Evaporators: Type, size, and speed to match application, volume, and throughput

Evaporation systems are those devices that are everywhere. They are not flashy, complex, or pricey, yet they are often an essential component of work horse applications like proteomics research or biosynthesis core laboratory operations. Although the methods and underlying chemistry of both the evaporator type and the sample type can be unique, the basic premise is universal – to remove as much unwanted solution as possible while leaving the sample analyte material intact.

Rotary Evaporators (rotovaps) allow the gentle removal of solvents from aqueous samples and can range in size from analytical benchtop to 20-50L process scale. There are several main components:

  1. A vacuum system to lower the pressure in the chamber.
  2. A heated bath for controlled heating of the sample vessel.
  3. A condenser with a “cold finger” that allows condensation of the removed vapor.
  4. A condensate-collection vessel which removes the condensate from circulation.

Other elements include a motorized rotary unit that rotates the sample vessel and a motor to lift and lower the vessel into the bath.

The basic theory of the technique involves several steps. The vacuum lowers the system pressure below the liquid sample’s vapor pressure, thus causing the sample to boil and vaporize. This boiling is facilitated by the heat bath, which along with the vacuum can be adjusted to suit the application and sample type. The condenser coil, sometimes called a cold finger because it is filled with a cooling liquid or submerged in dry ice, functions to lower the temperature of the removed vapor thereby causing it to condense. The condenser vessel then collects this condensate and removes it from circulation through the vacuum pump.

Rotovaps are commonly used to remove solvents and other volatiles that have a significant difference in vapor pressure compared to the analyte liquid meant to remain intact. In other words, low boiling point solvents are best removed from high boiling point liquids to avoid co-evaporation, and potential loss of sample product. This is a major disadvantage of these systems, and alternative techniques, such as centrifugal evaporation methods, are much more successful in the minimization of co-evaporation. One such stringent application includes removal of solvent or concentration of aqueous peptide solutions subsequent to mass spectrometry.

Centrifugal Evaporation devices (also called speedvacs or lyophilyzers) are very commonly seen in many types of life science labs. There are several differences and advantages over the rotovap. The first difference is the ability to process many samples in batch in microtube or microtiter plate format. Another difference is the ability to remove high boiling point solvents while keeping the sample temperature low, a feature that may be critical to temperature sensitive analytes like proteins and enzymes. This is possible because the centrifugal force of the centrifuge creates a pressure gradient within the liquid sample, thus causing the sample to boil top to bottom. The denser material is captive in the bottom of the vessel allowing vaporization of the less dense solvent top level.

A third type of evaporation system involves blowdown or gas vortex shearing technology. This technique is fundamentally distinct from the previous two in that a gas is blown over the sample under such conditions and in such a way to facilitate evaporation without the requirement for vacuum. An inert gas such as nitrogen is blown in a helical fashion into the sample vessel which is placed in a water bath. The helical flow sets up a vortexing action that provides continuous sample mixing and rinsing of the vessel wall. The vapor rises from the center of the vortex and exits by means of an exhaust fan vented to a fresh air outlet.

The advantages of this type of system include time of operation, bath temperature, and gas flow rate; parameters which are typically controlled electronically by an integrated microprocessor, allowing high level precision and quality control. The evaporator also enables unattended operation and flexibility, such as pausing or stopping and temperature or sample adjustments.

Evaporators are those devices that often get taken for granted… That is, of course, until the pump goes down or the system looses vacuum. Then operations stop and the frantic search begins for a backup… And of course all of the other machines are in use… So your relegated to the “spare” system which only limps along with questionable effectiveness.

It’s best to have a greater idea of the range and models of evaporators in the market to be sure the lab has the most effective and reliable unit in the first place.