Chemistry: analytical and immunological testing – Nitrogen containing – Oxides of nitrogen
Patent
1997-06-23
1999-09-28
Soderquist, Arlen
Chemistry: analytical and immunological testing
Nitrogen containing
Oxides of nitrogen
422 8201, 422 88, 73 2406, 73 2401, 436135, G01N 2700, G01N 2902, G01N 3300
Patent
active
059587873
DESCRIPTION:
BRIEF SUMMARY
The invention describes a sensor for detecting oxidizing agents, such as oxides of nitrogen (NO.sub.x), nitrogen dioxide (NO.sub.2), ozone or per acids, the sensor comprising an oxidizable aromatic polymer, such as a polyarylene ether or polyarylene thioether, as the active component.
It is known (Analyt. Chem., 57(13), 2634-8, 1985), that ozone can be detected using a piezoelectric sensor coated with 1,4-polybutadiene. The production of the polymer coating by application with a brush presents a problem in the process described here. The contact surface may be damaged. Moreover, the homogeneity of the layer may not be reproducible by this process, which is confirmed by the stated range of frequency change (2000 to 10000 Hz). Furthermore, the observed frequency changes owing to the ppb quantities of ozone which come into contact are so small that they are of the order of magnitude of the noise of the natural frequency of the piezoelectric crystal (3 to 30 Hz). In addition, the reaction of the ozone with the 1,4-polybutadiene results in the formation of low molecular weight compounds which may partially evaporate. This leads to an opposite change in mass and hence to an error in the determination of the concentration of the gas.
The detection of NO.sub.2 in the sub-ppm range in a mixture with pure nitrogen, in which a dual arrangement of the quartz-SAW components (SAW=surface acoustic wave) having a resonant frequency of 600 MHz is used (M. Rapp et al., Sensors Actuators B 1991, 103-108), has also been described. The coating materials used are ultrathin layers (1 to 15 nm) of lead phthalocyanine and iron phthalocyanine derivatives, applied by vapor deposition or by the Langmuir-Blodgett technique. For example, 15 nm thick lead phthalocyanine films permit a limit of detection of 5 ppb for NO.sub.2 within a response time of a few minutes.
It is also known that electronic frequency generators for generating oscillations use a piezoelectric element of quartz or PZT ceramic. One of the resonant frequencies is selected for the detection of changes in mass and is amplied by the connected external frequency generator, the oscillation in question in the frequency range up to about 20 MHz being the fundamental oscillation generated by resonance excitation.
In the case of piezoelectric materials, the following function (Sauerbrey equation) is applicable for the frequency change .DELTA.f: .DELTA.m is the change in mass. If an oscillating surface (for example a quartz disk) is provided with a coating, the frequency of the sensor system changes owing to the increase in mass.
If the coating has absorptive properties, with respect to one or more substances in the surrounding medium, the oscillating system reacts with a change in frequency to the resulting absorption. The properties of the sensor (selectivity, sensitivity, regenerability, cumulability) can be adjusted within wide limits by an appropriate choice of the absorber.
However, it should be noted that the oscillation properties of the piezoelectric materials must not be adversely affected by the coating. Furthermore, the absorber must not react with the substances to be detected, with formation of volatile substances. Furthermore, rapid reaction with the material to be detected is essential for reasonable use.
The oscillatory capability of the piezoelectric crystal is generally lost if the applied absorber on the piezoelectric crystal is crystalline or semicrystalline. However, a prediction is never possible. Even when organic substances are used, the required properties cannot in general be established exactly in a reliable manner. The substance to be used is therefore chosen by a more or less empirical procedure.
It is the object of the invention to avoid the stated disadvantages and to provide a simple and reliable sensor for detecting oxidizing agents, such as ozone, oxides of nitrogen (NO.sub.x), nitrogen dioxide (NO.sub.2), hydrogen peroxide and per acids.
By using oxidizable aromatic polymers, such as polyarylene ethers or polyarylene thioethers, in sensors,
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Feucht Gernot
Frank Georg
Rieger Heinz
Schleicher Andreas
Schonfeld Axel
Soderquist Arlen
Ticona GmbH
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