Determination of untargeted compounds in e-liquids using high-performance liquid chromatography-time of flight-mass spectrometry (LC-ToF-MS)
Techniques like headspace-solid phase microextraction-gas chromatography-mass spectrometry (HS-SPME-GCMS) can identify a wide range of volatile and semi-volatile compounds in e-liquids associated with flavor active compounds of different classes. The objective of this study was to determine what additional, or characteristic, compounds of the e-liquid can be identified using LC-ToF-MS.
The ToF-MS was operated with electrospray and atmospheric pressure chemical ionization (ESI; APCI), both in positive and negative modes. Operating parameters included an acquisition mass range of 70-750 m/z, typical of most small molecules. Samples were prepared using 100 mg of e-liquid diluted with 25 mL of 5 mM ammonium acetate and isotopically labelled compounds acting as internal standards. This ratio provided a balance between sensitivity for detection of trace-level analytes, and the risk of detector or column saturation from compounds in greater abundance. Five replicates of four e-liquids were evaluated.
Preliminary evaluation of positive mode ESI shows approximately 200 features, or distinct compounds, between masses 70-300 da. Many additional features, including hundreds of features above 300 m/z, typical of phthalates or other common plastic contaminates, were detected. These are suspected to have originated with sample preparation materials. Negative mode ESI shows > 400 distinct features between 70-400 m/z. Many of these are isomeric compounds corresponding to chemical formulas typical of modified sugars such as sugar alcohols and acids, as well as, C6-C18 fatty acids. Positive mode APCI shows approximately 60 distinct features between 70-500 m/z while negative mode shows approximately 150, over the same range using a lower corona voltage condition. Few features above 500 m/z were detected.
LC-ToF-MS can identify different compounds present in e-liquids compared to HS-SPME-GCMS, based on the mode of operation of the ToF-MS. The combination of both techniques is useful to determine characteristic profiles of the e-liquids. This can be a valuable tool to identify changes in the characteristics of an e-liquid or product.