THE ROLE OF MASS FRACTIONATION PROCESSES IN Hg ISOTOPE RATIOS MEASUREMENTS

Among the global pollutants, Mercury could be considered one of most harmful. For this reason, the study of the distribution and of the sources of this element is relevant for the aims of environmental protection and the human health safeguard. In these contexts, the determination of mercury stable isotopes ratios and, in particular, the identification and the study of fractionation processes seems to be an extremely interesting and challenging field of investigation to verify the “provenance” of the element. Mercury, in fact, undergoes to both mass dependant, MDF, and mass independent fractionation, MIF, processes. In particular the MIF, involving only the odd isotopes (199Hg and 201Hg), appears to be a characteristic fingerprint of the process and the pathways involved in the Hg transformations [1]. Thus, the study of both fractionation phenomena can be a powerful tool to identify its natural or anthropogenic source. In order to have the required precision, the determination of the Hg isotope ratios is commonly conducted by means of HR-MC-ICP/MS instruments. The low magnitude of the MDF and MIF makes the correction of the instrumental mass-bias (MB) a critical step in the determination chain. The most common method for the MB correction assumes an exponential fractionation law and uses for the normalization the Tl isotope ratio [2]. The use of a different element for the MB correction could introduce some errors in the correction step, due to different factors such as the high difference in mass between the normalization couple and the corrected ratio (205Tl/203Tl for 199Hg/198Hg) and differences in the MB factors of the two elements. In order to minimize the possible errors in the correction step, for the Hg some authors report different applications of the exponential law to the MB problem [3]. The evaluation of the influence of the different methods on the data was conducted applying them on the same dataset and comparing the results with the values reported in literature. The dataset was composed by standards (NIST SRM3133) acquired in different session of measurement and coming from the bracketing sequences. Moreover, the influence of the sample matrix and sample preparation technique on the fractionation processes was also investigated acquiring the Hg ratios of muscle samples spiked with the SRM3133 at different concentration.