Patent Publication Number: US-10328432-B2

Title: Independent heating of samples in a sample holder

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of patent application Ser. No. 14/466,117 filed on Aug. 22, 2014, which is a divisional of patent application Ser. No. 13/025,469 filed on Feb. 11, 2011, which claims the benefit of U.S. Provisional Patent Application No. 61/304,387, filed on Feb. 12, 2010, the contents of which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present application relates to the field of sample holder arrangements and systems used in digestion and/or extraction for processes such as analytical spectroscopy and chromatography. 
     BACKGROUND OF THE ART 
     In order to perform digestion of a sample, the sample is usually placed in an open-ended recipient which is then closed and heated in a microwave oven. Some digestion systems only allow the heating of a single sample at a time and therefore a single sample holder is used. This practice is particularly time-consuming. 
     Other digestion systems allow several samples to be concurrently heated and so a multi-sample holder is used. These types of sample holders are usually generic racks that receive multiple open-ended recipients, such as test tubes. 
     For some digestion processes, heating may result in a large excess pressure in the recipient. To prevent damage or explosion, a valve is provided that automatically opens if a given internal pressure exceeds a threshold. Special sealing caps are used on the open-ended recipient to provide this function. However, having to manipulate such a sealing cap for each sample recipient is also time-consuming. 
     Therefore, there is a need for an improved system that is adapted for the specific needs of a digestion process for multiple samples concurrently. 
     SUMMARY 
     In accordance with a broad aspect, there is provided a heating system for independently heating at least one sample recipient containing a sample, the sample recipient provided in a sample holder, comprising: a heating chamber having at least one opening and adapted to receive the sample holder; at least one microwave generator for generating microwaves; at least one microwave applicator inside the heating chamber connected to the at least one microwave generator, the at least one microwave applicator comprising an oven cavity portion for creating a mini microwave cavity around a single sample recipient in the sample holder, and for applying microwaves generated by the at least one microwave generator directly to the single sample recipient; and a control unit for controlling the at least one microwave generator. 
     In accordance with a second broad aspect, there is provided a method for heating a sample material in a sample holder, the method comprising: receiving the sample holder in a heating chamber of a heating system, the sample holder having at least one sample recipient with the sample material therein; dynamically forming an individual mini microwave cavity around the sample recipient; and applying microwaves generated by at least one microwave generator directly to the sample. 
     In accordance with a third broad aspect, there is provided a sample holder for decomposition or extraction of a sample material, the sample holder comprising: a frame having a top plate and a base, the top plate having at least one aperture for receiving a sample recipient holding the sample material; at least one partial mini cavity provided between the top plate and the base, the at least one partial mini cavity having reflecting material on a sample facing surface to reflect microwaves towards the sample material in the sample recipient, and shaped to mate with a complementary partial mini cavity in a heating chamber of an oven such that when combined, the partial mini cavity and the complementary partial mini cavity form a complete and substantially hermetic mini microwave cavity around the sample recipient. 
     In one embodiment, the term “sample” refers to a mixture of material to be decomposed and at least one chemical decomposition reagent. In another embodiment, the term “sample” refers only to the material to be decomposed. While the sample recipients may sometimes be referred to as “tubes”, it should be understood that they should not be limited to circular in shape. In addition, the term “digestion” should be exchangeable with the term “extraction” throughout the description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a sample holder system, in accordance with an embodiment; 
         FIG. 2  is a perspective view of a rack of the sample holder system of  FIG. 1 , in accordance with an embodiment. 
         FIG. 3  is a perspective view of a rack cover of the sampled holder of  FIG. 1  with compression caps attached thereto, in accordance with an embodiment; 
         FIG. 4  is a bottom perspective view of a cap-receiving plate of the rack cover of  FIG. 3  with compression caps attached thereto, in accordance with an embodiment; 
         FIG. 5  is a bottom perspective view of a clamping bar of the rack cover of  FIG. 3 , in accordance with an embodiment; 
         FIG. 6  is a perspective view illustrating the securing of the clamping bar of  FIG. 5  to the rack of  FIG. 2 , in accordance with an embodiment; 
         FIG. 7  is a perspective view of a rack cover provided with a safety mechanism, in accordance with an embodiment; 
         FIG. 8 a    is a partial side view of the rack cover of  FIG. 7  secured to a rack, in accordance with an embodiment; 
         FIG. 8 b    is a blown-up perspective view of the rack cover of  FIG. 7 , showing the safety mechanism in more detail, in accordance with an embodiment; 
         FIG. 9 a    is a partially sectional perspective view of a compression cap to be secured in the cap-receiving plate of  FIG. 4 , in accordance with an embodiment; 
         FIG. 9 b    is another embodiment of a partially sectional perspective view of a compression cap, with a lock washer added; 
         FIG. 9 c    is yet another embodiment of a partially sectional perspective view of a compression cap, with the order of parts reversed; 
         FIG. 10 a    is an exploded perspective view of the compression cap of  FIG. 9 a   , in accordance with an embodiment; 
         FIG. 10 b    is an exploded perspective view of the compression cap of  FIG. 9 b   , in accordance with an embodiment; 
         FIG. 10 c    is an exploded perspective view of the compression cap of  FIG. 9 c   , in accordance with an embodiment; 
         FIG. 11 a    is a perspective view of the rack of  FIG. 2  accommodating tubes closed by sealing caps, in accordance with an embodiment; 
         FIG. 11 b    is a perspective view of the rack without any tubes, with microwave reflecting cylinders, in accordance with an embodiment; 
         FIG. 12  is a cross-sectional view of a sealing cap, in accordance with an embodiment; 
         FIG. 13  is a cross-sectional view of a test tube covered by the sealing cap of  FIG. 12 , in accordance with an embodiment; 
         FIG. 14  illustrates forces exerted on a slot member of the clamping bar of  FIG. 5  when inserted into a rectangular slot of a stud of the rack of  FIG. 2 , in accordance with an embodiment; 
         FIG. 15  illustrates a cross-sectional view of a slot member of the clamping bar of  FIG. 5  when inserted into a slot having a matching shape, in accordance with an embodiment; 
         FIG. 16  is a graph of coefficient (μ, α) as a function of coefficient of friction μ and angle α, in accordance with one embodiment; 
         FIG. 17  is a perspective view of a rack comprising a removable transportation plate and covered by a rack cover, in accordance with an embodiment; 
         FIG. 18  is a perspective view of a transportation plate, in accordance with an embodiment; 
         FIG. 19A  is a cross-sectional view of a sample tube provided with a flange, in accordance with an embodiment; 
         FIG. 19B  is a cross-sectional view of a sample tube having a varying diameter, in accordance with an embodiment; 
         FIG. 20  is a perspective view of sample tubes when received on the transportation plate of  FIG. 18 , in accordance with an embodiment; 
         FIG. 21  is a perspective view of the transportation plate and the sample tubes of  FIG. 20  when received in a holding frame, in accordance with an embodiment; 
         FIG. 22  is a perspective bottom view of the rack of  FIG. 17 , in accordance with an embodiment; 
         FIG. 23  is a block diagram of an automated digestion system comprising a straight conveyor extending through a microwave oven, in accordance with an embodiment; 
         FIG. 24  is a block diagram of an automated digestion system comprising a U-shaped conveyor extending through a microwave oven, in accordance with an embodiment; 
         FIG. 25  is a block diagram of an automated digestion system extending through a microwave oven and a cooling chamber, in accordance with an embodiment; 
         FIG. 26  is a cross-sectional side view of a cooling chamber, in accordance with an embodiment; 
         FIG. 27  is a block diagram of an automated digestion system comprising a straight conveyor extending through a microwave oven, a cooling chamber, and an auto-venting chamber, in accordance with an embodiment; 
         FIG. 28  is a cross-sectional side view of an auto-venting chamber, in accordance with an embodiment; 
         FIG. 29  is a block diagram of an automated digestion system comprising a conveyor extending through a microwave oven, a cooling chamber, and an auto-venting venting chamber, in accordance with an embodiment; 
         FIG. 30  is a photograph of an automated digestion apparatus, in accordance with an embodiment; 
         FIG. 31  is a block diagram illustrating a first disposition of sample holders on a conveyor, in accordance with an embodiment; 
         FIG. 32  is a side view of a conveyor belt, in accordance with an embodiment; 
         FIG. 33  is a block diagram illustrating a second disposition of sample holders on the conveyor of  FIG. 31 , in accordance with an embodiment; 
         FIG. 34  is a block diagram illustrating a third disposition of sample holders on the conveyor of  FIG. 31 , in accordance with an embodiment; 
         FIG. 35  is a block diagram illustrating a fourth disposition of sample holders on a conveyor, in accordance with an embodiment; 
         FIG. 36  is a block diagram illustrating a fifth disposition of sample holders on the conveyor of  FIG. 31 , in accordance with an embodiment; 
         FIG. 37  is a block diagram of a heating chamber provided with microwave applicators comprising movable cavity portions in a retracted position, in accordance with an embodiment; 
         FIG. 38  is a block diagram illustrating a rack provided with cavity portions, in accordance with an embodiment; 
         FIG. 39  is a block diagram illustrating the rack of  FIG. 38  inserted into the heating chamber of  FIG. 37  when the movable cavity portions are in the retracted position, in accordance with an embodiment; 
         FIG. 40  is a block diagram illustrating the rack of  FIG. 38  inserted into the heating chamber of  FIG. 37  when the movable cavity portions are in the extended position, in accordance with an embodiment; 
         FIG. 41  is a block diagram illustrating a heating chamber provided with two cavity elements having cavity recesses, in accordance with an embodiment; 
         FIG. 42  is a block diagram illustrating a heating chamber having movable microwave applicators, in accordance with an embodiment; 
         FIG. 43  is a block diagram illustrating a rectangular microwave cavity, in accordance with an embodiment; 
         FIG. 44  is a block diagram illustrating a circular microwave cavity provided with a protective element, in accordance with an embodiment; 
         FIG. 45  is a block diagram of an automated digestion system provided with a mini cavity microwave oven, in accordance with an embodiment; 
         FIG. 46  is a top view of mini cavities provided with a temperature sensor, in accordance with an embodiment; 
         FIG. 47  is a side view of a rack provided with rack cavity portions and a rack cover having pressure-relief valve caps, in accordance with an embodiment; 
         FIG. 48  is a perspective view of a closing mechanism for forming mini microwave cavities, in accordance with an embodiment; 
         FIG. 49  is a perspective view of a rack provided with rack cavity portions, in accordance with an embodiment; 
         FIG. 50  is a block diagram of a directional coupler for measuring the power of a signal transmitted by a microwave generator to a microwave cavity, in accordance with an embodiment; 
         FIG. 51  is a block diagram of the directional coupler of  FIG. 50  when used for measuring the power of a signal reflected by the microwave cavity, in accordance with an embodiment; 
         FIG. 52  is a perspective view of the directional coupler comprising detecting diodes and connected to a coaxial cable, in accordance with an embodiment; and 
         FIG. 53  is a schematic representation of a directional coupler comprising Schottky zero bias diodes, in accordance with an embodiment. 
     
