Abstract:
One of the major problems encountered in qualitative and quantitative determination of
residual pesticides by gas chromatography is the matrix effects. Matrix components
have a considerable effect on the way analysis is conducted and the quality of results
obtained, introducing problems such as inaccurate quantification, low analyte
delectability and reporting of false positive or even false negative results. It was aimed
to develop and validate a suitable method for counteracting the matrix effects so as to
improve the detection and quantification of selected organochlorine pesticide residues
from real water samples. The real water samples used were sampled from three points
along the Jukskei River catchment area in Gauteng, South Africa for a period of 7
months from January to July 201 0 so as to create a representative sample.
An automated solid phase extraction (SPE) method coupled to Gas ChromatographyMass
Spectrometry (GC-MS) method for the analysis of 20 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. Endosulphan beta, 4,4'
Dichlorodiphenyldichloroethane, and Heptachlor-epoxide had the broadest linear
calibration ranges of 1 ppm- 0.0156 ppm. Benzene hexachloride (BHC) delta and
Lindane had the lowest statistical limits of detection of 0.018 ppm. Statistical hypothesis
testing indicated that there was significant linearity in all selected organochlorine
calibration curves.
Four different reversed sorbent phases, including LC18, SC18- E and Strata-X (styrene
divinyl benzene) were tested for organochlorine retention efficiency. The LC-18 200 mg
cartridge proved to be the most robust and effective sorbent phase as it produced better
recoveries varying from 90-130% for most analytes. A breakthrough volume of 100 ml
for the LC-18 200 mg cartridge was determined using an optimum matrix load curve. 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.
Four different calibration methods, namely matrix-free external standard, matrixmatched
external standard, matrix-free internal standard and matrix-matched internal
standard were applied to test the efficiency of computing recoveries. All calibration
curves for the 20 organochlorine pesticides showed significant linearity > 0.99 when
plotted on both Chemstation and Excel. The calibration methods were tested on three
different matrices composed of a high sample matrix (synthetic matrix), a low sample
matrix (real sample matrix) and a no sample matrix (ultrapure water).
Statistical hypothesis testing led to the decision that there are significant differences
between the mean recoveries of the three water sample matrices and also that the
differences in the mean recoveries of the three sample matrices are independent of the
both the two calibration techniques (internal standard and external standard) and
calibration types (matrix-matched and matrix-free) applied. This led to the overall
conclusion that the matrix effects have an overwhelming influence on the selective
determination of the selected organochlorine pesticides.
