Updated on June 9, 2013

Critical Point Dryer
    Make/Model: cdp image

  1.  Open CPD door by turning metal rod slid into the door handle.
  2.  Place samples in metal sample holder (boat) which has immersed in acetone (using mesh basket to separate samples when necessary; covered with mesh stripe-mat), taking care to prevent the tissue samples from drying.  Load the specimen boat and then screw the door back on.
  3.  Turn on cold water.  CDP
  4. Open the black knob (#)1 to let CO2 into the chamber.
  5. Open the black knob (#3) to let CO2 out of the chamber using the black knob nearer under the chamber, knob 3.
  6. Turn off the cold water or leave it trickle..
  7. Flush the apparatus as outlined above (step 4 to 5) every half- or 1-hour for 30 sec or so, depending on the size of specimen, to allow specimens to infiltrate with CO2 and remove dehydrating fluid. Large specimens will require about 2-3 hr total time, while small specimens will be done in about 1 hr. Remember to leave the room door wide open to dissipate the CO2 gas that is being vented. 
  8. After flushing, close the inlet valve and lower the liquid level to just below the top of the boat by venting off excess gas.
  9. Turn on the hot water (while leaving the cold water partially on if necessary).
  10. Wait while the temperature and pressure rise.
  11. Once the critical point (31.5oC and 1200 psi (lb/in2)) has been exceeded, shut off the hot tap water, and wait for 1-5 min depending on the size of specimen. Wait longer (up to 0.5-1 hour) for larger and/or thicker samples (hot tap water is required sometimes).
  12. Open the knob 2 on the top of the chamber to allow the gaseous CO2 to escape slowly.
  13. Open the door (sometimes it takes a while for the door to be opened), remove the sample and store the samples into sealed glass vials.
  14. Make sure the CO2 valve on cylinder is closed after finish.

        Background (http://www.polaron-range.com/)

Critical Point Drying is an established method of dehydrating biological tissue prior to examination in the Scanning Electron Microscope (SEM).

cdp-2 The phase diagram shows the pressure to temperature ranges where solid, liquid and vapor exist. The boundaries between the phases meet at a point on the phase diagram called the triple point. Along the boundary between the liquid and vapor phases it is possible to choose a particular temperature and corresponding pressure, where liquid and vapor can co-exist and hence have the same density. This is the critical temperature and pressure, or critical point.


Critical point drying relies on this physical principle by bring samples to the critical of a suitable inert fluid.  This inert fluid replaces water in the sample, and then vaporizes at its critical point without change of density and therefore without surface tension effects which distort morphology and ultrastructure. CO2 is universally used today as the inert fluid. 

The critical point of CO2 is at approximately 31.5oC and 1200 psi (lb/in2) or 78 bar.


Since liquid CO2 is not sufficiently miscible with water, it is necessary to use an intermediate fluid (such as acetone, which is miscible with both water and liquid CO2) during this process.