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.
A peak shape is observed as shown in figure 1. This was an analysis of 1,2 butadiene on an alumina PLOT column, run at 150C isothermally. We see 2 reaction platforms: A and B. The 1,2 butadiene is converted in 2 components that have more retention, while it is moving trough the column. Alumina remains a very active material and some products decompose or react.
If a component is formed that has less retention, the reaction platform will be in front of the main component. We see this for instance with brominated flame retardants, see http://blog.restek.com/?p=602
Any time such a “reaction platform” is seen, we need to find conditions to reduce this effect. The component must elute at a lower temperature. This can be done via:
Use lower analysis temperature, see fig. 2 At 110C the peak elutes quite good; It will take a lot of extra analysis time;
Higher flow rate: this is also easy to try. Increasing the flow a factor 4 roughly results in 30C lower elution temperature; This is not always applicable ( for instance when MS is used);
Use Thinnest possible Films/layers: possible for many liquid phases. In this case, using alumina, there are no thinner layers commercially available;
More inert alumina columns.: Often reactivity is caused by impurities in alumina matrix. We have seen differences in alumina, which f.i. has led to a special alumina that behaves better for CFC also. http://blog.restek.com/?p=1143