Keeping Normal Phase System Dry?

[rkh]

Is is possible to keep a Normal Phase System Entirely DRY... not to absorb any water from the atmoshpere? If so, how?

If it is not possible, then is it better to pre-saturate mobile phase with water, CH3CN, or MEOH? What about the column?

[Noser222]

I've heard of a procedure using "half-saturated" solutions. Take your water immiscible solvent and saturate it with water, then mix it with an equal volume of unsaturated solvent.

[tom jupille]

The column will equilibrate with the water content of the mobile phase.

A thirty-year old trick that I've used is to get an empty prep column and pack it with silica (you can use large-particle material) which has been oven-dried and then had a measured amount of water added (sorry, I don't recall the exact percentage). Plumb this into the system as a "saturator" column between the pump and the injector. Then begin to pump mobile phase. As I recall, it took quite a long time for things to equilibrate. Once equilibrated, the saturator acts as a water "buffer". On a damp day, the saturator will take excess water out of the mobile phase; on a dry day it will release water to the mobile phase.

It's a PITA to set up, but it does work.

[GaryR]

Hi

Every normal phase system I have encountered has a small amount of water added.

We were experiencing problems with equilibrating these systems, and for a while we thought we had some dodgy columns. The peak retentions were shifting around, and the peaks would start out nice and sharp, but after a while would tail quite badly. We went through quite a few 10µm silica columns.

Problem solved:
Place mobile phase reservoir on a magnetic stirrer and stir gently through-out the run. (Don't put this too close to detector as it can cause baseline ripples)
Recycle during equilibration stage (can take from a few hour to a few days to fully equilibrate).
We even recycle MP during the run for a couple of low-dose products, but you need to make sure the accumulated analytes in the MP don't affect the results.

We were even able to regenerate all the columns we once thought were dodgy, even those that had been used for reverse phase with aqueous buffers.

Cheers
Gary

[Victor]

Hi Gary-that's a good suggestion about the stirring.

Can you confirm that you sometimes use bare silica columns for RP chromatography- is that what you mean? If so, what applications (in general terms) do you find this works for you?

[GaryR]

Hi Victor

Yes, we sometimes use bare silica columns for reverse phase work.
I did a quick search and could only find one (very quick search, I'm pretty sure we have a couple of others but would take some time to find out)

The analysis is an assay on famotidine tablets.
Column: µPorasil 300*3.9mm, 10µm.
Mobile phase: [31 H2O + 6 MeOH + 3 Buffer (phosphate, pH7)], pH5 with H3PO4.

Not sure where the method came from - possibly the supplier.
Our HPLC assay methods are supposedly stability indicating, but I suspect that this one is just a glorified UV detection (not sure how anything would get separated).
Not having done this analysis myself (and not looking into the molecule at all) I'm not too sure just what is going on...

Regards
Gary

[Bruce Hamilton]

It's the standard USP method for Famotidine tablets.

Please keep having fun,

Bruce Hamilton

[AdrianF]

The use of bare silica using aqueos solvents is using mainly an ion exchange mechanism - I think all the separations are of basic compounds.