Speciation of arsenic, selenium, lead, and mercury with capillary electrophoresis

This work deals with speciation of elements of environmental concern (arsenic, selenium, lead and mercury) using a variety of capillary electrophoresis (CE) methods combined with various on-capillary approaches to enhance the method sensitivity. Capillary zone electrophoresis (CZE) with UV-detection was used for the determination of arsenite, arsenate, monomethylarsonic acid, dimethylarsinic acid, p-aminophenylarsonic acid, 4-hydroxy-3-nitrobenzearsonic acid, 4-nitrophenylarsonic acid, phenylarsonic acid and phenylarsine oxide. The influence upon the method performance criteria of operational parameters such as electrolyte pH and composition, applied voltage, as well as electroosmotic flow (EOF) reversal protocols was investigated. Both the counter-BOP separation mode applied in fused-silica (FS) capillaries, and the co-EOF separation mode performed in a poly(diallydimethylammonium) chloride (PDDAC) coated capillary with reversed EOF were used. The background electrolyte (BGE) composition found to provide optimal separation was 20 mM carbonate buffer at pH 10.0. This method was applied to the determination of four aryl arsenic compounds in animal feed. The co-EOF method was also used for the simultaneous determination of nine arsenic and selenium compounds (arsenite, arsenate, monomethylarsonic acid, dimethylarsinic acid, selenate, selenite, selenocystine, selenomethionine and selenocystamine). The BGE composition found to provide optimal separation was 15 mM phosphate buffer at pH 10.6. Arsenic and selenium species were detected at 195 nm and 200 nm, respectively. Dithizon sulfonate (DzS) and ammonium bis(2-hydroxyethyl)dithiocarbamate (HEDTC) were used to derivatise mercury species to form complexes which have strong UV absorbance. These complexes were separated using CZE and microemulsion electrokinetic chromatography (MEEKC). Microemulsions having a range of pH 3.0-11.0 were used in MEEKC to achieve the optimal separation conditions. A FS capillary and successive polymer and microemulsion dynamically coated (SPMEC) capillaries were used in MEEKC. However, a fully suitable derivatisation reagent for organolead species has not yet been found. In this research, stacking mechanisms and concentration sensitivity enhancement factors were discussed and compared for (1) normal stacking mode (NSM, sometimes also referred to as field amplified stacking) in an uncoated FS capillary in the counter-EOF mode, (2) large volume sample stacking (LVSS) with polarity switching, and (3) the less often applied stacking method of co-EOF NSM stacking with EOF reversal using a PDDAC-coated capillary. Of these three stacking methods, L VSS with polarity switching gave the lowest limit of detection (LOD), while co-EOF NSM stacking was used to simultaneously pre-concentrate both cationic and anionic target analytes. The use of a high sensitivity detection cell (HSDC) further reduced the LOD of the target analytes by a factor of 5-8 times. By combining LVSS and HSDC, LODs of arsenic species could be reduced by a factor of 218-311, being 1.74, 4.87, 1.99 and 2.80 ˜í¬¿g/L for arsenite, arsenate, dimethylarsinic acid and monomethylarsonic acid, respectively. The method was demonstrated to be applicable to the determination of the target analytes in tap water, sediment and a selenium nutrition supplement sample, the sample matrix did not interfere with the determinations. Sweeping was used in MEEKC to improve the sensitivity of those analytes which have a high retention factor in the mi cell e. In this work, the LOD of the HEDTC phenyl mercury complex was significantly improved by a factor of 5 .4.