Gas sampling glow discharge: a versatile ionization source for gas chromatography time-of-flight mass spectrometry

A gas chromatograph has been coupled to a direct-current gas sampling glow discharge (GSGD) ionization source for the mass spectrometric analysis of halogenated hydrocarbons. The continuous discharge is contained within the first vacuum stage of the differentially pumped spectrometer interface. The discharge can be operated statically or rapidly switched between atomic and molecular ionization modes; both atomic and molecular spectra could be generated in the helium-supported plasma. In the switched configuration, the duty cycle is 50% for each mode. The ionization mode is selected by application of either a positive (molecular) or negative (atomic) potential to the sample introduction electrode, and the two kinds of spectra can be sequentially collected by changing the voltage and current between two preset values. Similar ion-optical voltage settings could be employed for both modes of operation, with the exception of the steering-plate potential, which had to be modulated between two different values (at the plasma switching frequency) to obtain the greatest atomic and molecular signal levels. The source is capable of generating mass spectra resembling those from an electron-impact source while operated in the 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 in the static mode with single-channel gated ion counting. Atomic detection limits obtained with boxcar averager data collection were comparable for static and switched operation of the source (1-30 pg/s). Likewise, the molecular detection limits were similar for the static and switched modes and span the range of 7-140 pg/s (boxcar averagers). Precision was better than 7% RSD under all conditions. The atomic and molecular chromatographic peak heights were nearly unchanged over a range of modulation rates from 5 to 100 Hz. The elemental ratio (35Cl+/12C+) for chloroform was also measured over a range of plasma modulation rates (5-90 Hz) and found to be randomly distributed about the sample mean. Several chlorinated hydrocarbons were introduced into the discharge and could be successfully differentiated (or speciated) on the basis of their 35Cl+/12C+ ratios.