    
    
     It will be noted that throughout the appended drawings, like features are identified by like reference numerals. 
     DETAILED DESCRIPTION 
       FIG. 1  illustrates one embodiment of a sample holder system  10  ready to be used for decomposition or extraction of a sample material. Sample holder system  10  comprises a rack  12 , sample tubes  13  with sealing caps  14 , a rack cover  15  and compression caps  16 . The material to be extracted or decomposed is placed inside the sample tubes  13  with chemical decomposition agents such as acids, for example. The rack  12  is used to maintain the sample tubes  13  in an upright position. The sealing caps  14  hermetically close the opening of the tubes  13 . The compression caps  16  are secured in the rack cover  15 . The rack cover  15  with the compression caps  16  secured therein is placed on top of the rack  12  so that the compression caps  16  allow an exhaust of gas when the internal pressure in the tubes  13  exceeds a predetermined threshold value. 
     The assembly of the compression cap  16  and the sealing cap  14  forms a pressure-relief valve and the rim of a tube  13  is the seat of the pressure-relief valve, whereby excess gas can be evacuated from the tube  13 . The compression cap  16  is adapted to allow the opening of the pressure-relief valve when the internal pressure within the tube  13  exceeds the predetermined threshold value. 
     While  FIG. 1  refers to a sample holder arrangement  10  having twelve tubes  13 , twelve sealing caps  14  and twelve compression caps  16 , it should be understood that the number of these pieces is exemplary only. The sample holder system could be adapted to receive six tubes, twenty tubes, or any other suitable number of tubes. 
     Referring concurrently to  FIGS. 1 and 2 , in one embodiment, the rack  12  has a base  20 , a support plate  21  and three studs  22  as illustrated in  FIG. 2 . The number of studs  22  is exemplary only. The base  20  presents twelve recesses  23 . The support plate  21  is U-shaped and comprises twelve apertures  24 . Each aperture  24  is positioned on top of and in line with a corresponding recess  23  and the combination of a recess  23  with a corresponding aperture  24  allows a tube  13  to be maintained in the upright position. The recesses  23  and the apertures  24  are sized as a function of the dimensions of the tubes  13 , as the tubes will be received therein. The studs  22  each have a slot  25 . The studs  22  and their corresponding slots  25  allow the rack cover  15  to be releasably secured on top of the rack  12 . The support plate  21  is also provided with a handle  26  on each side. The handles  26  allow an easy transportation of the rack  12 . In one embodiment, the central stud is D-shaped at the top to ensure that the rack cover  15  and a transport plate  126  ( FIG. 18 ) are correctly oriented. 
     The studs  22  may be replaced by any system which allows the rack cover  15  to be releasably secured to the rack  12 . For example, the studs  22  may be replaced by a plate provided with notches to secure the rack cover  15 . 
       FIG. 3  illustrates one embodiment of the rack cover  15  in which the compression caps  16  are secured. The rack cover  15  comprises a cap-receiving plate  50  and a clamping bar  51  which are interconnected to one another through supporting brackets  52 . The clamping bar  51  comprises three slot members  53  which are designed to slide into the slots  25  of the studs  22  ( FIG. 2 ) in order to removably secure the rack cover  15  to the rack  12 . The slot members  53  are located according to the location of the slots  25  of the studs  22 . The clamping bar  51  slides relatively to the securing brackets  52  and the cap-receiving plate  50 , according to direction A. This translation movement allows the slot members  53  to be inserted into the slots  25 . When the clamping bar  51  is in a closed position (i.e., when the rack cover  15  is attached to the rack  12 ), the slot members  53  are locked to the studs  22 . When the clamping bar  51  is in an opened position (i.e., when the rack cover  15  lies on the rack  12  but the slot members  53  are not interlocked into the slots  25 ), the slot members  53  are not in line with the studs  22 , whereby the rack cover  15  may be separated from the rack  12  by being lifted away. 
       FIG. 4  illustrates one embodiment of the cap-receiving plate  50  in which the compression caps  16  are threadingly engaged. The cap-receiving plate  50  is provided with two types of apertures, namely stud-receiving apertures  54  and cap-receiving apertures  55 . The stud-receiving apertures  54  are provided in a number equal to that of the studs  22  and they are located according to the location of studs  22  in rack  12 . The dimensions of the stud-receiving apertures  54  are chosen according to the dimensions of the studs  22 . When the rack cover  15  is installed on top of the rack  12 , the studs  22  are inserted into the stud-receiving apertures  54 , in a direction corresponding to B. The studs are removable from the stud-receiving apertures in order to change the studs of given height for studs of a different height, thereby accommodating tubes of a different height. 
     The cap-receiving apertures  55  are designed to receive the compression caps  16 . They have a thread (not visible in  FIG. 4 ) so that the compression caps  16  are screwed therein. The cap-receiving plate  50  also has fixing holes  56  which are used to fixedly secure the supporting brackets  52  by way of bolts, for example. Alternatively, the supporting brackets can be attached to the cap-receiving plate  50  using an adhesive, or any other removable or permanent mechanical connector. In one embodiment, the compression caps  16  are permanently secured to the cap-receiving plate  50  while in another embodiment, they are releasably secured. 
       FIG. 5  presents a bottom view of one embodiment of the clamping bar  51  on which the supporting brackets  52  are attached. The supporting brackets  52  can be translated along the clamping bar  51  as illustrated by arrow C. This relative movement between the supporting brackets  52  and the clamping bar  51  (illustrated by direction A in  FIG. 3 ) allows the slot members  53  to be slid into the slots  25  of the studs  22 . In one embodiment, the slot members  53  may have an L shape and be designed with a wedged surface  57  for improving the securing of rack cover  15  to rack  12 . 
       FIG. 6  illustrates how the rack cover  15  is secured on the rack  12 . For simplification purposes, only the base  20  and the studs  22  are represented for the rack  12  and only the clamping bar  51  is represented for the rack cover  15  in  FIG. 6 . The slot members  53  slide into the slots  25  of the studs  22  according to direction D as a result of a translation of the clamping bar  51 , thereby allowing the rack cover  15  to be locked to the rack  12 . 
       FIG. 7  illustrates one embodiment of a rack cover  70  securable on top of the rack  12  and comprising a clamping bar  72  provided with a safety mechanism  74  for preventing the clamping bar  72  from unlocking from the studs  22  of the rack  12 . Similarly to the rack cover  15  illustrated in  FIG. 3 , the rack cover  70  comprises a cap-receiving plate  76  having cap-receiving apertures in which compression caps  16  are screwed. The clamping bar  72  is translationally secured to the cap-receiving plate  76  in order to slide slot members  78  into respective slots of the rack studs  22 , as illustrated in  FIG. 8 a   . The clamping bar  72  comprises a recess  80  in which the safety mechanism  74  is secured. The safety mechanism  74  comprises a locking member  84  provided with an aperture  86  in which a spring  82   b  is inserted and an abutment member  88  for mating with the locking member  84 . The abutment member  88  also has a spring  82   a  and a pin  83 . Pin  83  (see  FIG. 8 b   ) moves up and down and is completely engaged when the clamping bar  72  is locked. Pins  71   a  and  71   b  act as visual indicators of a locking and unlocking of the clamping bar  72 . In this embodiment, one of the pins  71   a ,  71   b  is green while the other one is red. Only the red one is visible when the clamping bar  72  is locked. Only the green one is visible when the clamping bar  72  is unlocked. Other ways of providing visual indication of a locked or unlocked state will be readily apparent to those skilled in the art. 
     The safety mechanism  74  is movable between a closed position and an opened position. The abutment member  88  mates with the locking member  84  and is made of flexible material such as plastic for example, in order to bias the safety mechanism  74  in the closing position. The abutment member  88  abuts against the bottom surface of the clamping bar  72  and is positioned in compression to exert a downward force on the rear end of the locking member  84  via the spring  82   a . As a result of the downward force, the rear end of the abutment member  88  engages the stud  22  when the safety mechanism  74  is in the closed position, thereby preventing the slot members  78  from dislodging from the stud slots. By exerting a lateral force on the front end of the locking member  84 , the safety mechanism  84  is brought into the opening position which allows the clamping bar  72  to rearly slide in order to dislodge the slot members  78  from the stud slots. As a result of a lateral force exerted on the front end of the locking member  84 , pin  83  is released and moves upwardly, thereby disengaging the abutment member  88  from the stud  22 . 
     While the present description refers to an abutment member  88  for biasing the safety mechanism  74  in the closing position, it should be understood that any adequate mechanical compression device may be used. For example, a coil spring may be inserted in compression between the rear end of the locking member and the bottom of the clamping bar  72  to directly exert a downward force on the rear end. 
       FIG. 9 a    illustrates one embodiment of the compression cap  16 . The compression cap  16  comprises a cap casing  60  which is a hollow cylinder having a screw thread  61  on its external surface in order to be screwed into the cap-receiving apertures  55  of the cap-receiving plate  50 . The compression cap  16  comprises a chamber accommodating a helical spring  62 , a pressure-adjusting bolt  63  and a piston  64 , and an aperture adapted to receive a pressure arm  65 . The chamber and the aperture are connected so that the pressure arm  65  travels through the aperture and part of the chamber. 
     The pressure arm  65  is secured to the piston  64  so as to be biased by the spring  62  to exert pressure on the sealing caps  14 . For example, the pressure arm  65  may present a screw thread on part of its external surface and can be screwed into the piston  64 , which has a threaded cavity for receiving the pressure arm  65 . Alternatively, the pressure arm  65  and piston  64  can be integrated into a single piston piece. The spring  62  is placed into the cap casing  60  in compression so that a biased force is applied by the spring  62  on the piston  64 . 
     The spring  62  may be of any shape and dimensions. While the compression cap  16  comprises a spring  62  to apply a biased force on the piston  64  and to prevent an exhaust of gas from the tube  13  before the internal pressure in the tube  13  has reached a threshold value, it should be understood that the spring  62  can be replaced by any piece that applies a biased force on the piston  64 . 
     In one embodiment, the spring  62  is made from metal and covered by an acid-resistant plastic sleeve to protect the spring  62  from acid vapours and avoid corrosion. 
     In one embodiment, the pressure-adjusting bolt  63  has a fixed position and no adjustment of the biasing force of the spring  62  is possible. In this case, the pressure adjusting bolt  63  may be integral with the casing  60  of the compression cap  16 . 
       FIG. 9 b    is another embodiment of the compression cap  16 . In this embodiment, a lock washer  68  has been added. This will be explained in more detail with reference to  FIG. 10 b   .  FIG. 9 c    is yet another embodiment of the compression cap  16 , where the order of the various parts has been reversed. This will be explained in more detail with reference to  FIG. 10   c.    
