Retention factor pressure

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... 

Mobile phases with some solvating potential, such as CO2 or ammonia, are necessary in SGC. Even though this technique is performed with ambient outlet pressure, solutes can be separated at lower temperatures than in GC because the average pressure on the column is high enough that solvation occurs. Obviously, solute retention is not constant in the column, and the local values of retention factors increase for all solutes as they near the column outlet. 

Column pressure usually has little effect on enantioselectivity in SFC. However, pressure affects the density of the mobile phase and thus retention factor [44]. Therefore, similar to a modifier gradient, pressure or density programming can be used in fast separation of complex samples [106]. Later et al. [51] used density/temperature programming in capillary SFC. Berger and Deye [107] demonstrated that, in packed column SFC, the effect of modifier on retention was more significant than that of pressure. They also showed that the enhanced solvent strength of polar solvent-modified fluid was nof due fo an increase in densify, caused by fhe addition of fhe liquid phase modifier, buf mainly due fo fhe change in composition. 

First, we look at isocratic separations. Let us assume that the analysis can be accomplished within a retention factor of 10. We also suppose that the analysis is carried out with a typical reversed-phase solvent and a sample with a typical molecular weight of a pharmaceutical entity. In order to manipulate the analysis time, we will keep the mobile phase composition the same and vary the flow rate. The maximum backpressure that we will be able to apply is 25MPa (250 bar, 4000psi). In Figure 1, we have plotted the plate count as a function of the analysis time for a 5 J,m 15-cm column. We see that the column plate count is low at short analysis times and reaches a maximum at an analysis time of about 1 h. A further increase in analysis time is not useful, since the column plate count declines again. This is the point where longitudinal diffusion limits the column performance. The graph also stops at an analysis time of just under 5 min. This is the point when the maximum allowable pressure drop has been reached. 

With pressurized HP-CEC, the electrophoretic migration rate and the mobile-phase flow rate can be optimized independently for peptide separations. Under such conditions, the retention factor, kof a peptide(s) is dependent on both the electric field and the applied pressure12"1 according to the following ... 

Figure 6.2—Density for carbon dioxide as a function ofpressure at four different temperatures. At the critical point, the density of CO2 is 0.46 g/cm3. The figure on the right represents the variation of the retention factor k for three alkaloids analysed under identical conditions and pressure, fixed downstream from the column by a restrictor (T = 4"C, modifier 5% water and 15% methanol 1 codeine, 2 thebaine, 3 papaverine). As the pressure increases, k decreases.

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 experimental apparatus and technique has been described in detail elsewhere.(20>21) The retention factors of naphthalene and biphenyl under isothermal conditions at various pressures were obtained using capillary columns coated with a cross-linked phenyl polymethylphenyl-slloxane stationary phase with carbon dioxide as the fluid mobile phase. A Varian 8500 syringe pump was operated under computer control providing accurate, pulsefree control of the fluid pressure. 

While for uncharged analytes the selectivity in CEC and LC is identical, a different selectivity can be expected for charged analytes. Therefore, Tsuda et al. [9,14,17,35,36] introduced an electrochromatographic retention factor for charged analytes. In pressure supported CEC the retention time is dependent upon the electrophoretic mobility uep, the electroendoosmotic velocity ueo, the velocity of the pressurized flow up and the chromatographic retardation. 

NC = no change j = a factor < f L = column length r/,- - column diameter N = column plate number / , = resolution V , = column hold-up volume k = retention factor /k = retention time (proportional to the run time) F = flow rate of the mobile pha.se

mean diameter of packing particles Ap = pressure drop across the column. 

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