Thanks for the answer Bruce! I think one thing I did not understand about the single fit is that it tries to use all standards (although some of the standard's weights can be equal to 0 in the results), which can be detrimental to the fitting. Thanks a lot for the precisions, things are clearer now! -- Adrien Couet Graduate Student Dept of Nuclear Engineering, Penn State University +1 814 865 9709 axc1019@psu.edu