Gradient CH3CN/H2O : Air bubbles

[Laurent]

Hello dear colleagues,

I'am doing some semi-prep applications with a Waters 600 pump and a Waters 486 detector.
When I try to scale up a gradient method from my Alliance 2695 (CH3CN/H20 10/90 to 100% CH3CN on an Xbridge C18) it seems that air bubbles appears just after the mixer.
I use a flow rate of 7ml/min
Line A with 100% mQ Water (degas in sonic bath 15 min)
Line B 100% CH3CN (also degas)

The baseline is terrific, and i have pressure ripples (of course )...

How can i avoid this ?
How to scale my method from the alliance ?
Maybe 90/10 H2O/CH3CN line A and 100% CH3CN line B will fix the problem ? because i think that mixing H20 and CH3CN is endothermic
But will a mix of 90/10 be enough ?

(maybe i should also try the sparge but bubbles appears after the mixer)

Thanks for your help,

Laurent

[AA]

It is unlikely that bubbles are being formed after the mixer as this part of the system sees system pressure. BTW, how can you tell, as far as I remember the mixer and tubings are made of stainless steel, tough to see through. In any case, bubbles will happen with that 600 at the GPV and will cause loss of prime. You will need to sparge (or in-line degass) with that 600 system to eliminate that if that is the problem.

Maybe you could describe what is happening with a bit more detail, it might help diagnose what is really going on.

AA

[juddc]

The line from the mixer to the pump is not stainless, it's translucent plastic - I have on 18" from my keyboard. I agree though - you need to sparge or use an inline degasser.

[Bruce Hamilton]

Premixing combined with allowing to reach ambient temperature before ultrasonication of both mobile phases will help a lot, even without sparging with He or an in-line degasser. Also ensure any mobile phase reservoir and pump sinters are not partially blocked.

Although more dissolved gases may appear during the gradient, the cooling from mixing is also reduced - so fewer bubbles appear when mobile phase gradually warms travelling along lines after the instrument mixer.

If you have access to sparging and in-line degassers then use them, but if not, your proposed strategy, combined with blending well in advance and allowing to reach ambient temperatures will help mitigate the problem.

When I'm in a hurry, I warm the premxtures in a microwave before ultrasonication - but there can be serious safety issues for the careless.

Bruce Hamilton

[juddc]

I don't want to sound like a jerk, but if he has a 600, he can sparge with He. I've never seen one that wasn't equipped to do that.

~$100 for a bottle is cheap money and takes less time and aggravation than pre-mixing, microwaving ,sonication, or whatever else one might do. Heck, the helium is probably cheaper than the acetonitrile he's using...Just my two cents.

[Laurent]

Thanks a lot everybody for your reply !

I have connected an old in-line degasser L7612 from merck hitachi but it seems that was not efficient enough at semi-prep flow rate.

So I will try to connect an He Bottle to my system and sparge the lines.

[AA]

Just to clarify this comment from a previous post:

The line from the mixer to the pump is not stainless, it's translucent plastic - I have on 18" from my keyboard.

The mixer is not the thing with the bit of plastic tubing attached. That device is the GPV (gradient proportioning valve). There is a mixer on 600 based systems, it looks like a block of stainless steel.

The GPV does not do any mixing, all mixing happens down stream of the GPV (starting in the tubing from the GPV to the inlet check valve).

AA

[Laurent]

Ok AA thx for your precision.
The bubbles are formed just after the GPV
I'll tryi to sparge but i'm not convinced