Lanthanide phosphate interferences in actinide determination using inductively coupled plasma mass spectrometry

The determination of actinides at low levels using ICP-MS can be interfered by polyatomic ions appearing at the same nominal mass-to-charge ratio. In this work, interferences initially found when analysing plutonium in soil and sediment samples were identified as lanthanide phosphates and the formation of these species examined. It was found that high sample gas flow rates and low rf powers enhanced the formation of lanthanide phosphates. All lanthanides studied (La, Ce, Pr and Nd) formed phosphates, albeit to various extents and of slightly different compositions. Furthermore, the lanthanide phosphate formation was verified by introducing the source of phosphorus, hydroxyethylidene diphosphonic acid (HEDPA), in an 18O enriched water solution. This experiment also revealed that the HEDPA is essentially completely dissociated in the plasma and that the interfering species are most likely formed during ion extraction.