Other modes of chromatography

As stated in the Introduction, the most widely used LC separation mode for polymer analysis is SEC. For low molecular weight species, however, where separation of similar size molecules is required, interactive liquid chromatographic modes (partition/absorption) are more suited. 

The main use of these more typical HPLC modes is in the analysis/ quantitation of polymer additives  

C atalysts (peroxides) and monomers/oligomers (acrylate and epoxy) can also be separated. 

For the reasons described in section 9.2, it is advisable to remove all polymer prior to any such analysis. The equipment used for such determinations can be described as conventional HPLC hardware the main difference from SEC being that solvent pumping system are normally employed that can generate solvent gradients with time and more sophisticated UV detectors (diode array) are often utilised which can provide not only quantitative data, but identification/peak purity information on the separated peaks. 

Glockner, G. (1987) In Polymer Characterisation by Liquid Chromatography, Jounml of Chromatography Library, Vol. 34, Elsevier, Amsterdam, pp. 404-405. 

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Reversed-phase HPLC can be compared in terms of its utility with other modes of chromatography and with other separation techniques such as gel electrophoresis, capillary electrophoresis (CE), and capillary electrochromatography. 

Size exclusion is the simplest form of chromatography, in which retention depends only on the permeation of analyte into and out of the pore system of the stationary phase. In contrast to other modes of chromatography, such as reversed phase or... 

If the sample is readily soluble in the mobile phase, GPC is unmatched by any other mode of chromatography for simplicity, since the entire analysis is accomplished in a column volume. The time and effort required to develop a separation is less than any other mode of HPLC. It can be of immense value in the purification or organic and inorganic synthesis reaction mixtures, purification of natural products extracts, and for the rapid clean-up of extracts (from plants, insects, soil, etc.) prior to the assay of small molecules. Aqueous size separation is referred to as gel filtration chromatography and is very useful for protein separations and the analysis of water-soluble polymers. 

Additives to enhance protein stability are generally compatible with the process. Contrary to its effect on other modes of chromatography, reducing the temperature decreases the retention in HIC due to its being an entropy-driven technique. 

Competitive Isotherms Models for Other Modes of Chromatography..186... 

With this respect exclusion chromatography basically diflers from all other modes of chromatography in that the analytes are not retained by the column packing and, therefore, do not need any special displacer or additional portions of the mobile phase, in order to be eluted from the column. Dilution of fractions separated in accordance with the size exclusion mechanism is no more unavoidable. (Dilution can be minimized to the diffusion effects at the front and tail of the analyte zone.) The absence of any supplementary matter in the frontal exclusion chromatography process relates ISE to the above-defined ideal separation process. 

The coupling of other modes of chromatography, such as affinity chromatography, supercritical-fluid chromatography, and planar chromatography, has also been demonstrated successfully. 

In HIC, molecules are bound with a high concentration of salt, usually ammonium or sodium sulfate (1-2 A/) in a buffer (0.02-0.05 M). Elution is attained by a gradient to a lower concentration of salt in the buffer. The pH is controlled and is usually in the range of 6-8, but it is not a critical factor in selectivity. Additives to enhance protein stability are generally compatible with the process. Contrary to its effect on other modes of chromatography, reducing the temperature decreases the retention in HIC due to its being an entropy-driven technique. 

This entry describes the classical and modem sample preparation methods that have been used prior to qualitative and quantitative analysis by thin-layer chromatography (TLC) and high-performance (HP) TLC. Extraction and cleanup methods that are covered include classical methods such as liquid-liquid extraction (LLE) and Soxhlet extraction, as well as modern methods such as solid-phase extraction (SPE), pressurized liquid extraction (PLE), and supercritical fluid extraction (SEE). Modern methods have not been as widely apphed in TLC as for other modes of chromatography, e.g., column high-performance liquid chromatography (HPLC). 

In contrast to other modes of chromatography—particularly adsorption chromatography—the enthalpy term can be neglected in this case as the solute and the stationary phase are assumed not to exchange any interaction. The equilibrium constant (K) thus depends exclusively on the entropy term ... 

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