Screw caps used on chromatography vials are either 8mm, 9mm, or 10mm, with 9mm being the most popular size. Vials can have a standard opening (5mm) or wide-mouth opening (6mm) depending upon the cap type to be chosen. Always choose a vial that is appropriate for the specific instrument in use. For example, 11mm crimp and 9mm screw-cap vials