Method for dehydration and critical point drying

Abstract

A method for dehydration and critical point drying of a sample in a single chamber is introduced, comprising the steps of (a) dehydrating the sample by replacing water by an intermediate fluid, (b) replacing the intermediate fluid by a transitional fluid, (c) pressurising the transitional fluid to or beyond its critical pressure and/or heating the transitional fluid (4 to or beyond its critical temperature, and (d) in response to gradually releasing the pressure, letting the transitional fluid gasify and escape from the sample. In step (a) and/or step (b), a ratio of the fluid to-be-replaced to the replacing fluid is measured and used to control a supply of the replacing fluid. The method reduces consumption of intermediate fluid and/or transitional fluid, making the process more efficient in terms of duration and user interaction while ensuring a high degree of dryness and the integrity of the sample.

Claims

1. A method for dehydration and critical point drying of a sample in a single chamber, the method comprising the steps of: a) dehydrating the sample by replacing water with an intermediate fluid, b) replacing the intermediate fluid with a transitional fluid having a critical temperature and a critical pressure, c) one or more of pressurising the transitional fluid to or beyond said critical pressure and heating the transitional fluid to or beyond said critical temperature, and d) in response to gradually releasing the pressure, letting the transitional fluid gasify and escape from the sample, and wherein one or more of: in step a) a first ratio of the water to the intermediate fluid is measured and used to control a supply of the intermediate fluid; and in step b), a second ratio of the intermediate fluid to the transitional fluid is measured and used to control a supply of the transitional fluid.

2. The method according to claim 1, comprising: a combination of two or more of leaving fluid present inside the chamber at rest, circulating the fluid present inside the chamber, and flushing the chamber with at least one of fresh intermediate fluid in step a) and fresh transitional fluid in step b).

3. The method according to claim 2, wherein the combination is repeated.

4. The method according to claim 1, comprising: one or more of continuing with step b) after step a) in response to the first ratio falling below a predefined limit, and continuing with step c) after step b) in response to the second ratio falling below a second pre-defined limit.

5. The method according to claim 1, comprising: one or more of stopping the supply of the intermediate fluid in step a) in response to the first ratio falling below a third predefined limit, and stopping the supply of the transitional fluid in step b) in response to the second ratio falling below a fourth predefined limit.

6. The method according to claim 1, comprising: sensing a temperature of the intermediate fluid outside the chamber in step a) and, respectively, of the transitional fluid outside the chamber in step b), and a temperature of fluid present inside the chamber.

7. The method according to claim 6, comprising: cooling the fluid present inside the chamber below the temperature of the intermediate fluid in step a), causing the intermediate fluid to stay liquid when entering the chamber, and cooling the fluid present inside the chamber below the temperature of the transitional fluid in step b), causing the transitional fluid to stay liquid when entering the chamber.

8. The method according to claim 7, wherein the temperature of the fluid present inside the chamber is controlled to a temperature level at least 5 C. below the temperature of the intermediate fluid in step a) and at least 5 C. below the temperature of the transitional fluid in step b).

9. The method according to claim 1, comprising: sensing a fill level of fluid present inside the chamber.

10. The method according to claim 1, comprising: heating fluid present inside the chamber to raise a pressure in the chamber and measuring the pressure in the chamber.

11. The method according to claim 1, comprising: sensing a fill level of fluid present inside the chamber, heating the fluid present inside the chamber to raise a pressure in the chamber and measuring the pressure in the chamber, and switching to step b), only if the fill level of the fluid present inside the chamber is equal to or greater than a predefined value in order to immerse the sample in the intermediate fluid, and if a pressure level of the transitional fluid outside the chamber and the pressure in the chamber match within a predefined margin, in order to avoid or reduce a pressure shock.

12. The method according to claim 1, wherein the intermediate fluid is one of ethanol, acetone, isopropanol, amyl acetate, or a solution of one of ethanol, acetone, isopropanol and amyl acetate in water, and the transitional fluid is liquid carbon dioxide.

13. A computer readable product, comprising computer readable code for causing a performance of a method according to claim 1 when operated on a control unit.

