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Even though mixtures of water/ACN alone may not bump, the addition of actual compounds sometimes changes this. Bumping is preventable by employing a controlled vacuum ramp to the centrifugal evaporator when initially pumping down, and increasing the centrifugal force to greater than 450G. Employing this technique creates a steep pressure gradient in the liquid and forces all boiling to take place cleanly in a narrow band at the surface of the liquid, eliminating bumping.
Large heat requirement
A second problem often encountered with drying water / ACN or water / methanol mixtures is because water has a high specific latent heat of vaporisation.
Consequently a relatively large amount of heat is required to dry fractions. Most modern evaporators apply heat to the sample containers using infra red lamps, so when selecting a system choose one with plenty of available lamp power. The difference between available systems on the market is significant, so verify actual run times before you invest. The system with the most “capacity” (i.e. number of samples that can be loaded) may not be the system with the best “throughput” of HPLC fractions.
Accurate temperature monitoring
Conventional temperature monitoring systems for centrifugal evaporation display the temperature of the radiant heat source (chamber wall) or the outside wall of the sample vessel. However, the solvents cooled by vacuum evaporation will be at a significantly lower temperature than the external wall. In order to achieve the fastest possible rate of evaporation without exceeding the maximum sample temperature, accurate temperature monitoring and control is essential.
When the samples are still drying, all the heat applied by the lamps will be used to boil solvent, and the sample temperature will be “held down” by the solvent boiling point. But when the samples reach dryness, if heat is still being applied at the same rate the temperature of the samples will rapidly rise and compounds may be compromised. For this reason it is necessary to have some sort of system to control the application of heat.
An example of such a system is SampleGuard™ (Figure 2) from Genevac . SampleGuard™ guarantees that the samples can never exceed a temperature you specify and thus the IR lamp power is safely applied. On evaporators without such a system, the only safe way to treat samples is to turn off lamps before any samples reach dryness. This has the disadvantage that the latter part of the run (without lamps) becomes extremely slow, relying on heat from the chamber walls passed via the vapour in the system.chemical synthesis, fractionation