Quantitative analysis of e-liquid solvent using ATR-FTIR and GC-FID: Method validation and regression modeling

The growing electronic cigarette market faces significant quality control issues regarding e-liquid compositions. Fast analytical methods are required to verify the proportions of the primary solvents, vegetable glycerin and propylene glycol. This study used Attenuated Total Reflectance Fourier Transform Infrared spectroscopy with partial least squares regression to quantify glycerin and propylene glycol in commercial e-liquids. Gas Chromatography with Flame Ionization Detection served as the reference method. A matrix-matched calibration model, built with real commercial samples, was developed to effectively account for spectral interferences. The optimized PLS model achieved high predictive accuracy (R² = 0.9995), strongly correlating with the chromatography results. Crucially, both techniques exposed severe discrepancies between the measured glycerin/propylene glycol ratios and the manufacturers declared labels. These findings validate this spectroscopic approach as a robust, non-destructive, and low-cost tool for the routine quality control and rapid screening of mislabeled formulations. Future work should expand the calibration dataset with a larger set of commercial samples to further enhance the model's universal applicability.