Critical Point Drying Principles

EMS Catalog #91090 Critical Point Drier

It may be apparent that we an utilize the Critical Phenomena as a drying technique, as it achieves a phase change from liquid to dry gas without the effects of surface tension and is therefore suitable for delicate biological specimens. However, it is not surprising that the initial investigations were CO₂ as will be apparent from a table of Critical Constants for some common substances. Even the practical achievement of the critical conditions would not assist the Biologist, as the specimens would suffer significant thermal damage if we attempted to apply the technique direct in the removal of water from the specimens.

CRITICAL CONSTANTS
Substance	Temp. °C	P.S.I
HYDROGEN	-234.5	294
OXYGEN	-118	735
NITROGEN	-146	485
CARBON DIOXIDE	+31.1	1072
CARBON MONOXIDE	+141.1	528
WATER	+374	3212

Therefore, CO₂  remains the most common medium for which to apply the C.P.D. procedure, and is termed the Transitional Fluid', however, it is not miscible with water and we have to replace the water in the specimen with another fluid which is miscible with CO₂, this is termed the Intermediate Fluid'. Ideally and often it can also replace the water in the specimen, also serving as the ''Dehydration Fluid'. 

(Wet Specimen) H₂O Acetone 30%---100%** CO₂ C.P.D. (Dry Specimen)

* 50/60/70/80/90 Typically 10 mins. each. 
**Flush Typically 3 times. 

The table gives an indication of some Intermediate Fluids. (Water is 73 Dynes/cm.)

DEHYDRATION INTERMEDIATE FLUIDS FOR C.P.D.
SUBSTANCE	SURFACE TENSION (DYNES/CM)
ETHANOL	23
ACETONE	24
FREON (113)	19

Having transferred the specimen to the chamber in the Intermediate Fluid, the chamber is flushed several times to replace it with the Transitional Fluid. The process from which the complete techniques derives its name C.P.D. can now be initiated. 

Fruit body neck with spores  adhering to the sides

Bacteria adhering to the tip of  a fungus.

Powdery Mildew, hyphal  filament on leaf surface.

Transitional Stage

The conditions for which the Critical Point Passage is obtained, (for CO₂, 31.1°C and 1072 p.s.i.). Pressure is obtained by the effect of applying heat and while it can be readily appreciated that we can take a liquid from below its Critical Temperature and obtain the Transition to gas above its Critical Temperature, and understanding of the relevant 'start' and 'end' points and the cycle involved is required in evaluating the Design and Performance of C.P.D. equipment. It is still useful however, to utilize these CO₂ Isothermal as indicated with the "superimposed arrows" showing differing conditions for the C.P.D. device.

 

Note: CO₂ and Acetone (or Ethanol) are recommended, while Freon has lower surface tension and has been used, it is not significant and is not recommended.

Online Ordering

EMS 850 Critical Point Dryers are available online from the EMS Catalog. For ordering or product information, click here.