Restricted surface access phase

Figure 7.17 Phase with restricted surface access.

The first attempts employing two Cjg columns showed that the selectivity was not high enough, although this improved when the first column was substituted by a 5 p.m GFF n internal surface rcversed-phase material. This is known as a restricted-access-material (RAM) column which, since it restricts some compounds because of their size and includes rcversed-phase interaction and ionic exchange, is very useful for analysing herbicides in samples with high contents of humic and fulvic acids (54). 

Silica gels with mean pore diameters of 5-15 nm and surface areas of 150-600 m /g have been preferred for the separation of low molecular weight samples, while silica gels with pore diameters greater than 30 nm are preferred for the separation, of biopolymers to avoid restricting the accessibility of the solutes to the stationary phase [15,16,29,34]. Ideally, the pore size distribution should be narrow and symmetrical about the mean value. Micropores are particularly undesirable as they may give rise to size-exclusion effects or irreversible adsorption due to... 

Restricted access phases are another approach to exploiting the differences in characteristics of analytes. Large analytes are excluded from an internal surface on which an adsorptive stationary phase is present. A herbicide analysis for Metsulfuron methyl, Bentazone, Bromoxynil, methylchlorophenoxy acid, and Mecoprop in the presence of humic acid was performed on restricted access reversed phase media.52 The cytostatic compound epirubicin and its metabolites were separated from plasma using a Pinkerton GFF II column.53 Gradient separations of polymers on reversed phase and on normal phase represent an alternative to gel permeation chromatography. Polyesters of noncrystalline materials were separated on a variety of such phases.54... 

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. 

The best approach to date for the analysis of basic compounds by CEC has been to incorporate a small basic compound such as triethylamine, triethanolamine [63] or hexylamine [64] into the low pH mobile phase. The small bases act in a competitive manner to restrict the access of the basic analytes to the silanol groups on the surface... 

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. 

The restricted-access solid-phase extraction may work in dual mode when it combines size exclusion and reversed-phase partitioning to isolate and clean up complex matrices such as serum, blood, and plasma (Haginaka, 1991). In this case, it is an internal-surfaced reversed phase (ISRP), as shown in Figure... 

Carbon Black is formed from the burning of gaseous or liquid hydrocarbons under conditions of restricted air access. According to electron micrographs taken with a phase contrast microscope, carbon black has a graphitelike microstructure with lattice distances of 0.35 nm. The layers lie parallel to the particle surface. Since discrete crystalline regions cannot be observed, the structure of carbon black is better described in terms of a paracrystalline state rather than a random distribution of graphite crystals. 

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... 

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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