Silica-based restricted access materials

Silica-based restricted access materials (RAM) have been developed for cleanup in bioanalysis, first for low molecular weight compounds in biofluids (Rbeida et al., 2005) and subsequently for biopolymers such as peptides (Wagner et al., 2002). A classification of different types of RAM has been given by Boos and Rudolphi (1997). Novel RAMs with strong cation-exchange functionality have been synthesized and implemented in the sample cleanup of biofluids. Racaityte et al. (2000) have shown that this type of RAM is highly suitable for the online extraction and analysis of... 

A wide variety of SPE materials and cartridges are commercially available for example, alkyl-diol silica-based restrictive access materials (RAMs) and a variety of silica- and polymer-based SPE materials of different binding abilities and capacities. Reversed-phase, size-exclusion, ion-exchange SPE, and turbulence flow methods will be discussed in this chapter related to real-world applications. 

Fig. 4. Principle of restricted access material (RAM) based on the silica LiChrospher 60.

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... 

The usual base materials for LC restricted-access media are porous silica supports, which limit the pH range of the mobile phases to 2-8. Polymer-based stationary phases have also been used as restricted-access media and are character-... 

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