Since its introduction more than 10 years ago, the e-cigarette has become a commonly used consumer product. To guarantee the users safety, analytical characterization of the refill liquids (e-liquids) used for e-cigarettes is essential. Different techniques have been used for the qualitative and quantitative chemical characterization of e-liquids of which gas-chromatography is the most often used technique as e-liquids mainly consist out of (semi)-volatile ingredients (i.e. nicotine and flavourings) dissolved in a propylene glycol and glycerol matrix. The e-liquid matrix is viscous and greasy which may result in a faster deterioration of the GC-columns and deactivate the interaction with the active groups. To overcome this issue, a proper sample preparation is needed, to improve the life span of the column and reduce the total number of cuts in the GC-column.
The aim of our study is to quantify potential harmful flavourings used in e-cigarettes using GC-method with an appropriate sample preparation. The group of flavourings are divided in two groups: high and low semi-volatiles. High volatile flavouring group contains the flavourings diacetyl and acetylpropionyl. The low and semi-volatiles contain potential genotoxic flavourings, selected through in-silico genotoxicity screening of 436 identified components in 77 e-liquid samples. These components are pulegone, estragole, 5-methylfurfural, 2-furylmethylketon, methylnaphthalene, trans-menthone and trans-2-hexanal.
For the high volatiles flavourings headspace is considered to limit the transfer of the matrix to the column. The headspace temperature was chosen in order to obtain an optimal transfer of the target components to the headspace with minimal transfer of the matrix components. The e-liquid was dissolved in water, to shift the equilibrium of the target components towards the headspace and to enhance the retention of the matrix components. The optimal headspace temperature was set at 85°. For the low and semi volatiles, full evaporation conditions (145°C) of the headspace were considered. In this case a sample clean-up is needed to avoid transfer of the matrix components to the column. Different sample clean up techniques were tested. Finally the liquid-liquid extraction (LLE) with freeze out using flash cooling technique was optimized. This sample preparation techniques can also be used with other GC-injection techniques. Both methodologies were further optimized and validated for quantification of the target components using matrix-matched calibration
The sample preparation of most quantification methods for e-liquids only consisted of simple dilute and shoot methodologies. In this study, we successfully developed a clean-up technique to eliminate most of the matrix components. The sample preparation of LLE with freeze out using flash cooling technique is a sample preparation which could be applied and optimized for other target components in e-liquids.