Application of a nitrogen microwave-induced plasma mass spectrometer as an element-specific detector for arsenic speciation analysis

Abstract

A high power nitrogen microwave-induced plasma (1.3 kW) mass spectrometer (N₂-MIP-MS) was successfully coupled with an HPLC system using a silica-based cation-exchange column. It was examined as an element-specific detector for its applicability to the optimization and determination of seven arsenic compounds [As(V), methylarsonic acid (MA), dimethylarsinic acid (DMA), arsenobetaine (AB), arsenocholine (AC), trimethylarsine oxide (TMAO) and tetramethylarsonium ion (TMI)]. The system is a promising alternative ion source for mass spectrometry for elemental speciation analysis. The MIP was stable with a pyridine mobile phase for up to 6 h. Replacing the MIP-MS fabricated nebulizer (concentric) and sample input tubing (PTFE) with an ICP-MS (PMS-2000) nebulizer (concentric) and PEEK tubing increased the ion signals for anionic and cationic arsenic compounds by 17–30 and 21–25%, respectively. PEEK tubing additionally increased the separation efficiency for the arsenic compounds. The detection limits of As(V), MA, DMA, AB, TMAO, AC and TMI obtained with the optimized HPLC-N₂-MIP-MS system were 0.68, 0.95, 2.01, 0.92, 22.1, 1.31 and 1.75 µg l^–1, respectively. The repeatability (RSD for three successive analyses) and reproducibility (RSD for three successive analyses performed on three different days) achieved were 0.7–9.22 and 6.5–11.4%, respectively, for the seven different arsenic compounds. No detectable spectroscopic interference of ⁴⁰Ar³⁵Cl⁺ was observed with a high chloride matrix (10 000 mg l^–1). The developed HPLC-N₂-MIP-MS method was successfully applied to the determination of arsenic compounds, principally AB, in NIES Candidate CRM-18 Freeze-Dried Human Urine (134 ± 6 µg l^–1). The results agreed reasonably well with the HPLC-ICP-MS values.