Good control of sample temperature during evaporation is required to achieve reasonably fast drying times and to ensure that the samples cannot be damaged by over heating. By Michael Cole.
Ideal control of sample temperature would adjust the heat input to the sample to keep it just below the maximum allowable temperature throughout the evaporation process. If the sample becomes too hot it could be damaged and if it is allowed to become too cool it will take longer than necessary to dry. Heat input is needed to offset the cooling which occurs during evaporation due to loss of latent heat of evaporation. If no heat is supplied to the sample it will cool to a temperature at which the vapour pressure is so low that evaporation would stop.
Heat from the chamber walls reaches the samples mainly through radiation which, at these temperatures, is weak and insufficient to prevent sample temperatures falling to -40oC or below at which evaporation of some liquids can be very slow.
The use of infrared radiation from lamps run at high temperatures (normally over 2500oC) can greatly speed up the drying process. While there is evaporating solvent in the sample tube the sample will remain cool but as soon as the sample dries its temperature can rise rapidly to undesirable levels. When infrared lamps are used, therefore, a means must be found to turn off the heaters before the sample is completely dry to prevent overheating it.
If the actual sample temperature sensor is used to control the heaters it will switch them off when the sample temperature reaches the set point usually just below the maximum permissible temperature. This is not normally satisfactory because the rotor assembly is generally hotter that the sample at this point and residual heat in the rotor can feed into the dry samples causing them to be overheated.
There is also another problem with using sample temperature to control sample heat which arises when the samples are not all of exactly the same size and composition. This is often the case when high performance liquid chromatography (HPLC) prep samples are being dried. If a quick drying sample is chosen for monitoring of sample temperature the heaters will switch off when that sample is dry and the slow drying samples will get no more heat and will take a long time to dry. This can be solved with some sacrifice of speed by holding all the samples in a large block of high conductivity material such as aluminium in such a way that no radiation from the lamps can reach the samples direct. The lamps are used to heat the aluminium block to the maximum permissible temperature for the samples.
Even this method is still not quite perfect because samples may be contained in several aluminium blocks loaded into an evaporator and with a single or two-channel controller the blocks being controlled may contain samples of different average composition so that samples in the controlled blocks may be still evaporating while other blocks have all dry blocks. This could lead to overheating of these all-dry blocks. This difficulty has been achieved successfully in a Genevac HT12 evaporator (Fig. 1). Three dish like plates of solid aluminium were joined together with heavy vertical cylinders of aluminium and four aluminium blocks were let into each aluminium plates so that 12 aluminium sample blocks were held in this way at virtually the same temperature.
Enquiry No 26
Michael Cole, PhD, is chairman and managing director at Genevac Ltd, Ipswich, Suffolk, UK.
