These 20mL 20mm Crimp-Top Headspace Vial are the best choice for GC. 20mL 20mm Crimp-Top Headspace Vialmade of borosilicate glass. 20mL 20mm Crimp-Top Headspace Vial ND20. The 20mm Crimp Top Aluminum Cap produced by Aijiren has a 9.5mm center hole. This large opening is used to facilitate the sampling of the autosampler.