Method for determining temperature of evaporating liquid samples

Abstract: A method is described for determining the temperature of evaporating liquid samples containing or comprising at least one volatile component and contained in at least some of a plurality of individual sample holders (4) which are mounted within a chamber (14) and rotated (5A,5B) during the evaporation process so that centrifugal force is exerted on volatile liquid contained therein. A source of heat (1,2) supplies heat to the sample holders (4) to heat the liquid therein whilst a pressure below atmospheric is maintained in the chamber in manner known per se. In the method a pressure sensing device is located in the chamber, sensing the pressure therein at least during the evaporation process. An electrical pressure data signal is generated which is proportional to the sensed pressure and the pressure data signal is conveyed along a signal path to electronic data signal processing means which is programmed inter alia with information relating to the volatile component or components present in the samples so as to convert the pressure data signal to a temperature value equal to that which equates to the measured vapour pressure for the known volatile component or components present.

Description: This application relates to the evaporation of samples comprising solid material dissolved or suspended in a liquid in a vacuum, and to an improved method for determining the temperature of such samples while they are evaporating. It is particularly applicable to monitoring samples in and control of centrifugal evaporators. In centrifugal evaporators samples are usually held in glass or plastic tubes or, sometimes, in a large number of small wells in plastic blocks. The sample holders are spun so as to arrange for a considerable centrifugal force to be applied to them in a direction which forces the liquid to the lower part of the sample tubes to prevent any frothing or spitting of the liquid out of the sample tubes when a vacuum is applied. The spinning samples are held in a vacuum-tight chamber (referred to henceforth as "chamber") which is connected to a vacuum pumping device. Evaporators of this type are well known and many types are available commercially. They all suffer from the difficulty of providing the latent heat of evaporation to the samples to allow evaporation at reasonable high speed without allowing the samples to reach temperatures which could damage or destroy the samples which are often thermolabile. No heat can be provided by conduction because the samples are held in a vacuum but microwaves or radiant heat from a high temperature source (500° - 3000°C) can be used to provide the heat required for evaporation. Radiation from a low temperature source, e.g. the chamber walls at 40°C, is frequently used but it cannot provide sufficient heat for rapid evaporation of anything other than small samples of highly volatile solvents. The use of radiant heat and of microwaves is known. Microwaves can cause damage to some samples and in the known embodiments that use radiant heat, the heat has been applied in a manner in which samples are not heated uniformly so that some samples can be dry whilst others are still liquid. This causes overheating of the dry samples if the heat is maintained long enough to dry the last samples. The present invention and the invention disclosed in the corresponding parent application, EP 99906352.2 aim to overcome some of the problems associated with the prior art. The invention provides a method of determining the temperature of evaporating liquid samples comprising at least one volatile component and contained in at least some of a plurality of individual sample holders which are mounted within a chamber and rotated during the evaporation process so that centrifugal force is exerted on volatile liquid contained therein, heat being supplied to the sample holders to heat the volatile liquid therein whilst a pressure below atmospheric is maintained in the chamber , and wherein a pressure sensing device is located in the chamber, comprising the steps of sensing the pressure therein at least during the evaporation process, generating an electrical pressure data signal which is proportional to the sensed pressure, conveying along a signal path the pressure data signal to electronic data signal processing means which is programmed inter alia with information relating to the volatile component or components present in the samples, to convert the pressure data signal to a temperature value equal to that which equates to the measured vapour pressure for the known volatile component or components present. The data signal processing preferably includes the step of addressing a look-up table containing temperature and pressure values for different liquids, and the further step of identifying to the signal processing means the volatile component or components present in the samples. In addition or alternatively the data signal processing means may include an algorithm, and memory means, and the method involves the step of storing numerical values for insertion in the algorithm depending on the volatile component or components present in the samples and the pressure determined by the pressure sensing device, to enable computation of the temperature to which the sample must have been raised for the particular pressure to be observed in the chamber given the presence of the volatile component(s) concerned. The heat source may comprise a source of infra-red radiation. Where the heat is infra-red radiation, a heat absorbing screen is preferably located between the source of infra-red heat and the samples, having a plurality of radiation conductive regions