Fig. 4.

Assessment of crystal flatness and embedding by electron diffraction. (a–d) The flatness of the crystal will affect the attainable resolution. If crystals are not flat the resolution will be cut off in the direction perpendicular to the tilt axis (tilt axis in dashed line, perpendicular to the tilt axis in solid line). Likewise, a well-embedded crystal will show strong and sharp spots in the diffraction pattern but a poorly embedded crystal will have limited resolution. (a) An example of a crystal that is not flat and also poorly embedded. No sharp spots are visible perpendicular to the tilt axis, indicating that this crystal is not flat. Only a limited number of spots are visible on the tilt axis, indicating that the crystal is not embedded properly and was damaged during grid preparation. (b) An example of a crystal that is flat but poorly embedded. It is flat because many spots are visible in all directions, even perpendicular to the tilt axis. The crystal, however, is poorly embedded because most of the spots are not sharp and have a weak intensity. (c) An example of a crystal that is not flat but is embedded well. The crystal is well embedded because most spots in the diffraction patters appear strong. However, the crystal is not flat because only the spots on the tilt axis are sharp, while the spots perpendicular to the tilt axis are smeared. (d) An example of a crystal that is both flat and well embedded. All of the spots in this diffraction pattern are intense and sharp. In all figures the beam stop is masked by the grey rectangle.