Evaluation of internal standards for inductively coupled plasma–mass spectrometric analysis of arsenic in soils

Arsenic (As) is a common contaminant in soils, and analysis of soils by inductively coupled plasma–mass spectrometry (ICP‐MS) is often used to detect As in soil extracts. Internal standards (ISs) are part of ICP‐MS analyses to enhance precision and accuracy by compensating for instrument variability; however, an improper choice of IS can result in negative analytical bias. The goal of this study was to develop a protocol for evaluating ISs commonly used in ICP‐MS. Three soils of varying clay content and total As were extracted with a dilute electrolyte [0.005 mol L–1 Mg(NO3)2] and an acid digest. Arsenic concentrations were quantified by ICP‐MS using typical ISs: 6Li, 45Sc, 69Ga, 89Y, 103Rh, 115In, 159 Tb, and 209Bi. Standard addition was used as a benchmark for As quantification. The most consistent IS was 115In. Elevated, naturally occurring concentrations were detected for several of the ISs, particularly in the total digests, emphasizing the necessity for screening soils prior to analysis. Core Ideas Addition of internal standards (ISs) is a normal protocol for analysis by inductively coupled plasma–mass spectrometry (ICP‐MS). If “native” concentrations of ISs are significant, analytical bias is likely. Most ICP‐MS analyses of soil extracts do not include quantification of native ISs. A protocol is presented for screening for the most suitable IS in soil analyses of arsenic.