       FIG. 10 a    illustrates an embodiment of the cap casing  60  without the pressure arm  65 . The cap casing  60  has thread  66  on part of its internal surface in the chamber. The pressure-adjusting bolt  63  has a screw thread  67  on its external surface. The screw thread  67  corresponds to the thread  66  so that the pressure-adjusting bolt  63  can be screwed into the cap casing  60 .  FIG. 10 b    illustrates another the embodiment shown in  FIG. 9 b   , whereby an additional lock washer  68  is present. The lock washer  68  is installed after the spring  62  and is screwed into the barrel of the cap casing  60  until the spring  68  reaches its appropriate tension. This level of tension is the maximum pressure on the compression cap  16  of the vessel that the safety mechanism must resist in order that the cap  16  does not release pressure from the vessel.  FIG. 10 c    illustrates the embodiment shown in  FIG. 9 c   . The cap casing  60 ′ is a combination of cap casing  60  and pressure adjusting bolt  63 . The spring  62 , piston  64 , lock washer  68 , and pressure arm  65  are all the same as in the embodiment illustrated in  FIG. 10 b   , but in a different order. This allows calibration to be done from the bottom of the cap (i.e. pressure arm  65 ) and adjustment of the cap casing  60 ′ from the top does not affect this calibration. 
     It should be noted that the shape and dimensions of the compression caps  16  may vary. For example, the compression caps  16  may have a square shape and the apertures  24  may be adapted to receive the compression caps  16 . The compression caps  16  may also be secured to the rack cover  15  by way of screws or clamps for example. 
     When it is screwed into the cap casing  60 , the pressure-adjusting bolt  63 /lock washer  68  compresses the spring  62 . It results in an increased biased force exerted by the spring  62  on the piston  64 . When the internal pressure increases in the tube  13 , an upward vertical force is applied on the piston  64  through the sealing cap  14  and the pressure arm  65 . The piston  64  cannot move as long as the biased force applied by the spring  62  is superior or equal to the upward vertical force resulting from the pressure increase in the tube  13 . The internal pressure in the tube  13  which creates an upward force applied on the piston  64  that is equal to the biased force applied by the spring  62  on the piston  64  corresponds to a threshold pressure. This threshold pressure can be controlled by adjusting the position of the pressure-adjusting bolt  63 /lock washer  68  within the cap casing  60 . 
     When the internal pressure in the tube  13  is inferior to the threshold pressure, the pressure-relief valve system constituted of the compression cap  16 , the sealing cap  14  and the rim of a tube  13  is in a closed position and the tube  13  is hermetically closed. When the internal pressure in the tube  13  exceeds the threshold pressure, the pressure-relief valve system is in an open position and gas can exhaust from the tube  13 . This relief of gas limits the internal pressure in the tube  13  and prevents damage to or explosion of the tube  13 . When the internal pressure goes back below the threshold pressure, the pressure-relief valve system hermetically closes back the tube  13  since the biased force applied by the spring  62  is superior to the upward force created by the internal pressure of the tube  13 . 
     In one embodiment, the compression cap  16  and the sealing cap  14  form a same and single piece. In this case, a disk  69  of the pressure arm  65  has a shape and a size adapted to act as a sealing cap in order to close the tube  13 . Having a sealing cap and a compression cap as two different pieces enables the compression cap  16  to be used with different sealing caps  14  independently of the shape and dimensions of the sealing cap  14 . 
       FIG. 11 a    illustrates one embodiment of the installation of the tubes  13  in the rack  12 . The material to be extracted or decomposed is placed inside the tubes  13  with appropriate chemical decomposition agents. The tubes  13  are positioned through the apertures  24  and rest on the recesses  23 . The sealing caps  14  are positioned on top of the tubes  13 . The compression caps  16  are threadingly locked into the apertures  55  of the rack cover  15  as illustrated in  FIG. 4 . The compression caps  16  are chosen in accordance with a desired threshold pressure. If an adjustment of the threshold pressure is required, the location of the pressure-adjusting bolts  63 /lock washer  68  within the cap casings  60  can be adjusted.  FIG. 11 b    illustrates a different embodiment for the rack, with microwave reflecting cylinders (explained in more detail below). 
     The rack cover  15  with the compression caps  16  thereon is positioned on top of the tubes  13  in the rack  12 . During the positioning of the rack cover  15  on top of the tubes  13 , the studs  22  are threaded into the apertures  54  of the rack cover  15  and the cap-receiving plate  50  slides down along the studs  22  in the direction of arrow B ( FIG. 4 ). The rack cover  15  with the compression caps  16  secured therein is locked to the rack  12  by inserting the slot members  53  of the clamping bar  51  into the slots  25  of the studs  22 . The insertion of the slot members  53  is achieved thanks to a translation movement of the clamping bar  51  in the direction of arrow D ( FIG. 6 ). As indicated above, the middle stud may have a D-shaped slot to provide better orientation between the rack cover and the transport plate. 
     The insertion of the slot members  53  into the slots  25  exerts a downward force on the rack cover  15  and on the compression caps  16  as they are secured to the rack cover  15 . This downward force is transferred to the springs  62  of the compression caps  16  via the bolts  63  or lock washer  68 . The downward force does not add any extra force adds a further compression to the springs  62 , which increases the biasing force exerted by the springs  62  on the pistons  64 . The downward force resulting from the locking of the rack cover  15  allows the tubes  13  to be hermetically closed. As a result, the threshold pressure at which the relief of gas occurs is the pressure corresponding to an upward force equal to the biasing force exerted by the springs  62  on the pistons  64  in addition to the (no extra force) downward force resulting from the insertion of the slot members  53  into the slots  25 . 
     Having the compression caps  16  already installed on the rack cover  15  before securing it to the rack  12  allows a gain in time, as each compression cap  16  does not have to be screwed and adjusted independently. It also allows automation of the assembly of the sample holder system  10 . When the sample holder system  10  is assembled, the cap-receiving plate  50  is at a predetermined distance from the base  20 . This predetermined distance enables the compression caps  16  to lie on the sealing caps  14  so that the tubes  13  are hermetically closed when the slot members  53  are inserted into the slots  25 . If a small adjustment is required, this can be achieved by turning the bolt  63 . Once the assembly is finished, the sample holder system  10  is ready to be placed into heating equipment, such as a microwave oven, when heat is required for decomposition of the material. 
     In order to dismantle the sample holder system  10 , the slot members  53  are dislodged from the slots  25  by translating the clamping bar  51  in the opposite direction of arrow D ( FIG. 6 ). The rack cover  15  is removed from the rack  12  by upwardly translating the rack cover  15  along the studs  25  of the rack  12 . The tubes  13  can then be removed from the rack  12 . 
     In one embodiment, the sample holder system  10  is placed into a conventional or microwave oven for decomposition of the sample material. After being taken out from the oven, the samples are cooled using air blowers for example. After a predetermined cooling time, the rack cover  15  is unlocked by translating the clamping bar  51  in the opposite direction of arrow D ( FIG. 6 ), thereby dislodging the slot members  53  from the slots  25 . This allows for an auto-venting of all of the tubes  13 . 
     In one embodiment, the rack  12  is provided with at least one temperature sensor positioned below the recesses  23  in order to measure the temperature of the tubes  13 . In this case, the rack cover  15  is unlocked when the temperature of the sample material contained within the tubes  13  is below a threshold value. In one embodiment, the rack  12  is provided with a single temperature sensor for measuring the temperature of a single tube  13 , namely a reference tube, and the rack cover  15  is unlocked when the temperature of the sample material within the reference tube is below the temperature threshold. 
     The different pieces of the sample holder system  10  may be made of heat-resistant materials if a conventional oven is used. If the heating equipment is a microwave oven, the different pieces of the system  10  may be chosen to be compatible with microwave heating. In one embodiment, the different pieces of the sample holder  10  are made from an acid-and-microwave resistant material such as plastic for example. 
     In one embodiment, at least the studs  22  are removable from the rack  12  so that studs of different height may be removably secured to the base  20 . The height of the studs  22  may be chosen as a function of that of the tubes  13 . For example, studs having a first adequate height may be used with 50 ml sample tubes and studs having a longer adequate height may be used with 75 ml sample tubes. By simply choosing studs having an adequate height, the sample holder  10  can accommodate sample tubes of different heights. 
     In one embodiment, the rack cover  15  is first secured to the rack  12  and subsequently, the compression caps  16  are individually screwed into the cap-receiving plate  50 . 
     While the description refers to sample tubes  13  to receive the material to be decomposed, it should be understood that any container having any shape and dimensions can be used as a receiving part. In this case, the rack  12  and the sealing caps  14  are adapted to receive the container and to hermetically close the container, respectively. 
     The sample holder system may be of any shape and size. In particular, any frame adapted to receive the sample tubes  13  can be used and any cover into which the compression caps  16  can be inserted may also be used. While the rack  12  is a hollowed piece, it could be replaced by a block having holes adapted to receive the tubes  13 , for example. 
       FIGS. 12 and 13  illustrates one embodiment of a sealing cap  100  for sealing a cylindrical sample tube  102 . The sealing cap  100  has a tube engaging side  104  provided with a circular groove  106  and a central conical protrusion  108 . The groove  106  and the protrusion  108  are sized and shaped so that the rim  110  of the sample tube  102  does not abut against the bed surface  112  of the groove  106  when the sealing cap  100  is positioned on top of the tube  102 , as illustrated in  FIG. 13 . Therefore, the tube is not hermetically close when the sealing cap  100  is positioned on top of the tube  102 . 
     The sealing cap  100  is made from a flexible material so that the groove  106  and the conical protrusion  108  may be deformed when the sealing cap  100  is positioned on top of the tube  102  and a downward force is exerted on top of the sealing cap  100 . The downward force may be exerted by a compression cap such as compression cap  16  for example. As a result of the downward force, the walls of the groove  106  hermetically engage the rim of the tube  102  to hermetically close the tube  102 . 
       FIG. 14  illustrates an example of forces in action when a slot member  53  is positioned in a slot  25  of a stud  22 . It should be noted that this example is illustrative only and that other scenarios involving same or different forces are also possible. Force T is the force used to push back the slot member  53  out of the slot  25 . Force R is the reaction force exerted by the stud  22  on the slot member  53 . Forces F 1  and F 2  are friction forces resulting from the friction of the slot member  53  on the stud  22  when the slot member  53  is pushed back. Force P is the force resulting from the increase of internal pressure in the tubes  13 . 
     Friction force F 1  can be expressed as a function of the force P and a coefficient of friction μ as shown in the following equation:
 
