Single wall carbon nanotubes (SWNTs) may be metallic or semiconducting, and may have varying diameters and lengths. Methods of forming SWNTs may produce mixtures of metallic and semiconducting SWNTs with a range of diameters and lengths. Applications using SWNTs may be improved if SWNTs of uniform conductivity are provided, for example all semiconducting SWNTs or all metallic SWNTs. Accordingly, a method and apparatus for sorting SWNTs is desired.