Direct determination of trace elements in sea-water using reaction cell inductively coupled plasma mass spectrometry

A method is described for the direct determination of several transition elements in sea-water by reaction cell (RC)-inductively coupled plasma mass spectrometry (ICP-MS) using ammonia as a reaction gas. Results show significant improvement over standard quadrupole (Q) ICP-MS performance for direct sea-water analysis and were comparable to high resolution (HR) ICP-MS. In this study, the RC-ICP-MS was used to determine chromium, copper, manganese, nickel and vanadium in sea-water. Ammonia gas was passed through the reaction cell to reduce molecular ion interference. These interferences normally lead to false positive results when low resolution quadrupole ICP-MS (Q-ICP-MS) is used to analyze sea-water. Good recoveries were obtained for V-51, Cr-52 and Mn-55 in the 1∶10 diluted sea-water, CASS-4, NASS-4 and SLEW-3. Molecular ion interference was reduced significantly but not completely for Ni-58 and Cu-63, giving rise to a positive bias typically no more than 0.3 µg l−1. A matrix blank subtraction or matrix matched external calibration can be used to further improve the accuracy for these elements at levels below 0.5 µg l−1. Detection limits in the undiluted sea-water range from 0.02 µg l−1 for chromium to 0.3 µg l−1 for copper. The precision and accuracy of the method were checked against three certified reference standards.