Separations molecular exclusion

Two classes of micron-sized stationary phases have been encountered in this section silica particles and cross-linked polymer resin beads. Both materials are porous, with pore sizes ranging from approximately 50 to 4000 A for silica particles and from 50 to 1,000,000 A for divinylbenzene cross-linked polystyrene resins. In size-exclusion chromatography, also called molecular-exclusion or gel-permeation chromatography, separation is based on the solute s ability to enter into the pores of the column packing. Smaller solutes spend proportionally more time within the pores and, consequently, take longer to elute from the column. 

In summary, silica gel can be an excellent stationary phase for use in exclusion chromatography in the separation of high molecular weight, weakly polar or polarizable polymers. It cannot be used for separating mixtures that require an aqueous mobile phase or operate at a pH outside the range of 4-8. Examples of the type of materials that can be separated by exclusion chromatography using silica gel are the polystyrenes, polynuclear aromatics, polysiloxanes and similar polymeric mixtures that are soluble and stable in solvents such as tetrahydrofuran. 

To ensure a better separation, molecular sieving will act much better This size exclusion effect will require an ultramicroporous (i.e pore size D < 0.7 nm) membrane Such materials should be of course not only defect-free, but also present a very narrow pore size distribution. Indeed if it is not the case, the large (less separative and even non separative, if Poiseuille flow occurs) pores will play a major role in the transmembrane flux (Poiseuille and Knudsen fluxes vary as and D respectively). The presence of large pores will therefore cancel any sieving effect... 

Fractionations. Following lyophilization of the polar leachate fractions, separations by exclusion chromatography suggest three major molecular weight regions, labelled peak 1A, peak 1B and peak 2 (Figure 3). Retention times on a G—15 Sephadex gel indicate molecular weights of peaks 1A and 1B compounds to be between 600 and 1000. 

Molecular exclusion chromatography. The stationary phase in molecular exclusion chromatography is a material containing pores, the dimensions of which are chosen to separate the solutes present in the sample based on their molecular size. This can be perceived as a molecular sieve allowing selective permeation. This technique is known as gel filtration or gel permeation, depending on the nature of the mobile phase, which is either aqueous or organic. The distribution coefficient in this technique is called the coefficient of diffusion. 

If the molecular masses of solutes are >2 000 and if they are soluble in organic solvents and their molecular diameter is >30 nm. Figure 25-14 tells us to try molecular exclusion chromatography.. Stationary phases for this type of separation are described in Chapter 26. If the molecular masses of solutes are >2 000, and they are soluble in water, but not ionic, and have diameters <30 nm, the decision tree says to use reversed-phase chromatography, or hydrophobic interaction chromatography. 

Figure 26-13 Separation of proteins by molecular exclusion chromatography with TSK 3000SW column. [Courtesy Varlan Associates. Palo Alto, CA.]...

Nanoparticles can be separated by molecular exclusion chromatography just as proteins are separated. Figure 26-15 shows the relation between measured size and retention time of CdSe quantum dots. These are particles containing 2 000 CdSe units in a dense, crystalline core capped by alkyl thiol (RS) groups on Cd and trialkylphosphine (R3P) groups on Se. 

Molecular exclusion chromatography is based on the inability of large molecules to enter small pores in the stationary phase. Small molecules enter these pores and therefore exhibit longer elution times than large molecules. Molecular exclusion is used for separations based on size and for molecular mass determinations of macromolecules. In affinity chromatography, the stationary phase retains one particular solute in a complex mixture. After all other components have been eluted, the desired species is liberated by a change in conditions. 

Ferritin (molecular mass 450 000), transferrin (molecular mass 80 000), and ferric citrate were separated by molecular exclusion chromatography on Bio-Gel P-300. The column had a length of 37 cm and a 1.5-cm diameter. Eluate fractions of 0.65 mL were collected. The maximum of each peak came at the following fractions ferritin, 22 transferrin, 32 and ferric citrate, 84. (That is, the ferritin peak came at an elution volume of 22 X 0.65 = 14.3 mL.) Assuming that ferritin is eluted at the void volume and that ferric citrate is eluted at Vm, find Kay for transferrin. 

This chromatographic technique concerns the separation of sample molecules on the basis of size and shape by the principle of molecular exclusion. The stationary phase consists of a cross-linked porous gel which permits the entrance into the pores of molecules having a certain size. Larger molecules are excluded, and thus these remain in the mobile phase and are eluted from the column with no retention. The smaller the molecules, the greater distance they can diffuse into the gel and therefore the slower is the migration. 

