Applications of Scanning Electron Microscope (SEM)
Samples for SEM need typically to be dry and conductive. Therefore, biological specimens are fixated, dehydrated and coated with thin layer of metal. Small specimens are dehydrated chemically, via ascending concentration series of alcohol and hexamethyldisilazane (HDMS), but larger specimens can be dried using a critical point drying (CPD) instrument. In either case, water within the specimen is substituted with a solvent with lower surface tension that helps keeping the specimen structure intact. Our new SEM (Zeiss Sigma HD|VP) has maximum reolution of about 1 nm.
However, also non-conductive specimens can be imaged with appropriate instrumentation and detection (so-called variable pressure mode). Utilizing an environmetal mode in our old SEM (Philips XL30 ESEM-TMP) even moist samples can be analyzed.
In addition to conventional secondary electron imaging, 3D reconstruction of the sample surfaces can be processed from data acquired with four-quadrant backscatter electron detector. At best, surface roughness and volumetric analysis is applicable.
Similarly to TEM, elemental composition can be analyzed at the same time with SEM imaging. However, SEM is especially useful to analyze elemental distribution (i.e. elemental mapping) in the sample. Keep in mind that typically the elemental information is collected from about 1 micron deep layer.
TEM grids can be inserted and analyzed with SEM, too! If low magnification is sufficient and you are more familiar to SEM than to TEM, you may use SEM for transmission imaging.
Correlative microscopy (CLEM)
Together with PhD Kirsi Rilla (Institute of Biomedicine), we have developed protocols to study first live cells with confocal microscopy and later, the very same cells with SEM.