Low-pressure helium ICP-MS for trace analysis

Abstract

Inductively coupled plasma mass spectrometry (ICP-MS) offers a powerful multielement analytical method. Polyatomic interferences, however, are a major problem for the analysis of some elements. For example, 40Ar 16O+, 40Ar35Cl+ and 40Ar2+ cause overlap interferences when detecting 56Fe+, 75As+ and 80Se+, respectively. To address this issue, we studied low-pressure helium-ICP-MS, where a highly energetic helium plasma was generated with an rf power of 500 W in a water-cooled plasma torch. A sampling orifice was fabricated from an aluminum metal to minimize the background spectra. Different sample-introduction techniques were also developed for extending the scope of the proposed method. A liquid sample, for example, was first converted into tiny droplets with an ultrasonic nebulizer and then desolvated in a heater. The resulting dry aerosol was successfully introduced in the low-pressure helium plasma through a PTFE capillary tube. A small amount of sample was transported to the plasma after electrothermal vaporization on a tungsten filament. Other sampling techniques examined include hydride generation and laser ablation. The detection limits were measured based on 3σ for a large range of elements in the periodic table. Improved lower detection limits were obtained for halogens, C, Cr, Fe, Se, and so on. The diagnosis of helium plasma with a Langmuir probe indicated that the plasma temperature was higher than the conventional argon plasma, which may contribute to an enhancement of the detectability. The analytical capability of the proposed ICP-MS was demonstrated by analyzing different types of certified reference materials.