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

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Abstract
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 µL 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.

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