During storage and when subjected to heat, unsaturated fatty acids of edible oils undergo oxidation forming hydroperoxides and secondary volatile oxidation products [1]. Oxidation has a great impact on the quality of food products because of off-flavours, of a decrease in the nutritional properties and especially because of the formation of toxic compounds and can increase propensities to various cardiovascular and metabolic diseases [2]. Thus there is a great interest on fast and simple detection of lipid oxidation. The evaluation of the oxidation level can be based on the quantity of hexanal as it is the main secondary oxidation product of linoleic acid which is one of the principle fatty acids of many edible oils [3].
The most complicated step of the determination of volatile compounds in complex matrices is the separation of analytes from the matrix. Traditional analyte separation methods (liquid extraction, Soxhlet extraction) require a lot of time and labour, moreover, there is a risk to lose the volatile analytes. In recent years, headspace gas chromatography has been used to determine the volatile compounds. To date, traditional extraction solvents have been used in headspace gas chromatography. As a rule, such solvents are rather volatile; therefore, a large amount of solvent vapour enters into the headspace together with the analyte. Because of that the determination sensitivity of the analyte is reduced, a huge solvent peak in the chromatogram can overlap with the peaks of the analyts. The sensitivity is also limited by the fact that the sample can't be heated at a higher temperature than the solvent boiling point.
Hexanal in fat-rich food can be determined by static headspace gas chromatography (SHS-GC). It includes a partial transfer of the volatile compounds into the gaseous phase and subsequently to GC system, meanwhile high molecular weight non-volatile molecules remain in the sample. For SHS extraction a solvent should be used to disperse the matrix and to prepare calibration solutions of hexanal. To promote the transfer of the volatile analyte into the headspace, the sample should be heated. Heating temperature should not reach the boiling point of the solvent, otherwise the vapour pressure becomes too great and leakages can occur or the vial could even burst. In order to increase heating temperature and to drive the equilibrium in favour of the gas phase, nonvolatile solvent should be advantageous.
In this work we suggest to replace traditional headspace gas chromatographic solvents by non-volatile, eco-friendly, biodegradable, inexpensive, easy to prepare deep eutectic solvents (DESs).
Eight hydrophilic and hydrophobic DESs have been synthesized and the influence of the temperature on their headspace gas chromatographic behaviour has been investigated. All the DESs studied were resistant to heating at 80 °C temperature. Among the solvents studied, DES composed of choline chloride, glucose and water showed the highest hexanal extraction efficiency. Using this solvent, headspace gas chromatographic conditions have been optimized for the determination of hexanal in potato chips. Under optimized conditions quality parameters of the prepared technique have been determined. The suggested technique was applied for the determination of hexanal in fat-rich food.