Vapouround magazine Vapouround Magazine Issue 26 | Page 55

blue, while under a pH of <6.0 it will appear a straw yellow. What other methods are there to test e-liquids? Nicotine is basic, which means it can receive protons (H+ atoms) whereas an acid can donate protons. So, by adding an acid, nicotine accepts protons, forming what is known as a salt. High Pressure Liquid Chromatography (HPLC) For example, with hydrochloric acid, nicotine will form a nicotine HCl salt. As the nicotine is more ‘reactive’ than bromothymol blue, it will consume all the H+ ions fi rst. Once all the nicotine has been converted to its salt form, excess H+ ions are then free in the solution to react with the indicator compound. Due to the chemical structure of this compound when it reacts, it changes colour from a deep blue to straw yellow, so we can determine at that point that all the nicotine has been neutralised. This process is also reversible: if we added more nicotine, the solution would turn back to blue, as nicotine is able to ‘pull’ the protons from the indicator, reversing the colour change. Problems can occur where the liquid under study contains acidic flavourings. This will affect the overall accuracy of the result. Bear in mind, this method would not work at all in the analysis of nicotine salts. Salt-based nicotine has already been neutralised and so will not react with an acid. In this instance, a method using a basic titrant such as sodium hydroxide could be used instead. Although this is a very basic description of titrations, more complex systems exist, which can determine nicotine content without the addition of water or an indicator and is determined potentiometrically. In this method, a portion of the substance under study is solubilised into a solvent (mobile phase) and passed through a column (stationary phase) while under pressure. Depending on the type of column and solvent utilised, the individual components separate at different rates and can then be detected and quantifi ed. After passing through the separation column there are varying methods that can be used for detection and quantifi cation of the components. Methods of detection include UV-VIS, Mass Spectrometry (MS) and fluorescence. The detection method used is dependent on the physical and chemical properties of the components being examined. This method is not just limited to nicotine. HPLC analysis can be used to detect other impurities in e-liquids, such as aldehydes, nitrosamines and other related substances. GC-MS Gas chromatography coupled with a mass spectrometer as the detector. GC is useful for volatile and thermally-stable organic compounds. This method uses an inert gas (such as helium) as the mobile phase. GC-MS is a commonly employed and reliable method for testing nicotine content and levels of other impurities. Analysis of e-liquids Titration, HPLC and GC-MS are the main techniques most commonly used. However, there are many more techniques than can be used in e-liquid analysis, such as infrared spectroscopy for nicotine, Karl Fischer analysis for water content and even NMR (nuclear magnetic resonance.) For TPD submissions, a combination of GC-MS and HPLC are used to get a full analysis of an e-liquid’s composition. Techniques such as titration, while limited, are useful for rapid analysis of nicotine content in e-liquids in production situations before having third party analysis carried out for verifi cation. VM26 49