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Centrifugation ensures that solvent boils from the sample surface downwards, thereby minimising boiling over and solvent bumping thereby preventing sample loss and cross- contamination. Solvent at the liquid surface is at the pressure of the equipment, whereas solvent below this level is at higher pressure due to the extra weight of solvent multiplied by the g force exerted by the centrifuge rotor. Systems with very high rotor speeds generating 500g or more are proven to prevent solvent bumping. The centrifugal evaporation technique


accommodates a wide range of solvents and can concentrate, dry to a film or freeze dry samples.


Advanced centrifugal evaporation systems can achieve rapid freeze drying by concentrating the majority of a larger volume before freeze drying the final few millilitres of sample. Samples must be processed in batches although large numbers of small samples can be processed at the same time. Although early systems were viewed as slow, recent developments in high-energy sample heating using low-temperature low-pressure steam, such as employed by the Genevac RocketTM Evaporator, enable rapid concentration of larger volumes at the speed of a rotary evaporator. Such systems operate without the solvent bumping and consequent need for continual monitoring associated with rotary evaporators.


Blow-Down Evaporation


In these evaporator systems, an inert gas such as nitrogen is blown down through needles onto samples in tubes, vials or microplates to create a flow over the liquid surface. This alters the equilibrium between the vapour and liquid phases to favour the vapour phase. Heat is normally applied to the samples to hasten evaporation and pre-warmed gases can also be used. The technique enables open access equipment use, and is comparatively inexpensive ranging from self-assembled apparatus to simple commercial systems. Although blow down evaporation is relatively fast for volatile solvents, it can be slow for


solvents with high boiling points or those that are difficult to evaporate such as water. Using blow down evaporation samples are prone to becoming hot during the process, being at the temperature of the heating block or bath during evaporation, and consequently the technique offers poor recovery of volatile analytes. As a manual process – blow down evaporation requires continuous monitoring by the user to detect the end point of the drying process. The technique typically achieves poor final dryness and splashing may occur, particularly if the gas flow rate is too high, leading to sample cross-contamination. A common use of blow down is for concentrating large volumes to just a few millilitres for subsequent processing by other techniques. Specialised teat-ended tubes are available for this procedure and some commercial systems have basic automatic stop mechanisms based on liquid level detection.


Vortex Evaporation


These systems boil batches of samples under vacuum, to keep the samples cold throughout vaporisation, while swirling the sample tubes to create a vortex.


A rotary evaporator is essentially similar, but is for a single sample contained in a flask. The vortex created generates a large sample surface area for evaporation, making the process relatively fast.


However, the resultant dried product is spread across the vessel walls, which can make sample recovery more difficult. Moreover, in contrast to centrifugal concentrators, the swirling movement generates insufficient g force to prevent solvent bumping and hence vortex evaporators are prone to sample loss and cross contamination.


In some vortex systems, evaporation is further accelerated by use of heating lamps directed into the sample tubes, but these systems are prone to overheating all or part of a sample as it becomes dry.


Factors Affecting the Speed of Evaporation


Three factors affect the speed of concentration: heat energy supply, vapour removal and solvent surface area. For boiling solvents, the faster heat energy is supplied the faster solvents boil. Similarly for evaporative systems, more heat equals faster evaporation, although samples are at the set system temperature rather than boiling where the boiling point of the solvent controls the sample temperature.


Heat is supplied by lamps, heated blocks/baths or, in the latest generation of centrifugal systems, by low-temperature low-pressure steam. In these latest systems, water takes heat from the chamber wall, turns to vapour, then condenses on sample vessels (which are cold due to solvent boiling within) to transfer the heat to the samples.


The heating vapour reaches all areas, minimum heat is wasted and the time to reach the required temperature is very rapid, thereby improving the efficiency of concentration.


In addition to the effects of heating rate, the faster vapours can be removed the faster solvents will boil. As a wet, boiling sample is at the boiling temperature of the solvent, then the better a system is at getting heat into the sample, the faster will concentration occur, and the more efficient a system will be at removing vapours by condensation in a cold trap. However, although speed of concentration can increase with higher vacuum level, this remains true only to a certain point. At the highest vacuum levels achievable by modern evaporator systems, a volatile solvent will boil at extremely low temperatures that are so cold that a cold trap cannot effectively catch the solvent and is hence rendered useless.


For optimal system performance, a balance is therefore essential between the heat input to the sample and the heat removal at the condenser. If a system is not balanced, either the cold trap does not capture solvent such that solvent goes to the pump, the pressure in the system rises and solvent recovery is lost – or the cold trap effectively controls the pressure such that the system operates at a slower rate than optimal. In a freeze dryer, vapour flow rate is the controlling factor, such that the faster vapour is removed, the faster samples dry. This is most affected by cold trap power and temperature. Finally, concentration or evaporation can be accelerated through generating a greater solvent surface area, and blow down systems speed up evaporation through a shift in equilibrium between phases.


Latest Developments in System Technology


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 therefore key to obtaining a balanced system that ensures optimum performance in solvent removal and solvent recovery.


Vacuum Pumps


Vacuum evaporative 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 both high water usage and solvent vapour 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 vapours leading to loss of pump performance or pump seizure.


The most modern diaphragm vacuum pumps are now very reliable and also solvent resistant, however their effective ultimate vacuum level is 1 mbar or 2 mbar which means they are ideal for


Figure 2. Scroll Pump


working with volatile solvents, but not suitable for drying high boiling point solvents or freeze drying as their vacuum is insufficient.


Newer, dry scroll pumps do not require any pumping fluid or lubricant and are suitable for use with evaporating and concentrating systems. Dry scroll pumps (Figure 2) 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 vapour flow, be easy to clean, and prevent solvent 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 vaporise there is huge volume expansion, of the order of 20,000 times. As vapours condense back to liquid at the cold trap, the volume reduction helps pull a vacuum.


Traditionally, a cold trap has comprised a stainless steel vessel with cooling coils around the outside that is positioned in the vapour path between the concentrator and the vacuum pump and 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, these cold traps must be defrosted before emptying, adding to the downtime of the system.


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 hence this method has not proved popular.


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