Quaternary pump problems

[angupa]

I'm developping a method that has a mobile phase whit 27% acetonitrile and 73% of buffer prepared with 7.452 g of KH2PO4 y 3.288 g of hexylamin in 1000 ml of water. At the beginning the mixture of mobile phase was made by the quaternary pump (Agilent 1100) but, after several weeks working, the system doesn't work properly: the quaternary pump don't make the mixture well.
The supplier of the pump can't solve the problem and said me that I need to make my mobile phase outside the quaternary pump but, I think that this kind of pumps are designed to do this kind of mixtures of mobile phase correctly. I would like to know if anyone has had the same problem.

Thanks,

[DR]

If that ratio of buffer & ACN is constant, you will get better performance if you premix the mobile phase. If you're using a gradient, you may need to add a little buffer to the ACN to get better mixing (straight ACN can be a problem, more typically in high pressure mixing systems).

Your buffer may be a little strong - it's ~55mM PO4, then add in the other stuff. Is there any hint of precipitation when you mix 3 parts buffer to 1 part ACN? Solubility shouldn't be a problem, but Murphy's law does apply in proportioning valves...

[Consumer Products Guy]

Your 1100 quat pump should be able to handle this. Are you (each day) purging (to waste) ALL four channels at 5 ml/min for 5 minutes each channel to get rid of bubbles? This is very important even though you're only using two channels for your assay.

[Consumer Products Guy]

This purging will help both the multi-channel mixing valve and the pump.

[mwoodman]

Hello;
How old is your quat 1100 pump? There was, I recall, a modification of the quat valve to minimize the chance of precipitation of buffer in the valve. This could behave like bubbles w/r/t retention time or pressure fluctuations outside of what you see when just pumping ACN/wa. Contact your local service engineer and see if this is a possibility.

Regarding the earlier comment about bubbles and high flow purging of the individual channels: The higher the flow the less gas removed by the degasser in your system. It might be wise to purge at a lower flow rate. I like to open the drain or purge valve (most quat systems have them) and run 3-4ml/min 25/25/25/25 if all four valves are active in the method. You can knock out bubbles as suggested and then reset to your operating flow and composition before closing the valve.
If you are changing solvents on a line, certainly run a high flow on that line to quickly change over. After that, though, lower the flow during the purge to allow the degasser to do its' job.

Best of luck. M

[John]

As mentioned above, a thorough purge of all the channels to be used would solve most problem cases.

Also, you could try a performance check of your gradient system. With 100% water in A and 0.3% acetone(in water) in B, run the following gradient at about 2mL/min and 254nm UV. Use a restrictor coil of low volume to obtain a back pressure close to your system working pressure:
Inject nothing or a small amount of water if an injection is necessary to start a run.
100% A for 10 min
10 %B for 6min
50% B for 6 min
90%B for 6 min
100 % B for 6 min

Your chromatogram should be a clean step profile.
for b = 10,50 and 90%, calculate the absorbance ratios.

%Ratio = 100*(abs(b)-abs(0%))/abs(100%)

do this for b,c,d vs a to determine which channels are not functioning.

Have fun,



John