Influence of chemical species on the determination of mercury in a biological matrix (cod muscle) using inductively coupled plasma mass spectrometry

Abstract

In common with most analytical techniques, inductively coupled plasma mass spectrometry (ICP-MS) is susceptible to instrumental drift and matrix effects which can result in a degradation of analytical performance. In order to minimize such effects an appropriate internal standard element can be incorporated in all the samples such that data can be normalized to a particular temporal sensitivity. The chemical species of the analyte and internal standard in solution is believed to be unimportant in ICP-MS since the high plasma temperature will reduce all chemical species to their constituent elements and then ionize them. Therefore, the internal standard and analyte elements will be affected in the same manner by instrumental drift and matrix effects and so the internal standard can be used for correction purposes. We have recently participated in an international exercise to certify the mercury content of a candidate reference material (Community Bureau of Reference, Certified Reference Material 422 Cod Muscle). The analyte was determined using cold vapour atomic fluorescence spectrometry (CVAFS) and ICP-MS. In both cases, the sample digestion was achieved using a closed vessel microwave digestion procedure. The results obtained by ICP-MS using direct calibration against aqueous standards and internal standardization were approximately half of the value obtained by CVAFS [328 ± 50 (n= 8) and 566 ± 46 (n= 7) ng g^–1 of Hg, respectively]. Good agreement between the methods was obtained when standard additions and isotope dilution techniques were used [567 ± 40 (n= 4) and 559 ± 13 (n= 5) ng g^–1 of Hg, respectively], but only if spike addition was made before a cold nitric acid overnight digestion stage. There is strong evidence to suggest that the chemical species of the mercury present in the sample and standards influences the results obtained by ICP-MS. The results of the certification exercise for the mercury content of the material are briefly discussed.