For reference, I'm pumping 50:50 water:methanol at 1.0 mL/min through an Agilent Zorbax Phenyl-Hexyl column controlled at 30 degrees C, monitoring 214 nm (necessary for two analytes in my analysis that do not have strong UV absorbance, and show virtually none past 225 nm).

So, I'm wondering if anyone has had this issue with the G1312A binary pump system from Agilent? If so, how did they fix it? Premixing mobile phase isn't really a great solution (my usual mobile phase on my primary analysis is 52:48 water:methanol, which is hard to mix reproducibly, IMHO, as I run alomst constantly and would be mixing mobile phase over and over and over....).
If, after having pondered over all of the intestines, you don't obtain an adequate baseline, I'd highly recommend using a top pan balance ( eg 20kg ) and preparing bulk solvent ( 10L Duran bottle is a good mixing vessel and reservoir and will last a week at 1 ml/min for 24/7 ). A suitable 20L drum is another option.

Preparation by weight ( allowing for density ) is precise and quick. However just remember to add the water to the methanol ( for improved mixing ), mix well, and allow to cool before use.

Please keep having fun,

Bruce Hamilton