particularly advantageous development of the sensor membrane according to the invention, the latter contains an indicator substance which is effective in gas-specific fashion. As a result, cross-sensitivities such as those which occur in particular with sensors based on metal oxides and mixed oxides are prevented in very simple fashion. It is thus possible, depending on the indicator substance, to measure each volatile compound and each desired gas, for example hydrogen, oxygen, water vapor, carbon monoxide, volatile amines, etc., without having the compounds mutually block or falsify their detection.
In a further advantageous embodiment, the indicator substance is present as an ion pair, the ion pair consisting of a cationic or anionic dye molecule, for example a phenolphthalein or eosine or fluorescein derivative, and at least one counterion compensating for the electrical charge of the dye molecule. The ion pair principle used here can make it possible to convert the gas into the corresponding ion, which is charged and consequently requires a c

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