I was asked to provide lower limits of detection of various oxides of micas, feldspars amphiboles, epidotes, spinels and chlorite analyzed with CAMECA SX50 microprobe
The detection limit of microprobe analysis depends on a number of factors such as probe current, background counting time and your hardware configurations...
It is impossible calculate the DL without knowing all those factors.
Following paper would be a good read
Precision and sensitivity in electron microprobe analysis, Thomas O. Ziebold