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 detectio n of free carboxylic acids, so that these polar and reactive substance s 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 secret ions using this method is not quantitative. Factors affecting the quan titative analysis of free fatty acids have been explored. Two sampling methods, either by solid-phase micro-extraction (SPME) or the solid i njector technique we developed and traditionally used, were found to b e reliable if particular precautions are observed. These are the use o f deactivated injection liners and glass wool in the injector. Deactiv ated pre- and post-columns prevent the detection of long-chain fatty a cids even if the latter are present in mu g amounts. By avoiding prehe ating of the samples in the injector and removing the glass powder of the sample capillaries after each injection, enhanced results have bee n observed with the Keele injector. For the SPME, absorption from head space and moderate sample heating temperatures showed better results c ompared with high-temperature sample heating. (C) 1998 Elsevier Scienc e B.V. All rights reserved.