I am running a method for EDDP with a normal linear range of 5ng/mL up to 1000 ng/mL on-column. Chromatography is an Acquity using a Phenomenex Kinetex column and the usual formic acid in Di water/methanol mobile phase. We use a single-point calibrator at 30 ng/mL and verify the linear range every 6 months. Right now I am having difficulty with the top point having -25% to -40% deviation. This seemed to point to a saturation effect, but decreasing the injection volume, diluting the samples, and even detuning the MS settings for that compound have had no effect on this other than to cause the low points to become nasty. :/ Any suggestions would be quite welcome! Thanks!