what is critical point drying?
In a Scanning Electron Microscope there is an electron gun that produces a narrow beam of electrons. The beam is scanned in a regular pattern over the sample. As the electron beam hits the sample it produces secondary electrons that are captured by a detector. Of course the detector captures more electrons from the side of the sample closest to the detector. That is why my amoeba has a bright and a dark side.
As the chemicals that make most lifeforms have a pretty low atomic number, the number of secondary electrons emitted from the sample are low, too. Thus a thin coat of gold which has a high atomic number is applied to the sample. Now the electron beam has a higher chance to cause secondary electrons to be emitted. The result is better contrast.
If we want to sputter coat the sample we will need vacuum. If our sample contains any liquid matter f.e. in cells they would literally explode while the sputter coater is evacuated.
If we would simply heat the sample for water or ethanol to evaporate, the surface tension at the phase transition would destroy our sample. But under high pressure (218 atm) and high temperature (374 °C) there is a so called critical point in the phase diagram. At and above that critical point there is no distinction between liquid and gas - it is called a supercritical fluid. As 218 atm and 374°C is a bit hard to achieve, CO2 is used. The critical point for CO2 is 31°C and 73 atm.
So we have our sample in 100 ethanol. The ethanol bathes (30%, 50%, 70%,90% 4x 100%) are used to exchange water against ethanol. Then we put our sample in the pressure chamber of the Critical Point Dryer. We cool the sample to 10°C and then open a valve to release some liquid CO2 into the chamber. Now we wait a little while so ethanol and CO2 mixes. Then we open another valve to release some of the liquid. This is repeated about 30 times. This causes the entire ethanol to be replaced with CO2. Then the temperature is increased to 40°C. Around 31°C the CO2 becomes supercritical. The valve is opened again and slowly the CO2 evaporates without any damage to the sample.
No we have a totally dry sample that is ready to be sputter coated.
Sorry for this lengthy explanation.
@David,
I totally agree. I wish less nasty chemicals would do a good job
Of course this was done under a fume cupboard. OsO4 and HgCl2 kill so fast that cilia etc. are "frozen".
regards
Ecki