Retention general elution problem

While such large changes in a are limited to compounds of quite different structure, it is likely that some preparative separations can be greatly facilitated by maximizing a in this way without limit. In analytical separations, however, values of a larger than 5-10 can actually be detrimental, because they result in too large a difference in retention times for the two solutes (the general elution problem, Ref. 1). 

A more definitive identification may be obtained by combining retention characteristics with more specific information from an appropriate detector. Arguably, the most information-rich HPLC detectors for the general identification problem are the diode-array UV detector, which allows a complete UV spectrum of an analyte to be obtained as it elutes from a column, and the mass spectrometer. The UV spectrum often allows the class of componnd to be determined but the... 

This problem has been discussed by Snyder (8,10) and Scott 9, II). A general solution is difficult to give since it would depend oh the composition of the sample mixture, e.g., the concentration of the la eluting components, and the detection limit, which varies in liquid chromatography with the chemical nature of the sample component. Therefore some arbitrary assumptions have to be made. From Eq. (26), using the maximum permissible sample size given by Eq. (73), we can write foi the retention volume, Vr, the following expression ... 

The difference in retention on phases of different structures are more pronounced in mobile phases of lower elution strength. The k values are governed by the solute structure. The results suggest that comparison of retention on the branched and unbranched phases could also be used for solving more general problems of the separation mechanism, based on study of molecular interactions. The phases with branched structure could also be useful in extending the application of HPLC. 

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