Lateral resolution as a function of the primary Cs + ion current with 16 keV impact energy was quantified for the NanoSIMS-HR and a NanoSIMS 50L with the same Al-Si sample (Fig. & S1; Table ). For the NanoSIMS-HR tests, the primary current on the sample was set to values from <0.2 pA Cs + (below Faraday Cup detection limit) to 600 pA Cs + . The best lateral resolution for the NanoSIMS-HR Cs + source was 25 nm, compared to 45 nm for the 50L. The specifications are set to 30 nm and 50 nm, respectively. The NanoSIMS-HR resolution at 600 pA was ~600 nm, compared to 100 pA for similar resolution with the 50L. At 100 nm lateral resolution, the data show a 2.5-fold improvement in current density for the NanoSIMS-HR over the NanoSIMS 50L. We tested the improved lateral resolution of NanoSIMS-HR on a biological sample by imaging P. tricornutum algal cells with the ultimate resolution of the two instruments at 16 keV (Fig. ). At ~30 nm lateral resolution, the ion images revealed small holes in the algal cell that would not be distinguishable with a standard NanoSIMS 50L, even at the ultimate resolution of 50 nm. These holes form during the sputtering of the P. tricornutum cell wall. We also tested the ability of the NanoSIMS-HR to deliver more current within a 100 nm spot. As expected, the secondary ion current increased 2.5x, proportional to the increase in current density. High-lateral-resolution NanoSIMS-HR analyses were also conducted on a 90-nm-thick section of a P. hallii root infected with R. irregularis, an arbuscular mycorrhizal fungus (Fig. &). Figure shows correlated TEM and NanoSIMS-HR 12 C 14 N -images of a subportion of an R. irregularis vesicle in the root tissue. The 12 C 14 N -image was collected with the 30 nm lateral resolution settings. The NanoSIMS-HR 12 C 14 N -image shows details that could be seen in the TEM image, as well as chemical contrast that could not. The TEM image shows sub-micron structures inside the vesicle that were visualized with the 12 C 14 N -ion in the NanoSIMS-HR, indicating that it was N-rich. The NanoSIMS-HR 12 C 14 N -image also shows that the vesicle wall is composed of multiple micron-scale layers of N-rich material. This material exhibited 100-nm-scale texture in the NanoSIMS-HR 12 C 14 N -image that is not visible in the TEM image, which we suspect was caused by extended ion bombardment.