Smoke from 11 brands of cigarettes was analyzed for a specific form of nicotine called "free base" that passes quickly into the bloodstream when it is inhaled.
American Spirit, a brand owned by R.J. Reynolds Tobacco Co., topped the list. It was followed by the French brand Gauloises Brunes, according to the study by Oregon Health & Science University chemist James F. Pankow. Their free-nicotine levels were around 25 to 35 times higher than those of the lowest-level cigarettes.
The free-base form of nicotine occurs naturally, but some varieties of tobacco contain far more than others.
The study adds weight to claims that cigarette makers blend tobacco varieties to manipulate the nicotine potency and boost sales, as some industry critics have charged. The tobacco industry has long claimed that it blends tobacco to adjust for taste, not to increase nicotine potency.
A spokesman for R.J. Reynolds, Seth Moskowitz, said Tuesday he had not reviewed the study and could not comment.
Acid levels in nicotine largely determine how quickly it can be absorbed. Free-base nicotine is much less acidic than other forms and thus gets to the brain more quickly.
Previous studies had measured the acid levels in cigarette smoke to indirectly test for free-base nicotine. Pankow's was the first to directly test for the chemical, according to Neal Benowitz, a nicotine addiction expert at the University of California at San Francisco.
"This is important," he said. "It's been suspected that cigarettes are manufactured in a way that optimizes the free-base."
Still, he said the study was not conclusive proof that manufacturers manipulate the level of free-base nicotine.
Pankow's study measured the first three puffs, which typically pack the biggest nicotine punch. His artificial inhaler smoked the cigarettes down to a roughly one-inch butt.
The nicotine delivered by American Spirits was 29 percent free-base nicotine in the first three puffs, and 36 percent for the rest — an exception to the general rule that the first puffs are stronger, the study showed.
Gauloises Brunes delivered an even dose of 25 percent free-base nicotine throughout the smoke.
The GPC brand made by Brown and Williamson had the lowest dose of free-base nicotine at 1.6 percent for the first puffs and 1 percent for the rest of the cigarette.
The study was published in the online version of the journal Chemical Research in Toxicology.
By Andrew Kramer