Chemical apparatus and process disinfecting – deodorizing – preser – Analyzer – structured indicator – or manipulative laboratory... – Means for analyzing gas sample
Patent
1986-03-27
1988-10-25
Marcus, Michael S.
Chemical apparatus and process disinfecting, deodorizing, preser
Analyzer, structured indicator, or manipulative laboratory...
Means for analyzing gas sample
250283, 250306, 250423P, 422 54, 422 89, 436 35, 436153, G01N 2300
Patent
active
047802847
DESCRIPTION:
BRIEF SUMMARY
This invention is concerned with gas chromatography, and is especially concerned with detectors for detecting low concentrations of a specific gas in gas samples in gas chromatography.
The photoionization detector is one type of detector which is used in this field and, typically, it is capable of detecting concentrations of about 10.sup.-4 to 10.sup.-10 of the gas to be traced. There are two versions of the photoionization detector, one of which, the "window-type" detector, is capable of operating at atmospheric pressure, and the other of which, the "windowless-type" detector, is designed to operate at low pressures, i.e., less than 50 Torr. It is the "windowless-type" of detector with which this invention is predominantly concerned.
The windowless-type photoionisation detector typically comprises a hollow vessel which may be a quartz tube having an outlet port for connection of a low pressure vacuum pump thereto, a gas source at one end of the tube for providing ultraviolet radiation, and an inlet for gas samples at the other end of the tube. Between the two ends of the quartz tube (glass or a suitable metal may also be used as the material of the tube), a collimator is provided for collimating the ultraviolet radiation directed along the tube towards the sample gas inlet. A cathode is provided at the gaseous source, typically in an inlet through which the source gas enters the quartz tube, and an anode is provided at the mouth of the sample gas inlet. An ion chamber defined by a metal cylinder encloses longitudinally the mouth of the sample gas inlet and is usually maintained at ground potential relative to the anode.
In the operation of the detector, the cylinder of the ion chamber collects the ions produced by the radiation acting upon the sample gas and is connected to monitoring and measuring equipment which translates the current due to the ions collected into a signal which provides an indication of the presence or absence of specific gases.
A drawback which has hindered the wider application of the windowless photoionization detector is the relatively high level of background current, which gives rise to noise in the received signal. The observed noise levels impose severe limitations upon the minimum detectable signal. The background current is caused by a number of different phenomena, namely
(a) photoelectric emission from the electrodes and from other internal surfaces of the detector which are either directly or indirectly exposed to the ultraviolet radiation;
(b) ionizing collisions between metastable atoms or molecules and the surfaces of the electrodes or other internal surfaces of the detector;
(c) ions which are blown into the ion chamber from the ultraviolet radiation source.
All of these effects magnify the noise due to instability of the ultraviolet radiation source.
The present invention provides apparatus for detecting gas in a sample gas in a carrier gas stream, the apparatus comprising a windowless photoionization detector comprising a vessel having a cathode, a first inlet through which a source gas for providing ultraviolet radiation can be introduced, a second, sample gas/carrier gas inlet, a port to which vacuum pump means can be connected, and collector electrode means for collecting ions produced by ionization within the detector; and vacuum pump means for maintaining pressure within the photoionization detector between 0.1 Torr and 50 Torr; the apparatus being characterized by means for alternately switching a power supply to the cathode on and off at a rate providing a period which is longer than the transit time of ions to be detected within the photoionization detector.
In apparatus as set forth in the last preceding paragraph, it is preferred that means is provided for maintaining ambient temperature of the photoionization detector at a temperature in the range from 200.degree. C. to 300.degree. C.
In apparatus as set forth in either one of the last two immediately preceding paragraphs, it is preferred that the port is located at a position such that any metastables pr
REFERENCES:
patent: 3626181 (1971-12-01), Wernlund
patent: 4028617 (1977-06-01), Kamo et al.
patent: 4063156 (1977-12-01), Patterson
Hewlett--Packard Company
Marcus Michael S.
Timbie Donald N.
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