On one of our LC-UV systems we have observed that the response factor (RF) of the analyte decreased when the analyte concentration was increased until a certain treshold concentration was reached after which the RF remained constant. We donīt think this is an absorption phenomena since the opposite would be expected (increased RF with increased concentration). No carry over have been observed and the injection sequence started from the standard of lowest concentration. When we changed the autoinjector in the LC system, the responsfactors remained constant. Hence, we have excluded that the phenomena is related to the detection.

Does anyone have any reasonable explanation to the above?

Do you think you might be swamping out the detector at the high concentrations?

If you use an internal standard, reconstitute the higher concentration samples in higher volumes and see if the ratio of area analyte to area ISTD increase linearly.