Atmospheric-Pressure Helium Inductively Coupled Plasmas for Elemental Mass Spectrometry

Analytical and fundamental characteristics of helium inductively coupled plasma mass spectrometry (He ICPMS) were explored for atmospheric-pressure plasmas generated in a 13-mm He ICP torch with the use of a prototype ICPMS equipped with an analogue detector. Four sets of operating conditions were identified for the detection of four categories of elements at levels lower than or similar to those of the Ar ICPMS. In general, detection limits of He ICPMS were improved by two to four orders of magnitude for metals, compared to results from our previous study with the same torch. For the easy-to-atomize elements (Li, Na, K, Ga, Rb, In, Cs, Tl, Pb, and Bi), the detection limits ranged from 0.1 to 7 pg/mL at 500–600 W forward power. Detection limits of the fourth-period transition metals (Cr, Mn, Fe, Co, Ni, and Cu), i.e., elements subject to the spectral interferences in Ar ICPMS, were 4 to 100 pg/mL at a power level of 560–700 W. The detection limits of hard-to-atomize elements (Ti, V, Sr, Y, Rh, Pr, Tm, Th) ranged from 4 to 90 pg/mL at 800 W. For hard-to-ionize elements (As, Se, Br, Sn Sb, I), detection limits of 0.05 to 1 ng/mL were obtained at 820 W. Ion kinetic energies were measured under two diverse sets of operating conditions. At 600 W, the ion kinetic energies were very low (2.6–6.6 eV for mass range 39–209 amu), indicating the absence of a secondary discharge at the sampler tip of the MS interface. The gas-kinetic temperature estimated in this measurement was 600 K for the 600-W He ICP. In contrast, very high ion kinetic energies (31–66 eV for mass range 51–232 amu) were obtained at 800 W. The estimated plasma potential and gas kinetic temperature were 22 V and 3500 K, respectively. Ion kinetic energies of polyatomic ions were slightly lower than those of atomic ions at the same mass, suggesting that polyatomic ions were formed in the low-temperature zones of the He ICP.