Sample Preparation & Processing
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 condensers 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 Speed Trap™ (Figure 3), have cold coils suspended in the vapour 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.
Power tails off at low temperatures
-60 -55 -50 -40 -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45 50 Gas Temperature
Figure 4. Compressor Power v Gas Temperature The Importance of Pressure Control
Pressure control in a vacuum evaporation system is critical for (1) ensuring optimum trapping of evolved vapours; (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 vapours, 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.
Figure 3. miVac SpeedTrap Design - 1. Hot vapours enter. 2. Condensing coils with ice shell. 3. Glass collecting flask. 4. Vacuum insulation. 5. Solvent collects as a liquid.
Condensing Power
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 vapour. 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 4); 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 optimise 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.
Summary
Today a wide range of evaporation and concentration systems are available 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.
Sample freezing is undesirable in concentrator systems as this slows down evaporation, therefore, pressure should be kept higher to attain an appropriate boiling point. For instance - water will freeze if evaporated below 6mbar, 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. high performance liquid chromatography (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 optimising 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 volatilise, and 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.
Compressor at maximum power
96-well SPE Cleanup and Analyte Enrichment The MicroluteTM
96 well SPE microplate from Porvair Sciences is designed to automate solid phase extraction cleanup and analyte enrichment in busy laboratories.
To provide an optimised solution to your application - Porvair Sciences offers a large number of sorbents from C18 to Mixed Mode in either silica or polymer format with a choice of 10mg to 100mg loading per well. Microlute plates are also available with a combination of sorbents to provide a tool for accelerating the SPE methods development process.
The innovative design of the Microlute™ offers all the advantages of automated and high throughput SPE sample preparation in a convenient microplate format capable of rapidly processing 96 samples in one go repeatedly and precisely. Constructed from a single piece of moulded high quality polypropylene, the Microlute™ plate will not bend or distort because individual SPE cartridges do not have to be repeatedly plugged in and out. Using a proprietary sorbent slurry loading technique, Porvair has eliminated the channelling effects often limiting the performance of dry powder loaded SPE columns. Each well on a Microlute™ plate has an individual drain spout ensuring 100% sample transfer and zero crossover contamination.
The Microlute™ plate is compatible with most liquid-handling robotic systems for automated, high throughput SPE. MORE INFO. 88
Compressor Power
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