The second installment of this paper describes the latest developments in pumps, cold traps, and condensers used in performing the various evaporation and concentration techniques described in Part 1 (http://www.americanlaboratory.com/914-Application-Notes/130187-Understanding-Evaporation-and-Concentration-Technology-Part-1-Basic-Principles-of-Commonly-Used-Evaporation-Techniques/).
An understanding of the evaporative process and factors affecting it, involving careful selection of a vacuum pump, cold trap or condenser, and consideration of pressure control, is key to obtaining a balanced system that ensures optimum performance in solvent removal and solvent recovery.
Vacuum evaporation systems, including freeze dryers and centrifugal concentrators, require a vacuum source. This may be a tap aspiration pump, traditional vacuum pump, or new-generation dry pump.
The use of tap aspiration pumps is declining due to their negative environmental impact; there is high water usage and solvent vapor condenses in and contaminates the water going to waste. Traditional vacuum pumps can provide good performance, attaining pressures below 0.02 mbar with a high flow rate. However, the mineral oil used to lubricate the pump vanes is messy and over time can be degraded by solvent vapors, leading to loss of pump performance or pump seizure. The most modern diaphragm vacuum pumps are very reliable and solvent resistant; however, their effective ultimate vacuum level is 1 mbar or 2 mbar, which means that they are ideal for working with volatile solvents, but are not suitable for drying high-boiling-point solvents or freeze-drying, since their vacuum is insufficient.
Figure 1 – Scroll pump.
Newer, dry scroll pumps do not require any pumping fluid or lubricant and are suitable for use with evaporation and concentration systems. Dry scroll pumps (Figure 1) also provide high performance, down to pressures of 0.07 mbar at high flow rates, but with very low maintenance and minimal environmental impact compared with traditional vacuum pumps or tap aspiration.
Cold traps and condensers
A cold trap or condenser is used to capture the solvent removed by concentration or evaporation systems. The cold trap should not impede vapor flow, should be easy to clean, and should prevent solvent from reaching and attacking the vacuum pump. Additionally, an efficient and well-designed cold trap offers the advantages of good solvent recovery and an accelerated evaporation process through its contribution to the vacuum generated. When solvents vaporize, there is huge volume expansion, of the order of 20,000 times. As vapors condense back to liquid at the cold trap, the volume reduction helps pull a vacuum.
Traditionally, a cold trap comprises a stainless steel vessel with cooling coils around the outside that is positioned in the vapor path between the concentrator and the vacuum pump and is chilled to below 0 °C by a gas compressor system. Although effective, these cold traps can be awkward and time-consuming to empty. To remove condensed solvent, particularly water that freezes to ice, the cold traps must be defrosted before emptying, adding to the system’s downtime. To overcome this obstacle, some systems use interchangeable glass flasks that are placed within the stainless steel vessel. At the end of the concentration process, the condensate-containing flask can be removed and substituted with a fresh flask. However, successful chilling of the glass flask relies on a silicon-based thermal transfer fluid between the stainless steel vessel and the flask that causes the flask exterior to become slippery and potentially hazardous to handle, and therefore this method has not proved popular.
These problems of defrosting and interchanging slippery flasks are avoided in simple evaporator systems. Rotary evaporators, for example, collect removed solvent as a liquid in a glass flask, using a glass condenser chilled with cooling water or dry ice. The principle of the simple Graham condenser has recently been applied to cold trap technology. These new-generation gas compressor cold traps, such as the miVac SpeedTrap™ (Figure 2) (Genevac, Suffolk, U.K.), have cold coils suspended in the vapor path; solvents condense on the coils and are collected directly as liquids into an insulated glass vessel on the front of the trap. These cold traps deliver up to 50% more condensing power than earlier designs, thereby providing higher solvent recovery, and require no cooling water or dry ice to operate. Moreover, the glass flask is easily removed for rapid transfer of solvent to waste, and can be replaced immediately without having to wait for the system to defrost.
Figure 2 – miVac SpeedTrap design. 1) Hot vapors enter, 2) condensing coils with ice shell, 3) glass collecting flask, 4) vacuum insulation, 5) solvent collects as a liquid.
For an efficient cold trap, condensing power is more important than low trap operating temperature. Traps running at very low temperatures (e.g., –80 °C to –100 °C) often consume almost all of the available power to attain these extreme temperatures rather than to condense solvent vapor. Such systems may be adequate for freeze-drying, which is a relatively slow process; however, they are inefficient as cold traps for high-speed concentration due to their limited condensing power. Gas compressors provide cold traps with maximum condensing power down to approximately –20 °C (Figure 3); beyond this, condensing power declines rapidly. Optimal performance from a cold trap is therefore best attained by controlling the boiling point of solvents to –20 °C or higher to ensure that the gas compressor system of the trap operates with full condensing power. Thus, to optimize the solvent recovery process in a vacuum system, it is critical to consider vacuum pump and cold trap function and to have a pressure controller and knowledge of the solvents used.
Figure 3 – Compressor power versus gas temperature.
Importance of pressure control in a vacuum evaporation system
Pressure control in a vacuum evaporation system is critical for 1) ensuring optimum trapping of evolved vapors, 2) speeding up the evaporation of complex mixtures, and 3) preventing sample loss by sublimation.
Control of solvent boiling point is achieved by controlling the pressure and manipulating the pressure to attain a boiling point of –20 °C (see above). A gas compressor cold trap operates at maximum condensing power and highest efficiency for trapping evolved vapors, and samples remain in a typically preferred cold state. Operation at higher temperatures is feasible although likely to be slower due to reduced heat input to samples. Sample freezing is undesirable in concentrator systems because it slows down evaporation; therefore, pressure should be kept higher to attain an appropriate boiling point. For instance, water will freeze if evaporated below 6 mbar. The optimum pressure for water concentration is 8 mbar; at this pressure water boils at +4 °C.
In the case of complex mixtures (e.g., HPLC fractions), where often both water and an organic solvent are present, the organic solvent must be removed without freezing the water, or evaporation is very slow. This can be achieved with correct pressure control. Detailed technical guidance for optimizing these specific types of applications is generally available from the leading evaporator/concentration system manufacturers.
Most samples can become volatile under the right conditions. Generally, the smaller the size of a molecule the easier it is to volatilize; this is especially true for organic molecules. However, when a sample is of low molecular weight (less than 300) and/or has high volatility, for example, a straight-chain organic molecule with few side groups, then some sample may also be lost through sublimation during the evaporation process. Good pressure control can prevent this sublimation, and it is important to stop the evaporation process as soon as the samples are dry. Control measures to achieve this have been developed for some systems.
A wide range of evaporation and concentration systems are available today to accommodate the diversity of applications and samples requiring solvent removal. The correct choice of vacuum pump and cold trap is critical to ensuring optimum evaporation and concentration performance. Pumps with appropriate vacuum level and having high flow rates are recommended. Highly efficient cold traps are now available that not only speed concentration and drying rates, but also recover solvents in liquid form, thereby reducing environmental impact and eliminating time lost to defrosting procedures.
Dr. Induka Abeysena is Application Specialist, and Rob Darrington is Product Manager, Genevac, The Sovereign Centre, Farthing Rd., Ipswich, Suffolk IP1 5AP, U.K.; tel.: +44 (0) 1473 240000; fax: +44 (0) 1473 461176; e-mail: email@example.com.