Nicotine is a synthetic challenge for chemists. Until recently, the downside of synthetic nicotine was the cost, and the most economical way to obtain the desired enantiomer of nicotine was through the separation of the racemic mixture (RS-nicotine). Another step, recrystallization, would allow the selection of the desired enantiomer (e.g. S-) and produce S-nicotine.
The main problem with the racemic mixture is that the presence of 50% of non-psychoactive R-nicotine and 50% of psychoactive S-nicotine splits the potential activity of the oil into two parts compared to natural tobacco extracts.
Since labels only show nicotine content, a racemic mixture of synthetic nicotine with a strength of 10mg/ml is actually equivalent to 5mg/ml of e-liquid.
After purification, fresh synthetic liquid is colorless, as is nicotine extracted from tobacco leaves. The brown color of liquid nicotine is a natural process produced by exposure to light, heat and oxygen, and sometimes it also occurs in the dark, even in a sealed bottle. Nicotine coloration is purely "cosmetic" and a clearer liquid does not mean it is purer or higher quality.