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: ⁶ Li, ⁴⁵ Sc, ⁶⁹ Ga, ⁸⁹ Y, ¹⁰³ Rh, ¹¹⁵ In, ¹⁵⁹  Tb, and ²⁰⁹ Bi. Standard addition was used as a benchmark for As quantification. The most consistent IS was ¹¹⁵ In. 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.