Chromatography Forum

Hi,

I am setting up a normal phase method for the first time. Right now, I'm just trying to repeat the chromatogram in the column insert. My mobile phase is 98% hexane/2% ethyl acetate. I'm using a 3.3cm x 4.6mm Supelcosil LC-SI silica gel column with a small silica gel guard cartridge. When I run at the recommended flow rate of 1 ml/min I get a lot of pressure fluctuation. This goes away if I run at 5 mL/min. I assume that this means that I'm getting air bubbles in my system somewhere. My instrument is an Agilent 1100 with a binary pump and an inline degasser. Any suggestions to get more constant pressures?

Thanks,

KarenJ

I'm assuming that you're mixing on-line. If so, try pre-mixing your mobile phase and then putting it in both the A and B reservoirs. Theres a chance that your "B" pump isn't happy at 20 μL/min.

If that does't fix the problem, try turning off the degasser. Outgassing is much less of a problem in NP, because you don't see the huge shifts in air solubility vs composition.

Have you change the plunger seal with normal-phase compatible one?

I have experience with our Shimadzu. When we run normal phase the seal was leaked so easily (though I had change it a week before I run normal phase LC).
When we bought Agilent (it was my experience with Agilent), I asked the technician if they have normal-phase compatible seal. They explained that they have two types of seals: for reverse phase (5063-6589) and for normal phase applications (0905-1420).
The seal leakage may be the source of your problem.