Size exclusion selectivity

Probably the most precious occurrence of anion recognition is that related to size exclusion selectivity. This takes place when the receptor, providing for instance a spheroidal cavity, includes only spherical anions of radius less than or equal to a definite value. In this context, the smallest anion, fluoride, has offered vast opportunities. 

In a more conventional example Pt/ZSM-5 is used to induce reactant (size exclusion) selectivity in the hydrogenation of substituted aromatic molecules. Over Pt/ZSM-5 the rate of styrene conversion is at least 25 times higher than that of methylstyrene, while over Pt/Al Oj similar rates of hydrogenation were observed for both these compounds. 

In addition to the applications of ICPs already mentioned (for a general review of and its applications, see [547]), PPy has been suggested as a matrix for a variety of sensor applications [548, 549,560-563]. (A special application where a sensor responds to the respective monomer was described by Vinokurov [564].) In a related concept, the size selectivity ( ion sieving ) of PPy membranes is utilized [565]. The influence of the electropolymerization conditions on the size-exclusion selectivity of various ICPs, includ-... 

Examples of the application of size-exclusion chromatography to the analysis of proteins. The separation in (a) uses a single column that in (b) uses three columns, providing a wider range of size selectivity. (Chromatograms courtesy of Alltech Associates, Inc. Deerfield, IL). 

Use the model for the size exclusion of a spherical solute molecule in a cylindrical capillary to calculate for a selection of R/a values which... 

Another example of vims clearance is for IgM human antibodies derived from human B lymphocyte cell lines where the steps are precipitation, size exclusion using nucleases, and anion-exchange chromatography (24). A second sequence consists of cation-exchange, hydroxylapatite, and immunoaffinity chromatographies. Each three-step sequence utilizes steps based on different properties. The first sequence employs solubiUty, size, and anion selectivity the second sequence is based on cation selectivity, adsorption, and selective recognition based on an anti-u chain IgG (24). 

Smaller pore size microfilters in single-pass systems which have pore sizes small enough to remove some vimses by size exclusion have been examined (26,37,38). Minimum levels of vims removal can be estabhshed for these systems if fluid and process conditions are employed which minimize removal of viral particles by mechanisms other than size selection. 

Conductivity detectors, commonly employed in ion chromatography, can be used to determine ionic materials at levels of parts per million (ppm) or parts per bUHon (ppb) in aqueous mobile phases. The infrared (ir) detector is one that may be used in either nonselective or selective detection. Its most common use has been as a detector in size-exclusion chromatography, although it is not limited to sec. The detector is limited to use in systems in which the mobile phase is transparent to the ir wavelength being monitored. It is possible to obtain complete spectra, much as in some gc-ir experiments, if the flow is not very high or can be stopped momentarily. 

Porous silica packings do, however, sometimes suffer from adsorption between the sample and silanol groups on the silica surface. This interaction can interfere with the size exclusion experiment and yield erroneous information. In many cases, this problem is easily overcome by selecting mobile phases that eliminate these interactions. In addition, the surface of porous silica packings is routinely modified in order to reduce these undesirable interactions. Trimeth-ylsilane modified packing is typically used with synthetic polymers. Diol modified packing is typically used with proteins and peptides. 

As with other size-exclusion techniques, the pore size of the selected Zorbax GF column should provide resolution over the molecular size range of the proteins that are to be separated. The Zorbax GF-250 column separates proteins in the range of 4000 to 400,000 Da. The Zorbax GF-450 provides separation over the range of 10,000 to 1,000,000 Da. When these two columns are coupled, they can be used to separate proteins with molecular weights of 4000 to 1,000,000. 

Let us consider the separation of polymethylmethacrylate (PMMA) on a nonmodified silica column as an example. In THE (medium polar eluent) the PMMA eludes in size exclusion mode because the dipoles of the methylmethacrylate (MMA) are masked by the dipoles of the THE. Using the nonpolar toluene as the eluent on the same column, the separation is governed by adsorption because the dipoles of the carbonyl group in the PMMA will interact with the dipoles on the surface of the stationary phase. The separation of PMMA in the critical mode of adsorption can be achieved by selecting an appropriate THF/toluene mixture as the eluent. In this case all PMMA samples... 

In size exclusion chromatography, the mobile phase must be selected to totally solubilize the sample and eliminate all interactions of the solutes with the... 

To select a column for a particular analytical problem, the first step is to make a choice about the pore size(s) to be used for the separation. In general, one cannot expect that a single pore size will fulfill the needs of a separation. In size exclusion chromatography, it is more common that columns of different types are combined with each other to deliver the separation range needed for a particular analysis. Therefore, column banks with different pore sizes are frequently combined with each other to maximize the separation power for... 

This chapter gives a summary of the size exclusion columns currently available from Waters Corporation. It includes a description of the principles used to select the column type as well as a discussion of the properties and use of different columns. A few application examples are included as well. This review is designed to give the user a good overview over the properties and capabilities provided by Styragel, Ultrahydrogel, and Protein-Pak columns. 

Most size exclusion chromatography (SEC) practitioners select their columns primarily to cover the molar mass area of interest and to ensure compatibility with the mobile phase(s) applied. A further parameter to judge is the column efficiency expressed, e.g., by the theoretical plate count or related values, which are measured by appropriate low molar mass probes. It follows the apparent linearity of the calibration dependence and the attainable selectivity of separation the latter parameter is in turn connected with the width of the molar mass range covered by the column and depends on both the pore size distribution and the pore volume of the packing material. Other important column parameters are the column production repeatability, availability, and price. Unfortunately, the interactive properties of SEC columns are often overlooked. 

This chapter makes no distinction between gel-permeation chromatography (GPC) and size exclusion chromatography (SEC). We make mention of specific analysis conditions wherever possible. We have attempted to include a variety of conditions but by no means should this chapter be considered a comprehensive review of conditions for analyzing polyacrylates. We have drawn extensively from our own experience in selecting examples. 

The instrumentation of HdC, including a pump, an injector, a column (set), a detector, and a recorder or computer, is very similar to size exclusion chromatography SEC). The essence of this technique is the column. There are two types of HdC columns open microcapillary tubes and a nonporous gel-packed column. This chapter emphasizes column technology and selection and the applications of this technique on the molecular weight analysis of macromolecules. 

A more complicated, but flexible, system has been reported by Blomberg et al. (46). Here, size exclusion chromatography (SEC), normal phase EC (NPLC) and GC were coupled for the characterization of restricted (according to size) and selected (according to polarity) fractions of long residues. The seemingly incompatible separation modes, i.e. SEC and NPLC, are coupled by using an on-line solvent-evaporation step. 

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