Retention factor gradient elution

The stationary phase is selected to provide the maximum selectivity. Where possible, the retention factor is adjusted (by varying the mobile phase composition, temperature, or pressure) to an optimum value that generally falls between 2 and 10. Resolution is adversely affected when k 2, while product dilution and separation time increase greatly when k 10. When this is not possible for all feed components and large differences exist among the -values of the different solutes, gradient elution should be considered. 

Similar considerations apply for the integration of the volume elements in gradient elution. Setting the net (corrected) elution retention times (or volumes), R = tR-t , Vr = (Vr- VJ as the upper integration limits in Equation 5.3 implies that the retention factor k relates to the actual position of... 

N is the average column isocratic theoretical plate number is the retention factor at the point of elution controlling the bandwidths in gradient elution— Equation 5.5... 

The effect of the dwell volume on the retention times of analytes increases with decreasing retention factor at the start of gradient elution and with increasing ratio VpIV, and becomes very significant in the instrumental setup with the dwell volume comparable to or larger than the column hold-up volume, which is more likely to occur in micro- or in capillary LC than in conventional analytical LC (see Figure 5.4) [12]. 

The retention factors of the polymers in 60 % THF were calculated from gradient elution and discussed together with those directly measured for oligomers with 2-12 repeat units or 2000, 4000, and 9000 g/mol molar mass. On a lOOnm-pore packing, the Martin equation was fulfilled for n = 19 repeat units. At higher values, the curve... 

Figure 11 illustrates the parameter space defined by the equilateral triangle. The initial pressure and conditions for the 3 vertices of the pressure gradient/ temperature triangle were determined arbitrarily from the critical conditions of the supercritical fluid (carbon dioxide), the retention characteristics of nitroaromatic compounds, and the following criteria (i) the first analyte should not co-elute with the sample solvent and (ii) the retention factor of the last analyte should not exceed 30. 

The variety of stationary phases available commercially and the lack of standardisation between different laboratories makes it difficult to compare the retention factors k directly as comparison of lipophilicity. Therefore different lipophilicity scales obtained from chromatographic data (isocratic or gradient elution) have been introduced. A calibration of the individual chromatographic system with compounds of known lipophilicity is used in all cases. 

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