Sequential injection absorption spectrophotometry using a liquid-core waveguide: Determination of p-arsanilic acid in natural waters

We describe the development, optimization, and application of a novel method for the quantification of p-arsanilic acid (p-ASA), an organoarsenical that is widely used as an animal feed additive, at μg L−1 levels in surface water samples. The method is based on sequential injection-long pathlength absorbance spectrophotometry (SI-LPAS). This method incorporates the selective reactivity of p-ASA with p-dimethylaminobenzaldehyde (DMAB, “Ehrlich's Reagent”) in the presence of other common environmental organoarsenicals, such as dimethylarsenic acid (DMAA), monomethylarsonic acid (MMAA), and 3-nitro-4-hydroxyphenylarsonic acid (3-NHPAA, Roxarsone). Optimal reaction conditions were studied, namely the pH dependence of the reaction (pH = 1.4), volumes of reagent and sample, reagent concentration, reaction time, reaction temperature, and flow rates. A calibration model that was linear over four orders of magnitude was prepared, where y (A450, peak height) = 0.139 × (mg L−1) + 0.0275 (R2 = 0.9984). The limit of detection was 21.2 μg L−1 (3 Ω), and the analysis time was <8 min/sample. A study of the response of the method to other organoarsenicals demonstrated that the method is selective for p-ASA (t-test, t = 0.622, tc = 3.707 at the 99% confidence limit). For an authentic surface water sample from a swine farm that was fortified with p-ASA to 0.500 mg L−1, a recovery of 100.5% and an average precision (n = 4) of 13.7% relative standard deviation were observed.