Suggestion: Try substituting Formic Acid in DI Water and adding Ammonium Acetate (10mM) and Acetic Acid (1%) to the Aqueous Phase. This should improve the Reproducibility of Peak Areas and Retention Times.

In addition Extensive Reconditioning of the Analytical Column with pure Acetonitrile in between Separations is Essential for good Reproducibility when working with Plasma Samples.