Chromatography Forum

My GC-MS system frequently has peak shape problem (tailing and fronting) from inlet and column head contamination after injection of protein precipitated serum spiked with glycols. I have to perform inlet maintenance and cut column head as often as every batch of 20 injections.

I am now using a HP-5MSI column for the analysis and am considering using a guard column.
I see that an integrated guard with analytical column is available with 5MS type column (VF-5MS) but not with 5MSI or 5MS UI column.

Does anyone know if this integrated type column is better than pressfit type guard column?
The VF-5MS is not inert or ultra inert. Will the shortcomings outweigh the benefit?

Thank you very much.

Hello Edde,

I've been using guard columns in my GC/MS since always. The use of integrated columns is much better than the press fit ones. With the latter I have occasionally experienced problems such as leaks and unexpected breakages; besides it takes some practice to install them correctly.

Hope it helps

in stead of using press fits we use an ultimate union http://www.chem.agilent.com/Library/Sup ... F03032.pdf
they work really well, don't leak and are not that difficult to use.

We use a Restek RXi 5 sil ms column with integra guard and use the guard side in the transferline to prevent column bleed in the ms. on the injector side we use a regulare (deactivated) guard column that is changed monthly and connected with an ultimate union.

Thanks both. I have no experience at all with use of guard column, so your experiences are really enlightening.

I want both the inertness of analytical column as well as cleanliness function of the guard. So my idea is using a fused silica tubing as inlet guard connecting to the HP-5MSI analytical column with Ultimate union and Ultimetal Plus ferrules.

I have no idea about using a guard column to transfer line. What is the benefit?

How long should the guard column be? Will that affect compound retention time a lot?

How much should the guard column be cut each time?

My heartfelt gratitude to all your comments.

we use the integraguard column in the transferline (total length of 'pre' column is 5m). only to prevent (minimise) column bleed from entering the MS. The transfer line is at 250 -300°C (depending on the method that is applied). With an analytical part of the column in the transferline this would cause more (I did not say a lot) column bleed into the MS.
We use a seperate pre-column because it is easy to replace. I don't take small parts of each time, I just replace it once a month. this way there is hardly any shift in retention time and for my application it's ok to leave it for a month without having to much problems related to activity. I do take a small piece off the analytical column about once every 2 months (e.g. 10-15cm).
The guard column does not have a huge effect on retention and seperation (at least not for my pesticides and PAHs).

I use a 10 m (integrated) guard column. I cut about 70 cm every 40 samples or so, and also when the injector liner is dirty.I guess you'll have to find your periodicity. Retention times are slightly affected (they are shorter) but it is easy to calculate the new pressure with the software (mine is Agilent's Chemstation)using the actual length of the column (nominal - length cut).
Using a guard column your peak shapes could improve if you injected at an oven temperature below your sample solvent's boiling point, held it for a couple of minutes and then increased it above it before running your customary temperature program. This will focus your analytes in a narrow band at the head of the analytical column.

Thanks again.

It sounds that the dirty non-volatile can still accumulate in the guard and affect peak shape and quantitation as well. Is my perception correct?

I think I will try 5m fused silica guard and test how frequent and how long I have to cut the guard and the influence on retention time and peak shape.

The solvent focusing approach is also useful to me.

A million thanks to you all.

Have you considered using a liner with wool to keep the "junk" in the injector?

I have been using a liner with glass wool. But that did not work.

The only downside to the integrated guard columns is that if there is something the passes through the guard column and onto the head of the analytical portion you have to remove and lose all that guard column just to trim a little off the coated portion. I have always had better luck using a 5m or less guard and on a periodic basis removing it, trimming maybe 10cm from the analytical column then reinstalling a new guard column. The guard is much cheaper than the analytical column and replacing it often makes the analytical column last much longer.

Thank you very much, James.
I appreciate your kind sharing of invaluable experience.