Scanning Transmission Electron Microscopy Mass Measurement-- PM28 isoforms solubilized in OTG were adsorbed for 1 min to glow discharged thin carbon films supported by a thick fenestrated carbon layer (directly after cation-exchange chromatography). The gold-plated copper grids were then washed on 8 drops of quartz double-distilled water and were freeze-dried at -80C overnight in the microscope. For mass analysis, annular dark-field images were recorded in a STEM (VG-HB5) at 80 kV and doses of 325 +/- 35 electrons/nm2. Digital acquisition of the images and microscope parameters, system calibration, and mass analysis were carried out as described previously. The total experimental error was calculated as the standard error of the mean, plus 5% of the measured particle mass to account for the absolute calibration uncertainty.