I am new to using a Waters 2695. The instrument has an inline degasser. I'm doing protein runs using a YMC protein-pac column. A=0.1% TFA in optima water, B=0.1% TFA in optima acetonitrile. TFA is newly purchased from pierce. Flow=1mL/min, 214nm detection. I'm getting a 0.5 mAU undulation in my baseline during the shallow part of my gradient, with a periodicity of 0.5min. Baseline is flat for 100%A (and B and C and D). I ran the GPV valve test and the proportioning is very good (the instrument just had a PM checkup). Undulations present for other combinations of mixing (eg A&D, B&C, when checked with the 0.1% acetone solution used for GPV test). I manually degassed solvents (vacuum sonication for 1min) and reflushed system but no improvement.
It looks to be a mixing issue. Another forum ref mentioned stroke volume to help. I checked and it is already set to maximum value (130uL). Another ref concerned backpressure after detector but the 2487 detector is set up per waters, with metal outlet tubing connected to waste via teflon tubing.
Is this just standard? I never had this problem with the 1100 system. Thanks for any advice?