Development of a direct current He atmospheric-pressure glow discharge as an ionization source for elemental mass spectrometry via hydride generation

A new direct-current glow discharge has been designed and interfaced with a time-of-flight mass spectrometer. This glow discharge is simple to operate, sustainable in a helium environment at atmospheric pressure at the 40-100 mA level with burning voltages of 0.9-1.2 kV, and capable of generating singly charged atomic ions observable by mass spectrometry. Because of its high first ionization potential (24.6 eV), the presence of He+ in background mass spectra suggests that a significant amount of energy is available for the ionization of gaseous analyte species. In addition, noise power spectra obtained from helium ions in an analyte-free discharge indicate that discrete-frequency noise sources are not present in the atmospheric-pressure glow discharge; only 1/f noise and white noise are found. When this detection system is coupled with hydride generation, the analytical performance of the hydride generation-atmospheric-pressure glow discharge system is comparable with that typically obtained with an inductively coupled plasma ionization source. Although the precision and sensitivity of this system are slightly poorer, the discharge spectrum is free from polyatomic background ions that could otherwise yield problematic isobaric interferences. In addition, a reduction in the overall continuum background enables limits of detection for three routinely-investigated hydride species (arsenic, selenium, and mercury) down to 10 ppt to be realized.