Lateral resolution was determined for a range of primary Cs + beam currents (~0.2 pA to 600 pA) and impact energies (16 keV to 2 keV). Primary ion current selection and focusing was performed as is typical for NanoSIMS instruments. Ion currents were controlled using Einzel lenses (L0 and L1) and a primary beam limiting aperture (D1) located in the coaxial optics column, astigmatism was corrected with an octopole, and the final focusing of the primary beam onto the sample surface was performed with E0P, a lens in the coaxial lens stack. The impact energy was adjusted by sweeping the potential of the source tube containing the Cs reservoir and adjusting the sample potential to the optimized value. High voltages on the lenses and deflectors in between for primary and secondary optics were scaled down with the energy decrease. Imaging was performed with electron multipliers in pulse counting mode and secondary electrons (SE). The mass spectrometer was operated without an entrance or aperture slit (~2000 mass resolving power (MRP)) for the highest lateral resolution (lowest primary beam current) analyses to achieve the most precise beam resolution measurements by optimizing ion transmission. The beam size was measured by imaging an Al-Si sample with Si -. Al 1 H -counts at mass 28 were negligible. Image collection parameters were set to provide a beam overlap from 75% for high lateral resolution image to 99% for large beam for NanoSIMS 50L and from 85% to 97n% for NanoSIMS-HR (2 x 2 to 20 x 20 µm 2 for NanoSIMS-HR and 5 x 5 to 12 x 12 µm 2 for the NanoSIMS 50L; all with 512 x 512 pixels; beam overlap defined as the [beam diameter -pixel width]  beam diameter x 100% ). The dwell time varied from 0.2 to 3 milliseconds per pixel. As the intensity profile across two chemically contrasting phases (Al and Si in this case) is a convolution between the sharp edge and gaussian beam intensity distribution, a cumulative gaussian distribution model was used to describe the 28 Si -profile. Beam size was defined as the distance represented by the 16 to 84% quantile of the intensity distribution containing 68% of the beam intensity. Ion count transects across selected features were extracted in WinImage ion image data processing software (CAMECA) using a region of interest (ROI) of 2 to 4 pixels wide line by 1 pixel deep to average adjacent pixels perpendicular. Lateral resolution was calculated in WinCurve data processing software (CAMECA) using a cumulative Gaussian distribution model to extract 16% and 84% beam intensity locations on the profile abscissa as the beam size or lateral resolution .