AWarningWhen Adjusting Temperature Programs


When changing a temperature program, confirmation of peak identities in the new chromatogram is essential. Peak retention orders can shift upon a change in the temperature program (called peak inversions). Peak misidentifications or an apparent loss of a peak (actually co-eluting with another peak) are common results of undetected peak inversions. This is especially true for the most polar stationary phases.


Most analyses require the use of a temperature program. A temperature program involves heating the oven at a controlled rate during the run. This allows the faster analysis of solutes with dissimilar retention, and there is very little peak broadening with an increase in retention (Figure 10b). The primary disadvantages of a temperature program are the more difficult method development process and the longer GC oven cool down time between analyses. There are no secrets or tricks for finding the best temperature program for an analysis. Usually some trial and error is involved.


If numerous attempts at different temperature programs have not resulted in satisfactory peak resolution, a different approach may be necessary. Some compounds cannot be separated with a particular stationary phase with any reasonable temperature program, thus a different stationary phase may be necessary. Sometimes, improving efficiency may be the answer. Optimizing the carrier gas average linear velocity, improving injector efficiency, or using a more efficient column dimension may provide the desired resolution.


Figure 10b: Temperature Program Condition


Column: DB-1, 15 m x 0.25 mm, 0.25 µm Carrier: Helium at 30 cm/sec Oven:


C10 C11 C12 C13 C14 C16 C15 60°C for 1 min, 60-180°C at 20°/min


02 4 68 Time (min.)


156


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