Instruction manual

Page 20 Leica EM CPD030 Operating Manual 09/09
5. Basic Operation
This chapter is directed toward users who operate the EM CPD030 critical
point dryer in normal (standard) operation.
As precondition for this work, the user must be familiar with Chapter 2 and 3.
5.1 Critical Drying Method
Drying water-containing biological specimens in air or under vacuum can drastically
alter their structures or even destroy them completely. They must therefore be dried
by a gentler method. One well-known method is "Critical Point Drying".
Phase diagram
The surface tension of the water in a specimen at the point at which it changes from
the liquid phase to the gaseous phase c can destroy a delicate specimen.
By increasing the pressure and temperature of the specimen it is possible to dry it
without crossing a phase boundary d. This is possible because once the critical
point has been passed, the density of the "liquid" and the density of the "gas" are
the same. The critical point for water is 228,5 bar and
374°C. However, this high pressure and extreme temperature would normally
destroy a biological specimen. For this reason the specimen must first be treated in
a suitable transitional fluid such as CO2 whose critical point of 73,8 bar and 31°C is
considerably more advantageous.
K= Critical Point
A= Initial state
E =Final state
T=Triple point
c Air-drying / vacuum-drying
d Critical point drying