Improved gas chromatographic analysis of organophosphorus pesticides with pulsed splitless injection

Gas chromatographic (GC) analysis of 6 organophosphorus pesticides (methamidophos, acephate, omethoate, diazinon, dimethoate, and chlorpyrifos) was performed with cool on-column, splitless, and pulsed splitless injections and with nitrogen-phosphorus or mass-selective detection. The pulsed splitless technique uses a high column flow rate during injection to sweep the sample out of the inlet rapidly, reducing analyte loss due to adsorption or thermal decomposition. After injection, the column flow rate is automatically reduced to normal values for chromatographic analysis. Pesticide recoveries for splitless and pulsed splitless injections were determined by comparison of GC peak areas with those obtained with cool on-column injection. With conventional splitless injection at a column flow rate of 5 mL/min, recoveries of acephate, omethoate, and methamidophos were only 57, 63, and 71%, respectively. Pulsed splitless methods with very fast injection flow rates dramatically improved recoveries, with all 6 pesticides falling in the 97-102% range. Because column flow rates are much less for GC with mass spectral detection (GC/MS), recoveries with splitless injection were lower and improvements with pulsed splitless injection were less dramatic for GC/MS. When splitless injection was used, recoveries of the 6 pesticides spiked into a green bean matrix were better than those of pesticides dissolved in pure solvent, presumably because matrix compounds compete with pesticides for active sites in the inlet. By using pulsed splitless injection of solvent standards with very fast initial column flow rates, systematic analyte losses in the inlet were eliminated, making recoveries of pesticides from solvent and green bean matrix very similar.