14. An apparatus to perform dehydration and critical point drying of a sample, comprising: a chamber configured to host the sample, a supply for an intermediate fluid into the chamber to dehydrate the sample by replacing water, a supply for a transitional fluid into the chamber to replace the intermediate fluid, the transitional fluid having a critical temperature and a critical pressure, a sensor configured to measure one or more of a first ratio of the water to the intermediate fluid, and a second ratio of the intermediate fluid to the transitional fluid, a heating element configured to perform critical point drying of the sample, and a control unit configured to control one or more of the supply of intermediate fluid into the chamber depending on the first ratio and the supply of transitional fluid into the chamber depending on the second ratio, control one or more of pressurising the transitional fluid to or beyond said critical pressure and heating the transitional fluid with the heating element to or beyond said critical temperature, and control, in response to gradually releasing the pressure, letting the transitional fluid gasify and escape from the sample.

15. The apparatus according to claim 14, comprising: a pump configured to circulate fluid present inside the chamber.

16. The apparatus according to claim 14, comprising one or more of: a temperature sensor as well as a pressure sensor arranged inside the chamber, a temperature sensor as well as a pressure sensor configured to sense one or more of the intermediate fluid and the transitional fluid outside the chamber, and a cooling element configured to cool fluid present inside the chamber.

17. The apparatus according to claim 14, comprising: a sensor configured to measure a fill level in the chamber for controlling the fill level of the intermediate fluid in such a way that a pressure shock is avoided or reduced when supplying pressurised transitional fluid.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The embodiments defined above and further aspects, features and advantages of the present invention can also be derived from the examples of embodiments to be described hereinafter and are explained with reference to the drawings. In the drawings it is illustrated in:

(2) FIG. 1 a block diagram of an apparatus according to an embodiment of the present invention;

(3) FIG. 2 a flow chart of a method according to an embodiment of the present invention.

MODES FOR CARRYING OUT THE INVENTION

(4) FIG. 1 shows a block diagram of an apparatus according to an embodiment of the present invention. The function and purpose of the different elements is detailed later in the description of FIG. 2. The apparatus of FIG. 1 comprises a chamber 1 to house a sample 2, a supply of intermediate fluid 3, a supply of transitional fluid 4, a heating element 5 to heat fluid present inside the chamber and a control unit 6. Preferably the supplied intermediate fluid 3 is one of ethanol, acetone, isopropanol, amyl acetate, or a solution of one of those in water, and the supplied transitional fluid 4 is liquid carbon dioxide. The heating element 5 is preferably combined with a cooling element 5a.

(5) The control unit 6 controls the process of dehydration and critical point drying. For that purpose it receives measurement values from sensors 21-26. Preferably the apparatus comprises a pressure sensor 21 and a temperature sensor 22 to measure the pressure and the temperature inside the chamber 1, respectively, as well as a sensor 23 to measure a liquid fill level in the chamber, and sensors 24 and 24a to measure a ratio of water to intermediate fluid, and a ratio of intermediate fluid to transitional fluid in the chamber, respectively. Preferably the apparatus also comprises a pressure sensor 25 and a temperature sensor 26 to measure pressure and temperature, respectively, in a supply line between the supply of intermediate fluid 3 and transitional fluid 4 and the chamber 1.

(6) The embodiment of the invention in FIG. 1 also comprises valves 11-15 which are controlled by the control unit 6. Valves 11-13 regulate the supply of fluids into the chamber 1, valves 14 and 15 regulate the outlet of the chamber 1. Valves 14 and 15, or an additional or valve at the outlet 9 are used to control an outflow of intermediate fluid 3 or transitional fluid 4, and to control a depressurising of the chamber 1. Preferably the apparatus also comprises a pump 7 which is also controlled by the control unit 6.

(7) FIG. 2 shows a flow chart of a method for dehydrating and critical point drying according to an embodiment of the present invention, preferably as conducted with an apparatus, e.g. as shown in FIG. 1. In a preparatory step S0, a water-containing sample 2 is put into the chamber 1. The sample 2 may for instance be a biological sample or a microelectromechanical system (MEMS) chip which has to be dried. In step S1 the chamber 1 is filled with an intermediate fluid 3 which dissolves and replaces the water from the sample 2, i.e. dehydrates the sample 2. Simultaneously the fluid inside the chamber may be cooled in preparation for connecting the supply of transitional fluid in step S7. To ensure sufficient dehydration of the sample 2, a ratio R.sub.wi of water to intermediate fluid 3 is measured with sensor 24 in the chamber in step S2. The value of R.sub.wi may for instance be determined by measurements of absolute or relative water content inside the chamber or by sensing one of these values at an outlet 9 of the chamber. The water content can e.g. be diagnosed through a density sensor, taking into account the known density of the intermediate fluid.

