Patricia A. Creed

Extraction and detection of a new arsine sulfide containing arsenosugar in molluscs by IC-ICP-MS and IC-ESI-MS/MS

Using IC-ICP-MS and IC-ESI-MS/MS, an unknown arsenical compound in molluscs has been identified as a new arsine sulfide containing analog of a known arsenosugar and is referred to as As(498). This species has been observed in four separate shellfish species following a mild methanol–water extraction. As(498) is unstable, especially in acid, and converts to the arsine oxide containing arsenosugar As(482) over time. Chromatographic retention of As(498) was observed on an anion exchanger ION-120 column but the species did not elute as a well defined peak from a PRP-X100. Mass spectrometric analysis of As(498) at pH 9.0 produced an [M–H]− species at a mass to charge of 497 in the negative-ion mode. A synthetic standard of As(498) was made by bubbling hydrogen sulfide into a stock solution of arsenosugar As(482). The retention time and ESI-MS/MS data were identical for the synthetic standard of As(498) and the unknown arsenical in shellfish extracts.

Investigation of the pH effects on the formation of methylated thio-arsenicals, and the effects of pH and temperature on their stability

Thio-arsenicals have been discovered in both food and biological samples. During analysis, the potential exists for interconversion between the oxide and sulfide forms, raising questions regarding analytical factors which may influence the observed speciation. Within this context, the conversion of trimethylarsine oxide (TMAO), dimethylarsinic acid (DMA) and monomethylarsonic acid (MMA) to their respective sulfide forms, in the presence of sulfide (Na2S) was investigated with respect to pH. All three arsenic oxides produced very little conversion to the corresponding sulfide above pH 8 while conversion to the sulfide form was observed at every pH ≤ 7. Less than 10% of the initial TMAO remained at every pH below eight, with the sulfide form trimethylarsine sulfide (TMAS) accounting for more than 90%. Likewise, the mono- and di-thiolated forms of DMA (dimethylthioarsinic acid (DMTA) and dimethyldithioarsinic acid (DMDTA) respectively) formed over the pH range 5–7, leaving less than 8% of the initial DMA in solution. In the pH range 5–7, MMA was converted to monomethylthioarsonic acid (MMTA), with more than half of the MMA converted to an unidentified peak. This peak is thought to be monomethyldithioarsonic acid (MMDTA) but a positive identification was not possible due to lack of a synthetic standard. The primary species detected at pH 3 were the mono-thiolated forms MMTA and DMTA. Given the range of extraction and separation procedures (specifically pH and temperatures) associated with speciation analysis, the stability of TMAS, DMTA and MMTA under such conditions is another area of concern. Room temperature sulfide to oxide conversion rates from 0.3% to 2.2% per week were observed in basic (pH 10), neutral (pH 7) and acidic (pH 3) solutions, demonstrating very little pH-induced instability for all three thio-arsenicals. Elevated temperature accelerated the sulfide to oxide conversion rates to ∼10% per week for all three thio-arsenicals, while refrigeration retarded it to <0.4% per week.

Detection and quantification of a thio-arsenosugar in marine molluscs by IC-ICP-MS with an emphasis on the interaction of arsenosugars with sulfide as a function of pH

The sulfur analog of As(328) (2,3-dihydroxypropyl-5-deoxy-5-dimethylarsinoyl-β-D-riboside), abbreviated As(328-S), was detected and quantified in five species of marine shellfish using IC-ICP-MS with structural verification via IC-ESI-MS/MS. The CAD spectra produced from the parent ion, m/z 345, in the extract contained two major daughter ions, m/z 253 and 235, closely matching the CAD spectrum of synthetic As(328-S). The ability of the oxide and sulfide forms of the arsenosugar to interconvert led to a series of fundamental studies in ideal solutions containing both the arsenosugar and sulfide. The conversion of As(328) to As(328-S) was found to be pH sensitive and promoted in the pH range where HS− is converted to H2S (pK1 = 7). The conversion was observed in both shellfish extracts and ideal solutions with comparable sulfide and arsenosugar concentrations. The conversion was further studied over a range of sulfur/arsenic molar ratios. At a 15-fold molar excess of sulfide at pH 4.8, the lowest pH experienced by an extract, >90% conversion to the sulfide was observed. In the context of shellfish, a molar ratio greater than 200:1 sulfide to arsenic was detected in all five extracts. These trends should prove useful in improving confidence in thio-arsenosugar speciation and predicting the extent of conversion under a given set of conditions.

An in vitro assessment of bioaccessibility of arsenicals in rice and the use of this estimate within a probabilistic exposure model

In this study, an in vitro synthetic gastrointestinal extraction protocol was used to estimate bioaccessibility of different arsenicals present in 17 rice samples of various grain types that were collected across the United States. The across matrix average for total arsenic was 209 ng/g±153 ([xmacr ]±2σ). The bioaccessibility estimate produced an across matrix average of 61%±19 ([xmacr ]±2σ). The across matrix average concentrations of inorganic arsenic (iAs) and dimethylarsinic acid (DMA) were 81 ng/g±67.7 and 41 ng/g±58.1 ([xmacr ]±2σ), respectively. This distribution of iAs concentrations in rice was combined with the distribution of consumption patterns (from WWEIA) in a Stochastic Human Exposure and Dose Simulator model to estimate population-based exposures. The mean consumption rate for the population as a whole was 15.7 g per day resulting in a 0.98 μg iAs per day exposure. The mean consumption rate for children 1–2 years old was 7 g per day resulting in a 0.48 μg iAs per day exposure. Presystemic biotransformation of DMA in rice was examined using an in vitro assay containing the anaerobic microbiota of mouse cecum. This assay indicated that DMA extracted from the rice was converted to dimethylthioarsinic acid, although a second oxygen–sulfur exchange to produce DMDTA was not observed.