 F 1 =μ*P   (Eq. 1)
 
     The friction force F 2  can be expressed as a function of the force R and the coefficient of friction μ as shown in the following equation:
 
 F 2 =μ*R   (Eq. 2)
 
     Force T is the force resulting from the friction forces F 1  and F 2  in the y-direction and is given by equation 3:
 
 T=μ*P+R *(μ*cos α−sin α)  (Eq. 3)
 
     where α is the angle of the wedge of wedged surface  56  of the slot member  53 . 
     The force R can be expressed as a function of the force P, the coefficient of friction μ and the angle α according to equation 4:
 
 R=P /(cos α+μ*sin α)  (Eq. 4)
 
     Substituting the force R by equation 4 in equation 3, the force T can be expressed as:
 
 T=P*μ+P *(μ*cos α−sin α)/(cos α+μ*sin α)  (Eq. 5)
 
     Equation 6 is a simplified expression of equation 5:
 
 T=P *coef(μ,α)  (Eq. 6)
 
where
 
coef(μ,α)=μ+(μ*cos α−sin α)/(cos α+μ*sin α)  (Eq. 7)
 
     Equation 6 shows that the force T is proportional to the force P. As a result, the force T, which is the force used to push back the slot member  53  out of the slot  25 , is proportional to the increase of internal pressure in the tube  13  and is also a function of the angle α. Therefore, it is possible to adjust the force T by controlling the angle α. 
     While  FIG. 14  illustrates a stud  22  having a rectangular slot, it should be understood that the slot may have any adequate shape. For example,  FIG. 15  illustrates a stud  22 ′ provided with a slot having a shape matching that of the slot member  53 . 
       FIG. 16  is a graph of coef (μ, α) as a function of the coefficient of friction μ and the angle α. Two observations can be made from  FIG. 8 : coef (μ, α) is proportional to the coefficient of friction μ and inversely proportional to the angle α. As a result, the force T is also proportional to the coefficient of friction μ and inversely proportional to the angle α. Decreasing the angle α implies a greater force T to push back the slot member  53  out of the slot  25 . The lower the angle α is, the higher the increase of the internal pressure into the tube  13  has to be in order to push the slot member  53  out of the slot  25 . As a result, it is possible to set the angle α to a value preventing the rack cover  15  from being removed from the studs  22  of the rack  12 . 
     In one embodiment, the spring  62  is enclosed in cap casing  60  in a compression state which sets a threshold pressure. For example, spring  62  has a length of 1 inch when no forces are applied to it. This spring presents a maximum load of 213.14 lb for a deflection of 37% of its length. Spring  62  is enclosed within cap casing  60  with a length deflection of 25%. This means that spring  62  presents a load of 144 lb. The internal pressure in tube  13  which can generate the same load is given by equation 8:
 
 P [psi]=Load[lb]/Surf[in 2 ]  (Eq. 8)
 
     where Surf is the internal surface of tube  13 . 
     For example, if the internal surface of tube  13  is equal to 0.76 in 2 , the internal pressure corresponding to a load of 144 lb is 189.26 psi. This internal pressure is the threshold pressure corresponding to a deflection of spring  62  equal to 25%. If the internal pressure in tube  13  is below 189.26 psi, tube  13  is hermetically closed, and if the internal pressure is superior to 189.26 psi, the internal pressure is sufficient to compress spring  62  and gas can escape from tube  13 . 
     The following example illustrates how the internal pressure threshold can be adjusted via the pressure-adjusting bolt  63  or the lock washer  68 . Table 1 presents the load of the spring  62  and the corresponding threshold pressure as a function of the displacement Dx of the pressure-adjusting bolt  63 /lock washer  68  within the cap casing  60 . When Dx is equal to zero, the pressure-adjusting bolt  63 /lock washer  68  applies no force on the spring  62 , which presents no additional deflection. In this case, the load of the spring  62  is 144 lbs, which corresponds to a threshold pressure of 189.47 psi. By screwing the pressure-adjusting bolt  63 /lock washer  68 , an additional compression is applied to the spring  62 , which increases its load. For example, by displacing the pressure-adjusting bolt  63 /lock washer  68  by 0.2 in, the total load of the spring  62  is increased up to 259.2 lb, which corresponds to a threshold pressure of 314.05 psi. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Dx 
                 Load 
                 Pressure 
               
               
                 [in] 
                 [lb] 
                 [psi] 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 0.000 
                 144.000 
                 189.26 
               
               
                 0.025 
                 158.400 
                 208.42 
               
               
                 0.050 
                 172.800 
                 227.37 
               
               
                 0.075 
                 187.200 
                 246.32 
               
               
                 0.100 
                 201.600 
                 265.26 
               
               
                 0.125 
                 216.000 
                 284.21 
               
               
                 0.150 
                 230.400 
                 303.16 
               
               
                 0.175 
                 244.800 
                 322.11 
               
               
                 0.200 
                 259.200 
                 341.05 
               
               
                   
               
            
           
         
       
     
