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

Abstract

A direct current, reduced-pressure, gas sampling glow discharge (GSGD) ionization source has been developed and interfaced to an orthogonally extracted time-of-flight mass spectrometer for the purpose of generating both atomic and molecular fragmentation mass spectra. The discharge is contained within the first vacuum stage of the differentially pumped interface of the mass spectrometer. The source is mechanically and logistically simple to construct, operate, and maintain. Switching between the atomic and molecular modes of operation is achieved by altering the discharge gas composition, the operating pressure, and the current. Helium was used to generate atomic mass spectra, whereas molecular spectra were produced by use of argon. Gas flow rates were less than 1 l min ^–1 for each mode of ionization. This report focuses primarily upon the atomic (elemental) analytical capabilities of the 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 exponential dilution device, and with boxcar averagers employed for data collection. Precision is better than 0.4% relative standard deviation (RSD) for measurement of the ⁷⁹ Br ⁺ / ⁸¹ Br ⁺ isotope ratio (presented to the source as bromoform vapor) over a period of 2.5 h. A variety of chlorinated hydrocarbons were introduced into the discharge via a flow cell, and it was possible to differentiate (i.e., speciate) the compounds based upon their ³⁵ Cl ⁺ / ¹² C ⁺ elemental ratios with a correlation coefficient (R) of 0.996.