Separation of small molecules

Sephasorb HP (ultrafine, prepared by hydroxypropylation of crossed-linked dextran) can also be used for the separation of small molecules in organic solvents and water, and in addition it can withstand pressures up to 1400 psi making it useful in HPLC. These gels are best operated at pH values between 2 and 12, because solutions with high and low pH values slowly decompose them (see further in Chapter 6). 

Righetti and co-workers [11] were one of the first to demonstrate the utility of classical isoelectric focusing for the chiral separation of small molecules in a slab gel configuration. In their system, dansylated amino acids were resolved enan-tiomerically through complexation with (i-cyclodextrin. Preferential complexation between the cyclodextrin and the derivatized amino acid induced as much as a 0.1 pH unit difference in the pK s of the dansyl group. 

Preparative continuous free flow electrophoresis was first reported in 1958 [15]. As in the case of classical gel electrophoresis, most of the work done in this area has been primarily in the purification of biopolymers. Continuous free flow electrophoresis for the separation of small molecules has remained relatively unexplored [16], although this is beginning to change. 

Metal nanotube membranes with electrochemically suitable ion-transport selectivity, which can be reversibly switched between cation-permeable and anion-permselective states, have been reported. These membranes can be viewed as universal ion-exchange membranes. Gold nanotube molecular filtration membranes have been made for the separation of small molecules (< 400 Da) on the basis of molecular size, eg. separation of pyridine from quinine (Jirage and Martin, 1999). 

Aqueous SEC is widely used for the determination of MWDs of a variety of synthetic and naturally occurring water soluble polymers, as well as for separations of small molecules. The column requirements for aqueous SEC are very demanding to eliminate ionic and hydrophobic effects. 

Advances in size-exclusion chromatography, coupled with refractive index, absorption, viscosity, and lightscattering detectors, and MALDI-ToFMS, have made it possible to accurately determine molecular weight distribution (oligomer profiling), even at the relatively low values of polymeric additives (up to about 5000 Da). Advances in column design, e.g. high-resolution PS/DVB columns (> 105 plates m-1) mean that SEC can provide a valuable alternative to conventional HPLC techniques for the separation of small molecules. 

Exclusion chromatography is also useful in the separation of small molecules from interfering matrices of larger molecules, for example in foods or other samples of biological origin. It can be used as the first step in the sequential analysis of complex unknown organic mixtures, which are first separated on a size basis by exclusion, then the collected fractions can be further separated by normal or reverse phase chromatography, where the separation is based on chemical differences. 

Exclusion chromatography separates solutes that differ in size and shape. The technique is used extensively in the investigation of macromolecules and in the separation of small molecules from an interfering matrix of larger molecules. 

Theoretically, chromatographic resolution depends on the square root of the column length. Separation of small molecules may be improved 40% by doubling column length. Retention time may also... 

The effect that the quality of the bed structure has on the chromatographic properties of columns packed with particles has been well known for a long time [1]. Similarly, the efficiency of capillary electrophoretic separations reaches its maximum for a specific capillary diameter, and then decreases steeply for both larger and smaller size [ 117]. Therefore, any improvement in the efficiency of the polymeric monolithic columns for the isocratic separations of small molecules is likely to be achieved through the optimization of their porous structure rather than their chemistry. 

Optimization of Resolution in Gel Permeation Chromatographic Separation of Small Molecules... 

For the optimal application of GPC to the separation of discrete small molecules, three factors should be considered. Solvent effects are minimal, but may contribute selectivity when solvent-solute interactions occur. The resolving power in SMGPC increases as the square root of the column efficiency (plate count). New, efficient GPC columns exist which make the separation of small molecules affordable and practical, as indicated by applications to polymer, pesticide, pharmaceutical, and food samples. Finally, the slope and range of the calibration curve are indicative of the distribution of pores available within a column. Transformation of the calibration curve data for individual columns yields pore size distributions from which useful predictions can be made regarding the characteristics of column sets. 

BMA/EDMA Thermal, AIBN 1-Propanol, 1,4-butanedioEwater 100pm I.D., HPLC separation of proteins, separation of small molecules [150,152,153]... 

In comparison to their silica counterparts, organic polymer monoliths generally exhibit lower efficiencies in the reversed-phase HPLC separation of small molecules. This reduced performance is primarily due to the lack of mesopores (see Section 1.3.2.5) and the presence of micropores in the polymer matrix, which cause slow internal diffusion. However, there are some promising approaches trying to accomplish the chromatography of small molecules on organic polymer monoliths. 

The equilibrium models of nonlinear chromatography assume that there always is an instantaneous equilibrium between the mobile phase and the stationary phase. That model is widely applied for the separation of small molecules, when mass transfer or diffusion in the stagnant pores of the mobile phase does not have a significant impact on the band profile. 

Under special conditions in which a reverse hydrodynamic flow was imposed to slow the passage of analytes through the capillary, up to 17 million plates were observed in the separation of small molecules 27... 

Because of the different interactions involved in partition and adsorption processes, they may be applied to different separation problems. Partition processes are the most effective for the separation of small molecules, especially those in homologous series. Partition chromatography has been widely used for the separation and identification of amino acids, carbohydrates, and fatty acids. Adsorption techniques, represented by ion-exchange chromatography, are most effective when applied to the separation of macromolecules including proteins and nucleic acids. 

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