Chemistry: analytical and immunological testing – Measurement of electrical or magnetic property or thermal... – Of an ionized gas
Patent
1996-03-14
1998-03-17
Le, Long V.
Chemistry: analytical and immunological testing
Measurement of electrical or magnetic property or thermal...
Of an ionized gas
436149, 436161, 422 83, 422 91, 422 98, 422 89, 73 2335, 73 234, G01N 3064, G01N 2100
Patent
active
057285861
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a photoionization detector and to a process of detecting effluents from a gaschromatographic column in photoionization mode.
BACKGROUND ART
Photoionization detectors (PIDs) are known in the art and are generally used to detect gaschromatograph eluates. Basically, these detectors use the radiant energy generated by a UV lamp to ionize at least part of the compounds eluting from the GC column. Ionization is carried out in an ionization chamber containing two electrodes, anode and cathode, acting as ion accelerator and collector, and a window, sealed to the lamp, through which the lamp radiation passes into the ionization chamber.
Present PIDs have several drawbacks, resulting from the basic requirements of a PID, i.e. that the lamp window and ion chamber must be as close as possible, to have high irradiance and good ionization of the effluents, and that the lamp and ionization chamber must be heated to avoid condensation of the effluents exiting the GC column. The main drawback is due to the fact that heating of the detector base body to temperatures of up to 350.degree.-400.degree. C., i.e. typical detector temperatures, results in shortened lamp lifetime due to deterioration of the seal between the lamp body and the lamp window. Furthermore, the lamp has to be sealed against the detector; this is usually done with a polymeric gasket or O-ring between the lamp window and the detector body. At high temperatures this polymer seal releases compounds which then deposit onto the window, decreasing lamp output.
This means that, because of the above cited requirement of closeness of the window to the detector base body, actual operating maximum temperature of the detector is about 250.degree.-280.degree. C., in order to avoid cited problems. This temperature limitation results in the possibility of analyzing volatile compounds only. High boiling compounds, i.e. those that require a detector temperature of up to 400.degree. C., cannot be analyzed in the PID mode--if state of the art PID detectors are used.
Another drawback is that the sample stream contacting the lamp window results in window fogging because of polymerization of the sample compounds by UV radiation: the window must therefore be periodically cleaned--or replaced.
A further drawback of the known PIDs is that many compounds eluting from the column, upon contact with metal electrodes are partially decomposed and are therefore lost for analysis by a further detector located downstream the PID, such as e.g. a Flame Ionization Detector (FID).
German patent DD-A-290296 discloses a photoionization detector having electrodes that are transversal to GC column and wherein the window is replaced by a light-guiding element of magnesium fluoride. This embodiment has the same problems above cited, in that the gaskets release some material and deposition of material on the light-guiding element occurs.
U.S. Pat. No. 4,804,846 discloses a PID provided with a duct feeding a purge gas to the ionization chamber space adjacent to the window of the hot lamp, in order to fluidically sweep away the sample compounds that could contact the lamp window, to thus try to prevent its fouling by polymerization of sample compounds under UV radiation action. Anyway, material released from the lamp gasket may (and does) easily deposit onto the lamp window. In this embodiment, the ionization chamber volume is as small as possible to reduce dead volume, the sweep gas exits the chamber through an exit transfer tube, and the electrodes are housed in recesses of a doubly conical polyimide seal. The presence of this seal results in above cited problems of thermal decomposition of the seal and consequent high baseline current during analyses at elevated temperature.
More recently (Pittcon '94--Chicago) Wentworth et al. disclosed a windowless PID, namely a Pulsed Discharge Photoionization Detector--see Abstracts of the Pittsburg Conference, n. 660. This completely different solution results in some limitations if compared to classic PIDs w
REFERENCES:
patent: 4137750 (1979-02-01), French et al.
patent: 4804846 (1989-02-01), Hall
patent: 5561344 (1996-10-01), Hsi
patent: 5578271 (1996-11-01), Simon et al.
Fisons Instruments S.p.A.
Le Long V.
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