What is critical point drying (CPD)?
Biological samples to be observed by scanning electron microscopy (SEM) under high
vacuum must be very dry and conductive. After primary and secondary fixation and
dehydration biological samples must be thoroughly dried and coated with a conductive
metal prior to introduction into the high vacuum chamber of the SEM.
Drying methods, such as air drying by evaporating alcohol and Hexamethyldisilazane
(HMDS) usually create artifacts by altering the surface topography of sample. The
primary cause of such damage is changes in surface tension, which generates considerable
forces as the liquid evaporates from the surface boundary.
CPD reduces surface tension boundary distortions of biological tissue and is therefore
the method of choice to dry biological samples for SEM observations. This method is
based on classic experiments on the phase transitions during the liquefaction of gases.
When the 'critical point' of temperature and pressure is reached during such transition,
it is possible to pass from liquid to gas without any abrupt change in state thereby
avoiding the damaging effects of surface tension on the sample surface.
How does the Samdri 790 achieve CPD?
At heart of the Samdri 790 is a pressure-resistant sample chamber that can be hermetically
sealed with knurl nuts. The first step of the operation is to open the chamber, fill
it in with some ethanol, immerse the sample into the ethanol and seal the chamber.
Liquid CO2 is then introduced into the chamber and the temperature and pressure of the chamber
are then controlled in a series of heat and purge operations that eventually result
in critical point drying of the sample. A detail SOP of the procedure is available
next to the instrument.
NOTE: All Samdri 790 CPD users must be trained by dedicated IMC personnel before being
authorized using the Samdri 790 independently.