Concerning the PIXE analysis of Cr(VI) in water using ion-exchange filters, the limit of detection (LOD) and the influence of matrix anions were investigated. In order to look for the experimental condition for obtaining the minimum LOD, we measured the Cr-Kα X-ray counts and background counts under the Kα X-ray peak as a function of the incident proton energy and the thickness of the Mylar absorber foil in front of the detector. To investigate the interference by coexisting anions, each of PO4 3− , SO4 2− , NO3 − , Cl − , and F − ions and Cr(VI) ions were mixed in aqueous solutions and adsorbed on DE81-DEAE cellulose paper, a weakly basic anion exchanger with diethylaminoethyl functional groups. Then the filter samples were measured by PIXE using 2.5 MeV proton beams. We obtained a LOD of 0.16 µg or 8 ppb for 20 mL samples at a proton energy of 2.5 MeV and a Mylar film thickness of 50 or 100 µm. The experimental results on the mixed solutions indicated that NO3 − , Cl − , and F − as coexisting ions didn't interfere significantly with determination of a 50 µg/L Cr(VI) concentration for 40 mL total solution volume, despite the total amount of anions was about 90% of ion exchange capacity of a filter. On the other hand, slight interferences by PO4 3− ions were observed. However, under the same condition, we found that if the total amount of SO4 2− ions was higher than 20% of ion exchange capacity, they induced significant interferences in determining Cr(VI).
The most important species of chromium (VI) formed in water are anions such as CrO4 2− , whereas chromium (III) is mainly present in the form of cationic species such as Cr(OH)2 + . Pure water and tap water samples were artificially contaminated with mixture of chromium (VI) and chromium (III) standard solutions. Sequential filtration through commercially available Whatman P81 cellulose phosphate paper and Whatman DE81 DEAE cellulose paper was employed for separation and preconcentration of chromium (III) and chromium (VI) in these water samples, respectively. The PIXE measurements of these filter paper samples were performed using 2-MeV proton beams delivered by a tandem pelletron accelerator at Research Laboratory for Nuclear Reactors, Tokyo Tech. The beam current and the measuring time for each sample were 1 nA and 10 minutes, respectively. Instrumental detection limits were 60 ng for chromium (VI) and 120 ng for chromium (III). Detectable range for chromium (VI) and chromium (III) in water were ≧ 10 ppb and ≧ 50 ppb, respectively. For chromium (VI), the detectable range was considerably lower than the maximum allowed concentration of 50 ppb for chromium (VI) in both the drinking water standards and the environmental quality standards in many countries.
Recently, water contamination by Cr(VI) has resulted in serious environmental problems. Therefore, many countries are changing the regulation of water quality standard from total chromium to Cr(VI). This study was conducted to develop a sample preparation method for the determination of Cr(III) and Cr(VI) in water by energy-dispersive proton-induced X-ray emission spectrometry. Whatman ion exchange celluloses (P81 and DE81) were used for separation and pre-concentration of Cr(III) and Cr(VI) species by filtration, respectively. The dependences of the limit of detection for both chemical species on the incident proton energy, beam current, and measuring time were examined. The accuracy and precision of the proposed method were determined. The results showed that the proposed method allows the separation and determination of Cr(III) and Cr(VI) with a high accuracy and precision.
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