I need to select an appropriate size guard column for use with a 4.6x50 mm 5um 120a c18 column. Running acn and water at 2-3 ml/ min. I know that typically guards should have a larger pore size but no one ever talks about actual physical dimensions of the guards. Please help, Thanks.

I would use whatever the manufacturer of the analytical column suggested. Your analytical column is short -- for our Supelco 4.6 mm x 5 cm amide column, the manufacturer supplies a 4 mm x 2 cm guard.

What I heard once was that guard columns were loosely packed with media. So their presence or absence should have a negligible affect on separation.

The only effect should be an increase in dwell volume, so a long length is to be avoided.

The most important thing in selecting a guard column is to use the exact same material as in your analytical column. In this case, cheaper is not better, it is just plainly useless. The size of the guard column is not important, if it is well packed. If the supplier of your column does not provide high-performance guard columns, but only junk (you can tell from the price), you should attempt to select a guard column with a smaller i.d. It will work as a guard, but the pressure is a bit higher than with the larger i.d. But a small guard does not destroy your separation.

What is your experience with plate numbers when using a guard column? We have seen a drastic decrease from 10000 to 8000. Did we something wrong?

K.H.W.: This is what one gets when the guard column is either packed with a different material or is not packed to high quality.