The determination of very low concentrations of plutonium, in the femtogram per millilitre range, in environmental samples using ICP-MS can be interfered by polyatomic species containing heavy elements, from hafnium to bismuth. Mass-to-charge ratios of these polyatomic species are very close to the ones of the plutonium isotopes and cannot be separated from them in the low resolution mode currently used for trace analysis. In this paper, we present an evaluation of the extent and impact of these interferences on the background at masses of plutonium isotopes Pu-239 and Pu-240, based on measurement of environmental samples for three years using a double-focusing sector-field ICP-MS. We demonstrate that these molecular interferences, especially the ones involving lead, mercury, and iridium through species PbO2+, ArHg+, IrO3+, must be considered as the additional background induced by these interferences (on average for all interferences respectively similar to 1.2 and similar to 1.9 counts s(-1) at 239 and 240 atomic mass units) is often higher than the instrumental background measured with deionised water acidified at 2% by ultra- pure grade HNO3 (similar to 0.4 counts s(-1)), and are generally of the same order as other usual sources of background for low-level plutonium analysis by ICP-MS, i.e. Pu-242 tracer isotopic impurities, U-238 hydrides and peak tail. Thus, if not corrected, these polyatomic interferences may lead to false detection of femtogram amounts of plutonium or overestimation of results. The extents of formation of the main polyatomic species are highly variable from one analysis to the other, over one or two orders of magnitude, depending on instrumental settings. Chemical purification procedures are very efficient in eliminating the greater part of heavy elements present in the initial samples. However, these procedures also bring heavy elements into the final sample solutions through contamination by the atmosphere, glassware and reagents.
