Determination of 19 Organochlorine Pesticides in Soil by Gas Chromatography Coupled with Bi-columns and Bi-electron Capture Detectors with Matrix Solid Phase Dispersion

Sample of soil was collected and mixed thoroughly. After picking out foreign matters, the soil sample was dehydrated, homogenized by grinding and sealed up to keep in a desiccator for use as analytical sample. 20.00 g of the sample were intermixed with 3.00 g of diatomite and 3.00 g of the adsorbent, Magnesol and then the sample mixture was transferred into the extraction cell upon a layer of silica-magnesium adsorbent (3.00 g) which was spread on the botton of the cell preliminarily. After spreading a layer of quartz sand and a piece of filter paper on the surface of the sample, ASE was carried out twice by the ASE extractor according to the predetermined program using a mixed solution of dichloromethane-n-hexane with the volume ratio of 1:1 as extraction solvent at 90℃. The extracts were combined and evaporated by N₂-blowing to a volume less than 1 mL, and after cooling, the volume of the solution was made up to 1.0 mL with n-hexane. 19 organochlorine pesticides (OCPs) were enriched in this solution and to be determined by the gas chromatography method. A sample injection volume of 1.0 μL was adopted in the gas chromatography procedure and 2 chromatographic columns with different polarityi.e. HP-5 LTM chromatographic column and DB 1701 LTM chromatographic column, each of the chromatographic columns was connected with an electron capture detector (ECD) were used in the gas chromatography separation. Standard curves were prepared with mixed standard solution of the 19 OCPs by gas chromatography separation and determination on each of the 2 chromatographic columns, giving same linearity ranges in 2 mass concentration sections of 5-100 μg·L^-1 and 100-1 000 μg·L^-1. Detection limits (3.143s) found for the 19 OCPs were in the range of 0.15-0.56 μg·kg^-1. If the difference between results determined by the 2 chromatographic columes for a same analyte came cut to be larger than 5 times, the result should be judged as "false positive"; and if the difference was less than 5 times, the analyte was determined to be existing, and the smaller one of the 2 data should be taken as the result of determination. Values of RSDs (n=6) found by precision test for the 19 OCPs were ranged from 2.7% to 15%. A CRM of soil (CRM 818-50G) was analyzed by the proposed method, results of the 19 OCPs found were all within the limited ranges of certified values.