Discharge ionization detectors

After the flov/ measurement, sample the discharge stream to a continuous Flame Ionization Detector (FID) that works as an Analyzer Indicator Transmitter CYIT). The Flow Controller (FC) reads the TCE concentration signal and adjusts the flow to keep the TCE at the set level of, say, 250 ppm. 

An important feature of all gas ionization detectors is their dead or paralysis time, which has a direct bearing on their utility. Once initiated a voltage pulse takes several hundred microseconds to die away, and may be prolonged by the discharge of secondary electrons from the cathode as the argon ions are reduced. Until this first pulse is terminated the tube is dead to further radiations and the recorded count CM will be less than the true count CT. The relation between the two values will depend upon the length of the dead time t... 

The gases from the vacuum pump are either directed to a gas chromatograph (GC) through an automatic injection valve whose inside loop was 0.250 mL or discharged to gas collection burettes. The GC is a Varian 3400 with a flame ionization detector (FID)... 

The pulsed discharge electron capture detector is an extension of the previously discussed pulsed discharge helium ionization detector, a... 

The sensor consists of two sections the upper section where the discharge takes place has a small diameter and the lower section where the column eluent is sensed and the electron capturing occurs has a wider diameter. As with the pulsed discharge helium ionization detector, the potential across the discharge electrodes is pulsed at about 3 kHz with a discharge pulse width of about 45 psec for... 

Figure 22 shows results obtained in the gas chromatography of a silylated mixture of aliphatic acids, phenylarsenic acid and hydrocarbons using a flame ionization detector and a glow discharge detector set at the silicon and arsenic wavelengths. 

FIGURE 22. Gas chromatograms of a silylated mixture of aliphatic acids, phenylarsonic acid and hydrocarbons. Top curve flame ionization detector (FID). Centre and bottom curves glow discharge detector (GDD), with monochromator set as indicated. Reprinted with permission from Reference 161. Copyright (1977) American Chemical Society... 

The metastable atoms that must be produced in the argon and helium detectors need not necessarily be generated from electrons induced by radioactive decay. Electrons can be generated by electric discharge or photometrically, which can then be accelerated in an inert gas atmosphere under an appropriate electrical potential to produce metastable atoms. This procedure is the basis of a highly sensitive helium detector that is depicted on the left-hand side of Fig. 1. The detector does not depend solely on metastable helium atoms for ionization and, for this reason, is called the helium discharge ionization detector (HDID). 

Fig.l The discharge ionization detector (courtesy of GOW-M AC Instruments) and the pulsed helium discharge detector (courtesy of Valeo Instruments). 

Wentworth has been a member of the American Chemical Society, the American Solar Energy Society, and Alpha Chi Sigma. He has published over 125 papers, written three textbooks, and received numerous patents. One of the publications from his doctoral dissertation was selected as a pioneer paper in analytical chemistry. His papers on the chemical applications of nonlinear least squares became Citation Index Classics. His patent for pulse discharge ionization detectors was selected as one of the 100 most significant inventions of 1997. He retired after four decades of teaching and research, having about forty graduate students, half of whom received doctorates. The Robert A. Welch Foundation supported his efforts every year. 

Abbreviations GC, Gas Chromatography IRMS, Isotope Ratio Mass Spectrometry ECD, Electron Capture Detector FID, Flame Ionization Detector TCD, Thermal Conductivity Detector MS, Mass Spectrometry PDHID, Pulsed-Discharge Helium Ionization Detector mol. sieve, Molecular Sieve HP, Hewlett-Packard PE, Perkin-Elmer. 

Dojahn JG, Wentworth WE, Deming SN, and Stearns SD (2001) Determination of percent composition of a mixture analyzed by gas chromatography. Comparison of a helium pulsed-discharge photoionization detector with a flame ionization detector. Journal of Chromatography A 917 187-204. 

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