therein, each conductive region aligning with the position of one of the samples in the array of samples, and the thermal transmissivity of the regions increase towards the centre of the array so that samples located in the central region of the array receive more radiation per unit time than those in peripheral regions of the array. The samples may be contained in an array of tubes, bottles or vials held in holders which uniformly swing upwardly from a vertical position to a generally horizontal position during rotation of a platform on which they are mounted, or in wells in a microtitre plate which can also be swung upwardly as aforesaid. The source of heat may be situated at one radial position relative to the axis of rotation of the sample containers, and each sample is subjected to radiant heat energy as it passes the source of heat during its rotation around the said axis of rotation, or alternatively the source of heat may extend around an arcuate path extending around some or all of the circular path of the samples, so that the exposure of each sample to radiant heat is for a greater percentage of its circular path than if the heat source is located at only one point around their path. The invention will now be described by way of example, with reference to the accompanying drawings comprising Figures 1 to 8. Figure 1 illustrates a centrifugal evaporator embodying the invention described and claimed herein. The samples in Figure 1 are contained in blocks (4) in which there are numerous sample wells (not shown), commonly referred to as deep well microtitre blocks. When the sample holder rotor 5A and shaft 5B rotates, driven by a motor 5C, which may be inside but more usually external to the chamber (14), the sample blocks swing out to the position illustrated in which the sample wells are horizontal, under the influence of centrifugal force. The sample blocks are connected to pivots (13) and the blocks are held with the wells vertical for loading into a stationary evaporator. Vacuum is then applied to the evaporator chamber (14) via pipe (9) from the vapour condenser which in turn is pumped via pipe (10) by the vacuum pump. Heat is applied to the rotating sample blocks (4) by a high temperature infra-red radiation source (1), and radiant heat energy (2) passes through a window of heat transparent material such as quartz which is sealed into the wall of the vacuum chamber (14) and reaches the sample holder as illustrated. A temperature sensor or probe (15) is placed in one of the sample wells, or otherwise placed in close proximity to the wells in one of the sample blocks, and is connected to transmitter (11) which transmits signals corresponding to the sample temperature to an arial and feedthrough (6) inside and extending through the chamber wall, and which is connected to a receiver and decoder (16). This includes data processing and computing facilities as required, and indicates the sample temperature by a display (not shown) and, if required, can be programmed to generate electrical signals to control the operation of the heater increase or decrease the heat energy to keep the samples at desired temperatures during the process. Such control signals are supplied to the heater via path 17. Sample temperature uniformity It is important that as far as possible all the samples are evaporated at the same rate. To achieve this all samples should receive the same heat input by directing the heat to them, so as to heat all the sample containing tubes uniformly. A common form of sample holder is a deep-well microtitre plate (4), in which typically there are 96 wells. The plate is mounted on swivel pins (not shown in Figure 2) so that when it is initially loaded onto a stationary rotor 5A the open ends of the wells face upwards, but as soon as the rotor 5A is rotated at sufficient speed, the plates or blocks (4) swing into a position in which the wells are horizontal, as is in fact shown in Figure 1, and in Figure 2. It is known and convenient to apply radiant heat from the top of the chamber as illustrated in Figure 2a, (or from the bottom) but this does not provide for uniform heating of the wells when they are horizontal. This is partly because most of the heat is in the infra-red region which does not significantly penetrate plastic material from which holders are constructed. The top wells are therefore strongly heated whilst the lower wells get little heat. This can cause overheating of the top samples before the middle or lower samples are dry. In Figure 2b, the infra-red beam is directed horizontally onto the closed ends of the sample wells in which configuration it is possible to achieve uniform heating of the wells. Cold neighbour effect Even with perfectly uniform heat input the samples will not evaporate at a uniform rate because of "cold neighbour effect". If the samples are in thermal contact with each other as is the case for example in a microtitre plate or block (4), the outer samples only have evaporating (and therefore "cold") neighbours on three or (corner samples) two sides, and therefore do not lose as much heat to their neighbours as those in the centre which have four "cold" neighbours. Also two of an outside sample's neighbours will generally be less cold than those of the inner samples. Outside samples therefore can evaporate faster than centrally located samples. This effect can be reduced or eliminated by reducing the heat input to the outer samples, and in the (preferred) infra-red heating case, a simple way of doing this is to provide graduated shading from the infra red beam by, for example, placing a metal screen 18 (see Figure 3) between the sample holder and the heat source. The screen contains graduated perforations 20, 