Chromatograms are like fingerprints. If you can “read” chromatograms well, you often can find a plausible cause. In this series, we will show a series of GC-chromatograms that are obtained from users and discuss some potential causes for the phenomena. Then we can move into some solutions for improvement.
The separation process and resulting peak shape in a chromatographic system, remains a sum of interactions that take place from the moment the sample is injected into the injection port. Also when the sample is traveling through the column, different interactions take place. Sometimes the matrix of the sample can also play an important role.
Fig. 1 shows the analysis of low methanol levels in ethanol using a Rtx-Wax column.
The Rtx-Wax column is perfect for this separation, although the peak shape for methanol looks bad. Analysis temperature was 40C, which is a temperature the Rtx-Wax still is quite efficient. The tailing that is obtained on the methanol peak is different from the “normal” tailing we get. If active sites are in the columns, the peak usually shows a strong tail at the bottom of the peak. Here the tail starts clearly from the top of the methanol peak.
The process that causes this type of interaction is initiated by a large concentration of a component that elutes just behind the target analyte. When the separation process starts , the huge ethanol peak will saturate the stationary phase and the interactions of methanol do not go only with the stationary phase, but also with the stationary phase-saturated with ethanol. This process depends strongly on the polarity of both compounds.
The result is that the chromatography of the methanol peak is influenced by the large ethanol peak.
One way to minimize this effect is by introducing less sample. Fig.2 show the same sample, but now the injected amount was reduced by a factor 4 (2x smaller injection volume, 2x higher split). The difference in methanol peak shape is huge.
If situations like this occur make sure:
- inject smallest possible amount (less sample, higher split)
- Use higher analysis temperatures: the effect will be less, but only use this if the separation is still possible;
- Use a thicker film coated column: higher capacity will help in peak shape; thicker film, wider column ID, will also allow higher analysis temperatures;
- Use a more selective phase: by moving target analytes away from the “big” matrix peak. This is not easy is this case, as we use already a PEG;