18MM SCREW THREAD CAPS, MAGNETIC. HEADSPACE VIALS. The 18mm magnetic screw caps are designed for use with any autosampler using a magnet to transport the vial from station to station. The closures are Vials, 20mL Headspace vial only , Screw Cap Style, 75.5 x 22.5mm . 20mL headspace vial only, for use with HS-10, HS-20, AOC-5000, and AOC-6000.