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.