Non-spectroscopic suppression of zinc in ICP-MS in a candidate biological reference material (IAEA 392 Algae)

ICP-MS was used for the simultaneous determination of nine analytes in a candidate reference material, IAEA 392 Algae (Scenedesmus obliquus), following microwave digestion with nitric acid and dilution to volume with de-ionised water. For eight analytes, the results showed reasonable agreement with consensus mean results obtained from a world-wide intercomparison exercise on this material. However, the value obtained for Zn was 55.4% of the consensus mean result (69.6 versus 125.6 mu g g(-1), respectively). Analysis of the same digest solutions by flame AAS gave results which were in good agreement with the consensus mean (127.1 versus 125.6 mu g g(-1)), indicating that all the Zn was in solution. When the method of standard additions was employed, the result was overestimated by approximate to 17% by ICP-MS. A 1+9 aqueous dilution of the digests gave an acceptable result for Zn (122.0 mu g g(-1)). Evidence is presented to demonstrate that the underestimation of Zn by external calibration ICP-MS is due to a non-spectroscopic suppression of the Zn in the digest solution caused by nitric acid.