Speciation and preservation of Cr VI and Cr III in finished drinking water matrixes using collision cell ion chromatography-inductively coupled plasma-mass spectrometry.

The polyatomic background at the major isotope of Cr was evaluated as a function of collision cell gas flow rate using three different mobile phases. The stability of Cr VIwas evaluated as a function of solution pH by fortifying samples with enriched 53Cr VI. The 53Cr VI recovery was > or =95% at pH 7.8, but by pH 6.2 a significant amount of the enriched spike was recovered as 53Cr III-EDTA. The instability of Cr III at pHs above 7.8 required that Cr III be chelated prior to speciation. The concentration of EDTA was optimized by evaluating Cr III recoveries in six finished drinking waters, a U.S. Geological Survey reference water, and a challenge water fortified with competing matrix cations. If the Cr III was allowed to precipitate (characteristic of shipping a sample without field preservation), acceptable recoveries of Cr III required the sample to be heated to 70 degrees C in 10 mM EDTA to quantitatively produce the Cr III-EDTA complex. The overall preservation/speciation approach was evaluated using an enriched 53Cr VI and natural Cr III in seven drinking water matrixes over a 46 day period. A linear least squares analysis was performed on each water, and the corresponding P-values were estimated. The method detection limits for Cr III-EDTA and Cr VI were 0.06 and 0.1 pglL, respectively.

Preabsorptive metabolism of sodium arsenate by anaerobic microbiota of mouse cecum forms a variety of methylated and thiolated arsenicals.

The conventional scheme for arsenic methylation accounts for methylated oxyarsenical production but not for thioarsenical formation. Here, we report that in vitro anaerobic microbiota of mouse cecum converts arsenate into oxy- and thio- arsenicals. Besides methylarsonic acid (MMA(V)), arsenate was transformed into six unique metabolites: mono-, di-, and trithio-arsenic acid, monomethyldithio- and monomethyltrithio-arsonic acid, and dimethyldithioarsonic acid. Thioarsenicals were found in soluble and particulate fractions of reaction mixtures, suggesting interactions with anaerobic microbiota. Metabolism of ingested arsenate to oxy- and thio-arsenicals before absorption across the gastrointestinal barrier could affect bioavailability, systemic distribution, and resulting toxicity.

Fluorescence spectroelectrochemical sensor for 1-hydroxypyrene.

A spectroelectrochemical sensor was demonstrated for an organic compound whose oxidation proceeds through an electron transfer-chemical reaction-electron transfer (ECE) mechanism to generate new chemical species that are used for detection by fluorescence. The polycyclic aromatic hydrocarbon 1-hydroxypyrene (1-PyOH) served as a representative model analyte. The spectroelectrochemical properties of 1-PyOH in solution were explored with an optically transparent thin layer electrode. Electrochemical oxidation of 1-PyOH under acidic conditions proceeds via the ECE mechanism to a diquinonepyrene, which shows reversible electrochemistry and fluoresces at 425 nm in its reduced form, dihydroxypyrene. The sensor consisted of a tin-doped indium optically transparent electrode coated with a Nafion thin-film (20 nm) that rapidly preconcentrated the analyte at the sensor surface. Fluorescence in the film was excited by the evanescent wave from attenuated total reflection spectroscopy. Electrochemical modulation of dihydroxypyrene fluorescence at 425 nm in the 500 to -200 mV (vs Ag/AgCl) potential range was used for indirect detection of 1-PyOH. The spectroelectrochemical sensor calibration curve had a range of 5 × 10(-9) to 1 × 10(-6) M with a calculated detection limit of 1 × 10(-9) M.