Our Talos L120C with LaB6 filament and Ceta camera is suited for screening of single particle samples in cryo modality. The Gatan 626.6 single specimen cryo-holder allows the user to screen approximately 4 grids per day, with a 40 minute holder warmup cycle between each grid. The user can choose to acquire the images using the microscope software or EPU.
The mode of imaging typically used for cryogen electron microscopy, but also applicable to other delicate specimens, is called low dose imaging. A cryospecimen is typically damaged after it receives a total of approximately 50 electrons per Å2. For 1s acquisition exposures at the recommended doses of <40 e-/Å2/s which allow sufficient contrast , this gives the user no more than a couple of seconds before the specimen melts and means that each point of the specimen can only be imaged once.
In low dose imaging the sample is first browsed at a low magnification and at a very low dose of <0.5e-/Å2, and therefore using a very noisy image, just good enough to find potentially good areas of ice (A in the figure below). To see specimen details, for example distribution of particles in the support film openings, we acquire images of a very limited area which is irradiated by the beam only during acquisition (C in the figure below). The beam is then automatically blanked using the pre-specimen shutter and condenser deflectors. The focusing and beam alignments are performed on a neighbouring area of support film (B in the figure below). The beam diameter is typically kept no larger than 2 µm to avoid damaging surrounding areas. The specimen is therefore either irradiated with extremely low dose at low magnifications (when browsing), or with a very small beam diameter at low dose (<40e-/Å2/s) during acquisition. The beam size, dose settings and beam offset for focusing between holes are stored separately by the software as search, focus and acquisition settings.