Chromatography


Following the scent to scientifi c discovery Investigating how the chemical process of heat affects the volatile ingredients of e-cigarettes Waldemar Weber, Shimadzu Europa GmbH


E-cigarettes have become increasingly popular in recent years. Much testing has been done on the raw ingredients used in creating the vapour produced by e-cigarettes. Less research has been done on what happens to those ingredients through the process of heating. A recent set of experiments using gas chromatography/mass spectrometry (GC/MS) looked into the effects of the heating process on fl avour: for instance, how certain chemicals designed to imitate a raspberry fl avour might end up tasting like something else after heating.


The result was a clever new method to better understand and predict how to create and maintain better quality control for vaping fl avors. Of additional interest, the new method also revealed the outline of a new way to test for consumer safety in the rapidly developing market for e-cigarettes.


Vaping – the use of e-cigarettes – has provoked heated discussion since commercialised electronic cigarettes fi rst appeared over 20 years ago. Proponents have argued that e-cigarettes are a pleasant and effective way for consumers to wean themselves off tobacco cigarettes. Smoking tobacco, of course, is associated with all manner of health risks, and vaping has been promoted as a healthier alternative, either as a permanent solution or as a stop on the road to complete nicotine abstinence.


At the same time, vaping opponents have argued that the health risks of e-cigarettes have not been adequately tested and that further research needs to be done. That argument continues today, with some countries simply banning vaping, others restricting the use of e-cigarettes, and many others simply allowing them and hoping for the best.


Most of the countries who do allow fairly unrestricted vaping naturally place an age limit on it, and many countries have also conducted safety tests on the individual components involved in the process, such as batteries and the chemicals being used in what they call the ‘e-liquid’. But clearly, much more testing is required, if we are to ensure that vaping is as safe as it possibly can be.


Fascinated by flavours


Meanwhile, several lab researchers were dealing with a different vaping issue. E-liquids for e-cigarettes are designed to have a specifi c fl avour. E-cigarette fl avours generally tend to imitate sweet or fruity fl avours, in large part because those fl avours appeal to younger consumers – the market of today and tomorrow. But how do you ensure that the fl avour of the vapour produced is the same as the fl avour promised by the liquid? They began to run tests using the world-class assortment of Shimadzu scientifi c instrumentation that they had at their disposal.


The focus on these tests was not on the chemicals themselves but on the chemical process at the heart of vaping. Basically, vaping uses a power source (usually a battery) to run an atomiser – a heating element – that creates an aerosol of vapour out of e-liquid in a small tank which is then inhaled. Relatively little work has been done on the effects of heating on the base chemicals, and the researcher began to look at how the raw ingredients change through the process of heating the e-liquid.


Taking advantage of state-of-the-art


testing equipment The vapourisation ratio of aroma compounds in e-liquids changes with the temperature, since the vapour pressure of each aroma compound is different. Therefore, the fl avour of an e-liquid is expected to change depending on the temperature. Gas chromatography/mass spectrometry (GC/MS) analysis is ideal for objectively evaluating the correlation between fl avours and aroma compounds.


What eventually became an effective new method started with this experiment: The aroma compounds of the e-liquid were exposed to a simple pretreatment and then analysed in an environment close to actual use conditions. A fl avoured e-liquid was employed for the sample, of which 1 mg was directly weighed in a crimp vial for a headspace sampler, using a heat-resistant crimp cap for the vial cap and a highly heat-resistant septum.


Figure 1. Shimadzu GCMS-QP2020 NX with HS-20 NX


An HS-20 NX headspace sampler was connected to a GCMS-QP2020 NX gas chromatograph/mass spectrometer, and the progress mode of a connected HS-20 NX was selected. Finally, analytical conditions specifi c to the Smart Aroma Database were used. During batch analysis, the oven temperature was changed step by step by selecting the progress mode, running analyses at 20°C-increments at temperatures ranging from 150°C to 270°C.


Heating things up


As a result of this, 47 aroma compounds were identifi ed by the Smart Aroma Database. Next, in order to investigate the correlation between the peak area of each aroma compound and the temperature, the area value was standardised, and cluster analysis was performed using the ‘R’ statistical analysis software. The heat map obtained as the result of the cluster analysis is shown in Figure 2. Here, it is apparent that the vapour pressure of e-liquid increases with rising temperature. Therefore, the peak area value in Figure 2 is corrected by the dilution rate with the pressurised gas. The dilution rate by the pressurised gas was calculated based on the measured internal pressure of the vial.


Figure 2 also shows the correlation between the area value of each compound and the oven temperature. As one can see, the correlation can be confi rmed at a glance. Overall, it can also be observed that the amount of aroma compounds in the generated gas tends to increase with rising oven temperature. In particular, there is a signifi cant tendency for this to increase from 210°C. On the other hand, the tendency of the area of aroma compounds to change depends on the variety of compounds when viewed in detail.


Changes in aroma characteristics


Figure 3 shows the temperature dependence of the quantitative ion peak areas of raspberry ketone and benzonitrile. These two compounds are shown in the red frame in Figure 2. The area values in Figure 3 have been corrected to take into consideration the dilution ratio of the pressurised gas, as in the case of Figure 2.


INTERNATIONAL LABMATE - JULY 2025


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