Chromatography Forum

Hi,

Currently I'm using a Chromolith 100x4.6 mm column with a guard column, but I'm considering moving into a smaller dimension: 100x3 mm. However, guard columns of the same chemistry are only available in 5x4.6 or 10x4.6 mm dimensions. I would still like to use a guard column -the question is whether a 4.6 mm guard column coupled to a 3 mm analytical column will cause major band broadening?

Thanks for the help,

Martin L

I don't think it would be noticeable. I just did similar Wednesday, to change from 150mm x 4.6mm to a new 150mm x 3mm, but the 3mm guard cartridge didn't arrive yet, so I used one that was 4.6mm i.d. temporarily. Results looked very similar to those on the complete 4.6mm system. I halved the flow rate (to keep linear velocity the same) and the injection size too.

It is not essential to have a guard column with the same chemistry - as long as it is less retentive than the main column it should be fine.

As far as using a guard column with larger diameter than main column -try and see.

Alternatively, dispense with the guard columnn and use an "in-line" filter unit instead.

You generally loose more performance, if the diameter of the guard column is larger than the diameter of the analytical column. If the loss is not important, you can of course live with it.

I disagree strongly with using a different C18 in your guard than in your main column. For one thing, mixing different chemistries will result in larger losses in efficiency compared to the use of the same chemistry. The second element is that you will get a better protection by using the same surface chemistry. A less retentive guard column will give you also less protection.

Also, filters don't do the same thing as guard columns. Guard columns provide more protection.