     For a fixed initial compression of the spring  62 , it is possible to vary the threshold pressure at which the pressure-relief valve opens and gas exhausts from the tube  13  from 189.26 to 314.05 psi by screwing the pressure-adjusting bolt  63 /lock washer  68 . 
     While the present description refers to slot members  53  to be positioned in slots  25  in order to removably and fixedly secure the rack cover  15  to the rack  12 , it should be understood that any adequate fastener that allows removably securing the rack cover  15  to the rack  12  can be used. For example, bolts or screws may be used for securing the rack cover  15  to the rack  12 . 
       FIG. 17  illustrates a sample holder system  120  comprising a rack  122 , a rack cover  124 , and a transportation plate  126 . The rack  122  comprises a base plate  128  to which a U-shaped support plate  130  is secured. The support plate  130  is provided with twelve apertures each adapted to receive a sample tube  132 , and a pair of handles  134 . The rack cover  124  comprises a cap-receiving plate  136  to which a clamping bar  138  is translationally secured. The cap-receiving plate  136  is provided with twelve apertures each for receiving a compression cap  140 . 
       FIG. 18  illustrates one embodiment of a U-Shaped transporting plate  126  comprising twelve tube-receiving openings  142  and three stud-receiving openings  144 . The tube receiving openings  142  are each positioned to be aligned with a respective tube-receiving aperture of the support plate  130  of the rack  122 , and shaped and sized to receive a tube  132 . The stud-receiving openings  144  are positioned, shaped, and sized to each receive a corresponding stud of the rack  122 . The transporting plate  126  is further provided with a pair of apertures  146  each forming a handle. 
     It should be understood that the shape, dimensions, position, and number of the tube-receiving openings  142  and the stud-receiving openings are determined in accordance with the shape, dimensions, position, and number of the tube-receiving apertures of the support plate  130  and the studs of the rack  122 , respectively. 
       FIGS. 19A and 19B  each provide an example of a sample tube  132  which may be used with the transporting plate  126  and the rack  122 . The sample tube  150  illustrated in  FIG. 19A  comprises a cylindrical tube  152  having an opened end  154 , and a flange circumferentially extending around the tube  152  adjacent the open end  154 . It should be understood that the position of the flange  156  along the height of the tube  152  is exemplary only. For example, the flange  156  could be positioned at about half of the height of the tube  152 . 
     The sample tube  158  illustrated in  FIG. 19B  is a cylindrical tube having a diameter varying along its height. The tube  158  comprises a first section  160  having a constant circumference therealong and a wide-mouthed section  162  having an increasing circumference near the opening  164  of the tube  158 . 
     The circumference of the tube-receiving openings  142  is larger than that of the tube  150  or that of the section  160  of the tube  158  so that the tube  150  or  158  can be inserted into the opening  142 . The circumference of the tube-receiving openings  142  is smaller than that of the flange  156  of the tube  150  or that of the rim of the tube  158  so that the flange  156  of the tube  150  or the wide-mouthed section  162  of the tube  158  may engage the surrounding or the rim of the aperture  142  of the transporting plate  126 . As a result, the tube  150  or  158  may be supported by the transporting plate  126 . 
     It should be understood that the shape of the sample tubes  150  and  158  is exemplary only. A sample tube to be used with the transportation plate  126  may have any adequate shape as along as at least a portion of the tube passes through the tube-receiving aperture  142  while being supported by the transporting plate  126 . For example, an adequate tube can comprise two cylindrical section having different diameters. 
       FIG. 20  illustrates the transporting plate  126  supporting twelve sample tubes  132 . Each sample tube  132  is provided with a flange  170  near the opening of the tube  132 . The circumference of the tube  132  is inferior to that of the opening  142  so that it can slide therein, but the circumference of the flange  170  is superior to that of the opening  142  so that the flange  170  is supported by the surrounding of the opening  142 . 
     In one embodiment, a protective ring  172  is inserted in each tube-receiving opening  142  for protecting the rack  126  against the high temperature of the tube  132 . The protective ring  172  can be made from Teflon for example. 
     In one embodiment, the transporting plate  126  allows the grouping of a plurality of sample tubes  132  on a same structure. The transporting plate  126  facilitates the transportation of the sample tubes  132  since a user does not have to individually transport the sample tubes  132 . 
       FIG. 21  illustrates a holding frame  180  for holding the transporting plate  126 . The holding frame  180  comprises a top plate  182  and a pair of side plates  184 , thereby providing the holding frame  180  with a U-shape. The top plate  182  comprises twelve apertures  186  each sized to receive a sample tube  132  and positioned to be aligned with a tube-receiving opening  142  of the transporting plate  126 . The side plates  184  each comprise an opening  188  forming a handle. The transporting plate  126  comprising the sample tubes, as illustrated in  FIG. 20 , is deposited on top of the top plate  182  of the holding frame  180  so that each sample tube  132  is received in a corresponding aperture  186 . Alternatively, the transporting plate  126  may be deposited on the top plate  182  and the openings  142  of the transporting plate  126  are each aligned with a respective aperture  186  of the holding frame  180 . Then the sample tubes  132  are each inserted into a corresponding tube receiving opening  142 , which results in the assembly illustrated in  FIG. 21 . 
     The assembly illustrated in  FIG. 21  may be used in a preparation station in which a user fills the tubes  132  with a sample. Once the preparation of the samples is completed, the user may concurrently transport all of the tubes  132  by taking the handles of the transporting plate  126  and lifting the transporting plate  126 . The user may then insert the tubes  132  into the rack  122 . First the tubes are each aligned with a respective tube-receiving opening of the rack  122 , and then the transporting plate  126  is pulled down to insert the sample tubes  132  into their respective tube-receiving opening until the transporting plate  126  engages the support plate  130  of the rack  122 . Once the transporting plate  126  with the tubes  132  is deposited on the rack  122 , the rack cover  124  is deposited on top of the rack  122  to obtain the sample holder system  120  illustrated in  FIG. 17 . The sample holder system  120  is placed into a microwave oven where the samples are heated. Once the digestion is completed, all of the tubes  132  may be concurrently brought to an analysis station where the user may analyse the digested or extracted samples. 
     While in  FIG. 22  the handles of the support plate  126  are upwardly directed and engage the support plate  130  of the rack  122 , it should be understood that the tubes  132  may be inserted in the transporting plate  126  so that the handles of the supporting plate  126  are downwardly directed. 
     It should be understood that the shape of the holding frame  180  is exemplary only as along as it allows the transporting plate  126  to be supported. For example, the holding frame may comprise a top plate having tube-receiving apertures and four legs to have a table-like shape. 
     In one embodiment, the tube-receiving openings  142  of the transporting plate  126  and/or the tube-receiving apertures of the support plate  130  may be identified by an identifier such as a number for example. For example, a number comprised between one and twelve may be printed or engraved adjacent to the corresponding tube-receiving opening  142  of the transporting plate  126  and/or the tube-receiving aperture of the support plate  130 . In another embodiment, only one tube-receiving opening  142  of the transporting plate  126  and/or the first tube-receiving aperture of the support plate  130  is identified as being the first opening. 
     It should be understood that the shape of the transportation plate  126  is exemplary only as long as it allows at least one sample tube to be supported by a sample structure. For example, the transportation plate may be a rectangular and planar plate provided with twelve apertures, or it may be provided with a single aperture. In one embodiment, twelve individual transportation plates each holding a single tube are inserted into the receiving apertures of the holding frame  180 . 
     It should be understood that the rack cover  15  or  70  may be used in the sample holder system  120 . Similarly, the rack  122  may correspond to the rack  12  provided with studs  22  having an adequate height. 
       FIG. 23  illustrates one embodiment of an automated microwave oven  200  for heating a sample to be extracted or decomposed. The oven  200  comprises a heating chamber  202 , a microwave generator  204 , a conveyor  206 , and a control unit  208 . The heating chamber  202  is adapted to receive a sample holder  210  containing the sample to be heated. The microwave generator  204  is adapted to generate microwaves and operatively connected to the heating chamber  202  in order to propagate the generated microwave energy into the heating chamber  202 . For example, the microwave generator  204  may be positioned in the heating chamber  202 . In another embodiment, the microwave generator  204  is separate from the heating chamber  202  and a microwave waveguide connects the microwave generator  204  to the heating chamber  202  in order to transport and propagate the generated microwaves into the heating chamber  102 . 
     The conveyor  206  is adapted to receive and transport the sample holder  210  through the heating chamber  202  which is provided with an entrance opening  212  and an exit opening  214 . An entrance door  216  and an exit door  218  are provided for closing the entrance opening  212  and the exit opening  214 , respectively. The entrance and exit door  216  and  218  are made from a microwave-resistant material in order to prevent the microwaves from propagating outside the heating chamber  202 . The conveyor  206  extends through the heating chamber  202  via the entrance and exit openings  212  and  214 . It should be understood that the generation of microwaves is stopped when the sample holder  210  enters or exits the heating chamber  202 . 
     The control unit  208  is configured for controlling the conveyor  206 , the microwave generator  204 , and the entrance and exit doors  216  and  218 . The control unit  208  may be adapted to adjust the power or the duty cycle of the microwaves generated by the microwave generator  204  and/or the duration of the microwave generation in order to heat the sample contained in the sample holder  210 . The control unit  208  is further adapted to control the displacement of the conveyor  206  in order to control the speed of displacement and position of the sample holder  210 . The control unit  208  is also adapted to coordinate the opening and closing of the doors  216  and  218  with the entry and exit of the sample holder  210  from the heating chamber  210 . 
     In one embodiment, the microwave generator  204  is adapted to control the power of the generated microwaves. In this case, the control unit may adjust the power of the generated microwaves to a desired value comprised between 0% and 100% of the maximum power of the microwave generator  204 . The microwave generator  20  is then operated continuously during a predetermined period of time at a desired power to heat the sample at a desired temperature. 
     In another embodiment, the power of the microwave generator  204  is not controllable which means that only the maximum microwave power may be delivered by the microwave generator  204 . In this case, the microwave generator  204  operates according to a duty cycle. 
     In one embodiment, the doors  216  and  218  are each provided with a microwave quarter-wave trap for preventing any leakage of microwaves outside of the heating chamber. The oven may also be provided with microwave sensor for detecting any leakage of microwaves outside of the heating chamber  202 . In this case, the control unit  208  may be adapted to stop the microwave generator  204  upon detection of a microwave leakage. 