22, 24, so that those in the outer region transmit much less radiation than do those in the central region, and those in intermediate regions, such as 22, which have an intermediate size thereby transmit greater quantities of heat than do the outer ones 20. The inner opening 24 allows an uninterrupted path for radiation to the centre of the array of sample holders. Although the sample holder (4) illustrated is described as being a deep-well microtitre block or plate, the same techniques may be employed to obtain uniform temperature and graduated heating as described above, when using arrays of tubes, bottles or vials in holders which swing out on swivels in a similar manner. Heating control The heater power is controlled by measuring sample temperature or chamber pressure and taking appropriate steps to raise or lower the heater power. Thus at the start of the process a high heat input is required but as the samples approach dryness the evaporation rate will reduce and the sample temperature will start to rise so that the heat input must be reduced to avoid overheating the sample, and when the samples are dry, the heating must be discontinued. Vapour flow A vapour condenser is shown at 26 in Figure 1. These devices are used in centrifugal evaporation equipment to increase pumping speed for the liquid being evaporated and to protect the vacuum pump 28 from vapours which might impair its efficiency. Such condensers are vessels held at low temperatures at which the vapours being evaporated condense or solidify. If a vapour condenser 26 is located between a vacuum pump 28 and an evaporation chamber 14 as shown in Figure 1, the pressure in the chamber 14 cannot be reduced below the vapour pressure of any condensed liquid remaining in the condenser 26. This is due to the evaporation of condensed material which will take place in the condenser if the system pressure is reduced to a level approaching the vapour pressure of the condensed material left in the condenser 26. This phenomenon, especially if a more volatile material has been left in the condenser 26 from a previous run, can make chamber pressure a rather insensitive technique for sensing sample temperature at the end of evaporation to indicate when the samples are dry, and it may be unreliable as a means for determining when the equipment can be shut down. By monitoring vapour flow rate, information can be obtained about a process to indicate when to turn off the heater, since when the samples are nearly dry the flow rate will become low. This enables equipment to be reliably shut down when the process is finished (i.e. the samples are dry). Flow rate through the condenser or the pipe 9 between the chamber 14 and the condenser 26 can be monitored by any convenient technique. Heating multiple sample blocks In some cases several microtitre plates or blocks can be stacked either directly one above the other, or on thin separating shelves or trays, normally made of stainless steel, and located as a stack in the evaporator. Such arrangements are known but they suffer from the disadvantage that infra-red energy cannot be applied evenly to all the wells in the plates or blocks. If it is applied directly to the bottom of an assembly of plates or blocks stacked above each other as described, the lowest layer of wells will be heated evenly but little if any heat will penetrate to layers of wells in plates or blocks above it. If it is applied to the base of a holder having thin stainless steel walls, base and shelves, heat conduction to the higher shelves will again be poor and the wells in upper plates or blocks will receive less heat than those in lower ones. Stacked plates can be heated more uniformly if they are supported on relatively thick trays of a material having a high thermal conductivity, such as aluminium or copper, which in turn, are connected to a thick frame also made of similarly high thermally conductive material with a good thermal connection between the trays and the frame, and the latter is heated by the infra red radiation. Such an arrangement of shelves or trays is shown in Figure 4. This is formed from a heavy aluminium base 9 and similar heavy aluminium ends 30, 32 (the latter being shown as transparent in the Figure) with trays 34, 36, 38 etc. spanning the gap between the ends 30, 32. In the arrangement illustrated in Figure 4, heat has been found to be distributed evenly to all of the plates, if the thickness of the supporting trays 34, 36, 38 etc. is in the region of 2mm, and the base and ends 29, 30, 32 are of a similar, or greater, thickness. As illustrated in Figure 5, most microtitre plates 40 have a vertically extended downwardly protruding outer edge 42, and, if they are supported on a flat tray, a small gap of a few mm will exist between the supporting tray and the underside of the microtitre plate, and therefore also the bottoms of the wells. If each supporting tray is shaped as shown in Figure 6 so that when viewed in elevation as in Figure 6, its central area 44 is stepped relative to its periphery, or at least both ends 46, 48 this gap will be reduced or eliminated and better heat transfer from tray to plate/wells occurs, leading to faster evaporation. Figure 7 shows the important components of the monitoring system for a chamber such as shown in Figure 1. Each probe 15 connects to an input of a signal processor 50, the output of which is digitised by an A/D converter 52 for supply to a microprocessor 54 which handles the modulation of a radio signal in a transmitter 56 to which signals are supplied from the microprocessor for radiation by an antenna 58. A power supply 60 may comprise a battery. Except for the probe 15 and antenna 58, all the units shown in Figure 7 may