Shape selectivity retention mechanisms

The exact mechanism controlling shape-selective retentive processes is not fully understood, although it is clear that the pure partitioning and adsorption models cannot account for differences in retention for isomer separations or the range of selectivityobserved for columns of various surface coverages and alkyl chain lengths. 

The accuracy of this system is dependent on the correlation coefficient of a retention description obtained from studies of QSRR, therefore, the selection of descriptors is the most basic and important task to construct RPS. This selection could be done with statistical framework, even if such description is not clearly derived from theories. The retention description obtained from QSRR studies is more effective for a rapid and accurate prediction of retention than that derived from theoretical models, because the former is simple and does not require introduction of a number of physicochemical parameters (they are often not clearly known and are very difficult and time-consuming to determine) for the latter case. By contrast, the consideration of physical meanings of descriptors derived from QSRR studies gave the overview of retention mechanisms in reversed-phase LC (7-10). That is to say, hydrophobicity, size and shape of alkyl-benzenes and PAHs are dominate factors controlling their retention. 

The major issue found in testing is the corrosion of the foam material and resultant contamination of the membrane. The high manufacturing cost of the metal or carbon foam with the required pore shape, size, and distribution also is a challenge. Further study and testing of the corrosion mechanism, selection of appropriate coating, a capillary process involved in the tiny pores, and related water retention are necessary to identify whether the new material and concept can be finally applied in the plate. 

Since most readers will be experienced in general HPLC applications, major differences in the practice of GPC are outlined in Table 1. The separation in GPC is governed by the well-known size-exclusion mechanism its principles can be found in the relevant literature [1]. In the ideal case, only the conformational entropy change causes retention. Secondary enthalpic interactions should be avoided by appropriate selection of the phase system. Since GPC does not measure molar masses directly, the retention axis has to be calibrated with so-called narrow polymer standards in order to transform peak position to molar mass. The absolute position of the peak(s) and its shape(s) are evaluated to determine the molar mass average and molar mass distribution, respectively. 

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