Large-volume injection in residue analysis with capillary gas chromatography using a conventional autosampler and injection by programmed-temperature vaporization with solvent venting

Abstract

Large-volume injection in capillary gas chromatography may be used to compensate for the limited detection sensitivity of selective detectors when further evaporation of sample extracts is not feasible owing to increasing losses of volatile solutes or because the analytical method can no longer be performed automatically. Large-volume injection using programmed-temperature vaporization (PTV) with solvent venting has proved its worth as an on-line pre-chromatographic sample concentration technique that permits the reproducible analysis of pesticide residues over a wide volatility range. Automated sampling and injection of a 12.5-μl volume of toluene extract using an HP 7673A autosampler with a 25-μl Hamilton syringe is described. The various parameters, including design of injector inlet, speed of injection, retention gap and initial column temperature, are discussed and the optimization procedure for split flow-rate, solvent evaporation temperature, solvent venting time and splitless or transfer time is reported. A test mixture containing C 12, C 14 and C 16 hydrocarbons, 1,7-dibromoheptane, 3,4-dichloroaniline, pentachlorobenzene, hexachlorobenzene and the pesticides heptenophos, propachlor, naled, lindane, heptachlor, aldrin, tetrachlorvinphos, dieldrin, p,p′-DDE and endrin in toluene was used, representing moderately volatile to high-boiling solutes and thermolabile compounds. Complete recovery of all compounds in the test mixture was achieved with the method developed. No degradation of the thermolabile pesticides naled and endrin was observed. The precisions of quantitative determinations of 12.5-μl autosampler injections at different concentration levels were good to excellent down to concentration levels representing 1–2.5 ng per pesticide injected when applying atomic emission detection (AED). Data on standard deviations and the wide dynamic range of all test solutes are reported.