Single-walled carbon nanotubes are coated in soap-like molecules called surfactants, then spun at tens of thousands of rotations per minute in an ultracentrifuge. The resulting density gradient sorts the nanotubes according to diameter, twist and electronic structure.

Full story at:
http://nsf.gov/news/news_summ.jsp?cntn_id=108074

Also featured in:
Science Daily
http://www.sciencedaily.com/releases/2006/10/061004180041.htm

and Chemical Principles, by Steven Zumdahl