We found that the minimum raster size was approximately 20 to 30 times the beam diameter, and the optimum ROI was approximately 10% of the raster area (Fig. ). Figure . P detection limit for 10 to 8000 pA (16 keV) vs. ROI width. These ROIs are ~10% of the total raster size. The Cs + current in pA is on the sample. IMS-7f data for comparison.