[1] What do Chromatograms tell us? Peak Shape: Reactivity of Alumina

2011-jaap-pasfoto4Chromatograms 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 fot 1,2 butadiene observed on active alumina if analyzed at 150C

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 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;

Fig. 2  analysis of 1,2 butadiene at 110C. Due to lower temperature, recativity is greatly reduced

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.  http://blog.restek.com/?p=1143

 

One Response to “[1] What do Chromatograms tell us? Peak Shape: Reactivity of Alumina”

  1. Lars Kurstein says:

    Dear Jaap -

    Always look at the chromatograms! Jaap, this installment is pure gold … Chromatograms like the one in this example is a very tough one, especially if we are dealing with unknown unknowns in the sample matrices. Formaldehyde and Water are another tough components, causing a lot of chromatographic problems. Please go on with this nice lecture in interpretating symptoms and odd peaks in chromatograms.

    With kind regards -
    Lars Kurstein, Copenhagen

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