Steady state acid effects in ICP-MS

Abstract

The influence of concentrated nitric acid matrices in inductively coupled plasma mass spectrometry (ICP-MS) was investigated. Generally, analyte signal intensities decrease with increasing acid concentration. However, under certain conditions signal enhancements occur. Acid induced changes are highly dependent on the nebulizer gas flow rate and power, in both controlling the analyte and aerosol transport to the plasma and local plasma conditions. The largest decrease in signal occurs when the experimental parameters (power and ion optics settings) were optimized to produce the maximum signal for 2% HNO ₃ matrices at higher nebulizer gas flow rates and least pronounced when the experimental parameters were optimized at low nebulizer gas flow rates. This is consistent with smaller relative changes in the analyte and aerosol transport rates to the plasma at lower nebulizer gas flow rates as a function of increased acid concentration. Working under robust conditions for ICP-MS not only reduced the magnitude of the acid effect but often resulted in signal enhancements rather than suppressions with increased acid content. Because the acid effect changes the local plasma temperature within the sampling volume, it affects ion kinetic energies and therefore transmission efficiency from the plasma to the MS detector. For a given set of operating conditions, a cooler plasma results from increased acid concentration, which can reduce analyte ionization efficiency, enhance metal-to-oxide ratios and incise the contribution of molecular ions and isobaric overlaps. Acid effects originate in the sample introduction system and so occur independent of the method of detection. Consistent with reports in ICP-OES, the acid effect in ICP-MS can be minimized but not eliminated by switching to more robust conditions. Robust conditions in ICP-MS are generally characterized by a nebulizer gas flow rate less than that required to produce optimum sensitivity, or a low nebulizer gas flow rate and high power.