C18 column bleed at 100% organic?

[yuki_weber]

Hi All,

I'm seeing something that looks like column bleed on an ELSD detector at 100% organic (60:40 Methanol:Acetone), i.e., a strongly elevated background.

The column manufacturer does not state anything specific regarding solvent stability.

Anyone seen something similar before?

Thanks,
Yuki

[Gerhard Kratz]

I guess you are using an HPLC packing material with pores? Beside the pores the material has also micropores. During the manufacturing process a kind of catalysator is used, for example to enable the endcapping. During the manufacturing process this "catalysator" has to be washed out of the material. The one manufacturer does this very carefull, the other pay not so much attention. In UV you don't see that wash out! In NMR and ELDS you can see it.
It takes time to do that wash out because flow rate will not help, only diffusion. Manufacturers cannot tell you how they do it. You can test different manufacturers. Or reduce the sensitivity of your detector. Sorry for that.
Good luck.

[vmu]

There can be virtually no problems with a C18 column in 60:40 methanol:acetone. However, 1) this mixture is a strong eluent, 2) ELSD is a universal detector sensitive to any non-volatile substance and to microparticles (debris) eluting from the column.
A C18 column can contain some hydrolyzed C18 groups (e.g. octadecyl(dimethyl)hydroxysilane), which are hydrophobic and strongly adsorbed. The strong eluent can wash them out to the detector resulting in elevated background signal. Also the problem may be caused by impurities in acetone. Additionally, you can try to filter the mobile phase through a 0.1 μm filter (first check the filter material resistance to the eluent).
If the debris from the column is the cause, it may be necessary to wash the column for a long time to reduce the background or necessary to change the column.

[yuki_weber]

Thanks everyone for your responses! Indeed I saw a decrease in the 'bleed' after several injections, so what you suggested makes total sense.

I'm glad to be reassured that the solvent composition is dot detrimental to the stationary phase!