A green and cost-effective procedure for determination of anionic surfactants in milk with liquid-liquid microextraction and smartphone-based photometric detection

Abstract

Aiming at determination of anionic surfactants as milk adulterants, a novel procedure based on simultaneous protein precipitation and liquid-liquid microextraction (LLME) of the ion-pair formed between the analytes and methylene blue is described. Sample treatment, ion-pair extraction, and photometric detection with a smartphone camera were carried out in a single vessel, thus significantly simplifying the procedure and minimizing risks of contamination and analyte loss. The chloroform volume for both sample treatment and extraction of the ion-pair was only 150 μL and usual drawbacks of LLME, such as the need for phase isolation and dilution of the extract before spectrophotometric measurements were avoided. By using sodium dodecylsulfate as a model surfactant, and only 100 μL of sample, 25 μg of methylene blue, and 2.9 mg of EDTA, a linear response was achieved from 10 to 50 mg L−1, r = 0.999 (n = 5), with detection limit (99.7% confidence level) and coefficient of variation of 2.2 mg L−1 and 1.9% (n = 10), respectively. The proposed procedure is a simple, fast, cost-effective, and environmentally friendly approach for detection of milk adulterations by surfactants.