Investigation of arsenic speciation on drinking water treatment media utilizing automated sequential continuous flow extraction with IC-ICP-MS detection

Three treatment media, used for the removal of arsenic from drinking water, were sequentially extracted using 10 mM MgCl2(pH 8), 10 mM NaH2PO4(pH 7) followed by 10 mM (NH4)2C2O4(pH 3). The media were extracted using an on-line automated continuous extraction system which allowed the arsenic in each of the extraction fluids to be speciated on-line using IC-ICP-MS. The 10 mM MgCl2 preferentially extracted As(III) from each of the media. The percentage of the arsenic extracted by the MgCl2, relative to a HNO3/H2O2 digestion of the media, ranged from 0.1-2.3% for the three solids. The next sequential extraction fluid, 10 mM NaH2PO4, extracted some of the residual As(III) remaining on each of the media but the predominant species extracted was As(V). The 10 mM NaH2PO4 extracted 15.3 to 42.8% of the total arsenic relative to a total digested concentration for each of the media. The As(III) and As(V) stability studies conducted in these two extraction fluids indicated that conversion between As(III) and As(V) was not significant for the short extraction fluid sample contact time associated with the on-line continuous flow extraction cell. Finally, the 10 mM (NH4)2C2O4 extraction fluid was utilized in an off-line analysis mode because the Fe and As concentrations extracted from the media were not compatible with direct ICP-MS detection. The (NH4)2C2O4 extracted 2.9-29% As(III) for all three media and caused an oxidation of As(III) to As(V) during the extraction period for one of the three media. The sum of the arsenic from each of the three extraction fluids represented 92%, 44% and 53% of the available total arsenic for the three media, respectively. The speciation results for each media were obtained by adding all the speciation results from all three extraction fluids together and the resulting distribution of As(III)/As(V) compared well with the speciation results obtained via XANES.

Estimating Inorganic Arsenic Exposure from U.S. Rice and Total Water Intakes.

Background: Among nonoccupationally exposed U.S. residents, drinking water and diet are considered primary exposure pathways for inorganic arsenic (iAs). In drinking water, iAs is the primary form of arsenic (As), while dietary As speciation techniques are used to differentiate iAs from less toxic arsenicals in food matrices. Objectives: Our goal was to estimate the distribution of iAs exposure rates from drinking water intakes and rice consumption in the U.S. population and ethnic- and age-based subpopulations. Methods: The distribution of iAs in drinking water was estimated by population, weighting the iAs concentrations for each drinking water utility in the Second Six-Year Review data set. To estimate the distribution of iAs concentrations in rice ingested by U.S. consumers, 54 grain-specific, production-weighted composites of rice obtained from U.S. mills were extracted and speciated using both a quantitative dilute nitric acid extraction and speciation (DNAS) and an in vitro gastrointestinal assay to provide an upper bound and bioaccessible estimates, respectively. Daily drinking water intake and rice consumption rate distributions were developed using data from the What We Eat in America (WWEIA) study. Results: Using these data sets, the Stochastic Human Exposure and Dose Simulation (SHEDS) model estimated mean iAs exposures from drinking water and rice were 4.2μg/day and 1.4μg/day, respectively, for the entire U.S. population. The Tribal, Asian, and Pacific population exhibited the highest mean daily exposure of iAs from cooked rice (2.8μg/day); the mean exposure rate for children between ages 1 and 2 years in this population is 0.104μg/kg body weight (BW)/day. Conclusions: An average consumer drinking 1.5 L of water daily that contains between 2 and 3 ng iAs/mL is exposed to approximately the same amount of iAs as a mean Tribal, Asian, and Pacific consumer is exposed to from rice. https://doi.org/10.1289/EHP418

Investigation of sequential and enzymatic extraction of arsenic from drinking water distribution solids using ICP-MS

A sequential extraction approach was utilized to estimate the distribution of arsenite [As(iii)] and arsenate [As(v)] on iron oxide/hydroxide solids obtained from drinking water distribution systems. The arsenic (As) associated with these solids can be segregated into three operationally defined categories (exchangeable, amorphous and crystalline) according to the sequential extraction literature. The exchangeable As, for the six drinking water solids evaluated, was estimated using 10 mM MgCl(2) and 10 mM NaH(2)PO(4) and represented between 5-34% of the total As available from the solid. The amorphously bound As was estimated using 10 mM (NH(4))(2)C(2)O(4) and represented between 57-124% of the As available from the respective solid. Finally, the crystalline bound As was estimated using titanium citrate and this represented less than 1.5% of the As associated with the solids. A synthetic stomach/intestine extraction approach was also applied to the distribution solids. The stomach fluid was found to extract between 0.5-33.3 microg g(-1) As and 120-2,360 microg g(-1) iron (Fe). The As concentrations in the intestine fluid were between 0.02-0.04 microg g(-1) while the Fe concentration ranged from 0.06-0.7 microg g(-1) for the first six drinking water distribution solids. The elevated Fe levels associated with the stomach fluid were found to produce Fe based precipitates when the intestinal treatment was applied. Preliminary observations indicate that most of the aqueous Fe in the stomach fluid is ferric ion and the observed precipitate produced in the intestine fluid is consistent with the decreased solubility of ferric ion at the pH associated with the intestine.