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Study of matrix-induced effects in multi-residue determination of pesticides by online gel permeation chromatography-gas chromatography/mass spectrometry.

RATIONALE
Multi-residue determination of pesticides in a complex matrix by online gel permeation chromatography-gas chromatography/mass spectrometry (GPC-GC/MS) is increasingly concerned for its high throughput, efficiency and accuracy. Study of matrix-induced suppression or enhancement of the instrument signals is important for the application of this technique.


METHODS
Matrix suppression and enhancement effects for 176 pesticides were studied by online GPC-GC/MS and post-extraction addition. Suppression effects induced by co-eluted compounds were examined in the matrices tea and human breast milk. The prepared matrix was studied to confirm the results by deliberate addition of phthalates or caffeine to the matrix of Chinese cabbage.


RESULTS
Both suppression and enhancement effects induced by co-eluted matrix were found. The different results may depend on the concentration of the matrix. Enhancement effects for carbamate and polar pesticides were found because of the blocking of active sites in the instrument by the large volume injection (LVI) technique combined with on-column and programmed temperature vaporization (PTV) mode. The results can explain the significant difference in instrument signals between isomers or native and isotope labels in some matrices.


CONCLUSIONS
Online GPC-GC/MS with combined PTV and on-column LVI mode is presumed to be applicable for the multi-residue method including the studied pesticides if calibrated by matrix-matched standards. However, further cleanup steps should be deloped to remove co-eluted matrix if remarkable suppression effects are found.