Retention factor temperature programming

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. 

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. 

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... 

Retention factors in CEC are also reduced by increasing column temperature because of increased partition into the mobile phase van t Hoff plots of In k versus T l are generally [7,55] linear, and the slopes of such plots may differ sufficiently for column selectivity to be changed by temperature variation. For example, in the CEC of a number of diuretic drugs on ODS-bonded silica at temperatures between 15 and 60°C, the resolution of chlorothiazide and hydrochlorothiazide increases [7] with decreasing temperature, and the relative retention of chlorothalidone and hydroflumethazide is reversed with increasing temperature variations of k with temperature, which may make [56] temperature programming a useful technique in CEC. 

In GC capacity factors, retention and selectivity are controlled by adjusting the column temperature and stationary phase characteristics. In LC change in the composition of the eluant serves both purposes more effectively and thus solvent programming (otherwise known as gradient elution) is used in HPLC where temperature programming would be employed in GC. 

Tf = final program temperature. To = initial program temperature, t = time, tp = program time for a linear temperature program, tM = column hold-up time, k = initial value of the retention factor at the start of the temperature program, wp = peak width at base and N = column plate count. 

A computer program for the optimisation of ID GC separations [30] has been modified to include the additional variables required for GCxGC [31]. The estimation is also based on Equation (6), but the use of the distribution constant K instead of the retention factor k allows the extension of its application to different column geometries and to mixed stationary phases. The compounds in the Grob test mixture were used in the validation. The retention times of these compounds, obtained using two different temperature programs were the only experimental data required. Results were good except for the retention of a few compounds in the D column. 

Figure 11 (A) Isothermal temperature conditions iead to an exponential increase in retention times for successive members of a series of homologous compounds. Peaks get broader, and for approximately equal amounts injected, their peaks will get smaller (assuming similar response factors). (B) Linder temperature programmed conditions, peak widths are now similar over the elution range for the sample, and total analysis time can be reduced significantly compared with isothermal operation. Peaks should be approximately similar heights if the same amounts are injected. Here, it would be suspected that more of the later eluting compounds are present.

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