Measuring and testing – Gas analysis – Gas chromatography
Patent
1987-07-17
1989-08-29
Levy, Stewart J.
Measuring and testing
Gas analysis
Gas chromatography
G01N 2900
Patent
active
048605735
DESCRIPTION:
BRIEF SUMMARY
There is a large need for apparatuses for quantitative detection of a noxious gas or vapour in a mixture, e.g. in exhaust gases of internal combustion motors or chemical industries, poisonous vapours in containers, reaction chambers, hothouses or the like, as well as poisonous gases in the air, e.g. for military purposes.
Such apparatuses should be small, robust and relatively cheap, but should, nevertheless, operate dependably, and moreover requirements in respect of the sensitivity, response speed, stability etc. should be satisfied.
Moreover an apparatus intended for this purpose should be selective for a given component (or a restricted number of related components), and should also operate in such a reversible manner that concentration variations of the substance to be detected can be followed rapidly and dependably, and a continuous measurement can take place.
For a quantative detection of components present in the liquid or gas apparatuses are known, comprising a substrate in which, by means of a piezo-electric transducer, elastic vibrations can be generated, said substrate being provided with a surface layer which is able to bond said component if it is being contacted with said layer, so that the vibration behaviour of said layer is changed, and said elastic vibration is transformed again into an electric vibration by means of a piezo-electric transducer so as to detect the change in the vibration behaviour of said layer. To that end reference can be made to U.S. Pat. No. 4,312,228. In this known apparatus the component to be detected is absorbed in the surface region of the substrate, the absorbed mass changing the vibration behaviour of the surface layer of said substrate, and in particular the propagation velocity of the elastic surface wave generated in said layer, and said wave an be detected by means of a piezo-electric transducer which transforms said vibration into an electric signal. The frequencies used therefor can be very high, so that, in view of the propagation velocity of the elastic vibration in the substrate, a very small distance can contain a very large number of wave-lengths, so that the transit time variations can be determined very well.
In this known apparatus, a polymer layer is provided on the substrate surface which serves as an absorbent for gases.
This known apparatus is not suitable for the above-mentioned purposes. In the first place absorption is not very selective, and moreover, the reversibility of absorption is insufficiently fast for obtaining a quick adaptation to concentration variations. Moreover the polymer layer is relatively thick and does not form an essential part of the transfer path of the vibration which is mainly propagated along the substrate surface. This unfavourably influence the sensitivity. Moreover it is very difficult to produce such layers with a reproducable thickness and structure.
It is an object of the invention to provide a substrate for such an apparatus which is suitable for the above-mentioned purposes, said substrate being characterised in that at least the surface portion thereof consists of a substance with active sites for covalent bonding situated at its free surface, and in that, in said sites, molecules of a substance are covalently bond, said molecules possessing, on the other hand, active sites for selectively and reversibly bonding molecules of the component to be detected by means of a coordinative bond.
The coordinative bond with the component to be detected is relatively weak, so that the molecules of the component to be detected can be released relatively easily when the concentration of said component in the vicinity of the substrate is reduced, but said bond is strong enough for fixing said molecules, during the measurement, to the vibrating substrate. Such a layer is extremely thin, and comprises a large number of very regularly distributed bonding sites for said component, and said bond can be made very selective, and the mass variation in respect of the mass of the layer is substantial so that a sensitive and also r
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Levy Stewart J.
Nederlandes Organisatie voor Toegepast-Natuurwetenschappelijk On
Roskos Joseph W.
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