Temperature programming in GC

We have seen in section 3.1 that the primary parameter in both GLC and GSC is the temperature. We have also seen that retention in GC varies very strongly with the temperature. The following equation was found to describe the relationship in quantitative terms  

This is further illustrated by the two (almost) horizontal lines, which enclose the optimum elution range (1 k 10). Apparently, there is no single temperature at which all components can be eluted from the column under optimal conditions. 

Harris and Habgood, in their standard work on programmed temperature GC [605] have shown that the retention time of a component under programmed temperature conditions is a function of the retention behaviour of the solute under isothermal conditions and the programming rate. The latter they defined as the heating rate (rT  

Resolution in programmed temperature GC is enhanced if the programming rate (rr/F) is decreased and if the initial temperature (T) is decreased. Giddings [606] suggested that the first peak in a programmed analysis should not appear within about five times the hold-up volume of the column. Since the temperature has little effect on the selectivity in GC (see section 3.1.1), the optimization of temperature programs is a process that may be seen as resolution optimization rather than as selectivity optimization. 

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Peak capacity can be very effectively improved by using temperature programming in GC or gradient elution in LC. However, if the mixture is very complex with a large number of individual solutes, then the same problem will often arise even under programming conditions. These difficulties arise as a direct result of the limited peak capacity of the column. It follows that it would be useful to derive an equation that... 

How is gradient elution HPLC similar to temperature programming in GC ... 

Note Here, the gradient elution may be simply compared to the temperature programming in GC. 

In some cases we may speed up the selection of appropriate primary parameters with the help of programmed analysis, i.e. temperature programming in GC or solvent programming in LC. Another useful scouting technique may be thin layer chromatography (TLC). Possibilities for establishing the appropriate values of the primary parameters will be discussed in section S.4. 

The inclusion of programming options (temperature programming in GC, solvent programming in LC) in the instrument may also be helpful, not only if a programmed analysis may be the result of the optimization procedure (chapter 6), but also to provide a scanning (or scouting ) facility for unknown samples (section 5.4). 

The overall density of the mobile phase is one of the most important parameters used to optimize separations in SFC with density programming as common in SFC as temperature programming in GC and eluent composition in HPLC [5]. Capacity ratios, k decrease roughly linearly at higher densities with different slopes for different classes of compounds, thereby affording changes in selectivity [5]. A similar effect is seen for the supercritical fluid elution of analytes from octadecylsilica sorbents, as seen in Fig. 2 [6]. 

Pumps can deliver solvent isocratically or via a gradient program (akin to temperature programming in GC). With a gradient program, the usual situation is to gradually increase the amount of organic component in the mobile phase over time so that the more nonpolar compounds are eluted in the same run as the more polar compounds. 

Programmed chromatography analyses in which the factors which determine the partition or distribution equilibrium are varied, e.g. temperature programming in GC, solvent programming in HPLC. 

The analogue of temperature programming in GC or gradient elution in HPLC is density programming [25] the pressure is changed so that a linear or asymptotic density profile is achieved according to the above polynomial. From eqn (9.3) it can be shown that there is a threshold density which all members of the homologous series co-elute. 

Peak capacity can be effectively improved by using temperature programming in GC (Section 4.4.3b) or... 

For complex mixtures, especially those analyzed with temperature programming (in GC) or gradient elution (in LC), it is impossible to select a single reference compound. The Kovats retention-index system [39], based on homologous series of reference compounds, provides an elegant solution in GC, one which has been widely accepted. In LC, the absence of suitable homologous series, and the fact that retention depends more than in the case of GC on the polarities of compounds and less on molecular weight makes the use of an index system impractical. 

Pressure changes in SFC have a pronounced effect on the retention factor k and thus the retention time l. The density of a supercritical fluid increases rapidly and nonlinearly with increases in pressure. Such density increases cause a rise in the solvent power of the mobile phase, which in turn shortens elution time. For example, the elution time for hexadecane is reported to decrease from 25 to 5 min as the pressure of carbon dioxide is raised from 70 to 90 atm. An effect similar to that of temperature programming in GC and gradient elution in FfPLC can be achieved by linearly increasing the column pressure or by regulating the pressure to create linear density increases. Figure 29-2 illustrates the improvement in chromatograms realized by pres-... 

What is meant by temperature programming in GC Why is it frequently used ... 

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