Many potential applications for carbon nanotubes depend on the lengths of the microscopic cylinders, said National Institute of Standards and Technology scientists. And the researchers said one of the most important features of their new technique is that it should be easily scalable to produce industrial quantities of high-quality nanotubes.

In 2006, researchers discovered nanotubes could be separated by "chirality" -- a measure of the twist in the carbon atom sheet -- by spinning them in a dense fluid in an ultracentrifuge tube because of a relationship between chirality and buoyancy.

In the new study, a team of NIST researchers demonstrated a variation of the same technique can separate nanotubes by length.

"When we spin the centrifuge, it turns out that the longer ones move faster," said researcher Jeffrey Fagan. "We basically just run a race and the longer ones move farther in the same amount of time. Eventually they get separated enough in position that we can just pull off layers and get different lengths."

The new technique is reported in the journal Advanced Materials.