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.

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.

A new optically transparent thin layer electrode (OTTLE) cell and holder have been designed to facilitate spectroelectrochemical measurements in standard bench-top absorbance and fluorescence spectrophotometers. The use of rapid prototyping for the OTTLE cell holder combined with the selection of inexpensive OTE materials results in a practical, low-cost spectroelectrochemical cell. The cell was characterized by thin-layer cyclic voltammetry and coulometry of ferricyanide/ferrocyanide. Spectroelectrochemistry of tris-(2,2'-bipyridine) ruthenium(II) chloride (Ru-(bpy) 3 Cl 2 ) and 1-hydroxypyrene (1-pyOH) was done with commercially available bench-top absorbance and fluorescence spectrophotometers. The good correlation between the results obtained and the known properties of each compound demonstrate that the OTTLE cell and holder provide an effective means for making spectroelectrochemical measurements in bench-top absorbance and fluorescence spectrophotometers.

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.

The spectroelectrochemical sensor concept uses electrochemistry to modulate an optical signal to give improved selectivity. The sensor consists of an optically transparent electrode/waveguide coated with a thin film that preconcentrates the analyte. The sensor concept was first demonstrated with ferricyanide and later it was shown to work on a complex sample by detecting ferrocyanide in radioactive Hanford tank waste. Nafion, partially sulfonated polystyrene-block-polyethylene-ran-butylene)block-polystyrene (SSEBS), and quaternized poly(vinylpyridine) (QPVP) are examples of polymers used for the thin film. Detection limits achieved by the sensor are 10-5 to 10-8 M using change in absorbance as the mode of detection and about 10-10 M using fluorescence. A sensor has been developed for detecting [Re(dmpe)3]+ where dmpe = dimethylphosphinoethane, which serves as a nonradioactive analogue for complexes of radioactive 99Tc. Some polycyclic aromatic hydrocarbons have appropriate electrochemical and spectroscopic properties for spectroelectrochemical detection as demonstrated for 1-hydroxypyrene and its conjugate 1-hydroxypyrene-glucuronide.