Effect on retention

In general, retention decreases as the modifier concentration increases because the modifier competes with the analytes for sites on the stationary phase. The effect on retention of changes in modifier concentration seems to be more pronounced for CSPs than for achiral stationary phases in SFC, and peak shapes are apt to degrade rapidly at low modifier concentrations [12]. Efficiency tends to decrease as the modifier concentration increases because analyte diffusion is slowed by the increased viscosity of the eluent [39]. 

Temperature variation may also be a relevant factor in flowrate stability. Since the viscosity of the solvent is temperature dependent, wide swings in the ambient temperature can directly affect pump performance. The direct effects of temperature on pump performance usually are far smaller, however, than the effects on retention and selectivity therefore, control of column temperature is generally sufficient to obtain high reproducibility. 

Lochmiiller, C. H., Moebus, M. A., Liu, Q., and Jiang, C., Temperature effect on retention and separation of poly(ethylene glycoljs in reversed-phase liquid chromatography, /. Chromatogr. Sci., 34, 69, 1996. 

Trying to determine which column is ideal for a specific analysis can be difficult with over 1000 different columns on the market [74]. A proper choice implies a definition of parameters such as column material, stationary phase (polarity), i.d., film thickness and column length. Guides to column selection are available [74,75]. The most important consideration is the stationary phase. When selecting an i.d., sample concentration and instrumentation must be considered. If the concentration of the sample exceeds the column s capacity, then loss of resolution, poor reproducibility and peak distortion will result. Film thickness has a direct effect on retention and the elution temperature for each sample compound. Longer columns provide more resolving probe, increase analysis times and cost. 

Variable Effect on retention time Effect on peak width... 

Obviously, the magnitude of the temperature effect on retention depends on the difference in the enthalpy of the solute in either phase, and is specific for each solute. Therefore, it also changes the column selectivity. There is no retention and no temperature effect for AH=0. 

FIGURE 1.5 pH -effect on retention factors k and separation factors a. CSP 0-9- tert-butylcarbamoyl)quinine bonded to sihca column dimension, 150 x 4 mm ID eluent, methanol-ammonium acetate buffer (80 20, v/v) (adjusted with acetic acid) temperature, 25°C 1 mL min sample, N-benzoyl-leucine (Bz-Leu). (Reproduced from M. Lammerhofer et al., American Laboratory, 30 71 (1998). With permission.)... 

Carr and co workers [77] studied the retention of different amines on PBD-Zirconia phases. On PBD zirconia, they found that steric hindrance has a much greater effect on retention of bases in comparison to silica ODS phases thus, quaternary amines have substantially less retention than the corresponding less hydrophobic primary amine. This result agreed with previous findings of Sokolowski and Wahlund [78]. Thus, secondary and tertiary amines such as dimethylbutylamine and TEA were less effective silanol blockers. Only ammonia and primary amines were able to improve the chromatographic properties of basic solutes on PBD-zirconia. It would be interesting to check the application of these results also to silica-ODS phases. 

Column stability, 161 Column temperature, 267 effect on retention, 192,200 effect on viscosity, 192 nonuniform temperature profile, 194 optimization of, 193 Column tubing, 145 cleaning of, 145 irmer surffice of, 145... 

Eluent strength, 4, 70, 169, 173, 181 adjustment of, 73-77 calculation of, 87 of commonly used solvents, 72 dependence on composition, 73-77 effect on retention, 102 modulation of, 39 of mixed eluents, 73-77 of neat eluents, 72 optimization, 87... 

Eluite, chemical structure, effect on retention in chromatography with polar sorbents, 90-92... 

Changing the column temperature can produce a variety of additional effects. Temperature changes the balance between enthalpy and entropy effects on retention mechanisms. Changing the temperature changes the equilibrium constants of both solvent and solutes, and it changes the... 

Plots of In k vs. 1/r are expected to produce linear relationships. This assumes that the thermodynamics of the retention process are independent of temperature and that neither the solute nor the stationary phase changes with temperature. For a large proportion of systems and moderate temperature ranges, these assumptions are met. But there are many factors that can complicate this and lead to non-linear van t Hoff plots. Guillarme et al. [16] studied van t Hoff plots for various mobile phases over a very wide temperature range, from 30°C to 200°C. They found that methanol-water systems were more linear than acetonitrile-water systems and that the effects were not due to pressure. They also developed a rule of thumb that a 30°C-50°C increase in column temperature had the same effect on retention as a 10% increase in the organic component of the mobile phase. 

The possible flow rate effects on retention volume (Section 16.3.4). 

Sun, Y. and Stalcup, A. M., Mobile phase effects on retention on a new butylimidazolium-based high-performance liquid chromatographic stationary phase, /. Chromatogr. A, 1126,276-282, 2006. 

The effect of the percentage of 2-propanol on the enantiomeric resolution of benzetimide is shown in Figure 13a. It may be concluded that retention factor values decreased with an increase in 2-propanol content. According to Wainer et al. [87], the diminution of retention factor values with increases in the content of polar modifiers indicates that competition for the binding sites on the CSP is a saturable process and that a maximum effect on retention factor will be reached at a certain polar modifier concentration. It is interesting to note that although only a... 

In addition to the antineoplastic activity, camptothecin was found to be an effective inhibitor of adenovirus replication [268, 269] and herpes virus replication [252, 270]. 10-Methoxycamptothecin is about 8-times more potent than camptothecin as an inhibitor of herpes virus [252]. A combination of camptothecin and dimethyl sulphoxide is very effective for the topical treatment of psoriasis [271]. Since, in the goldfish brain, camptothecin blocks RNA synthesis in eucaryotic cells by blocking the incorporation of uridine into RNA, this alkaloid can block the memory of conditioned avoidance and produces no measurable effect on retention of the learned response [272]. 

Figure 6.3 Ultrafiltration membranes are rated on the basis of nominal molecular weight cut-off, but the shape of the molecule to be retained has a major effect on retentivity. Linear molecules pass through a membrane, whereas globular molecules of the same molecular weight may be retained. The table shows typical results obtained with globular protein molecules and linear polydextran for the same polysulfone membrane [8]...

The temperature of the column is the most important parameter in GSC. Its effect on retention can be described by the same equations as were used in GLC, as can be seen from a comparison of eqns.(3.6) and (3.19). According to eqn.(3.10) a straight line will be obtained by plotting In (k/ T) vs. (1 / T). However, a plot of In k (or In V R) vs. the reciprocal temperature also yields approximately straight lines, as is illustrated in figure 3.5. 

Choosing the mobile and the stationary phase of roughly the same polarity (i.e. Sm x Ss). While this has the desired effect on retention, the remedy is equally effective for all solutes, independent of the value of 5, and therefore it creates a very non-selective phase system (see the discussion on selectivity below). 

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