Matrix effects during magnetic sector-field inductively coupled plasma mass spectrometry uranium isotope ratio measurements in complex environmental/biological samples

Sample matrix effects on mass discrimination during inductively coupled plasma mass spectrometry (ICP-MS) isotope ratio measurements are rarely reported. However, they can lead to errors larger than the uncertainty claimed on the ratio results when not properly taken into account or corrected for. For instance, up to 1% matrix specific effects were experienced during an isotope dilution mass spectrometry campaign we carried out for the certification of the Cd amount content in some food digest samples (7% acidity and salt content around 450μg g-1). Specific nuclear safeguards programs were designed for the monitoring of declared and non-declared nuclear activities and important efforts are currently deployed to better understand the consequences on human health of the dispersion of depleted uranium in the environment. The interest in developing and/or improving measurement capabilities for uranium isotope ratios and uranium content in environmental and biological samples has therefore considerably increased in the last decade. However, procedure validation is rarely addressed with these developments even though, for instance, non-disputable uncertainty statements are absolutely crucial to underpin correctly the important decisions of political, economical, military or medical nature that can arise from these results. This is why we produced simulated urine samples (complex matrix made of organic and inorganic components) with certified n(234U)/n(238U), n(235U)/n(238U) and n(236U)/n(238U) ratios. These, which will eventually be commercially available for validation purposes, will first be used as test materials for an international interlaboratory comparison organised by IRMM and this exercise, named NUSIMEP-4 and open for participation to anyone. This presentation will introduce magnetic sector-field inductively coupled plasma mass spectrometry (ICP-MS) uranium isotope ratio measurements on real human urine samples and in the NUSIMEP-4 test materials. These were carried out respectively with two different types of instruments, a single detector (SD) 'high resolution' ICP-MS and a multiple collector (MC) ICP-MS. Our results illustrate the importance of carefully adapting the sample preparation and measurement calibration strategies to meet the uncertainties targeted for the end results. The analytical protocol originally developed included a microwave assisted acid digestion step, a matrix separation step by extraction chromatography on U-TEVA resin and an acquisition step by SD-ICP-MS using the IRMM-184 isotopic certified reference material to correct externally for mass discrimination effects. This combination proved to be fit for the purpose of getting ∼ 2.5% combined uncertainty (U, k = 2) on n(235U)/n(238U) ratio determination in real human urine samples at ultra low uranium levels (∼ 5 - 20 pg g-1). The same protocol was investigated for MC-ICP-MS measurements and simulated urine samples containing ∼ 250 to 1000 times more uranium. Repeatability on MC-ICP-MS measurements can be as good as 0.02% (simultaneity of the signal acquisition) and thus the expectation was that this would result into an important reduction of the overall measurement uncertainty. However, our results show that it is not possible to get even 0.2% U. As described in the Figure below, this is mostly due to uncorrected small size matrix effects becoming 'visible' thanks to the performance achievable with the MC-ICP-MS. Hence, the strategy of sample preparation and/or of instrument calibration must be reviewed to account for these effects, correct for them and quantify the uncertainty arising from these corrections