Low Pressure Inductively Coupled Plasma Ion Source for Molecular and Atomic Mass Spectrometry: The Effect of Reagent Gases

Abstract

A low pressure inductively coupled plasma (LP-ICP) source, sustained at only 6 W and utilising 6 ml min^–1 helium, has been investigated as an ionisation source for molecular and atomic mass spectrometry. Iodobenzene and dibromobenzene were introduced to the LP-ICP via gas chromatography and yielded purely atomic ion signals for the iodine and bromine present, with detection limits of 4 and 76 pg for iodobenzene and dibromobenzene, respectively. The addition of nitrogen to a LP helium ICP increased the molecular ion signal for chlorobenzene, with a detection limit of 2 pg. However, the addition of nitrogen did not aid the production of molecular ions of iodobenzene and dibromobenzene. A study of the effect of skimmer spacing and forward power revealed considerable spatial separation of ionisation processes within the expansion chamber of the molecular beam interface. On the addition of 0.07 ml min^–1 isobutane the LP-ICP yielded mass spectra similar to those obtained by an electron impact source. However on the addition of more isobutane only the molecular ions (M⁺) for chlorobenzene, iodobenzene and dibromobenzene were observed. The detection limits for the instrument operating in the molecular mode were 100, 140 and 229 pg for chlorobenzene, iodobenzene and dibromobenzene, respectively.