In the preparation of pharmaceuticals and drugs it is a common requirement to separate unwanted volatile solvent components from less volatile materials and one technique which has been developed involves centrifuging the mixture whilst simultaneously evacuating the chamber containing the centrifuged material so as to draw off from the mixture the more volatile component and leave the less volatile material behind. Thus chemists and biologists frequently need to remove liquids in which the solid matter in which they are interested is dissolved or suspended. The solid matter may be potential new drugs, biological samples or other materials. They are frequently sensitive to heat, so that the mixture cannot be boiled off at atmospheric pressure because this would involve excessively high temperatures. Boiling, or evaporation under vacuum is often the preferred process because this can be done at low temperatures which do not harm the samples. If samples in liquids are exposed to vacuum they tend to boil vigorously and this activity can lead to liquid containing valuable sample material being spilled or lost, or worse, to cross-contamination of samples which may have been expensively purified.
It is therefore well known to spin such samples in a closed vacuum chamber so as to subject them to rotation generated centrifugal forces which suppress the spitting or frothing of the liquid while it is boiling under vacuum. This process is known as Centrifugal Evaporation, or Concentration.
If such a Centrifugal Evaporator is to achieve rapid evaporation of solvents it is necessary to heat the samples to provide the energy necessary to sustain evaporation. One well known method of heating is by the use of infra red radiation from lamps located in the wall of the vacuum chamber. Once the solvent within the receptacle is boiling, the rate of evaporation is governed only by the rate of heat input to the solvent.
One known method of operation is to locate the receptacle in which the sample is contained in a holder that will allow infra red radiation from the lamps to heat the solvent in the receptacle directly. This method has the disadvantage that when the solvent in the receptacle is all evaporated, the temperature of the remaining solid compounds cannot be controlled and will increase very rapidly unless the infra red lamps are turned off. Many of the biological compounds that are regularly dried by these evaporators are highly temperature sensitive. A further disadvantage is that the solids while in solution and when dry are subjected to possibly damaging levels of radiation in wavelengths from ultra violet through visible to infra red. With the development of genetic testing using Oligonucleotide Probes it is becoming increasingly common for such probes to contain a “marker”, and these markers are often sensitive to radiation and can therefore be damaged by a broad range of wavelengths including the range from ultra violet through visible to infra red.
An alternative known method aimed at overcoming the problem of temperature control highlighted above is to locate the receptacle in one or more solid aluminium blocks. In this case the block will protect the dried compounds from direct infra red radiation. The radiation from the lamps will heat the block and in turn heat will be transferred to the solvent by conduction between the sample receptacle and the aluminium block. This method gives good temperature control of the samples but has the disadvantage of slow evaporation with some formats of sample receptacle. Receptacles such as Microtitre plates give particularly slow evaporation when conduction is used to transfer the heat required for evaporation into the plate.
It has been proposed in GB 2,334,688 to provide a platform on a tray on which a microtitre well plate is located to engage a recessed underside of the microtitre plate which will otherwise be spaced from the surface of the tray, to improve heat transfer between tray and plate. However the provision of such a platform has not been found to satisfactorily increase heat transfer in practice due to the shape of the ends of the wells.
Damage to the samples by UV, visible and infra red radiation can be reduced by not using infra red lamps at all, but this has the disadvantage of increasing the length of time required for evaporation.
An alternative approach is to use a filter positioned between the IR source and the aperture into the chamber. Such filters are practical in filtering out harmful radiation in the range of wavelengths from 200 nm through to 600 nm but above this figure such filters start to significantly reduce the energy transfer from the source into the evaporation chamber.
It is an object of the present invention to provide a method and apparatus which will allow use of infra red lamps to speed the evaporation of sample solvents especially when contained within wells in microtitre well-plates, or in other sample holders having external undersides which are not substantially flat.