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GC Oven temperature and its effect on chromatography

When running a chromatogram there are 2 options for oven temperature. Either isothermally, which is defined as constant temperature for the entire chromatogram, or it can be a temperature programme. Temperature programme is when the temperature of the oven changes over the course of the run. The temperature can be increased at a constant rate, or by increasing to a particular temperature then holding constant for a given period of time. All of these differences in oven temperatures will have a different affect on the elution of the compounds.

Looking at temperature effects on non-polar columns for an isothermal run we notice that increasing the temperature has two effects on the chromatography. (see Figure 1) The run time is shortened and the resolution is reduced. Therefore there is a compromise between the oven temperature that can be used and the desired resolution. As a general rule of thumb, a difference of 2°C in boiling point between compounds will be enough to separate them.

Figure 1

Temperature programming can help with this compromise as it allows the user to increase temperatures at certain intervals to achieve the resolution required in the shorter runtime. The simplest way to determine if the temperature can be increased in a run is the determine the resolution of the two peaks. Figure 2 shows you the resolution equation. An R value of greater than 2 means that the two peaks are well separated and could be eluted closer whilst still maintaining good resolution. Therefore the temperature could be increased and still obtain good separation with a reduced run time.

Figure 2

Athough using different temperatures during a single chromatography run can be beneficial in reducing run time, it can also decrease the lifetime of your column. As mentioned earlier a higher temperature will reduce the analysis time, but it will also increase “column bleeding”. An increase in column bleed will reduce the life of the column as this bleed is actually phase being removed from the column. Therefore the maximum temperature limits need to be taken into account.

The above information is only valid for non-polar capillary columns as they separate predominately on boiling point. Therefore changing the temperatures will not alter the elution order. When using mid polar and polar columns (which separate primarily on polarity) changing temperature programmes can alter the elution order.

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