Direct analysis of radium-226 in sediment by ICP-MS: an analytical challenge?

Abstract

²²⁶Ra analysis by inductively coupled plasma mass spectrometry (ICP-MS) in sediment after acidic mineralization was shown to be a fast, convenient and reliable method once polyatomic interferences were taken into account. Long-term instrumental drift and matrix effects could be efficiently corrected with ¹⁰³Rh, ¹¹⁵In or ¹⁹⁵Pt acting as internal standard. Polyatomic interferences at m/z of 226 were identified and quantified at levels encountered in natural sediment digest. Using a collision cell and a Kinetic Energy Discrimination (KED) mode, most spectral interferences were efficiently reduced with the exception of Pb/O and W/Ar which remained and had a significant impact on the signal measured at m/z of 226 for uncontaminated sediment. Consequently, a matrix-matched solution of the sediment digest should be analyzed, and the apparent Ra concentration subtracted from the real sample for better accuracy. Alternatively, an off-line mathematical correction based on the interfering element concentration in the sediment digest could be applied. ²²⁶Ra concentration was measured by ICP-MS in both standard (STD) and KED modes for 5 sediment samples after an acidic digestion. Once polyatomic interferences were taken into account, ²²⁶Ra ICP-MS concentration matched ²²⁶Ra concentration measured also by ICP-MS after the radionuclide extraction on ion exchange resin and corresponded to the activities measured by α and γ spectrometry. For a 0.10 g sediment sample, detection and quantification limits were evaluated at 0.11 and 0.38 pg g⁻¹, respectively. Reproducibility of the method based on sediment mineralization and analysis of replicates was higher than 83%, even for sediment nearing ²²⁶Ra's natural background.