Sampling techniques for gas chromatographic mass spectrometric analysis of long-chain free fatty acids from insect exocrine glands

Many features of gas chromatographic equipment can hinder the detection of free carboxylic acids, so that these polar and reactive substances do not lend themselves well to gas chromatography through adsorption and peak tailing. Therefore the normal practice is the derivatization to esters. The analysis of small quantities of insect exocrine secretions using this method is not quantitative. Factors affecting the quantitative analysis of free fatty acids have been explored. Two sampling methods, either by solid-phase micro-extraction (SPME) or the solid injector technique we developed and traditionally used, were found to be reliable if particular precautions are observed. These are the use of deactivated injection liners and glass wool in the injector. Deactivated pre- and post-columns prevent the detection of long-chain fatty acids even if the latter are present in mu g amounts. By avoiding preheating of the samples in the injector and removing the glass powder of the sample capillaries after each injection, enhanced results have been observed with the Keele injector. For the SPME, absorption from headspace and moderate sample heating temperatures showed better results compared with high-temperature sample heating. (C) 1998 Elsevier Science B.V. All rights reserved.