Shielded hydrophobic phases

Column Packing Proprietary "shielded hydrophobic phase"... 

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... 

The sample homogenization with an MeCN THF mixture was used for the simultaneous determination of SMM, miloxacin, and oxolinic acid. The supernatant was filtered and injected directly into the ion-pair chromatographic system using a shielded hydrophobic phase. This method did not require time-consuming and complex extraction procedures moreover, the use of a restricted-access-material column prevented both column clogging and peak broadening throughout the analysis. On the other hand, no preconcentration of the sample affected the LOD... 

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. 

Figure 12.8. Schematic representation of a shielded hydrophobic phase (SHP). [Reproduced with permission from Haginaka (1991).]...

Santasania, C.T. Direct injection analysis of diuretic and anti-inflammatory drugs on a shielded hydrophobic phase column. J.Liq.Chromatogn, 1990, 13, 2605-2631 (serum direct irvjection gradient horse extracted furosemide, oxyphenbutazone, phenylbutazone]... 

Hisep shielded hydrophobic phase (Supelco, BeUefonte, Pennsylvania, U.S.A.)... 

Guard column Present but not specified Column 150 x 4.6 Hisep shielded hydrophobic phase... 

An important example of a non-ISRP phase is the shielded hydrophobic mixed funchon material known as Capcell Pak MF . This phase consists of porous silica derivahzed with a silicone polymer. The polymer is referred to as a mixed phase, since it contains hydrophobic moiehes dispersed at intervals in the polymer chain. Unlike small molecules, matrix proteins are unable to penetrate the hydrophilic polymer and are eluted to waste. A recent appUcahon by Hsieh et al. demonstrated the utility of this phase for direct plasma analysis without the use of a secondary column [70]. 

Figure B3.6.4. Illustration of tliree structured phases in a mixture of amphiphile and water, (a) Lamellar phase the hydrophilic heads shield the hydrophobic tails from the water by fonning a bilayer. The amphiphilic heads of different bilayers face each other and are separated by a thin water layer, (b) Hexagonal phase tlie amphiphiles assemble into a rod-like structure where the tails are shielded in the interior from the water and the heads are on the outside. The rods arrange on a hexagonal lattice, (c) Cubic phase amphiphilic micelles with a hydrophobic centre order on a BCC lattice.

These are molecules which contain both hydrophilic and hydrophobic units (usually one or several hydrocarbon chains), such that they love and hate water at the same time. Familiar examples are lipids and alcohols. The effect of amphiphiles on interfaces between water and nonpolar phases can be quite dramatic. For example, tiny additions of good amphiphiles reduce the interfacial tension by several orders of magnitude. Amphiphiles are thus very efficient in promoting the dispersion of organic fluids in water and vice versa. Added in larger amounts, they associate into a variety of structures, filhng the material with internal interfaces which shield the oil molecules—or in the absence of oil the hydrophobic parts of the amphiphiles—from the water [3]. Some of the possible structures are depicted in Fig. 1. A very rich phase... 

Feibush, B. and Santasania, C. T., Hydrophilic shielding of hydrophobic, cation- and anion-exchange phases for separation of small analytes direct injection of biological fluids onto high performance chromatographic columns, /. Chromatogr., 544, 41, 1991. 

As an alternative, stable high-coverage nonpolar RPC sorbents phases have been prepared by cross-linking hydrophobic polymers at the silica surface, either via free radical 143 or condensation 101 polymerization chemistry. In this case, the underlying silica becomes partly protected from hydrolytic degradation due to the presence of the hydrophobic polymer film coating that effectively shields the support material. Similar procedures have been employed to chemically modify the surface of other support materials, such as porous zirconia, titania, or alumina, to further impart resistance to degradation when alkaline mobile-phase conditions are employed. Porous polystyrene-divinylbenzene sorbents, be-... 

Liposomes and micelles are lipid vesicles composed of self-assembled amphiphilic molecules. Amphiphiles with nonpolar tails (i.e., hydrophobic chains) self-assemble into lipid bilayers, and when appropriate conditions are present, a spherical bilayer is formed. The nonpolar interior of the bilayer is shielded by the surface polar heads and an aqueous environment is contained in the interior of the sphere (Figure 10.3A). Micelles are small vesicles composed of a shell of lipid the interior of the micelle is the hydrophobic tails of the lipid molecules (Figure 10.3B). Liposomes have been the primary form of lipid-based delivery system because they contain an aqueous interior phase that can be loaded with biomacromolecules. The ability to prepare liposomes and micelles from compounds analogous to pulmonary surfactant is frequently quoted as a major advantage of liposomes over other colloidal carrier systems. 

Comparable to the bonded-micellar phases are the binary-layered phases. These phases are also covered by a ligand that possesses both hydrophilic and hydrophobic functions. Diol groups at the outside prevent adsorption of matrix proteins, and methoxypropyl chains shielded by the diol functionalities serve as adsorption sites for very hydrophobic analytes (120). 

Both [5-12SI]iodonaphthylazide (Karlish et al. 1977) and [3H]adaman-tane diazirine (Farley et al. 1980) labeled the a-subunit strongly and the (3-subunit hardly at all. The simplest interpretation of this result is that the (3-subunit is a peripheral protein, tightly attached to the a-subunit. The possibility remains, however, that the (3-subunit is relatively unreactive towards hydrophobic reagents even though the two used in these studies have quite different properties. A further possibility that must be taken into account when considering multisubunit membrane proteins is that (3 penetrates the bi layer but is shielded from the hydrocarbon phase by neighboring a-subunits. ... 

This proposed mechanism for protein separations is supported by the recent theoretical studies of Horvath ef al. (29) and Horvath and Melander (28). In these studies, the hydrophobic effect in aqueous-organic systems (termed the solvophobic theory) was used to predict the retention of peptides on a nonpolar column. These authors found that the dominant interactions were between the mobile and stationary phases and between the mobile phase and the sample molecules. The driving force in both interactions was the shielding of a nonpolar region of either the column or sample molecule from the polar aqueous phase. 

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