Extraction of the toluene exposure biomarkers hippuric acid and methylhippuric acid using a magnetic molecularly imprinted polymer, and their quantitation by LC-MS/MS

A magnetic polymer was molecularly imprinted with hippuric acid (HA) to obtain a nanomaterial with an architecture of type Fe₃O₄@SiO₂@MIP. It was used as a sorbent for magnetic solid phase extraction of HA and methylhippuric acids (2-MHA, 3-MHA, 4-MHA) from urine samples. The respective imprinting factor are 4.6, 2.7, 2.0 and 1.8, respectively, and the selectivity coefficients are 1.7, 2.3 and 2.6. The effects of adsorbent amount, extraction time, pH value, ionic strength, desorption solvent and desorption time on the extraction of HA and MHA were investigated. Following elution with 1 mM NaOH solution, the 4 HAs were quantified by ultra-performance liquid chromatography with mass spectrometric detection. Under the optimal experimental conditions, the limits of detection (at S/N = 3) range between 89 ng·L^-1 (for HA) and 170 ng·L^-1 (for 4-MHA), the limits of quantification (at S/N = 10) range between 300 ng·L^-1 (for HA) and 570 ng·L^-1 (for 4-MHA), the linear range extends from 500 ng·L^-1 to 10 mg·L^-1, the relative standard deviations of intra-day range between 6.4 and 9.6% (for n = 6 at 10 μg·L^-1) and inter-day range between 9.2 and 11.5% (for n = 6 at 10 μg·L^-1). The method was successfully applied to the analysis of HA and MHA in (spiked) urine, and good recoveries were obtained. Graphical abstract Schematic presentation of a method for the determination of hippuric acid (HA) and methylhippuric acid (MHA) in urine sample. The preparation of Fe₃O₄@SiO₂@MIP includes three steps: (1) Preparation of Fe₃O₄ nanoparticles (NPs), (2) Preparation of Fe₃O₄@SiO₂ and (3) Preparation of a molecular imprint in the surface of the nanoparticles (Fe₃O₄@SiO₂@MIP). The MIP was used as a sorbent for magnetic solid phase extraction (MSPE) of the toluene exposure biomarkers.