Elution eluent requirements

In on-line elution, the kinetic features of the process are much more important than for off-line batch procedures. Weak eluents requiring long equilibrium periods may be used successfully for off-line procedures, but caimot be used for on-line applications, since slow elutions may significantly degrade the enrichment factors and/or the concentration efficiencies. 

In gradient elution, detection requires quantitative measurement of the sample components in an eluent whose composition and, hence, physical properties alter in the course of the analysis. The detection problem in gradient elution can be solved by either using a selective detector sensitive to a property of only the solute or stripping off the solvent with subsequent measurement of non-volatile residues. 

Elution. Elution is necessary if the sorbed ions are required for examination, and were not removed just to facilitate analysis of the effluent. The volume of eluent required depends on both the resin and the nature of the sorbed ions see later for guidelines. 

Since from an ideal point of view polymer solubility and adsorption are not occurring in this chromatographic process, the volume of eluent required for the elution of any macromolecules species must essentially be dependent on chain length and appears to be insensitive to structure (universal calibration). However, this seems to be not always the case (non-exclusion effects). 

The ISO method prescribes polystyrene standards with tetrahydrofuran as the eluent, but this equation can also be used with other narrow distribution standards, provided the same elution solvent and the same standards are used for a comparison. Further, the ISO method requires the result to be greater than 6 for one decade of the molar mass. Because calibration curves are usually not linear, this decade should lie nearly symmetrically around the peak maxima of the samples in question. The required value of 6 is easy to fulfill, as results of 10 or more are usual with modern columns. If so-named linear or mixed... 

As stated in Section I, columns should be selected so the low molar mass portions of the samples in question can be sufficiently separated from the elution interval of the system peaks. This task cannot always be accomplished, e.g., dimethylacetamide often replaces dimethylformamide as a GPC eluent the analyzed, mostly polar, samples require a neutral salt (e.g., FiBr) (7). The calibration is usually carried out with poly(methylmethacrylate) standards... 

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