SPE is a form of chromatography, and, as with other chromatographic techniques, control of flow rate is critical for maintaining reproducible extractions. Unlike other vacuum manifolds, the Visiprep system contains a patented valve system that allows for precise flow control through each SPE tube via rotating, independent, screw-type valves situated in each port within the manifold cover. Visiprep vacuum manifolds allow you to process up to 12 (12-port version) or 24 (24-port version) SPE samples simultaneously. Spacing between ports are slightly smaller on the 24-port versions than the 12-port versions so consideration of tube sizes is recommended.