The STEM facility at Brookhaven National Laboratory has been in operation since Oct. ‘77, using a custom-built instrument (STEM1) with cold field emission source, 2.5Å probe, -150°C cold stage, efficient dark field detectors and computer control & data acquisition system. A specimen changing air lock and several portable vacuum chambers permit vacuum transfer of specimens from a separate vacuum system where they were freeze dried overnight.
The large angle dark-field signal produced by the STEM is directly proportional to the total mass within the probed area. STEM mass mapping is based on this linear relationship and the fact that only specimen-specific atoms remain on the substrate after washing with volatile buffer and freeze drying. All images are digital and available via Internet. PC software can be provided for analysis.
STEM mass accuracy ranges from a fraction of a percent on well-defined individual particles such as viruses in the 50 MDa to 10 GDa range, to ∼1% around 1 MDa and ∼10% in the 50 kDa range.