Effects of the sampling interface in MC-ICP-MS: Relative elemental sensitivities and non-linear mass dependent fractionation of Nd isotopes

Relative elemental sensitivities are reported for a number of sample and skimmer cone combinations, with standard and enhanced pumping of the interface region, for the Thermo Scientific Neptune MC-ICP-MS. An approximate two-fold sensitivity enhancement was observed for Sr, Nd, Hf, Pb and U using the Jet sample cone and an enhanced pumping configuration, compared to the standard arrangement. In the case of Li, the standard sample cone and enhanced pumping configuration gave the maximum sensitivity. In addition to improved ion transmission, the X skimmer cone geometry is also associated with additional contributions to the instrumental mass fractionation (at least in the case of Nd) that do not have a simple exponential dependence on the isotope mass. Neodymium isotope ratios measured using the X cone displayed large deviations (up to similar to 750 ppm) from the reference (i.e. 'true') values. Similar deviations have been reported previously when using high sensitivity skimmer cone geometries on the Nu MC-ICP-MS instruments (K. Newman et al., J. Anal. At. Spectrom., 2009, 24, 742). The reported non-linear contribution to the instrumental mass fractionation with respect to Nd is not specific to a particular MC-ICP-MS instrument; it is associated with an increase in the NdO(+)/Nd(+) ratio, due to either a change in the plasma operating conditions (e.g. an increase in the sample gas flow, absence of a secondary discharge at the interface, change in plasma gas composition) or modification of the cone geometry. The role of secondary discharge formation and the physical and chemical processes occurring in the supersonic expansion with respect to NdO(+) formation are discussed.