be housed with a housing located on the sample holder rotor 5A. So that there is no relative movement between it and the probe 15, the chamber 14 must be constructed so that at least part of its wall is capable of transmitting the radio signals from the antenna. A receiver and control system for locating outside the chamber 14 is shown in Figure 8. Here a receiver antenna 62 feeds radio signals to a receiver and decoder 64 which supplies decoded digital data signals (corresponding to those from 52 in Figure 7), to a second microprocessor 66. This controls the supply of digital signals to a motor controller 68 which controls the speed of rotation of the drive motor 5C (also shown in Figure 1). A tachogenerator 70 is attached to the motor shaft 72 and provides a speed signal for the microprocessor 66. An infra-red heating 1 (see also Figure 1) is controlled by a power controller 74 which in turn is controlled by signals from the microprocessor 66, to reduce the heat output from 1 as an evaporation process progresses, so as to reduce the risk of overheating as samples dry and are no longer cooled by evaporative cooling effects. The vacuum pump 28 of Figure 1 is shown associated to the chamber 14 via a pipeline 76 which includes a valve 78 also under the control of signals from the microprocessor 66. The latter includes memory in which operating system software and data relating to different volatile liquids are stored and a data entry keyboard or other device 80 allows data to be entered initially and volatile components to be identified to the system. A screen 82 assist in the entry of data and the display of monitored values of temperature from probe 15 and pressure for a probe 84 in the chamber. Power from the system of Figure 8 may be from a battery or mains driven power supply 86.

Claims: A method of determining the temperature of evaporating liquid samples comprising at least one volatile component and contained in at least some of a plurality of individual sample holders which are mounted within a chamber and rotated during the evaporation process so that centrifugal force is exerted on volatile liquid contained therein, heat being supplied to the sample holders to heat the liquid therein whilst a pressure below atmospheric is maintained in the chamber, and wherein a pressure sensing device is located in the chamber, comprising the steps of sensing the pressure therein at least during the evaporation process, generating an electrical pressure data signal which is proportional to the sensed pressure, conveying along a signal path the pressure data signal to electronic data signal processing means which is programmed inter alia with information relating to the volatile component or components present in the samples, to convert the pressure data signal to a temperature value equal to that which equates to the measured vapour pressure for the known volatile component or components present. A method of determining the temperature of evaporating liquid samples as claimed in claim 1, wherein the data signal processing means includes the step of addressing a look-up table containing temperature and pressure values for different liquids, and the further step of identifying to the signal processing means the volatile component or components present in the samples. A method of determining the temperature of evaporating liquid samples as claimed in claim 1, wherein the data signal processing means includes an algorithm, and memory means, and the method involves the step of storing numerical values for insertion in the algorithm depending on the volatile component or components present in the samples and the pressure determined by the pressure sensing device, to enable computation of the temperature to which the sample must have been raised for the particular pressure to be observed in the chamber given the presence of the volatile component(s) concerned. A method as claimed in any one of the preceding claims, wherein the source of heat is a source of infra-red radiation. A method as claimed in claim 4, in which a heat absorbing screen is located between the source of heat and the samples, having a plurality of radiation conductive regions therein, each conductive region aligning with the position of one of the samples in the array of samples, and the thermal transmissivity of the regions increasing towards the centre of the array, so that samples located in the central region of the array receive more radiation per unit time than those in peripheral regions of the array. A method as claimed in any one of the preceding claims, wherein the samples are contained in wells in a microtitre plate. A method as claimed in any one of the preceding claims, wherein the samples are contained in an array of tubes, bottles or vials held in holders which uniformly swing upwardly from a vertical position to a generally horizontal position during rotation of a platform on which they are mounted. A method as claimed in any one of the preceding claims, wherein the source of heat is situated at one radial position relative to the axis of rotation of the sample containers, and each sample is subjected to radiant heat energy as it passes the source of heat during its rotation around the said axis of rotation. A method as claimed in any one of the preceding claims, wherein the source of heat extends around an arcuate path extending around some or all of the circular path of the samples. A method as claimed in claim 1 in which the heat supplied to the samples is controlled in response to the value of the said pressure data signal.

Inventors: COLE MICHAEL, COLE, MICHAEL
Applicants: GENEVAC LTD, GENEVAC LIMITED
Application year: 1999
Main classification: B01D3
IPC codes: B01D3, B01D1