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.
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