The controlled evaporation of chemicals and biological molecules in solution is crucial for many applications. Investigators use it to concentrate analytes to threshold detection levels prior to environmental testing, control purity at multiple steps of pharmaceutical production, and preserve biologically active reagents for investigative workflows. The type of evaporator used for each process depends on a delicate balance of ridding solutions of volatile solvents while protecting non-volatile, temperature-sensitive solutes. Although water is the most common solvent and testing matrix, semi-volatile organic compounds often have comparatively higher boiling points, and concentration procedures have to be planned accordingly.
Laboratory evaporators manipulate heat, gas, vacuum, and motion separately or in combination. They are used to steadily remove solvent and therefore concentrate a previously dilute solution, or to eliminate solvent altogether and generate a powder suitable for long-term storage. The widely used rotary evaporator typically employs motorized systems to rotate a sample flask, and manipulate it to and from a heated liquid bath. A vacuum system and vapor duct additionally divert solvent vapors into a collection flask. The reduced internal pressure created by vacuum above a solution functions to lower solvent boiling point and distill it out at sufficiently low temperatures to preserve the chemical structure of the solute or analyte. As an added benefit, recovered solvent can be recycled into downstream procedures, mitigating waste and expense.
During the initial stages of solvent phase change, natural evaporative cooling properties are at their peak, and the greatest amount of heat can be applied with the least risk to the sample. Subsequently, as volume decreases, the sample itself attains greater risk of molecular damage, tragic events such as bumping or foaming, and the fouling of heat transfer surfaces with thin concentrated films. Optimal transition and timing between high and low temperatures can most efficiently yield the highest quality analytes. These principles extend into other evaporation/concentration platforms that fit different needs intrinsic to sample size, number, and desired outputs.
Gas blowdown evaporators introduce heat and nitrogen through vertical needles to volatilize solvent and promote evaporative cooling. This procedure is especially useful with multi-well plates in preparation for HTS or GC/LC-MS protocols, although it is applicable for multiple samples up to about one liter in volume.
Vacuum centrifugation-based concentration can handle up to hundreds of smaller samples, and can maximize throughput for extraction procedures, or speed evaporation by distributing starting volumes across many smaller vessels. Again, there is a delicate balance between evaporative cooling and heat sensitivity as cooling plateaus and then subsides. Monitoring correct temperature and flow of nitrogen is critical. Centrifugal rotor hubs generate heat at high speed that can damage samples unless instrumentation is refrigerated with a controlled cooling system.
Finally, for preservation of biological activity, especially for large-volume and highly heat-sensitive samples, lyophilization is often optimal. Here, patience is paramount, as samples must first be fully frozen before applying heat and vacuum to sublimate water or other solvents.
There is a versatile array of instrumentation to evaporate solvent and concentrate chemical and biological samples, for which considerations of heat sensitivity and volatility must coordinate with sample number and size.