Gass Bubbles From MSG Valve When Mixing Mobile Phase

Hello Agilent Community,

I have a problem with MCG valve on quarternary pump Agilent 1100 G1311A. Almost every method we use has a gradient elution. The problem is, if the pump is pumping a mixture of mobile phases, a large number of small bubbles come out of the MCG valve outlet tubing. If it pumps 100% of just one mobile phase (A, B, C or D), this problem disappears and bubbles are not present. 

Mobile phase A = phosphate buffer
Mobile phase B = acetonitrile
Mobile phase C = isopropanol
Mobile phase D = mixture acetonitrile:water (20:80)

No matter what mixes with what, whenever two channels are turned on and not just one, bubbles are present at the outlet of the MCG valve. We had a technician here who performed a gradient test on the pump while mixing water and water with the addition of acetone. Despite the fact that he agreed that there were gas bubbles in the outlet from the MCG valve, the gradient test came out according to the guidelines, so he did not see a problem. He also tested the pressure test of the pump, which also came out according to the guidelines. His recommendation was therefore to degas the mobile phase properly because we do not have a degasser on the HPLC (Agilent 1100). He identified the problem as an insufficiently degassed mobile phase, or an inappropriate change in the solubility of gases in the mixed liquid compared to separated liquids.

The mobile phase is usually degassed by sonication for about 15 minutes. After his recommendation, I extended the sonication time to 30 minutes and also tried degassing on a vacuum pump. None of this helped, which disqualifies the cause of the problem of insufficient degassing. To verify that the problem is not due to the change in the solubility of the gases in the mixed liquid stream, I connected the used mobile phases to a second instrument we own (Agilent 1260). On this instrument, when mixing the mobile phases, there was no occurrence of the bubbles at the MCG valve outlet, so even the second possibility of the problem cause will probably not be correct. 

Therefore, I think the cause of the problem is the MCG valve itself. However, replacing the valve is a very expensive affair, so I would like to ask if you have a recommendation for user repair before we buy a new one. Although all defined tests have been performed according to the guidelines, the presence of bubbles causes a large pressure ripple, which leads to signal oscillations in the baseline and a reduction in the detection limit. Furthermore, the retention times of the analytes are shifting. 

Any suggestions?

  • Hi

    First thing, check inline filter of mobile phase.

    2nd you can purge MCG valve by flowing IPA or Acetonitrile plus water( All 4 bottles) for 4 to 5 hours.

    If still problem then no choice of replacement.

  • Hello,

    Sonicating mobile phases will remove the dissolved gases but as soon as you stop, air will begin to dissolve again into them.  The G1311A pump requires an external degasser and you compared it to the 1260 but you didn't mention the model. The newer pumps have built in degasser so you may not be doing a fair comparison.

    When you mix in the solvents, they combine at atmospheric pressure so if two solvents results in a warming effect [water and methanol], then you are very likely to outgas.  Instead of sonicating, try vacuum filtering your mobile which does 2 things:  filtering out particles as well as degassing.  My practical experience shows that everything will be fine for about 2-3 hours then it will begin to outgas again.

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