A comparison of urinary arsenic speciation via direct nebulization and on-line photo-oxidation - hydride generation with IC separation and ICP-MS detection

Abstract

Urinary arsenic speciation provides information on recent arsenic exposure. The literature reported analysis of NIST SRM 2670 Freeze-dried Urine indicates considerable discrepancy in species specific concentration. In this study, two complementary sample introduction pathways, direct nebulization (DN) and hydride generation (HG), were utilized and compared for urinary arsenic speciation via ion chromatography (IC)-ICP-MS. The retention characteristics of arsenobetaine (AsB), arsenite [As(III)], dimethylarsinic acid (DMA), monomethylarsonic acid (MMA), arsenate [As(V)] and Cl− were systematically evaluated with respect to column temperature and the (NH4)2CO3 eluent molarity using the DN method. This characterization indicated that three early eluters [AsB, As(III) and DMA] were best separated at a higher column temperature and lower eluent molarity, whereas MMA, As(V) and Cl− were best separated at a lower column temperature and higher eluent molarity. From these observations, a gradient elution program was developed using 40 and 70 mM (NH4)2CO3 (pH 10.5) at 60 °C. This gradient condition produced satisfactory resolution for all five arsenic species with a Cl− tolerance up to 0.3% w/w. In the membrane hydride generation (HG) configuration, a photo-reactor interface was installed between the column and the HG device to facilitate the detection of non-hydride active arsenic species. Isocratic elution using 40 mM (NH4)2CO3 was adequate in resolving all five arsenic species while the chloride interference was removed by a gas–liquid separator. NIST SRM 2670 Freeze-dried Urine was analyzed using the DN and HG methods and the sum of the arsenical concentrations was 77.7 ± 3.5 and 71.1 ± 2.8 ng mL−1, respectively.