Polypropylene caps are chemically inert and suitable for most chromatography applications Shop Fisherbrand 18mm PP Screw Cap, Black, Center hole, 18-400 thread Fisherbrand™ 18mm PP Screw Cap, Black, Center hole, 18-400 thread: Autosampler Vial Caps and Closures Autosampler Vials, Caps, and Closures | Fisher Scientific