Retention time changes in LC-(MS/MS) separation due to conc

[ChromEnthusiast]

We are performing traditional gradient elution and UPLC-QQQ-MS for some pharmaceuticals with 2 mmol ammonium formate, 0.05% formic acid buffer (A) and acetonitrile (B). Retention times are stable for the four lowest concentration levels, but there are significant retention time changes for some compounds in higher concentrations. Run-to-run variations injected from the same vial are negligible (so the problem is concentration dependent). Column is typical C18 2.1 x 50 mm, 1.7 µm.

Is this all about too weak buffering or could there be some other reasons?

[Magister]

very interesting...

What is your concentration range?
Are significant retention time changes in higher concentrations for both standard solutions and real samples or just for real samples? How much do they differ? Is there any splitting?
What is the injection volume?
What is peak asymmetry at 1/10 of height for high concentrations?