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.

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 (r ≥ 0.995) are attained for all the herbicides. The limits of detection of the method range between 7.20 and 58.1 ng L while the limits of quantification (LOQs) range between 24.0 and 194 ng L. 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.