Is surface-enhanced Raman spectroscopy (SERS) a good alternative to separation techniques for nicotine dosage in e-liquid boosters?

Abstract

Since 2014, electronic cigarettes must follow the European Directive on tobacco products. In Belgium, the transposition of this directive requires that nicotine-containing e-liquid boosters cannot exceed a concentration of 20 mg mL⁻¹ to ensure consumers safety. Nowadays, accurate analytical methods available to measure nicotine levels in e-liquid products involve chromatography. The development of alternative analytical tools being faster, greener and adaptable to in-field analyses are therefore required. Surface-enhanced Raman scattering is a spectroscopic technique that significantly enhances inherent Raman scattering signals, improving detection limits, when analytes are adsorbed onto metallic nanostructures such as gold nanoparticles (AuNPs). This study introduces new SERS methods for quantifying nicotine in e-liquid boosters using two different Raman spectrophotometers based on a transmission (SETRS) and a backscattering detection mode. The transmission Raman spectrophotometer has a better sample representativity, which is very interesting to perform SERS on liquids samples, and an autosampler offering facilities for routine analyses as a benchtop equipment while the second spectrophotometer was a handheld Raman device allowing to expand the use of the developed SERS method to in-field analyses. These SERS analyses were performed using lab-synthetized AuNps and by adding an isotope-edited internal standards (IEISs) being nicotine-d4 to mitigate some repeatability issues. These methods were finally validated according to the ICH Q2 (R2) guidelines for a working range from 100 to 300 µg L⁻¹ of nicotine concentrations using a total error risk-based approach considering the acceptance limits fixed at 15 % and a risk level of 5 %.