The Study Of On-line Solid-phase Extraction For Five Avermectins Residues In Foodstuffs Using Hydrophobic Monolithic Extractor

Objectives The direct determination of avermectins in food matrices is usually difficult because of the complexity of the matrices and the extremely low concentrations of the analytes. Therefore, solid-phase extraction(SPE) is used for both isolation and/or preconcentration of the compounds from food samples. Generally, off-line SPE as a separate procedure prior to the final instrumental analysis is a complex procedure with poor reproducibility and the SPE sorbents are disposable. Here, a hydrophobic monolithic extractor(10 mm × 2.1 mm, i.d) is developed as a reusable on-line SPE sorbent coupled with liquid chromatography tandem mass spectrometry(LC-MS/MS) for the rapid,automatic and simultaneous determination of 5 avermectin residues in foodstuffs of both plant and animal origin.Methods 1 The poly(butyl methacrylate-co-ethylene glycol dimethacrylate)(poly(BMA-co-EDMA)) monolithic extractor was fabricated via thermally initiated polymerization in the stainless steel column(10 mm × 2.1 mm, i.d.). With the composition of the polymerization mixture fixed, the effects of the reaction time(2-24 h)and reaction temperature(55-65 ? C) on the hydrophobicity of the monoliths were investigated. 2 The optimization of on-line extraction parameters including loading solvent, eluting solvent and flow rate was conducted. The influence of the loading solvent was tested using 0.1% and 0.5% formic acid, and 5 and 10 mmol/L ammonium acetate(NH4Ac) solution. Acetonitrile(Me CN) and methanol were each evaluated for use as the eluting solvent and the flow rate(0.3-0.8 m L/min) of eluting was evaluated. 3 After the loading and washing steps, the extracted analytes were transferred from the poly(BMAco-EDMA) monolithic extractor into a C18 analytical column for LC-MS/MS separation.In this work, Me CN-0.1% NH4 OH solution and Me CN-0.5% formic acid were used as solvent for LC-MS/MS separation and the starting composition of Me CN for the gradient was investigated in the range of 80-10%. 4 The method was validated with matrix effect,specificity, limit of detection, limit of quantization, recovery and precision. The reusability and reproducibility of the poly(BMA-co-EDMA) monolithic extractors were also assessed by calculating the relative standard deviations(RSDs) of the extracted peak area of abamectin while performing the on-line SPE.Results 1 On the basis of the results, the monolithic column prepared at 55?C for 4 h was chosen for the experiment. The scanning electron microscope image demonstrated the distinct continuous porous structure of the monolith. A good linear(r=0.999)dependence of the column pressure on the flow rate was obtained. 2 The SPE was achieved using a 10 mmol/L NH4 Ac solution as the loading solvent and acetonitrile was selected for the washing step. 3 After being transferred from the monolith to a C18 analytical column using acetonitrile, the analytes were analyzed by LC-MS/MS using10% acetonitrile-90% NH4OH(0.1%) as the mobile phase. 4 For the proposed method, a good linear relationships for 5 avermectins were achieved over the concentration range of1-100 ng/m L with a correlation coefficient(r) value higher than 0.995. The detection limit was 2 μg/kg and the quantitation limit was 5 μg/kg for 5 avermectins. The recoveries in fresh pear, chili, bovine muscle and milk ranged from 71.8% to 101.3%with the relative standard deviations ≤ 8.94%. The on-line SPE and determination were achieved within 15 min and the monolithic extractor was reusable for more than 500 injections. The RSD of peak area for the first, the 250 th and the 500 th injection was 5.4%.The intra-batch and inter-batch RSDs were 6.3% and 9.3%, respectively.Conclusions A hydrophobic monolith is developed as a reusable on-line SPE sorbent coupled with LC-MS/MS for the rapid and simultaneous determination of 5 avermectin residues in foodstuffs of both plant and animal origin. An off-line process using the conventional SPE sorbents requires several steps and takes more than 2 h while the time for on-line SPE and analysis of this work is only 15 min. The results clearly indicate that the easy preparation, high mechanical strength and permeability allow the monolithic extractor to be available for on-line SPE, thus shortening the whole analysis time, and the monolithic extractor is reusable for more than 500 experiments.