Dear Mark Tracy and Chris Pohl,

Thanks for your valuable input.

I have solved my problems with your proposals.

Below is the summary of what you told me and is, I think, helpful to other DX600 users.
1. "be sure to use the seal wash each day before you start the pump and again when you shut down the system" if you are pumping an alkaline eluent. The pressure would be very high if the seal/check valve is not clean
2. It's not necessary to "do frequent pH calibrations with the electrochemical detector" if the detector works well
3. "Regarding carbonate contamination, the magnitude of the problem is usually small with carbohydrate separations provided." Always to "make up your eluents from 50% NaOH and use good quality DI H2O."

Thanks again.

And any other good suggestions from you are very welcome!

zhuwj007 with best regards