Nitrogen production using carbon molecular sieves is the only known commercial process using differences In lntrapartlcle diffusivity, rather than inherent adsorbent selectivity or selective molecular exclusion, as the basis for the separation ... 

Gel filtration chromatography (also known as size or molecular exclusion chromatography) separates molecules based on their ability to penetrate into the pores or channels in agarose or dextran beads. As a mixture of molecules in a fluid permeate through the beads of gel the volume available for diffusion is determined by their diameter and the size of the channels in the gel beads. The... 

Micro-ultrafiltration and reverse osmosis are mature technologies for separations based on molecular exclusion and solution-diffusion mechanisms, respectively. Cleaning and maintenance procedures able to control fouling to an acceptable extent have made these processes commercially suitable. 

Ion exclusion chromatography provides a convenient way to separate molecular acids from highly ionised substances. The separation column is packed with a cation exchange resin in the H+ form so that salts are converted to the corresponding acid. Ionised acids pass rapidly through the column while molecular acids are held up to varying degrees. A conductivity detector is commonly used. 

Molecular exclusion chromatography, also known as gel filtration, size exclusion chromatography, gel permeation chromatography, or simply gel chromatography, is another separation method based on differences in molecular size. 

Schematic illustration of the separation principle involved in molecular exclusion chromatography.

Antibodies and related molecule to separate Molecular size" (average) Exclusion limit of GF medium Fractionation range b Example of GF medium... 

Viscotoxin, a basic peptide of molecular weight ca. 9000 (Samulsson, 1961), moves unretarded in strongly cross-linked gels such as Sephadex G-25 (Fig. 4a) (in phosphate buffer, ionic strength 0.05, pH 6.8. When filtered under similar conditions in weakly cross-linked dextran, viscotoxin behaves quite differently (Fig. 4b). In fact it moves behind isoleucine. The gel of the first kind can be used to remove solutes of lower molecular size, the purification being based on molecular exclusion. Filtration in the second kind of gel may be used not only for separating solutes of different molecular size but also to separate peptides and other substances of similar molecular size when they differ in certain structural features. 

Column choice, (a) The octapeptide AVGWRVKS was digested with the enzyme trypsin. Would ion exchange or molecular exclusion be most appropriate for separating the products Explain, (b) Suppose that the peptide was digested with chymotrypsin. What would be the optimal separation technique Explain. 

We have divided the applications of zeolite membranes for gas separations into two main groups based on and depending on whether there is molecular exclusion or not in one of the components to be separated. 

Disc gel electrophoretic patterns show large differences between human serum, the two non-Antarctic fish sera and the two Antarctic fish sera (Fig. 2). Large differences were also noted on comparison of patterns of electrophoretic separations of human serum and the serum of T. borchgrevinki by molecular exclusion on Sephadex G-200 (Fig. 3). These differences were reflected in the amounts of proteins in the different fractions for these separations of D. mawsoni and the king salmon sera (Table III). The AFGP were found in fractions III from the sera of both T. borchgrevinki and D. mawsoni. Since fractions III also contained the albumins, it is evident that the albumin contents of these sera were much lower than those of human serum. Their sera... 

Size-exclusion chromatography, also known as gel-filtration, gel-permeation, steric-exclusion, molecular-exclusion, or molecular-sieve chromatography, separates solutes on the basis of their molecular sizes (Figure 6-5 see Fig, 6-3). Molecular shape and hydration are also factors in the process. 

GebFiltration Chromatography. More-discriminating separations on the basis of size can be achieved by the technique of gel-filtration chromatography, also known as molecular exclusion chromatography (Figure 3.3). 

The size of the pores determines the molecular weight range of the compounds that can be separated. Size exclusion often is used to clean up an extracted solution. For example, it is used to remove the fats and waxes from the pesticide residues that are extracted from a food composite. 

Gel filtration/permeation chromatography (also known as molecular exclusion chromatography) is a form of partition chromatography in which the solute molecules are partitioned between solvent and a stationary phase of defined porosity without an attractive interaction between the two phases. Gel filtration generally refers to aqueous systems while gel permeation is used in nonaqueous systems. The technique is normally used for the separation of biomacromolecules on the basis of size. Solutes are eluted in the order of decreasing molecular size. Gel filtration chromatography is not used as the first step in... 

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