High resolution ID-ICP-MS certification of an estuary water reference material (LGC 6016) and analysis of matrix induced polyatomic interferences

Reliable trace metal analysis of environmental samples is dependent upon th e availability of high accuracy. matrix reference standards. Here, we prese nt Cd, CU, Ni, Pb and Zn isotope dilution determination for an estuary wate r certified reference material (LGC 6016). This work highlights the need fo r high-accuracy techniques in the development of trace element CRMs rather than conventional inter-laboratory trials. Certification of the estuary wat er LGC6016 wits initially determined from a consensus mean from 14 laborato ries but (his was found to be unsatisfactory due to the large discrepancies in the reported concentrations, The material was re-analysed using isotope dilution ICP-MS techniques, Pb and Cd were determined using a conventional quadrupole ICP-MS (Elan 5000). Cu, Zn and Ni were determined using a magne tic sector ICP-MS (Finnigan Element), which allowed significant polyatomic interferences to be overcome. Using the magnetic sector instrument, precise mass calibration to within 0.02 amu permitted identification of the interf erences. Most interferences derived from the sample matrix. For example, th e high Na content causes interferences on Cu-63, due to the formation Ar-40 /Na-23 and Na-23(2) O-16 H-1, which in a conventional quadrupole instrument would relate to an erroneous increase in signal intensity by up to 20%. Fo r each analyte a combined uncertainty calculation was performed following t he Eurachem/GTAC and ISO guideline. For each element a combined uncertainty of 2-3% was found, which represents a 10-fold improvement compared to cert ification by inter-laboratory comparison. Analysis of the combined uncertai nty budget indicates that the majority of systematic uncertainty derives fr om the instrumental isotope ratio measurements.