Addressing matrix effects for 193 nm excimer LA-ICP-MS analyses of Fe-rich sulfides and a new predictive model

摘要

Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is one of the most popular techniques for determining trace element concentrations in sulfides. Due to the lack of matrix-matched standards, standardization of sulfide analyses are usually based on silicate glass calibrant materials. Matrix effects during ns-LA-ICP-MS analyses of Fe-rich sulfides were quantified for many trace elements by comparison of elemental concentrations obtained by LA-ICP-MS and electron microprobe (EPMA) for many synthetic sulfides. The data was used to obtain the fractionation indices (F_i, the ratio between the EPMA- and LA-ICP-MS-determined concentrations of element i) for many elements while considering Fe, Cu and Ni as internal standards. The results show that significant (>15% RD) matrix effects arise during ns-LA-ICP-MS analyses of Ti, Zn, Ge, Se, Mo, Cd, In, Sb, Te, Pb, Bi in sulfides when using Fe as the internal standard. The use of Ni as an internal standard yields on average higher F_i, values for most elements, resulting in more pronounced matrix effects for refractory elements and less so for volatile elements, relative to Fe. The use of Cu as an internal standard yields overall more significant matrix effects for volatile elements (i.e., lower F, values). The Ft values for most elements remain constant with increasing concentrations, and matrix correction factors for these elements can therefore be applied across the ppm to wt% range. In agreement with previous observations for Fe-rich metals and silicate glasses, the magnitudes of the matrix effects for the various elements are strongly correlated with elemental volatility. This correlation was used to obtain a predictive model for describing F, for Fe-rich sulfides. The results were used to assess the effects of matrix effects on calculated sulfide liquid-silicate melt partition coefficients derived from experiments. Matrix effects arising through the use of non-matrix-matched standards will result in significant discrepancies between measured and true partition coefficients, the extent mainly depending of the volatility of the element considered. Corrections on ns-LA-ICP-MS derived element concentrations therefore need to be performed to obtain true abundances in the absence of matrix-matched standards.