Use of Ar-He mixed gas plasmas for furnace atomisation plasma ionisation mass spectrometry (FAPIMS)

The effect of Ar-He mixed gas plasmas on analyte signal intensities generated in a furnace atomization plasma ionization mass spectrometry (FAPIMS) source is presented. Analyte is introduced as a volatile headspace gas effluent (I-2(g) and Hg-(g)) and in discrete liquid sample volumes (Fe, Rb, Pd, In, Cs, Yb, Pt, Pb and Bi). The presence of as little as 2-10% (v/v) Ar increases the signal intensity of analytes having first ionization potentials greater than 6 eV by up to 10-fold. This may be attributed to the formation of more energetic plasmas whose electron density, ionization temperature and gas kinetic temperatures increase with Ar content. Consistent with this, elements with the highest ionization potentials showed the greatest enhancements. Analytes with ionization potentials of less than 6 eV, which are already presumed 100% ionized, are unaffected. Further increases in Ar content (50-100% v/v) can lead to plasma instability and accelerated erosion of graphite surfaces within the source. Most of the analytes studied showed slight improvements in the limits of detection for plasmas containing approximate to 5% v/v Ar; the increased signal intensity was accompanied by little or no increase in the background signal. Although Ar can significantly affect conditions within the source, the composition of the Ar-He mixture can also influence the transmission of analyte into the free jet expansion and to the MS detector. The appearance and intensity of Ar-based spectral interferences, such as ArC+, ArO+ and Ar-2(+), increase with Ar content and source temperature, degrading the determination of species such as Cr-52(+), Fe-56(+) and Se-80(+). The amount of Ar required to generate significant signal enhancements is small (< 10% v/v) and therefore does not significantly ease the pumping requirements on the interface.