They electrostatically attach radiolabeled protein to dextran chips and stablish the relation 100mº=1ng/mm^2 but as far as I understand, this wouldnt hold for non-dextran chips like the C1 (MUA or similar SAM). In this case the proteins would be much closer to the surface and the SPR signal (or SPR angle) must be bigger for the same surface density.
In the Handbook of SPR by Schasfoort & Tudos you can read that for the Autolab SPRit and IBIS systems the relation is 122mº=1ng/mm2, but again, they are using dextran chips (and cite no source for the relation).
I often see people stating their results in ng/mm2 even when they dont use dextran chips... I would appreciate some discussion on this.