How to Evaporate or Concentrate Heat Sensitive Samples
The evaporation or concentration of samples is a common step in many sample preparation methods. Concentration of the sample happens when the sample undergoes evaporation and is often required as a final step before analysis.
Although there is a wide variety of laboratory equipment that evaporate samples in different ways, the common denominators are that evaporation is occurring as a result of heat input, increased surface area, and vacuum or vapor pressure differential. The best evaporation method for your sample is dependent on the characteristics of your sample — size, solvent, number of samples, and ability for the sample to be heated.
The easiest way to evaporate a sample is to simply heat the sample to boiling. The higher the temperature of the sample, the greater the kinetic energy of the molecules at its surface and therefore the faster the rate of evaporation. Unfortunately, many samples are heat sensitive and the simple addition of heat is not possible as the heat denatures the sample or reduces analyte recovery rates. For heat sensitive samples a vacuum is used to lower the boiling point of the solvents so evaporation occurs with a minimum amount of additional heat.
Bumping
When a sample is placed in vacuum as a liquid, volatile samples will undergo “bumping” which is the boiling of samples resulting in solvents bubbling or splashing uncontrollably due to the reduced boiling points. Bumping causes sample loss, low analyte recovery and sample cross contamination. Evaporation systems that utilize vacuum often create a force within the sample to eliminate bumping. Without the centrifugal force, the only other way to maximize evaporation and eliminate bumping is to precisely control the vacuum level of the system throughout the entire evaporation process, which is much too difficult.
Types of Vacuum Evaporation Systems
Rotary Evaporators are for single, large to medium sized liquid samples and Vortex Evaporators are for multiple, large to small sized liquid samples. Both systems utilize a controlled vacuum while mechanically creating, with physical movement, a centrifugal force within the sample that also increases the surface area. Rotary Evaporators utilize a water bath while Vortex Evaporators utilize a dry block for a controlled heat source.
Centrifugal Concentrators evaporatemany, small to medium sized samples by spinning the sample tubes in a rotor under vacuum creating centrifugal force. Heat is supplied through the chamber walls and very little reaches the samples as it is difficult to move heat through a vacuum atmosphere.
Lyophilizers or Freeze Dyers evaporateany size or number of samples under deep vacuums that must be below 0.400 millabar. In lyophilization the frozen sample goes from a solid to gas without passing through the liquid phase. As a result, not only are samples evaporated without heat damage, they acquire unique qualities that allow them to be stored without degradation at room temperatures.
Maximizing Evaporation Rate
The goal of evaporating heat sensitive samples is to evaporate them as fast as possible without damaging the samples. Many end users fail to recognize the effect of evaporative cooling and could greatly reduce the evaporation time required by adding heat into the samples at the appropriate time. Heat added to a sample early and in the middle of the evaporation process will drive a faster rate of evaporation without heat damaging the sample. Samples CAN BE heat damaged when heat is added at the end of the evaporation when evaporative cooling is no longer occurring. Systems that control the heat time independently of the run time maximize evaporation rates, reduce run times, and ensure samples are not heat damaged.
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