Colorimetric assay for the rapid determination of free-base nicotine in e-liquid

Nicotine exists in e-liquids primarily as the monoprotonated form and free-base form, the former is absorbed by the smoker relatively slowly and the latter is considered the bioavailable form of nicotine. Nowadays e-liquid manufacturers tend to increase nicotine in smoke aerosols, up to a content comparable to conventional cigarettes. Organic acids are added to suppress nicotine in free-base from, because the quick absorption of free-base nicotine (FBN) by the upper respiratory tract produces more bitterness and harshness to smokers. Although several methods have been developed to access FBN in conventional cigarettes or electronic cigarettes, spectrometric methods have rarely been reported. A water-solubility indicator Alizarin Red S (ARS) was introduced for the measurement of free-base nicotine. Since ARS exhibits lower acidity than organic acids, it does not compete for the tertiary amine with organic acids, but can only interact with FBN. The ARS turns from pale yellow to pink once it has been deprotonated by nicotine, and the binding constant between ARS and nicotine was determined to be 1.08 × 10⁶ M^-1. A linear calibration curve A = 0.0056c + 0.3309 with r² = 0.9984 as a function of FBN was constructed, and applied for the evaluation of FBN in prepared e-liquid samples, with RMSE 1.12 mg g^-1 for the 20 mg g^-1 liquids, and 1.37 mg g^-1 for the 50 mg g^-1 liquids. The evaluation of FBN in commercial e-liquids agreed well with published e-liquid values. It is believed that the convenient method herein developed will be useful for manufacturers to balance the strength and harshness levels of nicotine in e-liquids.