The Semipermeable surface, a new restricted access medium

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Abstract
The Internal Surface Reversed Phase (ISRP)-an elegant invention of Dr. Thomas C. Pinkerton–was the first example of a Restricted Access Medium (RAM). These materials allow the determination of small molecules in the presence of large biomolecules, for instance the direct injection of serum into an HPLC column without requiring prior removal of protein. A new RAM is the Semipermeable Surface (SPS), based on the doctoral research of Carla Desilets under Dr. Fred Regnier at Purdue University, developed to its present form at the Regis Chemical Company. The SPS has two phases: an outer and an inner, each independently synthesized, each covalently bound to the silica surface of the packing particle. The outer is a hydrophilic polyoxyethylene polymer; the inner is any of several common hydrophobic reversed phases (at present, C8, C18, nitrile, phenyl). The outer phase forms a semipermeable surface that prevents large biopolymers from reaching the inner phase. Small molecules, however, can and do interact with either the outer or the inner phase. They are retained by a unique mechanism–a combination of hydrogen bonding at the outer polyoxyethylene surface and hydrophobic interaction at the inner reversed phase. Aside from its experimentally variable selectivity-a completely new property-SPS also shows excellent chromatographic efficiency and roughly twice the retentivity of comparable RAM materials.