Photothermal lens spectroscopy in near-infrared region for trace arsenate determination in water samples

Arsenate (AsO₄^3-) is a toxic form of arsenic and a potential carcinogen whose presence in water sources can endanger human health. Therefore, detecting trace amounts of this ion requires a rapid, non-destructive, and highly sensitive method. In this work, double crossed-beam photo thermal lens spectroscopy (PTLS) was developed to detect arsenate in water samples based on the molybdenum blue (MB) reaction. After optimizing the chemical parameters (acid, reducing agent concentration and time) using UV-Vis-NIR, a calibration curve with a linear dynamic range of 2.4-48 μM, and a detection limit of 2.1 μM was achieved. Then, PTLS with an 850 nm diode laser was used for arsenate determination after optimizing device parameters (sample flow rate and excitation laser power). A calibration curve with a linear dynamic range of 0.48-9.6 μM, and a detection limit of 0.3 μM was obtained. Interferences such as phosphate can be a limitation of this environmentally friendly method. Its determination was investigated by reducing arsenate to arsenite using dithionite and samples heating. Overall, this study, for the first time, demonstrates the potential of PTLS for arsenate detection in water as a solvent with 7-fold sensitivity compared to NIR absorption spectroscopy and the possibility of working with microliter volumes (≈ 3 μL cell detection zone) of samples due to the presence of a microfluidic cell.