Arsenic speciation analysis for water samples: How long can samples be stored ?

Contaminated drinking water is one of the main sources for arsenic exposure on a global scale. Arsenic speciation is mainly driven by the biogeochemistry of the aquifer and the water geochemistry and significantly affects its transport, fate, toxicity as well as the technological possibilities for treatment. While inorganic arsenic (arsenite and arsenate) result from the contact of the water with mineral phases in the aquifer, organic species such as dimethylarsinate (DMA) and monomethylarsonate (MMA) have historically been used in agriculture but can also be produced during microbial processes. Chronic exposure to these species via drinking water is a human health concern due to risks for certain cancers and other health effects. Since the toxicity of arsenic is strongly dependent on the species being present, knowing the distribution of species is a prerequisite to evaluate the health risk. The World Health Organization’s guideline for drinking water As concentration is 10 micrograms per liter (μg/L) and the US Environmental Protection Agency’s (EPA) Maximum Contaminant Level (MCL) is the same. The MCL is enforceable at public water systems, but not at domestic (private) wells. While effective domestic well drinking water treatment systems are commercially available, their efficacy is dependent on a careful adaptation to the particular water geochemistry. The knowledge of the arsenic speciation as well as co-occurring constituents (e.g. iron, manganese) are crucial for proper selection of water treatment technology and effective operation. Although numerous laboratory methods have been developed for As speciation analysis, species preservation efficacy is highly dependent upon the water geochemistry, as well as the sample handling, sample storage, chemical preservation and storage time.

In order to investigate the stability of arsenic species in surface water and groundwater samples with variable chemical compositions, the team of scientists from the U.S. Geological Survey and the Minnesota Department of Health compared three preservation approaches.

Arsenic species standards were prepared in polypropylene graduated centrifuge tube for a concentration of 200 µg/L of each As species and 2.5 mM EDTA in ultrapure water. The standards were stored at 4°C ± 2 in the dark.

Surface water and groundwater samples collected from different locations in the US were field-filtered using a 0.45 µm filter.  The pH and specific conductance were measured in the field at the time of sample collection.

Aliquots of each sample were transferred to opaque polyethylene bottles and amended with an aliquot of 250 mM EDTA to achieve a final EDTA concentration of either 2.5 mM or 5.0 mM EDTA. In all cases, the final molar concentration of EDTA exceeded the sum of the molar concentrations of Al, Fe, Mn, Ca, Mg and Sr. Samples collected for total aqueous As and other selected elements were field-preserved with nitric acid.

Some groundwater samples were divided to evaluate the effects of two EDTA concentrations at 2.5 mM EDTA and 10 mM EDTA. 10 mM EDTA represented a molar excess of EDTA for each of the wells, while 2.5 mM was sufficient only for one sampling site.

Surface water and groundwater samples were then analysed by a LC-ICP-MS method using a Hamilton PRP-X100 column and a mobile phase containing 6-mM ammonium phosphate/ 6-mM ammonium 1 nitrate/ 20 μg/L germanium/ 2 % methanol, adjusted to pH 6.2. Species detection by an Agilent 8900 ICP-QQQ was performed with the oxygen shift mode. Measurements were repeated at different times up to 181 days after sample collection.

All  the As species in laboratory standards prepared with ultrapure water and 2.5 mM EDTA were stable for 180 days, establishing a shelf life of 180 days when stored at 4 °C in the dark. The authors are also confident that with careful attention to the elimination of potential sources of microbial activity and Fe, standards can be stored for extended periods of time, even at low As concentrations.

The groundwater samples of the hold time study showed no sign of degradation of any analyte of interest for the full 180 days of the hold time study. The ratio of As(III) to As(V) in one of the surface waters changed over time, indicating that there was likely oxidation of As(III) to As(V) occurring over time. If limits of 95 % to 105 % recovery are used, the regression lines exceed the limits at around 15 days for As(V) and 30 days for As(III). The authors speculate that microbial oxidation was the cause for the changes over time. Anyhow, the process is slow enough under the preservation and storage conditions used for this portion of the study (storage at 4 °C) that if the sample was analysed within 15 days of collection, only about 5 % bias would have to be introduced.

Groundwater samples used to compare arsenic speciation preservation methods had native As(III) concentrations ranging from non-detected to 132 µg/L and As(V) ranging from  1.5 µg/L to 19.0 µg/L. Samples preserved with 2.5 mM EDTA in white HDPE bottles were not stable for any hold time. Also, the species stability of samples preserved in Vacuette tubes was inconsistent. Only samples preserved with 10 mM EDTA and having Fe concentrations lower than 1,000 µg/L showed stable As(III) concentrations for at least 30 days.

The authors indicate that the proportion of As(V) (the easier to remove As species) would be overestimated if sample preservation is inadequate or inappropriate, or if speciation sample hold time is long.

Sarah J. Stetson, Melinda L. Erickson, Jeffrey Brenner, Emily C. Berquist, Christopher Kanagy, Susan Whitcomb, Caitlyn Lawrence, Stability of inorganic and methylated arsenic species in laboratory standards, surface water and groundwater under three different preservation regimes, Appl. Geochem., 125 (2021) 104814. DOI: 10.1016/j.apgeochem.2020.104814.

A.R. Kumar, P. Riyazuddin, Preservation of inorganic arsenic species 1 in environmental water samples for reliable speciation analysis. TrAC Trends Anal. Chem., 29 (2010) 1212-1223. DOI: 10.1016/j.trac.2010.07.009

 Birgit Daus, H. Weiss, Jürgen Mattusch, Rainer Wennrich, Preservation of arsenic species in water samples using phosphoric acid - Limitations and long-term stability, Talanta, 69/2 (2006) 430-434. doi:10.1016/j.talanta.2005.10.012

A.G. Gault, J. Jana, S. Chakraborty, P. Mukherjee, M. Sarkar, B. Nath, D.A. Polya, D. Chatterjee, Preservation strategies for inorganic arsenic species in high iron, low-Eh groundwater from West Bengal, India. Anal. Bioanal. Chem., 381 (2005) 347-353. DOI: 10.1007/s00216-004-2861-1

Patricia A. Gallagher, Carol A. Schwegel, Amy Parks, Bryan M. Gamble, Lar Wymer, John T. Creed, Preservation of As(III) and As(V) in Drinking Water Supply Samples from Across the United States Using EDTA and Acetic Acid as a Means of Minimizing Iron-Arsenic Coprecipitation, Environ. Sci. Technol.,  38/10 (2004) 2919-2927. DOI: 10.1021/es035071n

Related EVISA Resources

Link Database: Analytical Methods for Arsenic Speciation Analysis

Brief Summary: LC-ICP-MS: The most often used hyphenated system for speciation analysis

Brief Summary: Error Sources in speciation analysis - Overview

Brief Summary: Sample preservation  for speciation analysis - General recommendations