Development of a green deep eutectic solvent-based thin film solid phase microextraction technique for the preconcentration of chlorophenoxy acid herbicides in drainage ditches and river waters using a central composite design

Herbicides are chemicals commonly used for the control of the environment’s biodiversity. Many of them are soluble in water, thus they end up as water contamination. It is necessary to establish an analytical technique that can accurately measure the level of concentration of pollutants in water samples, while being time efficient, inexpensive and easy. Moreover, it is important for that technique to be green and sustainable. In this paper, the thin film solid phase microextraction (TF-SPME) technique is proposed with the use of a deep eutectic solvent (DES) as a novel sorbent. DES was a mixture of trihexyl(tetradecyl)phosphonium chloride: octadecanoic acid in a 1:2 M ratio. Stainless steel mesh was used as a support for DES sorbent. Concentrations of chlorophenoxy acid herbicides 3,6-dichloro-2-methoxybenzoic acid (DC), 2-methyl-4-chlorophenoxyacetic acid (MCPA), and 2-methyl-4-chlorophenoxypropionic acid (MCPP) were measured. To select the optimal extraction conditions, five factors such as volume of samples, amount of salt, extraction time, desorption time, and pH of samples were tested in the central composite design. According to the analysis of statistical data (Statistica), the volume and the pH of the sample were significant factors. By using DES sorbent on a mesh support, the determined herbicides were preconcentrated in samples DC 69 times, MCPA 65 times, and MCPP 71 times, respectively. Samples were taken from drainage ditches and rivers located close to farmland. The analysis was performed with the use of the HPLC-UV technique. The presence of DC and MCPA was found in two out of five samples, in amounts of 22.4 ng mL−1 and 40.3 ng mL−1. The presence of MCPP was not detected in any of the analyzed samples. Due to the high recovery (72–94 %) and precision (as RSD, 3.2–11.1 %), the developed method can be successfully used for the preconcentration and determination of chlorophenoxy acid herbicides.