QUANTITATIVE ANALYSIS FOR Cr(III) AND Cr(VI) IONS IN ENVIRONMENTAL WATER SAMPLES BY USING PIXE

Abstract

An enhanced sample preparation method for PIXE analysis is described allowing to separate and concentrate chromium ions of different valence in water samples. Cr 3+ ions are selectively adsorbed by ferric hydroxide colloids generated in the solution, and a PIXE target for analyzing the total concentration of chromium of hexavalent and trivalent states is prepared by depositing 0.15 ml of sample solution on a user-made thin polycarbonate film. PIXE analyses for the two kinds of targets reveal the fractions of chromium of different oxidation state. The standard method for collecting the colloids adsorbing Cr 3+ ions on Nuclepore filter of 0.2 μm pores is based on an investigation of the pH-dependence of the recovery of dissolved Cr 3+ ions and the calibration curve is measured. The prepared targets were examined for 5 to 10 minutes by 3 MeV proton beams (0.7-5 nA beam currents). The lower detection limit of chromium in a 25 ml aquatic sample is 1 ppb for the Cr 3+-precipitated targets and 3.4 ppb for the CrO 42--deposit targets based on the 3σ error of background counts integrated over the FWHM of chromium peak in the PIXE spectrum. The applicability of PIXE using these sample-preparation techniques for determining chromium oxidation states was confirmed using river water samples to which Cr 3+ and CrO 42- ions were added in 50 ppb concentrations.