The presence of a sample matrix is one of the most important practical considerations in gas chromatography analysis as there are potentially numerous problems associated with matrix based injections. This paper aims to highlight the distinction between blank sample analysis and real sample analysis using automated solid phase extraction (SPE) and gas chromatography-mass spectrometry. Four reversed sorbent phases, including a Supelco LC-18, Strata C-18-E and Strata-X (styrene divinyl benzene) were used for SPE method development using an automated Gilson GX-271 Aspec TM liquid handling instrument to determine the best solid phase and treatment for optimum organochlorine determination. The method developed proved to be valid when tested against parameters such as calibration range, coeffi cient of regression, linearity, repeatability and sensitivity. The StrataX and LC-18 cartridges produced the best recoveries, varying between 90% and 130% for most analytes. The LC-18 was selected for further analysis of the matrix effects as it showed greater reproducibility and method parameter robustness. Various real matrix sample volumes were tested on the selected LC-18 cartridge to determine its optimum maximum matrix load for effi cient recoveries (breakthrough volume equivalent). A 100 ml sample volume was determined as the optimum matrix load volume as it produced more precise recoveries than other spiked sample matrix volumes. Visual comparison and analysis of selective ion monitoring chromatograms of both matrix based and matrix-free extracts indicate that there are signifi cant matrix effects potentially capable of adversely affecting the chromatographic system from producing accurate identifi cation and quantifi cation of target analytes.

An automated solid phase extraction (SPE) method coupled to GC-MS method for the analysis of selected organochlorine pesticides was developed and validated for the purposes of studying the matrix effects. The analytical method showed a significant degree of validity when tested against parameters such as linearity, repeatability and sensitivity. Four different reversed sorbent phases, including a Supelco LC18, Strata C18-E and Strata-X (styrene divinyl benzene) were tested for organochlorine extraction efficiency. The LC-18 proved to be the most robust and effective sorbent phase as it produced better recoveries varying from 90-130% for most analytes. It was then concluded that the method developed was suitable for further research towards the influence of the matrix on selective determination of the selected organochlorine pesticides.