Now the electronic cigarette industry has had quality issues but nicotine strength was not one of the problems that seemed to have serious issues. Yes, if the cartridge stated 16mg it might be +/- 2mg. According to an unofficial test done on another forum the nicotine strength can vary quite a bit.
http://www.e-cigarette-forum.com/forum/diy-e-liquid/239728-houston-we-have-problem-nic-titration-results.html#post4564211
Now what does this mean?
1) Buy cartridges and not liquid – companies who sell cartridges naturally need to perform more Quality Assurance than e-liquid sellers. It takes more steps and hence better Quality Assurance checking to manufacture ecig cartridges.
2) If you must buy e-liquid, give it a few test puffs before you blow away like a chimney. If that nic strength is 100 mg instead if 16 mg your brain will get the few minutes it needs to warn you of impending disaster. Remember nicotine may be ok in small quantities but in larger strength it is absolutely fatal. In fact a nicotine overdose is more lethal than cocaine one.
http://en.wikipedia.org/wiki/Nicotine_poisoning
Customers shouldn’t have to worry about this. Companies should. The reality is that many electronic cigarette companies rely totally on their vendors and focus on the marketing aspects. Top ecig companies included.
As your ecig writer/contributor I will follow up by contacting ecig companies regarding how they test their nicotine strength and will keep you updated.