Key applications include sample concentration, solvent recycling, extractions, and separation of solvent mixtures
In their simplest embodiment, “rotovaps” consist of a temperature bath, rotating flask, condenser, collection flask, and vacuum source. Solvent distills from the sample under the combined effects of heat and vacuum, and collects after condensation in the collector. Recovered single-phase organic solvents may be dried and re-used; binary, tertiary, or quaternary solvent mixtures are also re-used but may need adjustment for composition.
Water is the fluid of choice for the bath, but some laboratories use oils to reach heating temperatures of up to 180°C. Several choices are available for the condenser coolant. Until about 10 years ago, almost everyone used house water. Concerns over water consumption caused many labs to switch to a chiller to recirculate coolant into and out of the condenser coils. Chillers provide more precise cooling, greater control over condensation, a greatly reduced environmental footprint, and are overall less expensive to use than water. High-efficiency trapping of low-boiling solvents is achieved with a “cold finger” charged with dry ice and acetone.
Of all the rotovap options, vacuum is probably the broadest. At one time, water aspirators were the most common vacuum sources. That practice has gone by the wayside for the reasons given for water coolers and due to environmental concerns related to solvent vapors venting down the drain.
The next most common source is house vacuum, which is limited but inexpensive and reliable. Users typically insert a Woulff bottle or cold trap between the vacuum spigot and the rotovap, to trap volatiles that the condenser missed and protect the house vacuum system.
Increasingly, users employ vacuum pumps to achieve reproducible and rapid solvent removal. The vacuum must be applied carefully, however, to avoid bumping and foaming.
Traditionally, vacuum control was achieved by slowly closing a glass stopcock. Increasingly, users are turning to more sophisticated vacuum control, which Kristof O’Connor, product manager at Heidolph Brinkmann (Elk Grove Village, IL), describes as “probably the number-one improvement in rotary evaporators over the past two decades.” Control became necessary, he says, because “vacuum pumps are very stupid machines. They try to achieve as high a vacuum as they can, as quickly as possible, which often results in bumping and foaming.”
Lisa Sprenger, account manager at IKA Works (Wilmington, NC), agrees on the importance of vacuum control and the full and volumeregulated distillation method implementations, and adds the following:
- Larger graphic displays with increased data storage capacity
- Additional safety features to regulate heating baths and motor units
- Implementation of full and volume- regulated distillation methods
- Motorized lift systems
- Clockwise and counterclockwise rotation of the distilling flask to increase speed and efficiency for powder drying applications
Newer rotovaps may incorporate thermocouple-controlled operation, in which a pump integrates with a controller and a thermocouple located in the vicinity of the condenser coils. As the coil temperature rises through heat transfer between the condenser and the evaporated solvent, the vacuum is bled out through valving to maintain steady distillation. Another technique, known as “RPM” control, speeds or slows the pump’s inner workings to control the delivered vacuum.
Whereas valve control is generic, RPM control locks users into pumps from a single manufacturer. “Our RPM control pump only works with our precision controller,” Mr. O’Connor tells Lab Manager Magazine.
Coupled with software and solvent databases, vacuum control allows users to plug in bath temperature and solvent for ultimate fine-tuning of evaporation rate, to the point of allowing fractional distillation of multi-component solvents, followed by final drying.
“These features provide reproducibility, and make it easier and faster to optimize operating parameters,” Mr. O’Connor says.
An important new concept in laboratory rotary evaporation is systems integration. To put it simply, in such an arrangement, the main components of the evaporation solution--the evaporator, the vacuum pump, the vacuum controller, and the recirculating chiller--are fully integrated with respect to parameters and control. The benefits of such an integrated solution include: up to 75 percent reduction in energy consumption, reduced heat emissions in the lab, optimized distillation capacity, and increased process safety due to the connectivity of all the components. “Chillers that are not fully integrated with the entire evaporation system waste a lot of energy and generate heat between the distillation runs because they do not shut off automatically,” advises Michael Stern, director of marketing at BUCHI Corporation (New Castle, DE). “On the other hand, fully integrated systems provide many performance, safety, and sustainability benefits,” he adds.
Potential purchasers should consider several vacuum and condenser cooling options before buying a rotovap.
Cooling method, which dictates the type of condenser used, should be decided based on the expected solvent load. More vigorous cooling is demanded for high volumes, rapid distillation, and very low-boiling solvents like ethyl ether.
Purchasers should consider whether automated, volume-dependent rotary evaporation is desired. This feature combines precise distillation with walk-away automation.
Angelo DePalma holds a Ph.D. in organic chemistry and has worked in the pharmaceutical industry. You can reach him at firstname.lastname@example.org.
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