Gas Sampling Glow Discharge: A Versatile Ionization Source for Gas Chromatography Time-of-Flight Mass Spectrometry
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- 作者：John P. Guzowski ; Jr and Gary M. Hieftje
- 刊名：Analytical Chemistry
- 出版年：2000
- 出版时间：August 15, 2000
- 年：2000
- 卷：72
- 期：16
- 页码：3812 - 3820
- 全文大小：112K
- 年卷期：v.72,no.16(August 15, 2000)
- ISSN：1520-6882
文摘
A gas chromatograph has been coupled to a direct-currentgas sampling glow discharge (GSGD) ionization source forthe mass spectrometric analysis of halogenated hydrocarbons. The continuous discharge is contained within thefirst vacuum stage of the differentially pumped spectrometer interface. The discharge can be operated statically orrapidly switched between atomic and molecular ionizationmodes; both atomic and molecular spectra could begenerated in the helium-supported plasma. In the switchedconfiguration, the duty cycle is 50% for each mode. Theionization mode is selected by application of either apositive (molecular) or negative (atomic) potential to thesample introduction electrode, and the two kinds ofspectra can be sequentially collected by changing thevoltage and current between two preset values. Similarion-optical voltage settings could be employed for bothmodes of operation, with the exception of the steering-plate potential, which had to be modulated between twodifferent values (at the plasma switching frequency) toobtain the greatest atomic and molecular signal levels. Thesource is capable of generating mass spectra resemblingthose from an electron-impact source while operated inthe molecular ionization mode (both static and dynamic).The best atomic detection limits (1-25 fg of analyte/second) were obtained when the plasma was operated inthe static mode with single-channel gated ion counting.Atomic detection limits obtained with boxcar averager datacollection were comparable for static and switched operation of the source (1-30 pg/s). Likewise, the moleculardetection limits were similar for the static and switchedmodes and span the range of 7-140 pg/s (boxcar averagers). Precision was better than 7% RSD under allconditions. The atomic and molecular chromatographicpeak heights were nearly unchanged over a range ofmodulation rates from 5 to 100 Hz. The elemental ratio(35Cl+/12C+) for chloroform was also measured over arange of plasma modulation rates (5-90 Hz) and foundto be randomly distributed about the sample mean.Several chlorinated hydrocarbons were introduced intothe discharge and could be successfully differentiated (orspeciated) on the basis of their 35Cl+/12C+ ratios.