Preliminary investigation of direct sea-water analysis by inductively coupled plasma mass spectrometry using a mixed-gas plasma, flow injection and external calibration

A method is described that allows the direct analysis of sea-water by inductively coupled plasma mass spectrometry (ICP-MS) using a simple external calibration without internal standardization. A 50 muL aliquot of sample is introduced by flow injection and diluted 2-fold with deionized distilled water on-line to an ICP-MS instrument. A 4% nitrogen - 96% argon plasma, operated at a 2 mm greater sampling depth than optimal for sensitivity, provides robust operating conditions that greatly reduce non-spectroscopic interferences (i.e. matrix effects) while still maintaining sufficient sensitivity for analyte quantitation. This simple approach allowed the accurate determination of Mo in a sea-water certified reference material (NASS-5) using a 150 ms dwell time on a second-generation ICP-MS instrument. The determination of other elements should be possible with more recent and more sensitive ICP-MS instruments. Although this mixed-gas plasma also reduced the ArCl spectroscopic interference on As, the residual interference prevented accurate quantitation of As in sea-water. However, the approach may still be suitable for the determination of As ( and other elements that suffer from spectroscopic interferences arising from the sample matrix) in samples where the troublesome matrix elements are less concentrated.