Journal of Chromatography A, 873 (2000) 129–135 www.elsevier.com / locate / chroma Speciation of dimethylarsinic acid and monomethylarsonic acid by solid-phase microextraction–gas chromatography–ion trap mass spectrometry * ´ Zoltan Mester, Janusz Pawliszyn Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada Abstract A solid-phase microextraction (SPME) method has been developed to determine two methylated arsenic species in human urine samples by GC–MS. The direct extraction of the methyl arsenic compounds by SPME after thioglycol methylate derivatization was studied. Direct extraction with SPME was suitable for the determination of trace levels of dimethylarsinic acid (DMA) and monomethylarsonic acid (MMA) in urine samples. Four different commercial SPME fibers were tested for the extraction of methyl arsenic compounds, and the best results were obtained using the polydimethylsiloxane coating. The extraction and desorption time profiles of DMA and MMA were determined. The detection limits for DMA and MMA using the SPME–GC–MS method were 0.12 and 0.29 ng/ml, respectively. The method is linear in the 1 to 200 ng/ml range.  2000 Elsevier Science B.V. All rights reserved. Keywords: Dimethylarsinic acid; Monomethylarsonic acid; Arsenic compounds; Organoarsenic compounds; Thioglycol methylate 1. Introduction arsenic in the body – in blood and urine – is in the form of dimethylarsinic acid (DMA) and mono- Speciation analysis has been one of the fastest methylarsonic acid (MMA). According to the litera- progressing techniques of modern instrumental ele- ture the majority of arsenic is in the form of DMA. ment analysis in the last decade [1–4]. This tech- Therefore, it is necessary to determine the exact nique aims at determining the different chemical quantity of the differently-methylated species as well forms of a given element. Generally, for the sepa- as the total amount of arsenic in body fluids [5]. ration of different species, a high-performance sepa- In the case of arsenic speciation the separation ration technique (e.g. HPLC, capillary GC) and an technique is typically liquid chromatography, while element selective atomic spectroscopic detector are the detection method is atomic spectroscopy. The coupled. In the liver of humans and mammals, there typical choices for the separation of anionic arsenic is a methylation mechanism for the detoxification of species [MMA, DMA, As(III), As(V)] are ion-ex- inorganic arsenic. As a result of this detoxifying change chromatography [6,7] and ion-pair chroma- process, the major part of the detectable amount of tography [7–10]. An interesting new approach was described by Taniguchi et al. [11] employing ion- exclusion chromatography for the separation of *Corresponding author. Tel.: 11-519-888-4641; fax: 11-519-746- As(V), As(III) and MMA. The advantage of this 0435. E-mail address: janusz@uwaterloo.ca (J. Pawliszyn) separation method is that it does not employ buffers; 0021-9673 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0021-9673(99)01350-3