Selenium determination in volcanic soils by ICP-QMS: influence of reaction cell pressurization and methanol addition on the occurrence of spectral interferences

Accurate determination of selenium in soils is a critical issue in geochemistry, because Se has a narrow range between dietary deficiency and toxic concentrations. However, its determination is difficult as Se commonly occurs at low levels. Se measurements by ICP-MS are hampered by Ar-based polyatomic ions overlapping Se isotopes. Moreover, in complex matrixes such as geological materials, additional spectral interferences might occur. In this work spectral inferences in volcanic soil digestions have been assessed using both synthetic solutions and real samples under four different measurement conditions that include the addition of an organic solvent (methanol) and the use of a pressurized octopole system. The results indicate that the traditional approach using Se-82 without collision-reaction cell significantly overestimates the Se concentration. With the use of a hydrogen pressurized collision-reaction cell an interference-free Se-78 signal could be obtained. Methanol was found to decrease the effect of interferences and additionally improves the count statistics due to higher sensitivity. Correspondingly, the best results were obtained using Se-78 with a hydrogen pressurized collision-reaction cell and a 2% methanol matrix.