Why So many Bubbles?

[bmoritz]

Just switched over our reverse phase 1100 HPLC to normal phase and I'm encountering a small problem. It seems that the hexane line leading OUT of the degasser into the quant pump is bringing in air bubbles. This only happens to the hexane solvent line. The other lines (IPA MeOH) have no air bubbles. What could be causing this? Thanks again for the responses. This forum is a HUGE help!!

BM

[mbicking]

There could be many reasons, but it may just take some additional purging, since this channel has the biggest change in solvent properties.

And I am assuming that you changed the solvent inlet filter during the change, but if you haven't, try that also.

[TimB]

Try purging the line longer. I've worked with HPLC's where the solvents have run dry before or the lines just haven't been used in a while, and I had to purge a lot longer than usual. It may take a half hour or so.

[adam]

We typically do not degass when we run normal phase.

Is it possible that you have a negative pressure in the hexane reservoir. Maybe it is not vented properly, and so when the liquid is "pulled" by the pump you get a negative pressure and this causes gas to form.

[bmoritz]

Well most of the bubbles have disappeared, but their are still some renegade bubbles coming from the degasser. I have line A delivering Hexanes and line B delivering IPA. Line B is fine, but the hexane line still is producing bubbles, and at a very specific rate. Basically every 20 seconds a new bubble appears from the degasser line to the quant pump. I've run the hexanes at 100% for a couple of hours at 5 mL/min and no bubbles appear, but when I decrease the flow rate to 1 mL/min I get the bubbles. Is there a problem with the degasser? Do I need to change out the vacuum chamber? Thanks for the replies everyone. I really appreciate it.

BM

[mbicking]

Odd problem. Did you replace or remove the solvent filter? That should not be the problem if you were OK at 5 mL/min, but some LC's are good at finding creative ways to fail, and confuse us in the process.

Also, loosen and tighten the plastic fittings going into and out of the degasser for channel A.

If these things don't solve the problem, then purge A with IPA. If you still have the problem, it's time for a service call.

[Mattias]

Very odd indeed. It is quite common to get bubbles when the flow is increased due to cavitation when the flow passes some narrow connection. That would especially be true fo a volatile liquid like hexane. But you see the opposite...

Is air at all soluble in hexane? Can you try off-line degassing with e.g. helium instead?