Arsenite, Asenate, MMA, and DMA 
by ion chromatography (IC) coupled with ICPMS

Forms of Arsenic

Arsenic (As) is found in the earth’s crust as arsenopyrite (FeAsS) and other sulfur and metallic containing minerals. Arsenic is used in a variety of forms: for doping electronic solid state devices (GaAs), pigments, pyrotechnics (As₄S₄), pesticides, metallurgy, wood preservative (chromated copper arsenate, CCA), chicken feed supplement (roxarsone ), herbicide (cacodylic acid).

Arsenic trioxide (As₂O₃ or As₄O₆) is one of the primary intermediates used to make other forms of arsenic. Dissolved in water it forms asenious acid although the pure acid has never been isolated. Salts are called arsenites, and these forms represent the +3 oxidation state of asenic, As(III).

Arsenates (AsO₄^-3) are salts of arsenic acid, or more formerly orthoarsenic acid.  This represents the +5 oxidation state of arsenic. Arsenite can be easily oxidized to arsenate, and arsenate can also be reduced to arsenite.

Arsenic can be found in water most commonly as arsenite or arsenate.¹ EPA currently regulates total arsenic in drinking water with a Maximum Contaminant Limit (MCL) of 10 ug/L. In biological systems, arsenic appears as a variety of organic arsenicals including anions monomethylarsonic acid (MMA) and DMA, the cation arsenocholine, the zwitterion arsenobetaine, and arsenosugars².

Some forms of arsenic are highly poisonous (arsenite) and thought to either cause or promote cancer.^3,4 Other forms, such as AsB found primarily in shellfish, are relatively nontoxic because it is rapidly excreted unchanged in urine. Thus, there is a big emphasis today on speciating the forms of arsenic.

Analytical Methods

This is a rapidly changing area; scientific papers are appearing at an ever increasing rate. The current state of the art uses ion chromatography (IC) to separate the common forms of arsenic followed by inductively coupled plasma-mass spectrometry (ICPMS) to detect the various species. The following chromatogram is an example of the IC-ICPMS analysis. Detection limits for the various species is in the range of ~0.2 ug/L in water. The method has been applied to water, soil, urine, and biological samples.

PE ELAN 6100 DRC ICPMS

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Water Samples

USGS found that inorganic arsenic as arsenite and arsenate were the predominant species in natural water samples with the highest concentrations being approximately 13 As mg/l in acid mine drainage samples. Typical surface water samples were reported near 5 As ug/L.. DMA and MMA were only found in surface waters at less than 6 As ug/L near agricultural areas using MMA as a herbicide.¹

Samples should not be preserved with nitric acid because this will oxidize arsenite to arsenate. A number of preservation techniques have been studied including refrigeration, opaque bottles to exclude light, dilute hydrochloric acid, ascorbic acid, and EDTA. Current recommendations are to field filter the water sample through 0.45 micron filters, minimizing any aeration, filter into opaque plastic bottles 

In addition, we recommend that spikes of each arsenic species be placed into separate sample bottles to be filled with sample at the site. In this way the stability of the various species are documented. Inter-conversion is common. In addition for mass balance, samples for total arsenic should also be filtered in the field prior to acidification with nitric acid.

Other Samples

Soils and biological samples are typically extracted with an alcohol and/or a sodium carbonate solution (pH 11). Samples should be simply refrigerated and analyzed as soon as possible.

Whenever speciating the various forms of arsenic, it is recommended that total arsenic also be determined to demonstrate that most forms have been detected. Total arsenic is usually measure by digesting the sample in concentrated nitric acid followed by ICPMS.

As Speciation by GC-ICPMS (update 1/18/2005)

Various arsenic species react with 1,3-propanedithiol (PDT) to form relatively volatile cyclic dithiaarsenolines.  Adapting the method of Roerdink and Aldstadt⁸ we have developed a GC-ICPMS procedure that can be applied to MMA, DMA, phenylarsonic acid (PA), and roxarsone.

An aqueous sample or extract is simply acidified with hydrochloric acid, reacted with a small amount of PDT, and the reaction products are then extracted.  

The chromatogram below shows the separation and quantitation of these compounds.  The ICPMS monitors mass 75 characteristic of arsenic.  Detection limits are ~1-10 ug/L using a 10 mL sample.  

One of the advantages of this method is the much greater resolving power provided by capillary gas chromatography (GC) over that possible with liquid chromatography (HPLC).  Peaks from LC or IC are generally 20-30 sec wide, while these capillary GC peaks are ~2 sec wide.

References

1.  US Geological Society, Water-Resources Investigations Report 02-4144.

2.  P.A. Galagher, et al., JAAS, 1999, 1829.

3.  Smith, A. Env. Health Perspective, 1992, 97, 259-267.  

4.  CA OEHHA, Developmental and Reproductive Toxicity of Inorganic Arsenic (Toxicology Review for Prop 65)

5. A.J. Bednar, et al., ES&T, 2002, 2213. (USGS study with EDTA preservation of water samples)

6.  P.A. Galagher, et al., J. Env. Mon., 2001, 371. (EPA study with EDTA)

7. X. Chris Le, et al., Anal. Chem., 2004, 27A. Recent Review.

8.  A.R. Roerdink and J.H. Aldstadt, J. Chrom. A, 1057 (2004), 177-183)

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