Pre-column and tailing

[ciubi85]

Hi!

I’m working in the organic gunshot residue (GSR) field and, currently, I’m developing a GC multi-residue method to analyse molecules such as nitriles (e.g., benzonitrile, tolunitrile, dicyanobenzene) and small (hetero-)PAHs up to 4 aromatic rings in GSR extracts. In order to avoid column degradation, I've installed a few days ago a pre-column on my Agilent GC/MS system. Unfortunately, after launching standards mix, I’ve noticed significant tailing on the peaks of some selected compounds. These are, especially, quinoline, all the isomers of dicyanobenzene, indole and the two isomers of naphthalenecarbonitrile (elution of PAHs was apparently normal). The analytical column is a 30m HP5-MS, while the pre-column I’ve just installed is a 4m de-activated uncoated column. I’ve have a PTV injector and a ramped injection method. Do you have any idea about what might be the cause? Is the pre-column too long, or maybe have I badly linked some connections? Or do the compounds interact with the uncoated column?

Thanks

[BMU_VMW]

I don't have any experience with the components you are analysing but the first thing that comes to my mind is a bad connection between pre-column and analytical column. If there is to much 'dead volume' it can cause tailing and peak broadening.

We always use a Agilent Ultimate union to connect both columns and it works really well
http://www.chem.agilent.com/Library/Sup ... F03032.pdf

[ciubi85]

Thank you for your answer!

I use the same union kit you have cited. Concerning the dead volume, I was reasoning about it but I think it doesn’t explain the limitation of the problem only to a restricted group of molecules (apparently, nitrogenated compounds). If a dead volume is present, would it affect ALL the compounds, right?

[James_Ball]

If the ends of the columns are not cut very square you can end up with more active sites and more dead volume which may be interacting with the specific compounds since the cut portion will not be deactivated.

Also you may try guard columns with different types of deactivation such as acid deactivated, base deactivated and Restek makes one called "Hydroguard" which helps when using samples with high moisture content but may help with these also.

[Rick]

Try 1m or 2m of pre-column...4m is very long for a pre-column.....