(8) If R.sub.wi is greater than a predefined value R.sub.1 (n), dehydration is continued in step S2a with two or more of the following possibilities: The fluid present inside the chamber 1 may be left at rest for some time to allow the water to diffuse out of the sample 2 and get dissolved in the intermediate fluid 3; the fluid may be circulated inside the chamber 1 by means of the pump 7 to avoid locally high concentrations of water that inhibit diffusion; or the chamber 1 may be flushed with fresh intermediate fluid 3. By monitoring R.sub.wi, it is ensured that only the necessary amount of intermediate fluid 3 is used. This reduces considerably the consumption of intermediate fluid 3 and also the process time compared to other methods.

(9) If the measured value of R.sub.wi is equal to or below a predefined value R.sub.1 in step S2 (y), the process is continued with step S3 which checks the fluid fill level F.sub.1 in the chamber 1 by means of sensor 23 to avoid air bubbles that can lead to pressure shocks when pressurised transitional fluid 4 is supplied, which would damage the sample 2. If F.sub.i is below a predefined value F.sub.1 (n), more intermediate fluid 3 is supplied in step S3a. Otherwise (y) in step S4, the temperature values T.sub.i inside the chamber 1 and T.sub.t of the supply of transitional fluid 4 are measured with sensors 22 and 26, respectively, to ensure that the transitional fluid 4 flowing into the chamber 1 in step S7 is always liquid. If T.sub.i is smaller than T.sub.t by at least 10 C., in particular by 10-12 C., the process is continued with step S5, otherwise more cooling is applied to the fluid inside the chamber 1.

(10) In step S5 the chamber 1 is sealed and pressurised to pressure p.sub.i by heating it with heating element 5, e.g. by at least 2 C. and up to 5 C. or 7 C., in order to reach the pressure level p.sub.t of the supply of transitional fluid 4. In step S6 the measured values p.sub.i and p.sub.t of sensors 21 and 25, respectively, are compared, and the process is only continued with the next step if the two values are approximately equal (y); otherwise more heating is applied (n). As a consequence of the cooling in steps S2-S4 and the heating in steps S5-S6, T.sub.i is smaller than T.sub.t by 5 C. or more, which ensures that the inflowing transitional fluid 4 in step S7 stays liquid.

(11) In step S7 the supply of transitional fluid 4 is opened to replace the intermediate fluid 3. The progress of the replacement is monitored by measuring the ratio R.sub.it of intermediate fluid 3 to transitional fluid 4 in the chamber 1 with sensor 24a in step S8. Possible realisations of the measurement are analogous to step S2. As long as R.sub.it is greater than a predefined value R.sub.2 (n), the fluid may again be left at rest for a while, or circulated inside the chamber 1, or the chamber 1 may be flushed with more transitional fluid 4 in step S8a. If R.sub.it is equal to or below R.sub.2 (y), the process of critical point drying is started. By monitoring R.sub.it, it is ensured that only the necessary amount of transitional fluid 4 is used. This reduces considerably the consumption of transitional fluid 4 and also the process time compared to other methods.

(12) In step S9 the chamber 1 is pressurised to or above the critical pressure p.sub.c corresponding to the critical point of the transitional fluid 4. Also, if necessary, the chamber 1 is heated to or above the critical temperature T.sub.c. These conditions are checked with sensors 21 and 22 measuring pressure p.sub.t and temperature T.sub.t, respectively. If pressure and temperature are both sufficiently high as measured in step S10 (y), the pressure is gradually released, letting the gasified transitional fluid 4 escape from the inside and/or the surface of the sample 2 and preferably the chamber 1 in step S11. During this step the temperature T.sub.t is maintained at or above T.sub.c by heating to avoid recondensation of the transitional fluid 4. When the pressure in the chamber 1 has gone down to ambient pressure, the dry sample 2 is taken out of the chamber 1 in the last step S12.