Looking Beyond the Rotovap for Evaporator Selection
Lab managers might consider designs other than standard, single-bulb rotary evaporators.
Evaporation, or sample concentration, is a staple of most biology and chemistry labs. Depending on sample size and composition, solvent type, and required throughput, laboratory managers might consider designs other than standard, single-bulb rotary evaporators.
Organomation (Berlin, MA) offers two core evaporative technologies, with nitrogen blowdown being their primary focus. The N-EVAP, MICROVAP, and MULTIVAP lines use nitrogen blowdown and heat to strip solvents from small evaporator vessels, while the parallel boiling-bath evaporator, S-EVAP, works much like standard rotary evaporators. “S-EVAP resembles a standard single-bulb rotary evaporator, but can process multiple samples simultaneously,” says Amy Valladares, sales technician at Organomation. S-EVAP holds up to 10 round-bottom or Kuderna Danish (KD) flasks of up to 500 mL.
Configurations differ depending on the sample flask. As the sample evaporates, solvent gases enter the condenser either through a Snyder column for KD flasks or via an adapter to a Hopkins condenser for round-bottom vessels.
One drawback of standard rotary evaporator models is that the evaporator, bath, hoses, etc. occupy too much bench space for just one sample. Multiflask designs multiply the sample-per-square-foot value proposition since they employ one bath, one condenser water source, and one solvent recovery flow path for up to 10 samples.
Nitrogen blowdown units use an entirely different process. Samples are heated in a water or dry aluminum bead bath. As the sample heats, nitrogen blows across the surface of the liquid. The N-EVAP line (the company’s best-selling product) features adjustable gas flow and sample sizes. Like S-EVAP, it makes optimal use of space, particularly for multiple samples of up to about 50 mL. “N-EVAP accepts a wide variety of tubes, a unique feature which is particularly useful when samples are coming in from different processes or workflows,” Valladares says, for example different-sized tubes or flasks. The MICROVAP and MULTIVAP lines are designed for batch processing.
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Valladares dismisses concerns that rapidly flowing nitrogen could sweep ambient air and moisture into the sample. “The nitrogen flow suffices to keep out other gases or impurities, particularly when samples are introduced in small-diameter tubes. The nitrogen flow is adjusted to the point where it disturbs the liquid surface without causing splashing.” Organomation evaporators enable individual control of nitrogen flow to each sample, which allows for greater or lesser flow depending on the solvent or solvent mixture. “There’s a controller for the main gas input, but the nitrogen flow to each tube is adjustable without affecting nitrogen flow to neighboring samples.”
As anyone who has tried one-off nitrogen-blowdown evaporation knows, the process can be slow, and it becomes even slower with high-boiling-point solvents. Valladares explains the significance of controlled heat in evaporation: “Blowdown by itself is definitely slower than vacuum alone, but it’s decently fast when you add heat. I would not recommend blowdown for very large samples, since the higher the boiling point, the slower evaporation will be.”
With water bath temperatures limited by the 100°C barrier and solid bath temperatures reaching about 120°C, users need to assess the suitability of blowdown evaporators for high-boiling solvents like toluene and dimethyl sulfoxide. Solvent removal in those instances is usually accomplished by addition of water to polar aprotic solvents followed by extraction, or, in the case of high-boiling hydrocarbons, by extraction alone.
Labconco (Kansas City, MO) sells vacuum and nitrogen blowdown evaporators that use dry block heat to keep sample flasks in direct contact with the heat source. “This eliminates the need for a water bath, reducing system maintenance and cutting down on potential contamination,” says Megan Zimerman, product manager.
The company also sells the CentriVap Centrifugal Vacuum Concentrator, a multiple-sample evaporator that combines centripetal force, dry heat, and vacuum to remove solvents quickly. CentriVap holds up to 132 samples of up to 25 mL in size. An option, Heat Boost, surrounds the chamber wall with heat to remove high-boiling solvents.
“Another very useful option with CentriVap is refrigeration capability, down to -4°C, when working with heat-sensitive samples,” Zimerman says. “Systems offer nine user-set programs which allow for independent control of run time and temperature settings, in single-degree or one-minute increments.” The temperature range is ambient to 100°C, and time range is up to 999 minutes.
Best practices
Operational and safety best practices are similar between waterbath evaporators and conventional rotovaps. Bumping—the sudden, nonevaporative flow of sample through the solvent recovery path—is a common problem when either heat or vacuum is applied rapidly. Splash or bump traps usually suffice to prevent the sample from traveling all the way to the solvent collection vessel.
Organomation’s Valladares suggests using a boiling chip when such problems are anticipated. “Also watch for nitrogen flow in blowdown systems. The surface of the fluid should be moving but not so strongly as to cause splashing, which can result in cross-contamination between samples.”
While solvent recovery from blowdown evaporators is impossible, solvent reservoirs for closed, bath-type systems fill very quickly with solvent of varying purity, dryness, and composition. This “product” can serve directly, or after rudimentary drying, in noncritical applications such as glassware washing or rinsing, or in extractions from aqueous suspensions. Given the price of high-purity solvents, many large research groups recycle evaporated solvents to reagent grade or above.
Labconco’s Zimerman advises lab managers to focus on sample needs when selecting an evaporator (or concentrator). The first consideration is sample volume, as systems using heat baths or blocks typically accommodate larger samples than nitrogen-blowback evaporators. Heat-sensitive or volatile samples will require a higher degree of temperature control, and possibly refrigeration as well, to remove solvents efficiently while preserving the sample.
“For safe and effective operation, we specify our products for operation only with nonflammable or Class D chemicals that have an auto-ignition temperature of 180°C or above,” Zimerman says. “It is also important to make sure you have the required accessories to aid in solvent recovery, for example cold traps, chemical traps, one or more trapping valves, etc.”
James Jacso, marketing director at Glas-Col (Terre Haute, IN), advises purchasers to look for flexibility in design and layout. “Also consider the system’s temperature range, footprint, and ease of use.”
Glas-Col’s nitrogen blowdown evaporators can hold 96-well plates, as well as tubes with volumes up to 50 mL, to accommodate evaporation from a wide variety of workflows. Their basic systems employ a stand/manifold and nitrogen flow regulation. Advanced models allow control of heat and gas flow to the sample, vertical movement of the sample, and proportional gas flow. All use a dry heat source instead of conventional water baths.
Jacso also suggests conducting evaporations in a fume hood and carrying out “periodic cleaning with the solvent you’re evaporating to prevent cross contamination in future runs.”
For additional resources such as useful articles and a list of manufacturers, visit www.labmanager.com/evaporators