     In one embodiment, the control unit  208  comprises a processor, a memory, and a command input device. A user enters parameters such an identification of the sample, a desired temperature, a desired microwave power, a heating time, and/or the like, into the control unit  208  via the command input device. 
     In one embodiment in which the user enters a desired temperature for the sample, the microprocessor is adapted to determine the microwave power or the duty cycle corresponding to the desired temperature for the sample. For example, the memory may be provided with a database of temperatures and corresponding microwave powers, or a database of desired temperatures and corresponding duty cycles. The processor may also be adapted to determine the microwave power or duty cycle in accordance with the type of sample contained in the sample holder  210  and/or the type of the sample tube. 
     It should be understood that any adequate conveyor system compatible with microwaves may be used. For example, the conveyor  206  may be a belt conveyor, a chain conveyor, a lineshaft roller conveyor, or the like. 
     In one embodiment, the sample holder  210  is provided with rolling elements rotatably secured therebelow. The conveyor  206  may comprise a planar surface extending through the heating chamber  202 , on which the sample holder  210  may roll, and a driving device adapted to roll the sample holder  210  on the planar surface. Any adequate driving mechanism may be used. 
     It should be understood that any adequate sample holder  210  adapted to microwave heating may be used. For example, the sample holder may be made from glass or Teflon. The sample holder may be adapted to receive a single sample or a plurality of samples. For example, the sample holder  10  or  120  may be used. 
     The heating chamber  202  may have any adequate shape and size for receiving the sample holder  210  and can be made from any adequate type of microwave-resistant material so that generated microwaves do not exit the heating chamber  202 . 
     In one embodiment, the conveyor  206  and the control unit  208  are adapted to stepwise transport the sample holder  210 . In this case, the sample holder  210  occupies a series of predetermined positions during a corresponding predetermined period of time. In another embodiment, the conveyor  206  and the control unit  208  are adapted to continuously move the sample holder  210  within the oven  200 . 
     While the present description refers to a single heating chamber  202 , it should be understood that the oven  200  may comprise more than one heating chamber each crossed by the conveyor  206  and each provided with movable doors and a microwave generator. The heating chambers may be physically secured together so that the sample holder  210  enters a second heating chamber while exiting a first heating chamber. Alternatively, the heating chambers may be physically spaced apart. 
       FIG. 24  illustrates one embodiment of an automated microwave  230  comprising a heating chamber  232  having a single opening  234  which is used for both entering and exiting the sample holder  210  and closed by a single door. In this embodiment, the conveyor  238  may be U-shaped. 
       FIG. 25  illustrates one embodiment of a microwave oven  300  provided with the same elements as the oven  200  and further comprising a cooling chamber  302 . The cooling chamber  302  is adapted to receive the sample holder  210  and the conveyor  206  extends through the cooling chamber  202 . The cooling chamber  302  is positioned adjacent to the heating chamber  302  so that the sample holder  210  may be brought into the cooling chamber  302  after leaving the heating chamber  202 . The cooling chamber  302  is provided with a cooling unit  304  adapted to cool the sample contained into the sample holder  210 . Any adequate cooling unit may be used. For example, the cooling unit may be a refrigerating unit. In another example, the cooling unit may comprise at least one fan positioned to blow air on the sample holder  210  or draw air outside of the cooling chamber  302  in order to remove heat from the sample holder  210  and cool the sample. 
       FIG. 26  illustrates one embodiment of a cooling chamber  320  provided with three fans in order to cool the sample holder  210 . An air outlet  322  positioned near the top of the cooling chamber  320  and connected to the outside of the oven  300  allows heated air to exit the cooling chamber  320 . Three fans  324  are located near the bottom of the cooling chamber  320  and adapted to draw air contained in the cooling chamber  320  outside thereof. When the fans  324  are operated, air contained in the cooling chamber  320  is expulsed outside and fresh air is drawn from the outside into the cooling chamber  320 , thereby creating an air current which cools down the sample holder. 
     In one embodiment, the user enters cooling parameters such a cooling duration, a cooling unit power, a desired end cooling process temperature, and/or the like in the control unit  208  which controls the cooling process in accordance with the cooling parameters. 
     In one embodiment, the oven  300  is free from any cooling chamber  202  and the cooling device  304  such as a fan is located at the exit of the heating chamber  202 . 
       FIG. 27  illustrates one embodiment of a microwave oven  350  comprising all of the elements of the oven  300  and further comprising a venting chamber  352  provided with an unsealing unit  354 . When the sample holder  210  is provided with a lid or cap for hermetically enclosing the sample into the sample holder  210 , the unsealing unit  354  is adapted to unseal the sample holder  210  so that pressurized gas contained in the sample holder  210  may exit the sample holder  210 . The control unit  208  is further adapted to control the unsealing unit  354 . It should be understood that any adequate cap for hermetically sealing the sample holder  210  and any adequate unsealing device adapted to unseal the sample holder  210  may be used. 
     In one embodiment, the sample holder  210  is provided with a thread so that a lid may be screwed therein. The lid is screwed in the sample holder  210  to hermetically close the sample holder  210  so that no gas may exit the sample holder  210  during the heating process. In this case, the unsealing unit may comprise an automated arm provided with any adequate mechanisms for unscrewing the lid such as pincers, a suction cup, or the like. 
     In another embodiment, the sample holder may be the sample holder system  10  and the unsealing device comprises a moving arm adapted to push on the front portion of the clamping bar  51  in order to at least partially dislodge the slot members  53  from the slots  25 , as illustrated in  FIG. 28 . The venting chamber  352  is provided with a movable arm  356  of which the displacement is controlled by a motor  358 . By actuating the motor  358 , the movable arm  356  is moved towards the clamping bar  51  of the sample holder system  10  as illustrated in  FIG. 28  (arrow E). The movable arm  356  then engages and pushes the clamping bar  51 , thereby dislodging the slot members  53  from the slots  25  and unsealing the tubes  13 . 
     In a further embodiment, the unsealing device may comprise a movable arm provided with pincers for pulling the rear end of the clamping bar  51 . 
     In a further embodiment, the sample holder may comprise a clamping bar having a safety mechanism such as the clamping bar  72  illustrated in  FIG. 8 . The unsealing device may comprise at least one moving arm adapted to downwardly push on the locking member  84  to disengage the locking member  84  from the stud  22  and horizontally push on the clamping bar  72  to dislodge the slot members from the stud slots. In one embodiment, the moving arm is beveled in order to concurrently engage the locking member  84  and the clamping bar  72  and exert a downward force of the locking member  84  and a substantially horizontal force on the clamping bar  72 . 
     In one embodiment, the venting process requires a precise positioning of the sample holder  210  with respect to the unsealing device  354 . In this case, position sensors such as mechanical position sensors or optical position sensors may be used by the control unit  208  to determine whether the position of the sample holder  210  within the venting chamber  352  is adequate. If the control unit  208  determines that the position of the sample holder  210  is inadequate, a sample holder positioning device controlled by the control unit  208  is used for moving the sample holder to an adequate position within the venting chamber  352 . It should be understood that any adequate mechanical positioning device adapted to move the sample holder to a desired position within the venting chamber  352  may be used. 
     In another embodiment, no precise positioning of the sample holder  210  with respect to the unsealing device  354  is required. 
     In one embodiment, the venting chamber  352  is fluidly connected to a cooling chamber provided with at least one fan adapted to draw air out of the cooling chamber. In this case, gases leaking out of the sample holder during the venting process are drawn out of the venting and cooling chambers by the fan. 
     In one embodiment, the heating chamber  202  and/or the cooling chamber  302  and/or the venting chamber  352  is(are) provided with a temperature sensor for measuring the temperature of the sample holder  210  and/or the sample contained in the sample holder  210 . In this case, the control unit  208  is adapted to control the microwave generator  204 , the cooling unit  304 , and/or the unsealing unit  354  in accordance with the temperature of the sample holder  210  and/or the sample in the respective chamber  202 ,  302 ,  352 . For example, if a temperature sensor is present in the heating chamber  202 , or is positioned in such a way or such a location to read a sample temperature in tube  132 , the control unit  208  can adjust the power and/or the duty cycle and/or the heating time of the generated microwaves in accordance with the sensed temperature to heat the sample up to a desired temperature. In another example in which the cooling chamber  302  is provided with a temperature sensor, the sample holder  210  may only exit the cooling chamber  302  when the temperature of the sample and/or the sample holder  210  has decreased below a predetermined temperature. The control unit  208  may also control the cooling unit  304  in accordance with the sensed temperature. In a further example in which the venting chamber  352  is provided with a temperature sensor, the unsealing unit  354  is activated by the control unit  208  only when the temperature of the sample holder  210  and/or the sample within the venting chamber  352  has decreased below a predetermined venting temperature. 
     In one embodiment, several sample holders  210  are positioned on the conveyor and are automatically brought to the heating chamber  202 , the cooling chamber (if any), and the venting chamber (if any) by the conveyor  206 . The control unit  208  may apply same parameters for heating, cooling, and/or venting all of the sample holders  210 . Alternatively, the control unit  208  is adapted to apply different parameters for each sample holder  210 . For example, a first set of parameters may be applied to the first sample holder, a second set of parameters may be applied to the second sample holder, etc. 
     In one embodiment, each sample holder  210  is provided with an identification (ID) device and the oven  200 ,  300 ,  350  is provided with an ID reader adapted to read the ID device. For example, the sample holder  210  can be provided with a bar code and the oven  200 ,  300 ,  350  can comprise a bar code reader. The user enters the bar code ID for each sample holder  210  and the corresponding heating and/or cooling and/or venting parameters into the control unit  208  before starting the heating process. When a sample holder  210  enters the heating chamber  202  or before entering in the heating chamber  202 , the bar code reader reads the ID of the sample holder  210  which is transmitted to the control unit  208 . The control unit  208  retrieves the heating parameters corresponding to the ID and controls the microwave generator  204  in accordance with the heating parameters. The control unit  208  also retrieves the cooling and/or venting parameters from the memory and controls the cooling and/or venting processes in accordance with the retrieved cooling and/or venting parameters. In one embodiment, the bar code may comprise bars inked on the sample holder. In another embodiment, the bar code may comprise slots made into the sample holder. In a further embodiment, at least one magnet is used for identifying each sample holder  210  and the ID reader is a magnetic reader. Alternatively, magnets are used to represent binary numbers, and more than one magnet is used. 
