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[18] What do Chromatograms tell us? Peak Tailing, Detector effects. What can a MS contribute?

13 Aug 2013

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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.

Peak tailing is often contributed to activity of injection ports or the column. When working with an instrument company very often the separation “columns” were blamed for activity.

I had this experience when working with a instrument company. When the results were discussed, there was always a fight between “MS-Instrument specialists” and “Column specialists”. For MS specialists, the GC column is basically a "sample introduction device" for the MS;  For the Column specialist, the MS is a "detection system", to detect eluting components and that has also capability to qualitate (identify) the components.

The last decade MS techniques have evolved and have become easier to use. Still, the basic chromatography needs to be respected as the MS results are strongly depending on what happens “before” the MS.Blog18-fig1
Fig.1 PAH detected by MS system. Note the tailing on later eluting PAH. Is this a bad performing column?

Fig. 1 shows an example of a PAH analysis using an ion trap detection system.  The column was installed via transferline in the optimal position for transfer to the ionization-source. The late eluting PAH peaks are strongly tailing and the MS-specialists blame the columns for being “active”.

The column was taken out of the MS and installed in a FID system and results look like fig 2.  We obtain perfect symmetrical PAH peaks.Blog18-fig2
Fig.2 The same column, but now installed with a FID. All PAH elute as perfect peaks. Detector and interface does not change peak shape

It was very clear that sometimes MS detection systems have a tendency to adsorb components, which results in tailing. Ion trap electrodes need to be deactivated, but also quadrupole systems can show tailing on PAH. This can also happen in the interface/transferlines if the column itself is not used as transferline.

It’s not easy to get gaussian PAH peaks in a MS, as activity will always be there. All ms systems will suffer more or less from this Deactivation of MS electrodes using Siltek will help, but is costly and will take extra service time.Transferlines must also be well deactivated and temperature needs to be high enough to prevent "cold-spots".

Alternate solution is to keep columns at a higher temperature for some time, to generate some level of bleed. The bleed products of siloxane stationary phases will deactivate electrodes/ionization source, which will improve peak symmetry.

Sometimes “column bleed” is not our enemy..
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