Mass Accuracy in the STEM
The scanning transmission electron microscope (STEM) has a number of features which make it ideal for quantitative studies: 1) large acceptance angle scintillator-photomultiplier detectors for low dose dark field imaging with linear electron detection, 2) simple contrast transfer function with no oscillations, 3) cold stage for zero contamination and reduced mass loss, 4) digital scanning for accurate magnification and positioning and 5) serial readout of several signals simultaneously for direct interfacing to a computer.For the past 10 years the Brookhaven STEM has been used for mass measurements on isolated individual molecules in freeze dried biological specimens. Interpretation of dark field STEM images is straightforward because of the clean background and linearity of the entire imaging process. Mass measurements provide a quantitative link to biochemical studies. Tobacco mosaic virus (TMV) usually is included along with the specimen of interest as a measure of the quality of the freeze drying and as a check on the microscope calibration, but is not necessary for absolute mass measurements.