Critical Point Dryer

 

The phase diagram (below) shows the pressure to temperature ranges where solid, liquid and vapour 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 vapour phases, it is possible to choose a particular temperature and corresponding pressure where liquid and vapour can coexist and have the same density, this is the critical temperature and pressure. Critical point drying relies on this physical principle.

Water in the specimen is replaced with liquid carbon dioxide (CO2) whose critical temperature for a realisable pressure of around 1,200 psi is just above ambient (around 320C). Therefore if the water is replaced with liquid CO2 and the temperature raised to above the critical temperature, liquid CO2 changes to vapour without change of density. This avoids the surface tension effects which distort specimen morphology and ultrastructure. Since CO2 is not sufficiently miscible with water it is necessary to use intermediate fluids, such as ethyl alcohol and acetone, which are miscible with both water and CO2.

Images (below) courtesy of the Advanced Microscopy and Bioimaging Institute of Biological, Environmental and Rural Sciences, Aberystwyth University. 

 

                      

                                                                 Head louse                                                                         Nematode worm

                      

                                                       Liver tissue and blood cells                                                                Fibroblast cells

                                                  

                                                          Rabbit articular cartilage                                                                E. coli bacteria