New Analytical Approaches to Determination of Linear Alkylbenzene Sulfonates in Natural Water at ppt Level Using HPLC-UV, NMR Spectroscopy, and GC-MS

Abstract

Sodium alkylbenzene sulfonates or linear alkylbenzene sulfonates (LAS), as they are commonly known, are among the most toxic anionic synthetic surfactants, which are stable in the freshwater environment. A new approach is proposed to determine these toxicants in natural water in the form of alkylbenzene sulfonic acid methyl esters using high-performance liquid chromatography (HPLC) with spectrophotometric detection in the UV spectral range. The limit of quantification (LOQ) estimated from the peak of the C10 homologue group is 16.0 ng/L under analysis on a Milikhrom™ А-02 chromatograph with a microcolumn 2 × 75 mm in size with a Nucleosil 100-5-С₁₈ sorbent. A new method is proposed for extraction and preconcentration of linear alkylbenzene sulfonates from filtered and ethyl alcohol-preserved water samples, which includes (i) solid-phase extraction on reversed-phase C18 cartridges at a flow rate of 30–50 cm³/min (recovery 100%) with subsequent elution of the analytes with methanol, (ii) re-dissolution of the analytes in distilled water and their re-extraction with chloroform in the form of an ion pair with methylene blue (recovery 98%), (iii) methylation of the extract with a new trimethyl orthoformate reagent in the environmental monitoring with a 98% yield of methyl esters of alkylbenzene sulfonic acids, and (iv) additional precleaning and preconcentration of the analytes by 20 000 times as compared with the initial sample on cartridge tubes with silica gel (100% recovery). The structure of methylated derivatives is confirmed by ¹H NMR spectroscopy and gas chromatography‒mass spectrometry (GC-MS) with electron impact ionization (70 eV) in the total ion current (TIC) mode (m/z 50‒500). The new approaches are tested by analyzing sodium alkylbenzene sulfonate homologues C10–C13 in the water of the Krestovka River, a tributary of Lake Baikal, during spring season when the floods associated with snow and ice melting have already complited by the sampling. The average (n = 3) total concentration of the analytes in river water is estimated as 2.1 ± 0.4 µg/L using the external standard calibration method.