Chromatography Forum

Rob,
did you ever figure out how the frit caused the air bubbles? Did it cause degassing, or were parts of the assembly above the liquid level? The latter has caused problems several times in this lab. These so called last drop frits are often not tight, for instance one by VALCO has the 1/8" tubing to the pump just stuck into the teflon assembly holding the metal frit. Air has been drawn in between the tubing and the teflon assembly, among other problems.
Thus if I need to go to "the last drop" the intake frit is usually removed.

(Mentioned the manufacturer here as several complaints to a representative did not produce any response, so I wonder whether others noted such problems with what frits)

Hi there,

I'm not sure what the problem with the ceramic frits. It certainly wasn't above the surface level of the mobile phase. It's a bit difficult to tell with these frits when they are "fouled" and not behaving as they should. Working with the "old style" metal sinker frits, you could sometimes tell if they were becoming clogged as they used to have a greasy sheen on their outside...

HW: I would guess that the problem isn't so much related to trying to get the "last drop", as a fouled intake frit adding sufficient flow resistance that air gets sucked in at some other point in the system. I'm having a harder time visualizing why it should only happen at certain flow rates on the "B" pump.

Hi tom,
I think it could be due to difference in surface tension of water versus organic solvents.

JM

Well, I was not talking about the frit itself drawing gas but the frit assembly, for instance at the connection to the tube. If all this was submerged (thus ruling out "sucking" in gas there), maybe outgassing could be the culprit, especially if He sparging is used? (The assumption is made here that Rob´s frit was only partially blocked).

sorry to get in this late while it sounds everyone else is satisfactory but my question is How the ceramic frit could draw bubble in the mobile phase B line in Rob's story. Thanks in advance for any one who can help me to understand.

sorry to get in this late while it sounds everyone else is satisfactory but my question is How the ceramic frit could draw bubble in the mobile phase B line in Rob's story.

That's actually what the last three or four posts are all about; we're speculating as to the mechanism.

I think HW and I are saying essentially the same thing: with a partially-blocked intake frit, the pressure drops below atmospheric during the pump's intake cycle (at least momentarily), with the result that air is forced in through one of the connections.

JM, that was my initial speculation, but then on re-reading Rob's posts, I realized that he's using a high-pressure mixing system, so that the composition seen by the B pump remains constant. In any case, the main thing is he got the problem fixed.