Applying heat to temperature sensitive samples during evaporation can help samples evaporate at a faster rate without a significant increase in sample temperature due to evaporative cooling. If you are looking to speed up evaporation there are other options, in addition to heat input, that may help.
Creating a lower pressure environment seems like it would help speed up evaporation, but sometimes lowering the pressure does the opposite of what you’d expect. If you run samples with limited heat input, the depth of vacuum may cause those samples to freeze and undergo sublimation instead of evaporation, which is a much slower process. Once the samples freeze and begin to sublimate, the drying rate of your samples will decrease substantially. Adding heat at the beginning of evaporation will help offset the freezing if you do not have vacuum control and must run at a deeper vacuum. Determining the correct temperature/pressure ratio is important to prevent sublimation and allow your samples to evaporate efficiently.
Maximized surface area equates to faster evaporation. With more sample surface exposed, the frequency of collisions with the surface by liquid molecules increases, which speeds up the evaporation rate. Maintaining sample tubes in a slanted orientation during evaporation helps maintain the greatest surface area as centrifugal forces push the samples against the walls of the container.
Most samples are sensitive to heat so scientists are cautious to add much or any heat to their samples during evaporation. However, studies show that the chamber temperature and the sample temperature can diverge significantly due to evaporative cooling.
Using ethanol as the solvent and a chamber set point of 45° C, ethanol slowly cooled during the initial evaporation and stayed at or below 0° C between 7 minutes and 22 minutes of the 35-minute run. After the 22-minute mark, the sample slowly warms up. This drop in temperature during the run is caused by evaporative cooling. Evaporative cooling is the reduction in temperature resulting from the evaporation of a liquid. This cooling effect is due to the latent heat of vaporization, which is the amount of heat required to transform a given quantity of a substance from a liquid to a gas at constant temperature and pressure.
Temperature probe readout between 7 and 22 minutes was below the 0° C sensing range.
As the sample dried, and less evaporation was occurring, evaporative cooling decreased and the sample temperature increased but never reached the chamber temperature set point.
Even for heat sensitive samples, heat can safely be applied during the first segment of the drying run. For faster evaporation rates, apply heat early in the run when evaporative cooling has the greatest effect.
For extremely heat sensitive samples that cannot be exposed to temperatures above 30° C a Refrigerated CentriVap is recommended. For precise chamber temperatures throughout the process, a Refrigerated CentriVap model adds the option of controlled refrigeration down to -4° C and up to 100° C in 1-degree increments.
The chart below was done with a standard, non-refrigerated CentriVap, and the heater was turned off after 20 minutes.
Temperature probe readout between 4 and 20 minutes was below the 0° C sensing range.
Evaporative cooling is overcome by the higher chamber set point and the samples increase in temperature, but still don’t approach the chamber temperature set point until they near dryness.
As seen in the tests above, adding heat at the beginning of evaporation, even high heat for a few minutes will help speed up the process without raising your sample temperatures.
Our testing has shown that for the fastest evaporation processing of temperature sensitive samples, high chamber temperatures can be applied at the beginning of the run, but the chamber temperature should be lowered towards the end of the run to prevent heat damage to the sample. Labconco CentriVap Centrifugal Vacuum Concentrators make this easy with independent timers for the heat and run time. If your evaporation step is the bottle neck in your lab, you can try any of the tips listed above to speed this process up.
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