Sensitive arsenic analysis by carrier-mediated counter-transport single drop microextraction coupled with capillary electrophoresis

Abstract

A sensitive analytical technique for arsenic compounds based on single drop microextraction (SDME) coupled in-line with capillary electrophoresis (CE) was developed. In SDME, a drop of an acceptor phase covered with an organic layer is hung at the inlet tip of a separation capillary. By adjusting the pH, analytes in the neutral form in an aqueous donor phase are first extracted into the organic layer, and then backextracted into the acceptor phase. However, the hydrophilic nature of the arsenic compounds, hampering the first extraction into the organic layer, lowers or even eradicates the efficiency of the SDME process. This problem can be solved by employing the scheme of carrier-mediated counter-transport using CH3(C8H17)3N+Cl− (Aliquat 336) as a carrier in the organic layer. Aliquat 336 enhances the transport of the arsenic compounds across the organic layer by forming hydrophobic complexes. The arsenic enrichment process is driven by the concentration gradient of hydroxide or chloride ion in conjunction with arsenic extraction from the donor phase to the acceptor phase of a high concentration of hydroxide or chloride. The gradient of hydroxide concentration yielded high enrichment factors for arsenic compounds, including As(III), which was not extracted well with the gradient of chloride only. After extraction, a portion of the enriched acceptor drop is injected and the arsenic compounds are separated by CE. Thus, the entire SDME and CE processes can be performed in an in-line mode using a commercial CE instrument. Using an acceptor phase at a pH of 13, the enrichment factors obtained for a sample in unbuffered water with extraction times of 15min were 390, 340, 1100, and 1300 for As(III), dimethylarsinic acid (DMA), monomethylarsonic acid (MMA), and As(V), respectively. The limits of detection (S/N=3) with absorbance detection at 200nm were 0.2, 0.7, 0.1, and 0.2μM for As(III), DMA, MMA, and As(V), respectively. Tap water spiked with 5μM of DMA and As(III), and 0.5μM of MMA and As(V) was successfully analyzed by standard addition.