Temperature Effect on GC Separations

What we really want in a good separation is:

1. complete separation of the compounds (base-line separation of the peaks in the spectrum)
2. narrow peaks so that the response of the system is large and noise in detection is relatively small

The quality and speed of separation of a component mixture on a GC is determined by both thermodynamic and kinetic factors

1. the thermodynamic partition coefficients of the components in the stationary phase
2. the speed with which the partitioning equilibrium can be established
3. the time the system has to establish the equilibrium

Of these three factors only the first two will be affected by temperature. The latter is more determined by the flow rate of the carrier and type of column used.

We will consider how the first two factors are influenced by temperature and as such influence the separation.

1. Temperature and Thermodynamic Partition Coefficient

The thermodynamic partition coefficient is really the equilibrium constant for the reaction

where the subscripts M and S refer to the mobile phase and stationary phase, respectively.

A value of K larger than one suggests that a preference exists for the compound to be in the stationary phase, while a K value smaller than one suggests that the compound prefers to stay in the mobile phase.

The partition reaction, as written above, is usually an exothermic reaction. This means that as the compound goes into the stationary phase heat is released. Le Chatelier's principle then says that the equilibrium will be shifted to the left if extra heat is supplied. Thus at higher temperatures a smaller K is expected and the more of the compound will be present in the mobile phase.

Note:
The above consideration only reflects equilibrium conditions on the column. Usually the conditions of the separation are such that equilibrium can not be established and the reasoning does not strictly hold for real life situations. The qualitative effect of higher temperature -> more in the mobile phase does still hold though.

2. Temperature and Establishment of Equilibrium

According to the kinetic molecular theory, temperature can be considered to be a measure of the speed of the particles (actually the root mean square speed). This means that at higher temperature the particles will have higher speeds and the diffusion rate of the compounds will be increased too. If we restrict ourselves to the simplest case of a wall-coated open tubular column, i.e. no particles packed in the direction of the flow of the gas, diffusion can occur in two directions:

1. Diffusion parallel to the stationary phase (longitudinal diffusion)
   This causes the broadening of the plug of material on the column independent of the interactions that it may have with the stationary phase. Thus a higher temperature will cause peak broadening and worsen the quality of the separation.
   
   
2. Diffusion perpendicular to the stationary phase
   This is the diffusion that allows the establishment of the partition equilibrium to be reached, and as such affects the retention of the material on the column. Thus a higher temperature will cause the equilibrium to be established faster and make for a better separation.

Both of these forms of diffusion are equally affected by changes in the temperature, but have opposite affects on the separation of two compounds.

These considerations only refer to the rate of diffusion. The total broadening of the plug of material on the column, and therefor the peak in the spectrum, is also determined by the amount of time that this diffusion can take place, i.e., how long the material is on the column.

Conclusion

Thus the effect of raising the temperature is that more of the compounds will be in the mobile phase resulting in less time on the column and less complete separation. The broadening of the peaks is affected by the fact that diffusion is faster (broader peaks), but less time is spent on the column (narrower peaks). Usually the latter of the two is dominant. In general one can expect narrower peaks but less separated peaks at higher temperature.

This means that in order to get good separation a balance must be found: the temperature must be low enough to achieve base-line separation, but not so low that the broadening of the peaks causes problems in the detection of the compound.

Below is an interactive movie animating the separation of a two component mixture on the same columns at different temperatures.

Things to note in the animation are:

1. From the spectra and the separation it is possible to see how the temperatures of the three columns are different.
2. The bottom column is too hot and the separation does not occur. The material does come off the column in a very short time (more in the mobile phase): too short retention times.
3. The top column is at a too low temperatures causing the separation to occur but in such a long period of time that the peaks are more broadened than necessary: too long retention times.
4. The middle column is just right. The peaks are just separated. Any increase in temperature would make them overlap more. Any decrease in temperature would separate them more, but would also broaden the peaks.