Stationary-phase immobilization, chemical

Andrisano, V. Bertucci, C. Cavrini, V. Recatini, M. Cavalh, A. Veroli, L. Fehx, G. Wainer, I.W. Stereoselective binding of 2,3-suhstituted 3-hyrox3fpropionic acids on an immobilized human serum albumin chiral stationary phase. Stereo-chemical characterisation and quantitative structure-retention relationship study. J. Chromatogr., A 2000, 876, 75-86. 

Finally, dipolar and chemical shift correlation (DIPSHIFT) NMR was used to investigate the molecular motion and dynamics of the immobilized alkyl ligands of poly(ethylene-co-acrylic acid) stationary phases [164]. Through the measurement of the dipolar couplings, it was possible to discern the geometry of motion of... 

The first report of surface-immobilized dendrimers was in 1994 [54]. Subsequently, our research group showed that the amine-terminated PAMAM and PPl dendrimers could be attached to an activated mercaptoimdecanoic acid (MUA) self-assembled monolayer (SAM) via covalent amide linkages [55, 56]. Others developed alternative surface immobilization strategies involving metal com-plexation [10] and electrostatic binding [57]. These surface-confined dendrimer monolayers and multilayers have found use as chemical sensors, stationary phases in chromatography, and catalytic interfaces [41,56,58,59]. Additional applications for surface-confined dendrimers are inevitable, and are dependent only on the synthesis of new materials and the development of clever, new immobilization strategies. 

The stationary phase can be bound to the tubing either as a physical coating on the wall, or can be chemically immobilized. The former type phases are called nonbonded phases, while the chemically bound phases, cross-linked within the tubing are known as bonded phases. The latter is preferred because it can be used at high temperatures with less bleeding and can be rinsed with solvents to remove nonvolatile substances that accumulate on the column. 

Affinity chromatography combines the analytical and chemical capacities of chemically bonded stationary phases and immobilized enzymes. Technology and methodology of both techniques are joined in the development of affinity stationary phases. Since steric requirements are even more determining than in simple immobilized enzyme systems, spacer molecules have great importance in these modifications. Commonly used spacer arms are summarized in figure 8.3. 

Such new RP-HPLC stationary-phase materials have been available for some years (Regis Chemical Company, Morton Grove, IL, USA). These so-called immobilized artificial membrane (IAM) columns consist of lipid molecules covalently bound to propylamine-silica. The unreacted propylamine moieties are end-capped with methylglycolate. The membrane lipid, phosphatidylcholine, possesses polar head groups and two non-polar hydrocarbon chains (C18). One of the alkyl chains is linked to the propylamine-silica surface. 

Silica gel can also be used as a solid support on which liquids are immobilized to produce LLC systems. Most common are nonpolar phases for reverse phase work. Other chemicals used as stationary phases include cellulose and cellulose ion exchangers, polyamide, magnesium oxide, and Kieselguhr. Silver nitrate is added to silica gel to retain olefins selectively, as noted earlier. The review by Scott15 can be consulted for further information on the stationary phase in TLC. 

The distinguishing features of gas chromatography are a gaseous mobile phase and a solid or immobilized liquid stationary phase. Liquid stationary phases are available in packed or capillary columns. In the packed columns, the liquid phase is deposited on a finely divided, inert solid support, such as diatomaceous earth or porous polymer, which is packed into a column that typically has a 2- to 4-mm id and is 1 to 3 m long. In capillary columns, which contain no particles, the liquid phase is deposited on the inner surface of the fused silica column and may be chemically bonded to it. In gas-solid chromatography, the solid phase is an active adsorbent, such as alumina, silica, or carbon, packed into a column. Polyaromatic porous resins, which are sometimes used in packed columns, are not coated with a liquid phase. 

Molecules set in motion by the mobile phase (eluent) move through the stationary phase, suitably immobilized on a medium. The higher the affinity for the stationary phase and the lower the affinity for the mobile phase, the slower the analyte. As in a race, the fastest chemical species cover a prearranged distance in the shortest time, arrive at the finish line, and produce a detector signal proportional to the amount of analyte. The aggregation state of the mobile phase enables us to differentiate liquid, gas, and supercritical chromatographic techniques. 

Despite the efficient separations, it has been noticed that some low-boiling compounds of alcoholic beverages coeluted because of the use of a polar stationary phase. This column separates mainly based on the boiling temperatures of chemical substances, but separation becomes very difficult if there are some compounds with similar boiling temperatures. A polar stationary phase like poly(ethylene glycol) (PEG) is a better choice for this sort of problem because these separations are based on compound structures. In all cases, cross-linked or immobilized phases are recommended because they are more ther-molabile and also resistant to most solvents. This is particularly important when splitless injection is used in combination with PEG-type phases because otherwise a severe column bleeding might be observed after --220°C. 

Recently, immobilized artificial membrane (lAM) stationary phases are becoming more popular for membrane simulationJ In lAM columns, silica gel is chemically bonded to phospholipids. There are various types of I AM columns. Among them, the most frequently used is lAMPC, which contains phosphatidylcholine. 

Polycarboxylate crown ethers such as (205) are suitable ligands for potentiometric studies of mixed-metal complexes of Al3+ and alkali or alkaline-earth cations.303 A similar (+)-18-crown-6-tetracarboxylic acid, chemically immobilized on a chiral stationary phase (CSP), can selectively recognize both enantiomers of some analytes.304 Calixarene polycarboxylates such as (206) and (207) are useful ligands toward alkali-305,306 and also transition-metal ions,307 308 with applications in... 

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