Studies On The Application Of Microwave-assisted Steam Diffusion In Analyzing Complex Matrix Samples

With the development of economy and society, the environment and food, whichare related to human health so close, have been regarded as going bad by variousdegrees of pollution. In most cases, the analysis of environment and food samples isdifficult as they are complex matrix samples, which are not suitable for beinganalyzed by the analytical instruments directly. Therefore, sample pre-treatmentprocesses are essential for complex matrix samples. In most of the samplepre-treatment processes, toxic organic solvents are used as the extractant, such asmethanol, acetonitrile, chloroform, ethyl acetate, dichloromethane, etc. The use ofthese toxic organic solvents may not only cause environmental pollutions, but alsothreat the health of analysts. Thus,“green extraction” methods have been paid moreand more attention.Microwave assisted extraction (MAE) was a commonly used sample preparationtechnique for extraction of organic contaminants from solid matrix samples. Due tothe advantages of short extraction time, high extraction efficiency and small amountof sample, MAE has been widely used for environmental pollutants analysis,pharmaceutical analysis, food analysis and natural products analysis. As one of thetrends in sample pretreatment is the elimination or minimization of organic solvents,some modified MAE processes utilizing water steam as extractant have been reportedrecently. Microwave assisted steam distillation, which is regarded as a modified MAEprocess, has been developed for the extraction of organochlorine pesticides andpyrethroids in Chinese teas. Additionally, another modified MAE process namedmicrowave steam diffusion (MSD) has been developed by Chemat and co-workers forthe extraction of essential oils from lavender flowers. In MSD method, steam does notabsorb microwaves and only sample is submitted to the microwave irradiation,resulting in a higher extraction temperature and hence increased extraction rates. Moreover, downward steam is used which allows the extract to diffuse outside thesample and drop out the extraction vessel.In this study, the application of microwave-assisted steam diffusion (MSD) inanalyzing complex matrix samples was investigated, MSD was combined with othersample pretreatment methods for the extraction of pesticide residues from differentenvironment and food samples,such as solid-phase extraction and disperseliquid-liquid microextraction. The main contents are as follows:1. Carbamate pesticides were extracted by microwave assisted steam extraction(MASE), and the extract was immediately applied on SPE cartridge for clean-up andconcentration. The eluate containing target compounds was finally analyzed by highperformance liquid chromatography with mass spectrometry. The parametersaffecting extraction efficiency were investigated and optimized. Under the optimizedconditions, the limits of detection ranging from1.1to4.2ng·g-1were obtained.Recoveries of eight carbamate pesticides ranged from66%to117%at three spikedlevels, and the inter-and intra-day relative standard deviation values were less than9.1%. Compared MASE-SPE with traditional methods, the extraction time wasshortened and the organic solvent consumption was reduced.2. The combination of microwave-assisted steam diffusion and disperseliquid-liquid microextraction was investigated, and the proposed method was appliedfor the extraction of triazine pesticides from tomatoes. Steam was produced undermicrowave irradiation, and the downward steam passed through the extraction vesselimmediately. After condensation of the extract, certain amount of acetone andchloroform was added to the aqueous phase as dispersion agent and extractant,respectively. Thereby, a cloudy solution is formed and the triazine pesticides inaqueous phase were extracted into the fine droplets of chloroform. After centrifugingat3500rpm for3min, the fine droplets of chloroform are sedimented in the bottom ofthe conical test tube. Then the chloroform was separated and vapored to dryness,200μL methanol was used to redissolution for HPLC-MS analysis. The parametersaffecting extraction efficiency were investigated and optimized. Under the optimizedconditions, the limits of detection ranging from0.35to0.79ng·g-1wereobtained.Recoveries of eight carbamate pesticides ranged from62.1%to108.1%atthree spiked levels. The proposed method combined the advantages ofmicrowave-assisted steam diffusion and dispersive liquid-liquid microextraction, which not only reduces the consumption of organic solvents, but also shortens thetotal sample pre-treatment time.3. In this study, a green, simple and sensitive sample pretreatment method wasdeveloped for the extraction of triazine herbicides from soil samples. The proposedmethod was based on the on-line coupling of microwave steam diffusion (MSD) withsolid phase extraction. Triazine herbicides in soil were extracted by MSD under themicrowave irradiation and then directly onto the SPE column. After being desorbedfrom the column, the triazine herbicides were analyzed by liquid chromatography-mass spectrometry. Critical factors influencing MSD efficiency, such as microwavepower, irradiation time, steam rate, were optimized using response surface designexperiments, and other related parameters were optimized subsequently. The limits ofdetection of6triazine were ranged from0.25to0.59ng·g-1. Recoveries ranging from80.3%to110.7%were obtained while the intra-and inter-day relative standarddeviation was less than8.3%. Compared with traditional extraction methods, theMSD process totally eliminated the use of organic solvent, moreover, the on-linecoupling of MSD with SPE were less labor intensive and reduced the lose of analytes.Through the investigation of the three sections, we have gradually built up agreen, fast and simple sample pretreatment method, microwave steam diffusionon-line coupled with solid-phase extraction, for the extraction of pesticides fromcomplex matrix samples. The extraction, purification and enrichment step arecompleted in one step, moreover, the consumption of organic solvents was reducedand the total sample pre-treatment time was shortened. In a word, the proposedmethod showed great promise on the extraction of pesticides in complex matrixsamples.