Abstract: To quantitatively measure the distribution of nicotine forms in e-liquids, a liquid-liquid extraction-GC-MS method was developed to quantify the levels of free-base and protonated nicotine in e-liquids. The mass fractions and distribution of free-base and protonated nicotine in propylene glycol-glycerol (PG-VG) samples and commercial e-liquid samples were studied. The results showed that: 1) The dichloromethane-H₂O liquid-liquid extraction could be used to quantitatively measure the distribution of free-base and protonated nicotine in e-liquids with good adaptability. 2) In laboratory PG-VG systems, the free-base nicotine fraction (α_fbn) was not affected by PG/VG ratio and the amounts of nicotine salts present, however it significantly positively correlated to the pH value of the system. 3) The α_fbn significantly positively correlated to the pH values of the e-liquids with same flavor formula, but no obvious correlation was found among the e-liquids with different flavor formulas. 4) The nicotine salts in the e-liquids did not present in protonated state totally, in fact they formed a dynamic equilibrium between free-base and protonated nicotine depending on flavor formula and the method of nicotine addition.