Introduction
A rotary evaporator, also known as a rotovap or rotavap, is a device used in chemical laboratories for isolation and distillation of large amounts of a single sample. The principle of rotary evaporation is to remove solution continuously and effectively by reducing pressure, which reduces boiling point (bp) of solution, increasing surface area, and optimizing temperature control depending on distilled solvent and analyte(s). Besides, it is recommended to use with a cold trap or condenser to minimize the risk of solvent emission.
Rotary evaporators are also used in molecular cooking for the preparation of distillates and extracts.
Evaporation under vacuum can also, in principle, be performed using standard organic distillation glassware — i.e., without rotation of the sample. The key advantages in use of a rotary evaporator are :
- that the centrifugal force and the frictional force between the wall of the rotating flask and the liquid sample result in the formation of a thin film of warm solvent being spread over a large surface.
- the forces created by the rotation suppress bumping. The combination of these characteristics and the conveniences built into modern rotary evaporators allow for quick, gentle evaporation of solvents from most samples, even in the hands of relatively inexperienced users. Solvent remaining after rotary evaporation can be removed by exposing the sample to even deeper vacuum, on a more tightly sealed vacuum system, at ambient or higher temperature (e.g., on a Schlenk line or in a vacuum oven).
The disadvantages in use of a rotary evaporator are:
A key disadvantage in rotary evaporation, besides its single sample nature, is the potential of some sample types to bump, e.g. ethanol and water, which can result in loss of a portion of the material intended to be retained.
Even professionals experience periodic mishaps during evaporation, especially bumping, though experienced users become aware of the propensity of some mixtures to bump or foam, and apply precautions that help to avoid most such events.
In particular, bumping can often be prevented by taking homogeneous phases into the evaporation, by carefully regulating the strength of the vacuum (or the bath temperature) to provide for an even rate of evaporation, or, in rare cases, through use of added agents such as boiling chips (to make the nucleation step of evaporation more uniform).
Rotary evaporators can also be equipped with further special traps and condenser arrays that are best suited to particular difficult sample types, including those with the tendency to foam or bump.
Methods
The preferred method for solvent removal in the laboratory is by use of a rotary evaporator, which is a device used in chemical laboratories for the efficient and gentle removal of solvents from samples by evaporation.
Vacuum evaporators as a class function because lowering the pressure above a bulk liquid lowers the boiling points of the component liquids in it. A rotary evaporator is essentially a reduced pressure distillation: a solution in a round bottomed flask is placed in the water bath of the apparatus, and rotated while the system is partially evacuated (by a water aspirator or vacuum pump). The reduced pressure in the apparatus causes the solvent to boil at a lower temperature than normal (see vacuum distillation), and rotating the flask increases the liquid’s surface area and thus the rate of evaporation.
Generally, the component liquids of interest in applications of rotary evaporation are research solvents that one desires to remove from a sample after an extraction, such as following a natural product isolation or a step in an organic synthesis. Liquid solvents can be removed without excessive heating of what are often complex and sensitive solvent-solute combinations. Rotary evaporation is most often and conveniently applied to separate “low boiling” solvents such a n-hexane or ethyl acetate from compounds which are solid at room temperature and pressure. However, careful application also allows removal of a solvent from a sample containing a liquid compound if there is minimal co-evaporation (azeotropic behavior), and a sufficient difference in boiling points at the chosen temperature and reduced pressure.
The solvent vapor condenses when it comes into contact with a water condenser and drips into a receiving flask. When the solvent is removed, the concentrated compound is left in the flask. One difference between distillation and rotary evaporation is that the distillate is most often retained in distillation, while the residue is retained in rotary evaporation.
The staple distilled solution of rotary evaporator is solvent and chemical solution, hence a chemical resistant vacuum pump made of polytetrafluoroethylene (PTFE) should be best choice. There are rules for pump selection, for example, the flow rate of pump is directly proportional to volume of evaporation flask. Moreover, vacuum requirement varies depending on boiling point (bp) of distilled solution. Therefore, vacuum controller is a recommended accessory for accurate vacuum controlling to prevent solvent bumping and shorten evaporation process significantly.
The main components of a rotary evaporator are:
- A motor unit that rotates the evaporation flask or vial containing the user’s sample.
- A vapor duct that is the axis for sample rotation, and is a vacuum-tight conduit for the vapor being drawn off the sample.
- A vacuum system, to substantially reduce the pressure within the evaporator system.
- A heated fluid bath (generally water) to heat the sample.
- A condenser with either a coil passing coolant, or a “cold finger” into which coolant mixtures such as dry ice and acetone are placed.
- A condensate-collecting flask at the bottom of the condenser, to catch the distilling solvent after it re-condenses.
- A mechanical or motorized mechanism to quickly lift the evaporation flask from the heating bath.
The vacuum system used with rotary evaporators can be as simple as a water aspirator with a trap immersed in a cold bath (for non-toxic solvents), or as complex as a regulated mechanical vacuum pump with refrigerated trap. Glassware used in the vapor stream and condenser can be simple or complex, depending upon the goals of the evaporation, and any propensities the dissolved compounds might give to the mixture (e.g., to foam or “bump”). Commercial instruments are available that include the basic features, and various traps are manufactured to insert between the evaporation flask and the vapor duct. Modern equipment often adds features such as digital control of vacuum, digital display of temperature and rotational speed, and vapor temperature sensing.
Applications
- Pharmaceutical industry, such as components development and cannabis extraction
- Environmental Analysis, such as TPHd, pesticide, etc.
- Concentration and solvent removal in lab research
