A simple LC-MS/MS alternative for arsenic monitoring in groundwater: validation and field application.

Monitoring inorganic arsenic in groundwater remains a major analytical and public health challenge in regions where access to advanced elemental instrumentation typically used for trace analysis is limited. Although LC-MS/MS platforms are widely available in environmental monitoring laboratories, standardized methods for inorganic arsenic are scarce, and most published approaches require derivatization or complex sample preparation. Here, we developed and validated a simple, derivatization-free LC-MS/MS method for total inorganic arsenic, based on a short oxidative conversion of As(III) to As(V) followed by a 4-min isocratic run on a conventional C18 column using a common mobile phase (methanol:water 10:90 with 0.1% formic acid). The workflow minimizes pretreatment, requiring only formic acid, hydrogen peroxide, and brief heating. The method met international validation criteria, with excellent linearity (R2 ≥ 0.999), LOD 0.08 µg/L, LOQ 0.3 µg/L, accuracy 94-106%, and intra/inter-day precision <6% RSD. Matrix effects were modest (+ 7%), oxidized extracts were stable during refrigeration or frozen storage, and robustness analysis showed little sensitivity to reagent lots, oxidation temperature, or moderate variations in reaction time. Comparison with ICP-MS using 20 groundwater samples showed strong agreement (slope 0.88; R2 0.93; no detectable bias). Application to 44 wells from peri-urban La Plata (Argentina) yielded 1.1-45.4 µg/L As, with 91% exceeding the 10 µg/L guideline. While not intended to replace established elemental methods, this method offers a practical analytical alternative for decentralized monitoring programs. Its compatibility with routine LC-MS/MS workflows enables accessible, high-throughput surveillance of arsenic-affected groundwater in resource-limited regions.