Development of a direct current gas sampling glow discharge ionization source for the time-of-flight mass spectrometer

A direct current, reduced-pressure, gas sampling glow discharge (GSGD) ioni zation source has been developed and interfaced to an orthogonally extracte d time-of-flight mass spectrometer for the purpose of generating both atomi c and molecular fragmentation mass spectra. The discharge is contained with in the first vacuum stage of the differentially pumped interface of the mas s spectrometer. The source is mechanically and logistically simple to const ruct, operate, and maintain. Switching between the atomic and molecular mod es of operation is achieved by altering the discharge gas composition, the operating pressure, and the current. Helium was used to generate atomic mas s spectra, whereas molecular spectra were produced by use of argon. Gas flo w rates were less than 1 l min(-1) for each mode of ionization. This report focuses primarily upon the atomic (elemental) analytical capabilities of t he GSGD interface. Atomic detection limits are in the range of 20-90 pg s(- 1) (as the halogen) for analytes introduced into the system with an exponen tial dilution device, and with boxcar averagers employed for data collectio n. Precision is better than 0.4% relative standard deviation (RSD) for meas urement of the Br-79(+)/Br-81(+) isotope ratio (presented to the source as bromoform vapor) over a period of 2.5 h. A variety of chlorinated hydrocarb ons were introduced into the discharge via a flow cell, and it was possible to differentiate (i.e., speciate) the compounds based upon their Cl-35(+)/ C-12(+) elemental ratios with a correlation coefficient (R) of 0.996.