Internal-surface reversed-phase packings

George, G. D.and Stewart, J. T. 1990. The use of internal surface reversed-phase packing for the solid phase extraction of drugs from serum, J. Liquid Chromatog., 13 3861-3889. 

Restricted access packing materials (RAMs) have both hydrophobic and hydrophilic sites. Also called internal-surface reversed-phase packing materials, RAMs have two functions the surfaces of particles (the end groups of bonded... 

Besides the above differentiation, restricted-access media can be further subdivided on the basis of the topochemistry of the bonded phase. Packings with a uniform surface topochemistry show a homogenous ligand coverage, whereas packings with a dual topochemistry show a different chemical modification of the pore internal surface and the particle external surface (114). Restricted-access media of the former type are divided into mixed-mode and mixed-function phases, bonded-micellar phases, biomatrix, binary-layered phases, shielded hydrophobic phases, and polymer-coated mixed-function phases. Restricted-access media of the latter type include the Pinkerton s internal surface reversed-phase, Haginaka s internal surface reversed-phase diol, alkyl-diol silica, Kimata s restricted-access media, dual-zone phase, tris-modified Styrosorb, Svec s restricted-access media, diphil sorbents, Ultrabiosep phases. Bio Trap phases, and semipermeable surface phases. 

Despite their distinct advantages, on-line SPE and column-switching proce-dures do not always represent ideal separation techniques. In many cases, only a small number of samples can be analyzed before contamination of the precolumn by proteins occurs. Alternative techniques that prevent the adsorption of macromolecules onto column packings and allow direct injection of sample extracts are those based on use of specific LC columns. Shielded hydrophobic phase (27), small pore reversed-phase (28), and internal surface reversed-phase (29, 30) columns can be used to elute proteins in the excluded volumes, allowing small... 

It is common for SPE users to create their own packings by combining materials with different modes of separation. A common mixed-mode system is a weak cation exchange packing with a typical reversed-phase packing (e.g., Cl 8). Variations of this mixed mechanism approach have been described as shielded hydrophobic phases, semipermeable surfaces, dual-zone phases, and internal-surface reversed phases.8,9 This type of methodology has been used... 

There are various types of restricted-access packings that have been developed for sample clean-up. Reviews by Unger (1991) and Haginaka (1991) discuss internal-surface reversed phases, shielded hydrophobic phases, semi-permeable surfaces, dual-zone phases, and mixed functional phases. The majority of these packings have been developed to purify drugs from body fluids. 

A synthesised stationary phase called internal surface reverse phase (ISRP) or Pinkerton column appeared in 1985 [35], The packing material operates with two mechanisms, size exclusion and reverse phase bonded sorption. The outside walls of the bead are non adsorptive whereas the small inner channels consist of the ISRP. The size exclusion part cleans up the sample to remove large proteins which tend to clog the reverse phase column and the ISRP allows small analyte molecules to penetrate and be separated. 

Nakagawa, T. Shihukawa, A. Shimono, N. Kawashima, T. Tanaka, H. Haginaka, J. Retention properties of internal-surface reversed-phase silica packing and recovery of drugs from hiunan plasma. 

Many other types of solid phase adsorbents, including those based on conventional and specialty materials like restricted access media (RAM), can increase analysis speed and improve assay performance. These types of materials, also known as internal reversed-phase packings, are especially useful for assaying target compounds in biological samples such as serum and plasma. They are chemically modified porous silicas that have hydrophilic external surfaces and restricted-access hydrophobic internal surfaces. The ratio of interior to external surface areas is large. Macromolecules such as proteins cannot enter the pores of the RAM (they are excluded from the hydrophobic internal surface) and they elute quickly through the column. However, the smaller analyte molecules that can enter the pores are retained via interactions with the hydrophobic bonded phase within... 

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