Deep eutectic solvent-based dispersive liquid-liquid microextraction coupled with HPLC and central composite design for biological monitoring of hippuric acid and methyl hippuric acid in urine

Exposure assessment of toluene and xylene as two hazardous compounds in occupational settings are great of importance. Hippuric acid (HA) and methyl hippuric acid (MHA) are common urinary metabolites for biological monitoring of toluene and xylene exposure, respectively. In this study a dispersive liquid–liquid microextraction (DLLME) method based on a deep eutectic solvent (DES), was suggested to determine the urinary MHA and HA simultaneously. Optimizations of all parameters affecting the DLLME-DES were done using response surface methodology. In obtained optimized conditions, a linear calibration range of 0.024–20 µg mL−1 for HA and 0.033–20 µg mL−1 for MHA were achieved and R2 was above 0.99 for these two metabolites. The detection limit of HA and MHA was 0.008 and 0.010 µg mL−1 respectively. The quantification limit was reported as 0.024 and 0.033 µg mL−1 for HA and MHA respectively. The suggested analytical technique was used for quantitation of target biomarkers in urine samples of occupational workers with high accuracy (97–103.6% for HA and 92–104% MHA) and precision (RSD ≤ 4.5% for HA (RSD ≤ 6.2% for MHA). Applying the proposed method for urine sample analysis of exposed-workers and comparing with the standard method (National Institute for Occupational Safety & Health standard method) showed that HA and MHA can be pre-concentrated and quantified by a simple, fast and environmental friendly method.