Elimination of interfering molybdenum oxyanion with an anion-exchange monolithic spin tip (AXTip) for precise determination of cadmium in human urine by ICP-MS

Abstract

A simple purification technique using an anion-exchange monolithic spin tip was described for the elimination of molybdenum oxyanion interfering with precise determination of cadmium in human urine samples by ICP-MS. The tip device named “anion-exchange tip-in monolith (AXTip)” was laboratory-prepared within the confines of a commercially available syringe filter tip by a two-step process: (1) in situ copolymerization of glycidyl methacrylate (GMA) and ethylene dimethacrylate (EDMA) and (2) its subsequent modification with trimethylamine (TMA), as a strong anion-exchange functional group, via ring-opening reaction of epoxide. The adsorption capacity of the AXTip was > 3.2 μmol/tip for molybdenum oxyanion. The performance of the AXTip was evaluated through an adsorption/non-adsorption experiment for molybdenum, cadmium and other metal and metalloid species, using a model solution and human urine samples to optimize the sample solution's pH and dilution factor for the AXTip treatment. In the case of urine sample analysis, 8 times dilution and pH adjustment to 4.0 prior to AXTip treatment were found to allow > 98% elimination of interfering molybdenum oxyanions and quantitative recovery (103 ± 7%) of cadmium. At the optimized conditions, the AXTip treatment reduces Mo/Cd ratio from ~400 in human urines to < 10, which is sufficient for conducting ICP-MS measurement of cadmium in no-gas mode (without collision/reaction cell technique). The proposed purification technique was validated through the analysis of an accuracy control material of human urine (Seronorm™ Trace Elements Urine L-1). Good agreement of the observed values with the reference values indicates that the proposed technique is practically applicable.