Retention factor temperature

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. 

In addition to the above strategies, the use of higher column temperatures is another approach that may decrease analysis time and improve sample throughput. The relationship between the chromatographic retention factor, k, and separation temperature is shown in Equation 13.1 ... 

This derivation shows that retention time is dependant on three factors temperature, energies of intermolecular interactions and flow rate. Temperature and flow rate are controlled by the user. Energies of intermolecular interactions are controlled by stationary phase choice. This theory is also the basis for the popular software programs that are available for computer-assisted method development and optimization [4,5,6,7]. More detailed descriptions of the theory behind retention times can be found in the appropriate chapters in the texts listed in the bibliography. 

Increasing the column temperature reduces the retention factor. The ion-pair formation is based on a chemical equilibrium therefore, temperature control is important to obtain reproducible results. 

Figure 6.7 Measurement of enthalpy using chromatography for the relationship between absolute temperature and retention factor.

FIGURE 1.4 Dependencies of retention factors k on counterion (i.e., phosphate) concentration [X]. Experimental conditions Mobile phase, methanol-sodium dihydrogenphosphate buffer (50 50 v/v) (pHa 6.5 adjusted in the mixture with sodium hydroxide) flow rate, 1 mLmin temperature, 25°C CSP, 0-9-[3-(triethoxysilyl)propylcarbamoyl]-quinine bonded to silica [30] column dimension, 150 x 4 mm ID. 

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.)... 

FIGURE 1.6 Effect of organic modifier (methanol) percentage in the elnent on the retention factors (a) and observed enantioselectivities (b) of Af-(2,4-dinitrophenyl)-a-(2-chlorobenzyl)-proline employing an 0-9-[(2,6-diisopropylphenyl)carbamoyl]quinine-based CSP. Experimental conditions Elnent, ammonium acetate buffer-methanol (total ionic strength = 25 mM pHj, = 6.5), methanol content varied between 60 and 90%, while ionic strength and apparent pH were kept constant temperature, 40 C flow rate, 0.8 mLmin . (Reproduced from A. Peter et al., J. Sep. ScL, 26 1125 (2003). With permission.)... 

FIGURE 1.8 Effect of the mole fraction of polar modifier (ethyl acetate) in n-hexane on the reciprocal of the retention factor for the separation of 3-chloro-l-phenylpropanol enantiomers on a 0-9-(terf-butylcarbamoyl)quinidine CSP. Temperature, 22°C. (Reproduced from L. Asnin, and G. Guiochon, J. Chromatogr. A, 1091 11 (2005). With permission.)... 

A parallel study [94] was performed by the same authors on the retention of model compounds (tryptophan, erythro- and t/ireo-P-methyltryptophan, A-carbobenzyloxy-tryptophan, A-(3,5-dinitro-2-pyridyl)-tryptophan, l-[5-chloro-2-(methylamino) phenyl]-l,2,3,4-tetrahydroisoquinoline, and y-phenyl-y-butyrolactone) on a ristocetin A-based CSP, using the three RP, POM, and NP elution systems. Also, in this case, the natural logarithms of the retention factors (In k) of the investigated compounds depended linearly on the inverse of temperature (1 /T). 

For a partial separation situation after screening, the organic modifier content and temperature are decreased according to a 2 full factorial design. When baseline separation is obtained, the retention factor can be further optimized by changing the... 

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. 

By simultaneous optimization of the percent organic modifier in the eluent and the column temperature to keep the retention factors fixed, very efficient, ultrafast separation can be achieved. The researchers conclude that for fast separations, the relationship between retention, temperature, and volume fraction of organic modifier needs to be taken into account. As the temperature increases, a lower volume of organic modifier is needed to speed up HPLC. Therefore, a highly retentive column... 

An increase in temperature decreases the viscosity and hence increases the EOF. Thus, for a given voltage, more rapid analysis is possible. Temperature also affects the solute partitioning between the mobile and stationary phases and therefore the chromatographic retention. The distribution of the solute between the mobile and stationary phases is a function of its solubility in the liquid phase and adsorption on the solid stationary phase. This is characterized by the distribution ratio K defined as the ratio of the concentration of the solute in the stationary phase to its concentration in the mobile phase. Retention factors are influenced by increasing column temperature because of the increased partition into the mobile phase according to the Van t Hoff equation ... 

Fio. 26. Methylene group selectivity, ocn,i of several hydroorganic mobile phases when octadecyl silica stationary phase is used. The selectivity is the ratio of the retention factor of a member of a homologous series to that of another member which differs in having one less methylene group. The solvents shown here are (A) acetone, (B) acetonitrile, and (C) methanol. The dau were taken at ambient temperature and the selectivity values are plotted on a logarithmic scale. Reprinted with permission ftom Kaiger et al. (/4S).. ... 

As a result the dependence of the logarithm of the retention factor on the absolute temperature is given by... 

Gant et al. (175) examined the effect of temperature on resolution and on selectivity, retention factors, and plate number, which determine the magnitude of resolution. They found that these data can be used together with the lempeniliire dependence of solvent viscosity to optimize iinaivsis rate with required resolution. This is of particular interest when RFC is used for automated repetitive analyses of lar e numbers of samples. 

Fig. S7. The logarithm of the retention factor of methyl red (cire ps) and methyl orange (squares) in 50 vol % methanol (1 atm, pH 7) increases with inlet pye sure. The data were obtained on Bondapak Cu/Corasil at room temperature. Reprinted Prukop and Rogers (28/), with permission from Marcel Dekker. f...

Solvent gradients are generally mnch more efficient to decrease the retention than programmed temperature. For example, the retention factors k of low-molecnlar-weight analytes in reversed-phase... 

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