Comparison of signal intensities and elemental fractionation in 257 nm femtosecond LA-ICP-MS using He and Ar as carrier gases

Signal intensities and elemental fractionation under different ambient gas conditions (He, Ar and a premixed He-Ar mixture) were investigated using 257 nm femtosecond (fs)-LA-ICP-MS. The experiments show that the change of ablation carrier gases from argon to helium increases the sensitivities of refractory elements (e.g. rare earth elements) in NIST SRM 610 by a factor of 1.05-1.20, whereas, the signal intensities of volatile elements (e.g. Tl, Cd, Se, Sn, Te, Zn, Pb, Bi, Ge, Ga, Sb, Ag, and Cu) were increased by a factor of 1.5-3.0. The much less deposited aerosol particles around the ablation craters produced in helium when compared to argon may partly account for this phenomenon. Our results also suggest that volatile elements may be enriched in small aerosol particles produced by femtosecond laser ablation whose transport efficiencies are increased when using He instead of Ar. The calculated elemental fractionation indices (with respect to Ca) of volatile elements (B, Cu, Zn, Ga, Ge, As, Cd, Mo, Ag, Sn, Sb, Pb and Bi) rapidly increased to 1.1-1.2 with increasing the spot size from 24 mu m to 60 mu m when using helium as the carrier gas. In contrast, they remain nearly constant at spot sizes of 24-60 mu m when using argon as the carrier gas. However, the indices of volatile elements (Co, Ni, Cu, Zn, Ga, Ge, As, Ag, Cd, Sn, Sb, Pb, and Bi) are less than 1 for all the spot sizes investigated. The signal intensities of all elements obtained in the pre-mixed He-Ar mixture is similar to the sensitivities acquired in pure helium. Meanwhile, the calculated elemental fractionation indices of all elements generally remain constant and closer to 1 than those obtained in pure He or Ar at different spot sizes. The technique of using the premixed He-Ar mixture as the carrier gas has been successfully used for the determination of major and trace elements in USGS and MPI-DING glasses by using fs-LA-ICP-MS.