When calibrating the flame AAS (AAnalyst400) with mixed-metal standards I receive an error message "No calibration because the signs of absorbance and concentration may differ". This message appears after my final calibration standard (30ppm) has been ran. Initially this only appeared when analysing for Zn, but now appears for every metal. I have tried using freshly prepared standards but have the same result.
Has anybody had this message before/knows how to overcome this issue?
Any help is appreciated!
Alex