Proton-induced X-ray Emission Analysis of Marine Particulates

We report a methodology used to analyze suspended marine particulates by particle induced X-ray emission (PIXE). Water samples from an estuary in Brazil were filtered soon after collection onto pre-weighed Nuclepore filters, washed with deionized water, dried to constant weight and analyzed as thin targets. Because of the relatively high mass loadings (0.1–1.0 mg/cm2) on the filters, proton bombardment times of a few minutes were adequate for maintaining good counting statistics. Precision and accuracy were determined by replicate analysis and intercomparison to geochemical standards. Suspensions of standards in deionized water were filtered, dried, weighed and analyzed in a similar fashion as our samples of marine particulates. Net X-ray intensities were related to mass by calibration against pure elemental standards. Initial experiments showed systematically low concentrations for all elements determined by PIXE relative to “known” values. Further experiments verified that this systematic error was due to an uneven distribution of mass on the surface of the filters. Improvements in the filtration technique have eliminated the topographic effect on our samples and the PIXE results were substantially improved. Variations in matrix and particle size of the samples analyzed did not cause any measurable analytical effect. PIXE thus seems well suited for providing rapid, multi-element data on samples of marine particulates if suitable precautions are made during the sample preparation process