Molecular-exclusion normal-phase

ASE using dichloromethane has been applied to extract alkylphenols and short-chain NPEO from sediment [8,47]. Samples of 2-5 g were extracted in two cycles of 30 mL at 100°C and 69 atm. Clean-up was performed using size exclusion chromatography to remove high molecular weight lipids, and then using normal phase HPLC. 

The elution volumes of polystyrene and benzene in the size-exclusion mode were 0.98 and 1.78 ml, respectively (Figure 1.4A). This means that separations by molecular size can be achieved between 0.98 and 1.78 ml in this system. In the normal phase mode the elution volumes of octylbenzene and benzene were 1.98 and 2.08 ml, respectively, in n-hexane solution (Figure 1.4B). This type of chromatography is called adsorption or non-aqueous reversed-phase liquid chromatography. These are adsorption liquid chromatography and non-aqueous reversed-phase liquid chromatography. The elution order of the alkylbenzenes in the reversed-phase mode using acetonitrile was reversed... 

Regarding the detection protocols used with capillary columns with molecularly imprinted stationary phases, on-column UV absorbance detection has been exclusively used. Some sort of open tubular area without imprinted polymer is normally prepared to perform detection. It has been shown, however, that UV detection can be performed through the imprinted polymer [39] in some cases. A nice feature with the photo-induced polymerisation procedure is that a part of the capillary column can be covered during the polymerisation reaction, thus preventing polymer being formed in that area. This is utilised to readily prepare detection windows on the MIP capillary columns. There is then no need for coupling of a second capillary to the imprinted polymer-filled capillary to be able to perform detection. 

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... 

The molecular size effect is used in size-exclusion liquid chromatography. The steric effect, a difference in molecular shape, is mainly applied to normal-phase liquid chromatography. Enantiomeric separation is usually carried out with normal-phase liquid chromatography, using specially... 

Hydroxyl number and molecular weight are normally determined by end-group analysis, by titration with acetic, phthaUc, or pyromellitic anhydride (264). Eor lower molecular weights (higher hydroxyl numbers), E- and C-nmr methods have been developed (265). Molecular weight deterrninations based on coUigative properties, eg, vapor-phase osmometry, or on molecular size, eg, size exclusion chromatography, are less useful because they do not measure the hydroxyl content. 

The fourth type of mechanism is exclusion although perhaps inclusion would be a better description. Strictly, it is not a true sorption process as the separating solutes remain in the mobile phase throughout. Separations occur because of variations in the extent to which the solute molecules can diffuse through an inert but porous stationary phase. This is normally a gel structure which has a small pore size and into which small molecules up to a certain critical size can diffuse. Molecules larger than the critical size are excluded from the gel and move unhindered through the column or layer whilst smaller ones are retarded to an extent dependent on molecular size. 

The factors that control separation and dispersion are quite different. The relative separation of two solutes is solely dependent on the nature and magnitude of the Interactions between each solute and the two phases. Thus, the relative movement of each solute band would appear to be Independent of column dimensions or particle geometry and be determined only by the choice of the stationary phase and the mobile phase. However, there is a caveat to this statement. It assumes that any exclusion properties of the stationary phase are not included in the term particle geometry. The pore size of the packing material can control retention directly and exclusively, as in exclusion chromatography or, indirectly, by controlling the access of the solute to the stationary phase in normal and reverse phase chromatography. As all stationary phases based on silica gel exhibit some exclusion properties, the ideal situation where the selective retention of two solutes is solely controlled by phase interactions is rarely met in practice. If the molecular size of the solutes differ, then the exclusion properties of the silica gel will always play some part in solute retention. 

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