Selective and sensitive determination of As(III) and tAs in Chinese herbal medicine samples using L-cysteine modified carbon paste electrode-based electrolytic hydride generation and AFS analysis.

A novel non-chromatographic speciation technique for ultra-trace arsenite [As(III)] and total arsenic (tAs) in Chinese herbal medicine (CHM) is developed and validated by electrolytic hydride generation (EHG) coupled with atomic fluorescence spectrometry (AFS). The studies show that As(III) can be converted efficiently to AsH3 on an L-cysteine modified carbon paste electrode (CMCPE), which has never been reported before. Significantly, other arsenic species such as arsenate [As(V)], monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) do not form any or only less volatile hydrides at low applied current mode (<1.0 A). The results also demonstrate that L-cysteine and graphite powder play different roles in the electrolytic generation of AsH3. Comparing with the traditional graphite electrode, CMCPE has better stability, sensitivity and interference tolerance. Under the optimal conditions, the limit of detection (LOD) of tAs and As(III) for this method are 0.087µgL-1 and 0.095µgL-1 respectively. The accuracy of the method is verified through the analysis of reference materials (CRM 08231 and SRM1568a), and the proposed method has been applied satisfactorily to the determination of As(III) and tAs in several CHM samples.