0-农药残留基础2007-7

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1972. Modification made to analyse even more polar compounds, particularly Methamidophos (Monitor). The petroleum ether partitioning is eliminated. Water is not added to acetone to obtain better recovery of the water soluble compounds. KCITD is used to analyse P and N containing residues. ECD is used after Florisil clean-up.
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1984-1988. Development of “twodimensional” capillary GC, which involves the use of two columns of different polarity and mulitiple detectors, hence eliminating the need in extensive cleanup.
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1977-1987. Further development for certain groups of compounds. Additional clean-up on C-18 for methyl carbamates, phenylureas, and benzimadazoles followed by determination with HPLC with different detection systems. 1992. Methylation procedure for chlorphenoxy acids determined with ELCD. Halogenated heat sensitive pesticides can be determined with by HPLC using a photoconductivity detector sensitive for halogenated compounds.
• 1981. Development the flame photometric detector (FPD) which could be used in P or S mode and the Hall electrolytic conductivity detector (ELCD) which could be used in a halogen or nitrogen detection mode. 1981. Elimination of the Florisil clean-up step for determination of the chlorinated pesticides since the use of ELCD in halogen mode.
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1982. Simultaneous extraction and clean-up in chromatographic columns. Mixing or blending aqueous samples with silica gel or alumina to remove lipids prior to liquid-liquid partitioning and Florisli cleanup.
After 1990’s
• The development of practical massspectrometry, a universal selective detection technique, in combination with capillary GC and HPLC for pesticide residue analysis, which made compound identification much more reliable.
In the 1970s’ Methylene chloride partition
• Methylene chloride partition after two petroleum ether partitions. More polar pesticides such as Mevinphos can be determined. Detection with newly developed potassium chloride thermoionic detector (KCITD) for phosphate and nitrogen residues.
2 Objectives of the cleanup procedure
• • • • • • • • • • • • When a sample is extracted, the resultant extract often contains a variety of coextractives such as oils, fats, waxes and plant pigments in addition to the pesticides of interest. If these co-extractives are not removed from the extracts they can interfere significantly with the determinative step of the analytical procedure. The removal of the extraneous co-extractives from the extract is known as sample clean-up. co-extractives 对仪器系统的可能影响: affecting the efficiency of the analytical column damaging and/or affecting the response of the detection system contaminating the injection system masking true positive results producing false positive results (contamination) enhancing or diminishing the GC response (matrix effect). 进一步对分析的影响: significant instrument “down-time” where essential maintenance must be carried out to correct the effect of co-extractives on the analytical instrumentation; increased consumable costs where affected parts of the analytical instrumentation have to be repaired or replaced; incorrect data being reported where co-extractives have masked the presence of pesticides (false negative results) or produced false positives. Effective clean-up of sample extracts is thus particularly important to minimise the risk of erroneous analyses and of damage to the analytical system.
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1985. Application of Gel Permeation Chromatography (GPC) for clean-up. More than 300 pesticides were reported to be cleaned simultaneously.
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1985. Development of Solid Phase Extraction (SFE). C-18 bonded silica gel cartridges, XAD resins, Tenax-GC cartridges were applied for extraction and preconcentration of pesticides from air and water samples.
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1993. Cairns method. Use of the solid phase extraction cartridges for the clean up. The change was necessary by the change in technology which had gone form packed GC columns to open bore columns which are more easily affected by the non-volatile sample extractives that are deposited in the injector port. Two points SFE is introduced. The first uses C-18 SFE to remove nonpolar sample compounds by a reverse phase clean-up. The second uses a strong and weak anionic extraction SPE cartridge to remove acidic phenols and saccharides. Methyl carbamate, phenylureas, and benzimadazoles can be determined without additional clean-up step.