     In another embodiment, the control unit  208  is provided with a clock which is used for identifying the sample holders  210 . The control unit can identify the different sample holders  210  using the heating times and the time required for transporting the sample holders  210  from one position to another in the oven  200 ,  300 ,  350 . 
     In one embodiment, the microwave oven  200 ,  300 ,  350  is sized and shaped to be portable. For example, in one embodiment, the entire system, including the heating chamber, the cooling chamber, and the venting chamber as illustrated in  FIG. 30 , is 26 inches in height×26 inches in width×23.5 inches in depth. The heating chamber is 21 inches in width×13 inches in height×4.5 inches in depth. The cooling and venting areas are each 20 inches in width×19 inches in height×13.5 inches in depth. In one embodiment, the rack is 14.7 inches in length×4 inches in width and can have varying heights, such as 10 inches, 12.5 inches, etc. These dimensions are exemplary only and should not be construed as limiting. 
       FIG. 29  illustrates one embodiment of a microwave oven  450  comprising all of the elements of the oven  400  but having a closed-loop conveyor  360 . The oven  450  may be used for heating a plurality of sample holders without any surveillance from a technician, over night for example. A same sample holder may pass through the heating chamber  202 , the cooling chamber  302  (if any), and the venting chamber  352  (if any) several times to be heated, cooled, and/or vented several times. 
     In one embodiment, each sample holder  210  is provided with an ID and the oven  450  is provided with at least one ID reader. The user enters the heating and/or cooling and/or venting parameters for each ID into the control unit  208  of the oven  450 . When a sample holder enters the heating chamber  202  and/or the cooling chamber  302  and/or the venting chamber  352 , the control unit  208  identifies the sample holder  210  using the ID and applies the corresponding parameters retrieved from the memory. In one embodiment, the control unit  208  is adapted to count the number of sample holders  210  and stop the conveyor  360  when the last sample holder  210  has completed the heating/cooling/venting cycle. In another embodiment, the control unit  208  is adapted to store the ID of the first sample holder entering the heating chamber  202  in order to identify it as being the number one sample holder and to stop the conveyor when the number one sample holder is about to enter the heating chamber for a second time. Alternatively, the control unit  208  is adapted to determine when the last rack of a series of pre-programmed racks exits the heating chamber  202  or the cooling chamber  302  or the venting chamber  352 . 
     While in the present description, the heating chamber  202  of the ovens  200 ,  300 ,  400 , and  450  is provided with an entrance and an exit doors for preventing the microwaves from propagating outside of the heating chamber  202 , it should be understood that the heating chamber may comprise a single door from allowing the entrance and exit of the sample holder  210 . In this case, the conveyor may be shaped to form a U-turn inside the heating chamber  202 . 
     While  FIGS. 25, 27, and 29  illustrate a microwave oven in which the heating chamber  202 , the cooling chamber  302 , and/or the venting chamber  352  are physically spaced apart, it should be understood that the chambers  202 ,  302 , and  352  may be physically regrouped to form a tunnel. The microwave generator  204 , the cooling unit  304 , and the unsealing unit  354  are positioned along the tunnel at different positions. At least two microwave-barrier doors are positioned on each side of the heating chamber  202  to define a heating station within the tunnel. 
       FIG. 30  illustrates an automated digestion system  400  comprising a microwave oven  402 , a cooling station  404 , a venting station  406 , a control unit  408 , and a conveyor (not shown). The automated digestion system  400  is adapted to receive fourteen sample holders  410  such as sample holder systems  10  or  120 , and successively heat, cool, and vent them. As each sample holder  410  comprises twelve sample tubes, up to one hundred sixty eight samples may be digested in an automated fashion by the automated digestion system  400 , thereby provided an automated digestion system having an improved throughput. 
     In one embodiment, the microwave oven  402  is removable from the automated digestion system  400  and may be used in a non-automated fashion. In this case, the user of the oven  402  manually inserts and removes the sample holder  410 . 
     In one embodiment, the control unit  408  applies the same heating and/or cooling and/or venting parameters to all of the sample holders  410 . In another embodiment, the user may enter different operating parameters for each sample holder  410 . 
       FIG. 31  illustrates one embodiment of a conveyor  403  which may be part of the automated digestion system  400 . The conveyor  403  comprises a planar plate  412  on which sample holders  430 - 456  are deposited. Low friction feet or balls are rotatably secured below each sample holder  430 - 454  so that the sample holder  430 - 456  may roll or slide on the planar plate  412 . The planar plate  412  is sized to receive fifteen sample holders. However, only fourteen sample  430 - 456  holders are placed on the planar plate  412  so that an available position  458  is free from any sample holder. The planar plate  412  is provided with four rectangular openings under which a corresponding conveyor belt  420 - 426  is positioned. 
       FIG. 32  illustrates one embodiment of the conveyor belt  422  which comprises a closed loop belt  470 , two driving wheels  472  and  474 , and two rack engaging members  476  and  478 . The wheels  472  and  474  are driven by at least one motor controlled by the control unit of the automated digestion system. When the wheels  472  and  474  are anti-clockwise rotated, the rack engaging member  478  is moved towards the sample holder  442 , abuts against the rear portion of the sample holder  442 , and exerts a force on the sample holder  442  which rolls to the next position. 
     Referring back to  FIG. 31 , the sample holder  430  is located in the heating chamber  402  while the sample holders  432  and  434  are in the cooling station and the venting station, respectively. Once the heating of the sample holder  430  is completed, the conveyor belt  424  is activated to move the sample holders  448 - 456  towards the available position  458 . The position on top of the conveyor belt  424  becomes the available position as illustrated in  FIG. 33 . The conveyor belt  422  is then activated to move the sample holders  442 - 446  towards the conveyor belt  424 . Once the sample holder  446  has reached the position on top of the conveyor belt  424 , the position on top of the conveyor belt  422  becomes the available position, as illustrated in  FIG. 34 . The conveyor belt  420  is then activated to move the sample holders  432 - 440  towards the conveyor belt  422 . Once the sample holder  440  reaches the position on top of the conveyor belt  422 , the position on top of the conveyor belt  420  is available and the sample holder  432  is in the venting station of the automated digestion system, as illustrated in  FIG. 35 . Then the sample holder  430  is moved from the heating chamber  402  to the cooling station  404  while the sample holder  456  enters the heating chamber  402 . It should be understood that any mechanical positioning device may be used for moving a sample holder inside the heating chamber  402  and moving a sample holder from the heating chamber to the cooling station on top of the conveyor belt  420 , and likewise from the cooling station to the venting station 
       FIG. 37  illustrates one embodiment of a microwave oven  500  adapted to independently heat six samples. The oven  500  comprises a chamber  502  adapted to receive sample holders and provided with a microwave barrier door  504  for opening and closing the oven  500 . The oven  500  also comprises six microwave applicators  506  each for individually applying microwaves to a different sample holder. Each microwave applicator  506  comprises a microwave generator  508  such as a magnetron for example, a flexible microwave waveguide  510  such as coaxial cable, and an oven cavity portion  512 . The oven  500  further comprises a control unit  514  adapted to control the microwave applicators  506 . In this embodiment, the oven cavity portion  512  is movable with respect to the microwave generator  508  between an extended position and a retracted position (illustrated in  FIG. 37 ) and the length of the flexible microwave waveguide  510  is chosen to allow the displacement of the oven cavity portion  512  between the two positions. 
       FIG. 38  illustrates one embodiment of a sample holder  520  adapted to the oven  500 . The sample holder  520  comprises a rack  522  adapted to receive six sample tubes  524 . The sample holder  520  also comprises six rack cavity portions  526 . Each rack cavity portion  526  is adapted to form a microwave cavity when physically connected to a respective oven cavity portion  512 . The rack cavity portions  526  are positioned on the rack  522  in accordance with the position of the oven cavity portions  512  when in the extended position. It should be understood that the sample tubes  524  may be removably secured to the rack  522 . 
       FIG. 39  illustrates the sample holder  520  received in the microwave oven  500 . The sample holder  520  is positioned within the oven  500  so that each rack cavity portion  526  faces a corresponding oven cavity portion  512 . 
     In one embodiment, a mechanical positioning device is used to precisely position the sample holder  520  within the oven  500 . Positioning sensors such as optical or mechanical sensors may be used to determine the position of the sample holder  520 . It should be understood that the mechanical positioning device may be controlled by the control unit  514  of the oven  500 . 
     In another embodiment, abutting elements are located in the oven  500  to precisely position the sample holder  520  with respect to the oven cavity portions  512 . 
     In a further embodiment, the sample holder  520  is positioned in the oven  500  by a user. 
     Once the sample holder  520  has been precisely positioned in the oven  500 , the oven cavity portions  512  are moved from the retracted position ( FIGS. 37 and 39 ) to the extended position, as illustrated in  FIG. 40 . In the extended position, each oven cavity portion  512  engages a corresponding rack cavity portion  526 . As the oven cavity portion  512  and the rack cavity portion  526  are complementary portions of a mini microwave cavity  528 , the mini microwave cavity  528  is formed when the oven cavity portion  512  engages the rack cavity portion  526 . It should be understood that the oven cavity portion  512  and the rack cavity portion  526  are made from microwave reflecting material such as metal, for example aluminum, and designed so that the mini cavity  528  is substantially hermetical to microwaves, i.e. so that substantially no microwaves exit the cavity at the junction of the oven cavity portion  512  and the rack cavity portion  526 . 
     While the present description refers to a rack  520  having six rack cavity portions  526  and an oven  500  having six oven cavity portions  512 , it should be understood that the number of cavity portions is exemplary only as along as the rack  520  and the oven  500  each comprise at least two respective cavity portions. 
     While  FIGS. 37 to 40  illustrate an oven  500  comprising two rows of physically spaced oven cavity portions  512  and a rack  520  comprising two rows of physically spaced rack cavity portions  526 , it should be understood that other embodiments are possible. For example,  FIG. 41  illustrates one embodiment of a microwave oven  530  comprising two oven cavity elements  532  each having three recesses  534  each forming an oven cavity portion. The oven  530  is adapted to receive a sample holder  536  comprising a rack  538  on which two rack cavity elements  540  are secured. Each rack cavity element  540  comprises three recesses  542  each forming a rack cavity portion. 
