Ion chromatography – nitrogen-sustained microwave inductively coupled atmospheric pressure plasma – mass spectrometry (IC-MICAP-MS) for arsenic speciation analysis in rice

Abstract

The performance of a nitrogen (N₂)-sustained Microwave Inductively Coupled Atmospheric-Pressure Plasma (MICAP) – Quadrupole Mass Spectrometer (QMS) was explored as an alternative to the traditional argon (Ar) equivalent for arsenic (As) analysis in rice. Conventional Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) can be limited by Ar-based spectral interferences depending on sample matrices. Specifically, for the analysis of As, argon chloride ions (⁴⁰Ar³⁵Cl⁺) can interfere with the detection of the monoisotopic ⁷⁵As⁺, and this interference can lead to systematic errors in As quantification. With N₂-sustained MICAP-QMS, major plasma background species are N₂-based, which eliminates the spectral interference possibility at mass-to-charge (m/z) 75 from ⁴⁰Ar³⁵Cl⁺. In this work, we present the first combination of the N₂-MICAP-MS with anion exchange chromatography for As speciation and quantification from several rice varieties. We used a slightly modified As-species extraction procedure per Food and Drug Administration (FDA) protocol EAM 4.11. In all rice samples, we quantified the As amount in the species arsenite (As(III)), dimethylarsinic acid (DMA), monomethylarsonic acid (MMA), and arsenate (As(V)), with detection limits of 0.26, 0.26, 0.13 and 0.22 μg kg⁻¹, respectively. We validated our experimental method with Standard Reference Material (SRM) NIST 1568b rice flour and obtained 85–110% recoveries for quantification of As from species. Speciation results were comparable to total As concentration determined by MICAP-MS. The use of N₂-MICAP-MS would allow for quantification of As in chloride containing matrices; here, we demonstrate strong matrix tolerance of the MICAP-MS for solutions with up to 1% (w/w) sodium chloride. These results suggest that the MICAP-MS is a promising approach for the analysis of As species in more complex matrices.