Speed up Separations and Maximize acetylene, propadiene and methylacetylene Response using High flow and Short alumina MAPD columns

Last week I had the chance to do some labwork at Restek on light hydrocarbon analysis looking at fast analysis of trace polar hydrocarbons (acetylene, propadiene).

For this application, a new column was developed : AluminaBOND / MAPD.  See also:  http://blog.restek.com/?p=2670   I always was amazed by the selectivity of the alumina adsorbent for hydrocarbon separation. Also the big impact of temperature on separation can be used to optimize separations.

As we wanted speed, we decided to use shorter columns.  I loaned a 15 m x 0.53mm fused silica Rt Alumina BOND /MAPD column from our PLOT specialist, Tom Vezza, and we ran this column at high flow-rate using an old 5890 GC. Alumina and high flow rate will result in low elution temperatures, which will make the alumina act as a more polar phase. The result is that polar hydrocarbons are moved backwards in the chromatogram and elute later in relation to the position of the saturated hydrocarbons.

Fast separation of C1-C5 on FS 15m x 0.53mm Alumina BOND / MAPD: Note high response and peaks symmetry for acetylene, methylacetylene and propadiene

I was amazed by the high response obtained on polar hydrocarbons, see figure 1. Propadiene, acetylene as well as methyl acetylene eluted with comparable response as the saturated hydrocarbons. Also the peak shape for these compounds will make Dr. Gauss very happy. In this case it is a result of the high inertness and loadability of the MAPD series of alumina columns. The test mixture used had all these components in 1.0 % w/v levels. We already knew surface deactivation was important, but elution temperature and residence time in the column should also not be underestimated.  Separation of all C1-C5 hydrocarbons on a 15 m x 0.53mm was very good and FAST. We got here down to 2 minutes separations. As we used high flow at relative low temperature, the alumina behaves very polar and we get the propadiene/acetylene separation after the n-butane. Limitation is however, that at very high flow rate the flame can be blown out. We could not exceed 100kPa for this column dimension.

In purity analysis of 1,3 butadiene, the methyl acetylene elutes after the 1,3 butadiene. As the 1,3 butadiene is the main peak it is broad. When a component is overloaded in adsorption chromatography, the peak will start to tail AND it will start to elute earlier. That’s why it’s preferred to have highest loadability and inject smallest sample size.

Despite of high loadability, the methyl acetylene elutes after the 1,3 butadiene on a skimmed baseline, see fig. 2. Longer columns will improve this separation, but also cost run time.

Crude 1,3-butadiene on 15m x 0.53mm Alumina BOND / MAPD; methylacetylene(MA) elutes just after 1,3 butadiene

If low levels of methyl acetylene are important one can also use the a lower polarity alumina column. We did some work on the Rt AluminaBOND / CFC and here the methyl acetylene elutes before 1,3- butadiene, see figure 3. Methyl acetylene is well separated from the 1,3 butadiene.

Crude butadiene on a 50m 0.53mm Alumina BOND/ CFC column; MA elutes before the 1,3 butadiene

The alumina BOND CFC is initially developed for eluting halogenated compounds, but due to its inertness, it does also very well for reactive hydrocarbons.

This type of alumina technology was successfully developed in fused silica and metal (MXT) columns.

4 Responses to “Speed up Separations and Maximize acetylene, propadiene and methylacetylene Response using High flow and Short alumina MAPD columns”

  1. Jack Cochran says:

    Hi Jaap:

    Great fast separations! Can you help the uneducated (that’s me!) understand why determination of methyl acetylene is important? Is it a unpreferred impurity? Does it have a negative effect on the use of 1,3-butadiene as a starting material?

    Regards,

    Jack

  2. Raymond says:

    Hi jaap:

    I am having problem analyzing propadiene using plot alumina deactivated with sodium sulphate. Does it react with the column or does it change into methyl acetlyene when the oven temperature is too high? Whenever i am analyzing propadiene, it doesnt return to baseline but will remain high till methyl acetylene elute when my sample doesnt contain at all. Currently using gs gaspro to analyze propadiene cause it doesnt have this issue. Do you have any idea whats wrong ?

  3. Hi Raymond
    I have seen this happening with several unsaturated hydrocarbons. The Na2SO4 deactivation shows the biggest reactivity. The KCl the lowest. Propadiene is a challenge on its own, as on Na2SO4 surfaces a % of the component just disappears. Looks like it is a tautorism where Propadiene converts in methyl acetylene and visa-versa, which is related to temperature and surface. The MAPD deactivation reduces this effect, but not totally. We also see reactivity on alumina with 1,2 butadiene and pentadienes. Silica BOND (like Gaspro) having silica surfaces, dos not show this behavior.
    regards
    jaap

  4. Raymond says:

    Hi jaap,

    You are good!! So far you are the only person that really know whats happening to my situation. I have asked so many experts but none gave me the answer i am looking for. The test mixture that you use was 1 percent concentration, does it happen too? Are there any other solutions other than using silica bond because the separation is not as good as using alumina column? I am ordering this column from restek but will shorter length mininise the activity? As i am analysing calibration mixtures for my customer which range from ppm to percent lvl, and usually from c1 to c9 alkanes, alkenes and isomers. I am looking forward very much to your advice. Thanks in advance.

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