Emulsification-assisted micro-solid-phase extraction using a metal-organic framework as sorbent for the liquid chromatography-tandem mass spectrometric analysis of polar herbicides from aqueous samples

Abstract

Two miniaturized sample preparation techniques, ultrasound-assisted emulsification microextraction (USAEME) and micro-solid-phase extraction (μ-SPE) have been integrated for the pre-concentration of four polar chlorophenoxy acid (CPA) herbicides from environmental aqueous samples. An metal-organic framework, MIL-101(Cr), characterized by its high porosity and large surface area, was explored as a sorbent. Despite the simplicity and convenience of μ-SPE, its use can potentially be limited by a long equilibration time, especially when applied to polar analytes. However, as demonstrated in this work, this drawback can be overcome by employing 1-octanol as a medium during the USAEME step to concentrate, and transport the polar herbicides through the polypropylene membrane of the μ-SPE device, onto the MIL-101(Cr). After μ-SPE, the herbicides were desorbed using 125 μL of organic solvent, and analyzed by ultra-high-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS). Under the most favorable conditions, wide calibration ranges with good linearity (r2 ≥ 0.995) are attained for all the herbicides. The limits of detection of the method range between 7.20 and 58.1 ng L−1 while the limits of quantification (LOQs) range between 24.0 and 194 ng L−1. The LOQs determined are lower than the guidelines set by the World Health Organization for the respective herbicides. USAEME-μ-SPE-UHPLC-MS/MS also shows good repeatability, with relative standard deviations ≤ 15.2%. Enrichment factors are between 21 and 70. The method was applied to the analysis of tap and reservoir waters, in which satisfactory relative recoveries ranging between 64.4% and 106% were obtained. The MIL-101(Cr) exhibited superior extraction capability in comparison to activated carbon and multi-walled carbon nanotubes.