Gas-liquid chromatography as a tool to determine vapor pressure of low-volatility pesticides: A critical study

The vapor pressure of pesticides at room temperature is one of the fundamental properties for predicting their volatilities and the likelihood of their movement into the environment. It is also highly valuable for decision making in agriculture and agro-industries. Direct methods applicable to low-volatile substances include gas saturation and molecular effusion techniques; however, they have several drawbacks. We propose an indirect method based on gas-liquid chromatography using short capillary columns coated with stationary phases with high phase ratio and operated at very high flow rates.

The method was applied to determine the vapor pressure of twenty pesticides, ranging from intermediate to very low volatilities (<0.133 mPa). Chromatographic assumptions were critically and comprehensively discussed, and the precision and accuracy of the method were evaluated using three fatty acid methyl esters (referred as validation solutes) with known and reliable vapor pressures available over a wide range of temperatures. The relative standard deviations of the primary data were below 4 % and increased to 18 % for extrapolated vapor pressures. The measurement conditions allowed for the extension of retention measurements to relatively lower temperatures while ensuring reasonable operational times, thereby, minimizing the extent of temperature extrapolation required. The target pesticides included widely used chemical compounds with multiple functional groups and molecules exhibiting configurational isomerism, whose individual vapor pressure could be distinguished provided that sufficient chromatographic selectivity was achieved.

This work provides a readily useable technique which employs instruments commonly found in both academic and industrial laboratories. Furthermore, the method is simple, robust, and reliable, allowing to estimate the vapor pressures of low-volatile and semi-volatile pesticides within reasonable analysis time and overcoming the main limitations associated with traditional methods.