Septa: Cross-Slit, Low Bleed, Non-Slit, Polyethylene, Pre-Slit, PTFE Only, PTFE/Silicone Rubber/PTFE, White Silicone Rubber/Red PTFE, TEF or Y-Slit. Soft-Guard™ Caps - Our most popular screw caps. Highest quality polypropylene caps with a proprietary blend of silicone rubber designed to minimize resistance for easy needle piercing, to reseal more and to prevent damage to the autosampler.