SPE is a form of chromatography and is used in other chromatographic techniques. Control of flow rate as well as repeatable extractions is critical for the preservation. Unlike other vacuum manifolds, Visiprep system located at each port in the manifold cap along each SPE tube field flow, precise flow control through independent, screw valves a patented valve system. Visiprep vacuum manifolds, 12 (12 port version) or 24 (24 port version) SPE samples at the same time allows you to Space between ports, 24 connections port versions are slightly smaller than the 12 port versions tube sizes are recommended.