Sideband absorbance integration: a spectrophotometric strategy for direct high‑concentration sulfate analysis

Direct quantification of sulfate at high concentrations by UV–vis spectrophotometry is hindered by severe absorbance–concentration nonlinearity and saturation at peak wavelengths. To address this issue, we propose a sideband absorbance integration strategy that enables direct, non‑destructive determination of sulfate without altering its original state. Wavelength ranges in the spectral sidebands where sulfate exhibits low absorption coefficients were selected to mitigate saturation effects, and the absorbance was integrated over these ranges to enhance analytical sensitivity. The effects of wavelength scanning interval and optical pathlength on the sensitivity and accuracy were systematically investigated and an adaptation relationship among these parameters was established. The proposed method achieves a wide linear measurable range of 4–150 g/L for sulfate, with sensitivity enhanced by 15.61‑ to 24.49‑fold compared with conventional spectrophotometry. It also yields reduced relative errors (RE < 5%, mean RE = 1.09% ± 0.33%), recovery rates within 90%–110% and a 15.72%–73.67% improvement in prediction accuracy (RMSEP) relative to conventional spectrophotometry. The method performs reliably in the presence of various metal ions (Zn2+, Ni2+, Mn2+ and Co2+), demonstrating its robustness for complex sample matrices. This sideband integration approach offers a simple yet effective spectrophotometric strategy for high‑concentration analysis and may be extended to other strongly absorbing species where conventional peak‑wavelength measurements are limited by saturation.