Dual mode sample introduction for multi-element determination by ICP-MS: the optimization and use of a method based on simultaneous introduction of vapor formed by NaBH4 reaction and aerosol from the nebulizer

Abstract

A method has been developed utilizing dual mode sample introduction of a commercial Multi Mode Sample Introduction System (MSIS^™) for simultaneous multi-element (As, Bi, Cd, Co, Cu, Ni, Pb, Sb, Zn) determination by ICP-MS. Vapor formed by NaBH₄ reaction and aerosol from the nebulizer were introduced simultaneously into the plasma. The effects of parameters that are expected to affect hydride generation of As, Bi and Sb, i.e. concentrations of HNO₃, NaBH₄ and thiourea, plus flow rates of sample and NaBH₄, were evaluated using Plankett–Burman experimental design. Then, the most significant of the parameters were optimized using a central composite design. Further, as optimum values were not the same for all the elements, a response optimizer was used to obtain a compromised optimum. Except for Pb, the sensitivity of the ‘classical’ hydride generating elements (As, Bi and Sb) increased significantly using the dual mode sample introduction, compared to pneumatic nebulization. The factors of sensitivity increase for As, Bi and Sb were 77, 33 and 56, respectively. For the other elements (Cd, Co, Cu, Ni and Zn), that are also reported to react with NaBH₄ forming volatile species, no significant change in sensitivity was observed. The limits of detection obtained for As, Bi, and Sb using dual mode sample introduction were 7, 15 and 10 pg mL⁻¹, respectively. The use of thiourea as a pre-reducing agent and masking agent was also investigated. The accuracy of the method was verified by analyzing various certified reference materials (CRMs): SRM1572 ‘Citrus leaves’, SRM1575 ‘Pine Needles’, SRM1643e ‘Trace Elements in Water’, GBW07602 ‘Bush Branches and Leaves’ and GBW07601 ‘Human hair’.