Plant-Derived and Synthetic Nicotine in E-Cigarettes: Is Differentiation with NMR Spectroscopy Possible?

To circumvent regulatory frameworks, many producers start to substitute plant-derived nicotine (tobacco-derived nicotine, TDN) by synthetic nicotine (tobacco-free nicotine, TFN) in e-cigarette products. Due to the higher costs of enantiomeric synthesis and purification of TFN, there is a need to develop an analytical method that clearly distinguishes between the two sources. To trace nicotine's origin, its enantiomeric purity can be postulated by ¹H NMR spectroscopy using (<i>R</i>)-(-)-1,1'-binaphthyl-2,2'-diyl hydrogen phosphate (BNPPA) as a chiral complexing agent. Low-field (LF) NMR conditions were optimized for this purpose even using a small amount of e-liquid sample (limit of quantification 8 mg/mL nicotine). All investigated products were found to contain one isomer (most likely (<i>S</i>)-(-)-nicotine). A direct ¹³C NMR method at natural abundance has been validated to differentiate (<i>S</i>)-TDN and (<i>S</i>)-TFN in e-cigarettes produced using nicotine of different origin. The method is based on calculation of the relative ¹³C content of 10 C-positions of the nicotine molecule with intraday and interday precisions below <0.2%. The method was applied to 12 commercial e-cigarette products labeled as containing TDN and TFN. Principal component analysis (PCA) was applied to the relative peak areas to visualize the difference between studied products. The LF ¹H NMR method is a good alternative to expensive high-field NMR to differentiate between a racemate mixture and single optical isomers, whereas only high-precision ¹³C NMR can be used to distinguish (<i>S</i>)-TDN and (<i>S</i>)-TFN in e-cigarettes after appropriate sample extraction.