Provide low background counts and excellent light transmission. Made of high-density polyethylene (HDPE) to resist leaking and swelling caused by solvent degradation. Fit all common counters. Caps fit GPI 22-400 thread, and are suitable for marking. Vials with caps attached are shrink-wrapped in five trays of 100 per case. When caps are separate, vials are shrink-wrapped in five trays of 100 per case, and caps are packed in five polybags of 100 each.