Possible interest of various sample transfer techniques for fast gas chromatography–mass spectrometric analysis of true onion volatiles

We improved GC–MS analysis of onion volatiles by comparing organic solvent partition with solid-phase microextraction (SPME) following cryo-trapping isolation and by comparing the same extraction methods on direct onion juice. Cryo-trapping produces very small quantities of volatiles and therefore is not a suitable extraction method for GC–MS analysis. We confirm that SPME accelerates the degradation of labile thiosulfinates but the lacrymatory factor remains intact. The identification of Allium thiosulfinates is only obtained on juice extracted by diethyl ether using a fast GC–MS analysis on a 10 m X 0.3 mm column of 4 μm coating, with routine splitless injection. The lacrymatory factor is best analysed directly on fresh onion juice by SPME with the same chromatographic conditions. To characterise and to quantify all the true onion volatiles, we propose to analyse the same sample by successive SPME–GC–MS and solvent extraction–GC–MS.