[1] What do Chromatograms tell us? Peak Shape: Reactivity of Alumina
28 Apr 2013
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.
Fig.1 Reaction platforms for 1,2 butadiene observed on active alumina if analyzed at 150C
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 Decomposition in the column? The Peak shape will tell !
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;
Fig. 2 analysis of 1,2 butadiene at 110C. Due to lower temperature, recativity is greatly reduced
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. New alumina-based PLOT column for high resolution separation of CFC / Refrigerants