Adsorptive stripping voltammetry as a sample pretreatment method for trace uranium determinations by inductively coupled plasma mass spectrometry

Adsorptive stripping voltammetry was investigated as a route for improving trace level quantification of uranium via on-line matrix elimination and analyte preconcentration. On the basis of prior literature reports, propyl gallate was selected as a chelating agent for adsorptive accumulation of uranium (VI) at a mercury thin-film electrode (MTFE). Off-line electrochemical studies indicated that the uranium-propyl gallate (U-PG) complex accumulated (-0.15 V) at a MTFE when the uranium containing sample was mixed (1:1 v/v) with 5 × 10^-5 M PG in 0.05 M sodium acetate buffer (pH 4.5), and could be stripped into 0.05 M sodium acetate or 0.1 M ammonium nitrate, an inductively coupled plasma mass spectrometry (ICP-MS) compatable matrix, by a potential scan to -1.4 V. A thin-layer, flow-by electrochemical cell was placed on-line with ICP-MS and the same basic stripping procedure performed, but ²³⁸U was not detected when the stripping potential was applied. Combining a potential step to -1.2 V with injection of 1% HNO₃ did, however, effectively release the uranium to the ICP-MS. Matrix elimination was successful, and 24-fold signal enhancement was achieved with a 10 min accumulation, consuming just 0.8 mL of a 0.5 μ/L uranium solution. Quantitative performance was tested on NASS-4 Open Ocean Seawater (2.68 ± 0.12 μg/L uranium) by using calibration plot and standard addition methods. Nonlinearities, as functions of both analyte concentration and deposition time, were observed and are consistent with saturation of the MTFE, suggesting that the technique is most applicable to ultratrace uranium analysis or appropriately diluted samples. (Int J Mass Spectrom 178 (1998) 51-63)