SPI Supplies Brand Ferritin Test Specimen for Transmission Electron Microscopes on 3 mm Grid
Resolution of the four points of the ferritin molecule, sometimes called the "tetrad" would indicate an instrumental resolution of at least 1.25 nm. While this particular specimen is not all that profound for today's most modern instruments, in former years, it used to be the "standard" used by the world's leading TEM laboratories. Today, its largest application seems to be in teaching settings where students are using older instruments with resolutions approaching 1.25 nm. The specimen is prepared on a carbon coated 400 mesh copper grid.
This is always hard to predict because it depends on the cleanliness of the TEM and the way the grid is being picked up by the operator and inserted into the TEM. It is reasonably robust but will not last forever. Those who teach EM courses tend to favor it because of its low cost replacement cost.
Other alternatives for documenting the operation of a TEM:
The ultimate test specimen on the market today is the Mag*I*Cal TEM Calibration Sample. While it is much more expensive, it will generally last much longer provided it is handled maintained properly.