Isotopic analyses of biological materials by single and multi-collector sector field ICP-MS

Abstract: Investigations concerning naturally occurring, mass dependent isotope fractionation of transition elements in biological materials have only been carried out during a very short period of time. Due to the lack of detailed descriptions of the analytical performance in previously reported work, the major focus of this study has therefore been to evaluate aspects of sample preparation and detection for obtaining precise and accurate isotopic ratios by multi-collector inductively coupled plasma mass spectrometry (MC- ICP-MS) using high mass-resolution. Quantitative determinations of elemental concentrations were performed by sector field ICP-MS using external calibration and internal standardization. Dissolution of whole blood and soft tissue reference materials in a micro-wave oven, or on a hot plate, using HNO3 resulted in concentrations of Zn, Fe and a host of other elements in agreement with certified values. This, together with high chemical yields of Zn and Fe (>93%) from the sample purification by anion- exchange chromatography or precipitation by NH3, limited the possibility for laboratory-induced isotope fractionation. Concentrations of the majority of potentially interfering elements were reduced to negligible levels, and most of the residual spectral interferences were resolved using medium (in the case of Zn) or high (Fe) mass-resolution settings of the MC-ICP-MS-instrument. To correct for differences in instrumental mass discrimination effects between samples and isotopic standards, concentrations were matched to within 10-20%, and Cu and Ni were employed as elemental spikes, for measurements of Zn and Fe, respectively. Almost all samples and standards were analysed in duplicate (or triplicate), and exponential correction for instrumental mass discrimination was performed by applying the exponential model. Precision at the sub-‰ level was achieved for both Zn and Fe delta-values, relative to a JMC Zn standard and an IRMM-014 Fe isotopic reference material, respectively. For the investigated reference materials, delta66/64Zn ranged from 0.07 ± 0.03‰ in human hair to 0.50 ± 0.03‰ in bovine muscle. Concerning Fe, a delta56/54Fe-value of -2.83 ± 0.06‰ was obtained for a human whole blood reference material. Variations in delta66/64Zn values, ranging from +0.56‰ in human whole blood to -0.60‰ in human hair were found, demonstrating fractionation of the Zn pool between body compartments. Where comparable isotopic determinations exist, the results were in good agreement.