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[15] What do Chromatograms tell us? First eluting peak is much too broad

29 Jul 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.

An analysis was done where a highly volatile component was measured in a matrix of air.  In this case it was ethylene.  This component has to be monitored in green houses at very low levels.  A chromatogram was obtained as shown in fig 1.Blog15-fig1
Fig. 1 500 ppb ethylene produces a "broad" peak on this system. The ethylene is already "moving" while injection is still not finished.

The ethylene peak is very broad and sensitivity is compromised.

It is clear that a huge injection error is made. The ethylene peak is already “moved” through the column while the sample is not injected totally.

For the best chromatography one needs to reduce injection band width. This can be done by reducing sample volume, or using a column with highest capacity and/or retention. Also temperature is an important factor. The lower the temperature, the higher the retention which reduces the injection band width.

Also make sure that the injection is as “smooth” as possible (no dead volumes):  For this, one may use the Uniliner type designs.

To retain ethylene, liquid phases like Rtx-1 will not deliver enough retention at temperatures above ambient.   One needs to consider PLOT columns for this. PLOT columns are based on adsorption chromatography and have approximately 50x more retention then liquid phases. For retaining volatile hydrocarbons there are several choices.

Rt Alumina BOND:  this phase is dedicated for hydrocarbon separations, highly selective. For example see: GC_PC01085   Challenge is that alumina also adsorbs water and polar compounds. As the matrix is air, water will be present.  This will result in variable retention times. If water is present, one needs a temperature program to 200-250C (250C for the Alumina BOND MAPD), to remove the water after each analysis.

Rt Q-BOND and U-BOND: These are porous polymers, highly retentive for volatiles, and a nice choice as it has little affinity for polar analytes (including water).  See "Light hydrocarbon analysis in water samples – not just a headspace method anymore"

Ethylene elutes relatively close to acetylene, so injection quality needs to be good or there may be a co-elution.

One can also use a material called “Shincarbon”. Here a huge retention is obtained for C2 compounds. This can be a benefit if large sample volumes are to be injected. Elution will take at least a temperature program to 200C. The Shincarbon is available in different dimensions, including 0.53mm MXT. The 0.53mm internal diameter fits also nicely in standard capillary GC systems, see "0.53mm wide bore MXT Metal MicroPacked Columns do Gases and Fit in any Standard GC

It still remains a packed column, meaning, efficiency (plate number) is relatively low. As there are only a few target analytes, high efficiency is not required.