Preservation of water samples for arsenic(III/V) determinations: an evaluation of the literature and new analytical results

Background: Stability of chemical speciation during sample handling and storage is a prerequisite to obtaining reliable results of trace element speciation analysis. There is no comprehensive information on the stability of common arsenic species, such as inorganic arsenite [As(III)], arsenate [As(V)], monomethylarsonic acid, dimethylarsinic acid, and arsenobetaine, in human urine. Methods: We compared the effects of the following storage conditions on the stability of these arsenic species: temperature (25, 4, and −20 °C), storage time (1, 2, 4, and 8 months), and the use of additives (HCl, sodium azide, benzoic acid, benzyltrimethylammonium chloride, and cetylpyridinium chloride). HPLC with both inductively coupled plasma mass spectrometry and hydride generation atomic fluorescence detection techniques were used for the speciation of arsenic. Results: We found that all five of the arsenic species were stable for up to 2 months when urine samples were stored at 4 and −20 °C without any additives. For longer period of storage (4 and 8 months), the stability of arsenic species was dependent on urine matrices. Whereas the arsenic speciation in some urine samples was stable for the entire 8 months at both 4 and −20 °C, other urine samples stored under identical conditions showed substantial changes in the concentration of As(III), As(V), monomethylarsonic acid, and dimethylarsinic acid. The use of additives did not improve the stability of arsenic speciation in urine. The addition of 0.1 mol/L HCl (final concentration) to urine samples produced relative changes in inorganic As(III) and As(V) concentrations. Conclusions: Low temperature (4 and −20 °C) conditions are suitable for the storage of urine samples for up to 2 months. Untreated samples maintain their concentration of arsenic species, and additives have no particular benefit. Strong acidification is not appropriate for speciation analysis.

Sample Preparation and Storage Can Change Arsenic Speciation in Human Urine

Arsenic is widely distributed in the environment by natural and human means. The potential for adverse health effects from inorganic arsenic depends on the level and route of exposure. To estimate potential health risks of inorganic arsenic, the apportionment of exposure among sources of inorganic arsenic is critical. In this study, daily inorganic arsenic intake of U.S. adults from food, water, and soil ingestion and from airborne particle inhalation was estimated. To account for variations in exposure across the U.S., a Monte Carlo approach was taken using simulations for 100,000 individuals representing the age, gender, and county of residence of the U.S. population based on census data. Our analysis found that food is the greatest source of inorganic arsenic intake and that drinking water is the next highest contributor. Inhalation of airborne arsenic-containing particles and ingestion of arsenic-containing soils were negligible contributors. The exposure is best represented by the ranges of inorganic...

https://www.ics.uci.edu/~bic/papers/Menzel-Arsenic.pdf

Estimation of Multimedia Inorganic Arsenic Intake in the U.S. Population

Arsenic and antimony geochemistry of mine wastes, associated waters and sediments at the Giant Mine, Yellowknife, Northwest Territories, Canada

Speciation of arsenic in body fluids.

Humans are unavoidably exposed to the arsenic compounds present in drinking water, in food items, and sometimes in respirable particles. Normally, the daily dose of total arsenic in the range of 10 to 50μg appears to be no threat to human health. However, in geographically limited regions Taiwan, India, Argentina, Chile) the concentrations of arsenic in drinking water and perhaps in food are elevated leading to excessive intake of arsenic and to arsenic-related health problems. Humans are exposed to inorganic arsenic compounds (arsenite, arsenate) and organic arsenic compounds methylarsonic acid, dimethylarsinic acid, arsenobetaine, arsenic-containing riboses, arsenolipids). These compounds are processed in the body quite differently. Arsenate might be reduced to arsenite, arsenite might be methylated to methylarsonic acid, and methylarsonic acid reduced and then methylated to dimethylarsinic acid. All of these compounds can be eliminated in the urine. Non-eliminated, trivalent compounds, such as arsenite and methylated compounds with trivalent arsenic, can interfere with biochemically important processes through reactions with thiol groups present, for instance, in enzymes. Information about the nature and the concentration of arsenic compounds excreted in the urine can be used to deduce exposure, gain insight into the chemical transformations of the arsenic compounds in the body, and estimate the methylating capacity of exposed persons.

Determination of arsenic compounds in human urine by HPLC-ICP-MS

Arsenic compounds present in human urine provide information on daily arsenic exposure, the type of arsenic compounds ingested, and human methylating capacity. The arsenicals most frequently determined in urine by anion-exchange chromatography are arsenite, arsenate, methylarsonic acid, and dimethylarsinic acid (Chana and Smith, 1987; Heitkemper et al., 1989; Sheppard et al., 1990, 1992). Whereas the methylated arsenic compounds are unlikely to change chemically during storage of the samples or during the analytical procedure, arsenite might be oxidized to arsenate. For example, Thompson et al. (1993) recently reported significant oxidation of arsenite to arsenate during chromatography on a Dionex AS7 anion-exchange column with a mobile phase changing from 1mM to 1000 mM NaOH. Consequently, the results obtained for arsenite and arsenate by anion-exchange chromatography could be erroneous. The use of ionpairing reversed-phase chromatography was recommended to obtain reliable results. Should the oxidation of arsenite during anion-exchange chromatography be commonplace, then all results for arsenite and arsenate obtained by this method are incorrect. To define the conditions under which arsenite is likely to be oxidized, the influence of pH, time, and the arsenite concentration on the conversion of arsenite to arsenate in aqueous solutions in contact with air was investigated.

The oxidation of arsenite in aqueous solutions

Concentrations of total arsenic in soil and water samples contaminated with old Arsenical Munitions are not very useful to characterize the potential risks. Knowledge of which arsenic compounds are present in such samples is absolutely necessary to define toxicity. The identification of arsenic compounds requires a separation step combined with a detection step. For separation, gas chromatography and high performance liquid chromatography are widely used. Atomic absorption spectrometers, inductively coupled plasma optical emission spectrometers, and inductively coupled plasma mass spectrometers may serve as arsenic-specific detectors.

W. Goessler

Papers

Determination of arsenic compounds in human urine by HPLC-ICP-MS

The oxidation of arsenite in aqueous solutions

HPLC-ICP-MS Methods for the Determination of Inorganic and Organic Arsenic Compound