Signal enhancement in laser ablation inductively coupled plasma-mass spectrometry using water and/or ethanol vapor in combination with a shielded torch

Abstract

To improve the precision and accuracy of in situ trace element and isotope analysis and to apply LA-ICP-MS to small or isotopically heterogeneous samples such as minerals, greater analytical sensitivity is required. In this study, we investigate the effects of the addition of water and/or ethanol vapor to the central gas flow (Ar + He) of an Ar plasma in combination with a shielded torch on the signal intensity of laser-generation using an 193 nm ArF laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) setup. A custom-built sample introduction system was developed to add water and/or ethanol vapor to dry aerosol at different and constant rates. The optimum makeup gas flow rate is negatively correlated with the water and/or ethanol load into the ICP. The effect of the shielded torch on the LA-ICP-MS sensitivity is minimal for dry plasma conditions but becomes significant in the presence of water and/or 1–4% (v/v) ethanol vapor. For most of the 60 investigated elements reported in this study, the addition of small amounts of water and/or 1–4% (v/v) ethanol vapor into the carrier gas increases the sensitivity by a factor of 2.5–3.0 or 1.2–1.5 with or without the shielded torch, respectively. The suppression of the ion kinetic energy distribution by the shield in the presence of water and/or 1–4% ethanol vapor may account for the increase in sensitivity. With the shielded torch, the oxide ratio increases from 0.47% in dry plasma to 6.41% in water plasma and decreases to 0.81% in 4% ethanol plasma. Hydride formation increases by a factor of 27 in the presence of 4% ethanol vapor relative to dry plasma. The yield of doubly charged ions decreases from 1.2% in dry plasma to 0.8% and 0.7% in water and 4% ethanol plasmas with the shielded torch, respectively. Relative to the spatial profiles of the ion distributions in dry plasma, the addition of water and/or 2% (v/v) ethanol vapor in combination with a shielded torch shows several effects: (1) axial profiles are significantly narrower; (2) the maximum ion density zone shifts away from the load coil; and (3) there are more uniform ion distributions for elements with different physical and chemical properties. The combination of changes in ICP geometry, enhanced vaporization, atomization and ionization of analyte, and efficient ion transmission with a shielded torch contributes to the sensitivity enhancement observed in this study.