     While  FIG. 37  illustrates an oven  500  comprising movable oven cavity portions  512 , it should be understood that other embodiments are possible. For example,  FIG. 42  illustrates one embodiment of a microwave oven  550  comprising six movable microwave applicators  552 . Each microwave applicator  552  comprises a microwave generator  554  and an oven cavity portion  558  connected together by a microwave waveguide  556 . The microwave waveguide  556  may be flexible. Alternatively, the microwave waveguide  556  may be rigid. The microwave applicators  552  are secured to two displacement plates  560  to form two rows of microwave applicators  552 . By moving one displacement plate  560 , three microwave applicators  552  are moved. Alternatively, each microwave applicator  552  may be independently movable. 
     While  FIG. 40  illustrates a mini microwave cavity  528  designed to match the shape of the sample tube  524 , it should be the mini cavity may have any adequate shape and size.  FIG. 43  illustrates one embodiment of a square mini microwave cavity  570  formed when an oven cavity portion  572  engages a rack cavity portion  574 . The internal perimeter of the square mini microwave cavity  570  is superior to the external perimeter of the cylindrical sample tube  524  so that the sample tube  524  does not engage the walls of the cavity  570 . 
       FIG. 44  illustrates one embodiment of a mini microwave cavity  580  formed by engaging an oven cavity portion  582  with a rack cavity portion  584 . The oven cavity portion comprises a U-Shaped microwave reflecting plate  586  having an internal groove and a protective element  588  positioned in the groove of the plate  586 . An antenna  590  having a shape matching that of the groove is inserted between the plate  586  and the protective element  588 . The antenna may be curved, vertical, or other. The antenna  590  is connected to a power generator via a microwave waveguide  592  and is used to emit microwaves in the cavity  580 . The rack cavity portion  584  comprises a U-shaped microwave reflecting plate  594  having a groove in which a protective element  596  may be inserted. 
     The protective elements  588  and  596  are made from a material transparent to microwaves such as Teflon for example, while the U-shaped plates  586  and  594  are made from a material capable of reflecting microwaves such as metal (aluminum, etc). 
     It should be understood that the mini microwave cavity may have any adequate height with respect to that of the sample tube to be received therein. For example, the height of the oven and rack cavity portions may be substantially equal to that of the sample tube. Alternatively, the height of the oven and rack cavity portion may be less than that of the sample tube. 
       FIG. 45  illustrates one embodiment of an automated digestion system provided with a control unit (not shown) and a closed loop conveyor for directing a plurality of sample holders  604  in a heating chamber  605 , a cooling station  606 , and a venting station  608 . Each sample holder  604  comprises at least one rack cavity portion  610  adapted to receive a hermetically closed or open sample tube  612 . The heating chamber  605  is provided with at least one movable oven cavity portion  614  connected to a microwave generator and adapted to form a mini microwave cavity when connected to a corresponding rack cavity portion  610 . In some embodiments, two or more rack cavity portions  610  are provided in the heating chamber  605 . 
     When a sample holder  604  enters the heating chamber  605 , a positioning device (not shown) precisely positions the sample holder  604  with respect to the position of the oven cavity portions  614 . Then, the oven cavity portions  614  are moved to their extended position in order to engage their respective rack cavity portion  610 , thereby forming a mini microwave cavity. 
     The sample contained in each sample tube  612  may be independently heated by applying sample specific parameters. The independent mini microwave cavities allow each individual sample to be heated to a sample specific temperature, for a sample specific amount of time. Therefore, each sample of a sample holder  604  containing a given number of samples may be different, and the sample specific parameters can be applied to each sample accordingly. Various heating programs may be created using a combination of heating and non-heating times and a plurality of heating temperatures. The sample holder  604  is maintained in the heating chamber  605  until the last sample has completed its heating program. 
     The sample-specific heating parameters may comprise a desired temperature, and/or a microwave power, and/or a duty cycle, and/or a heating time, and/or sample parameters such as an identification of the sample or the quantity of sample contained in the sample tube, and/or tube parameters such as the volume of the tube or the material of the tube, and/or the like. The automated digestion system  600  is adapted to identify a particular sample tube  612  and independently heat each sample tube  612  in accordance with the sample specific parameters. In one embodiment, the automated digestion system  600  is provided with a bar code reader and the sample parameters are retrieved by the control unit by reading the bar code of the sample container. 
     In one embodiment, the sample holder  604  is provided with an ID, such as a bar code or a RF ID for example, for each sample tube  612  and the automated digestion system  600  is provided with an ID reader adapted to read the sample tube ID. Alternatively, the sample ID may be located on the sample tube  612 . 
     In another embodiment, the rack is provided with an internal clock and the automated digestion system  600  is provided with a reader capable of identifying the sample holders  604  using the internal clock. One series of magnets are used to activate a sensor (the reader), and another series of magnets are used as the clock. The clock corresponds to an ID for the sample holder  604 . 
     Once the heating process is completed, the oven cavity portions are moved to their retracted position and the sample holder  604  is moved to the cooling station  606  to be cooled. Once cooled, the sample holder  604  is brought to the venting station  608  where an unsealing system unseals the sample tubes, thereby providing an auto-venting of the sample tubes. In one embodiment, moving the sample holder  604  from the cooling station  606  to the venting station  608  occurs when the samples in the sample tubes  612  at the cooling station  606  have reached a pre-determined temperature. 
       FIG. 46  illustrates one embodiment of mini microwave cavities  650  each comprising a temperature sensor  652  for sensing the temperature of a sample contained in a vessel or sample tube  654 . In this embodiment, the temperature sensors sit below each vessel underneath a floor of the heating chamber. A series of apertures are provided in the floor of the heating chamber to allow the temperature sensors to access the vessels. Individual temperature control of each sample in each vessel is provided. 
     Each mini cavity  650  is formed by a movable oven cavity portion  656  and a rack cavity portion  658 . An antenna  660  is connected to a microwave source  662  by a microwave waveguide  664 . For each mini cavity  650 , a proportional-integral-derivative (PID) controller  666  receives the sensed temperature from a temperature sensor  652 . In order to reach a desired sample temperature, the PID controller  666  adjusts the amount of microwave energy delivered by the microwave source  662  to the antenna  660  by controlling an adjustable high voltage current source  668  powering the microwave source  662 . Although  FIG. 46  illustrates a configuration comprising one microwave source per mini cavity, another embodiment may comprise one microwave source and a splitter for multiple cavities. 
       FIG. 47  illustrates one embodiment of a sample holder  670  comprising the rack cavity portions  658 . The sample holder  670  comprises a rack formed by a base plate  672  and a stud  674 , and a rack cover  676 . The rack cover comprises a cap-receiving plate  678  to which compression caps  680  are removably secured. The vessels  654  are received in the rack and sealed by a sealing cap  682 . Then the rack cover  676  is secured on top of the rack so that a compression cap  680  abuts against a corresponding sealing cap  682  for hermetically closing the vessels  654 . 
     In an alternative embodiment, open vessels are used that do not require the rack cover plate  676 . In this case, cap  682  may or may not be set on top of the vessel  654 . 
     While the sample holder  670  is provided with a rack cover  676  provided with a pressure-relief valve system, it should be understood that the vessels  654  may be closed by the sealing caps  682 . Alternatively, the vessels  654  may be left open during the heating process. 
     In one embodiment, each vessel  654  contains a sample  690  and a liquid solution  692  and is positioned into its respective mini cavity  650  so that the sample  690  and the solution  692  present an RF load matching that of the antenna  660 . This maximizes energy transfer to the solution  692  and minimizes energy reflection towards the microwave source  662 . 
       FIG. 48  illustrates one embodiment of a mini cavity assembly  700  in which the rack cavity portions are made of a single piece. The rack  702  comprises a cavity element  704  in which grooves  706  are made on opposite sides thereof. Each groove  706  corresponds to a rack cavity portion and a sample tube  708  is inserted into the grooves. Oven cavity portions  710  are regrouped into two rows and each row of oven cavity portions  710  is secured to a translation plate  712 . The translation plates  712  are activated by a motor (not shown) to engage the oven cavity portions  710  with the rack cavity element  704  to form the mini cavities. 
       FIG. 49  illustrates one embodiment of a rack  750  provided with rack cavity portions and a microwave cross-talk preventing device. The rack  750  comprises a base plate  752  to which a cavity plate  754  and a U-shaped plate  756  are secured. Recesses  758  are made in the cavity plate  754  to form six rack cavity portions on each side of the cavity plate  754 . Twelve openings  760  each aligned with a recess  758  and adapted to receive a sample tube  762  are made on top of the cavity plate  754 . The U-shaped plate  756  is provided with twelve tube receiving openings  764  each aligned with a respective opening  760 . The rack  750  further comprises twelve microwave reflecting cylinders  766  each secured on top of the cavity plate  754  and aligned with a respective opening  760  so that a sample tube  762  may be received in the openings  760  and  764  and the hollow cylinder  766 . The reflecting cylinders  766  serve as a microwave cavity extender to prevent cross talk and extend the microwave energy to samples that exceed the size (volume) of the microwave cavity. 
     The rack  750  is inserted into a heating chamber provided with oven cavity portions matching the rack cavity portions to form twelve mini microwave cavities. The cylinders  766  acts as a microwave barrier reducing or substantially preventing the propagation of microwaves from one mini cavity to another. It should be understood that the cylinders  766  are made from a microwave reflecting material such as metal or aluminum for example. 
     In one embodiment, an automated digestion system such as the system  400  is provided with a heating chamber comprising at least oven cavity portions and adapted to receive a rack comprising rack cavity portions. For example, the rack  750  may be used for heating samples in such an automated digestion system. 
     In one embodiment, the base plate  752  of the rack  750  is provided with a toothed groove  770  adapted to engage a gear having mesh teeth. The gear may be located in the heating chamber for precisely positioning the rack  750  in the heating chamber so that each rack cavity portion faces its respective oven cavity portion. The gear may also be used to bring the rack in the heating chamber and/or the cooling chamber. 
     In one embodiment, the base plate  752  is provided with at least four balls or low friction feet rotatably secured thereto for allowing the rack  750  to roll or slide on a substantially planar surface. In one embodiment, the front portion of the rack  750  must firstly enter in the heating chamber. In this case, only one side of the groove  770  is provided with teeth. This allows the gear not to engage with the groove if the rear portion of the rack is firstly presented to the gear. 
     In one embodiment, the base plate  752  is provided with twelve base plate openings each located beneath a corresponding sample tube  762  and the heating chamber is provided with twelve temperature sensors such as IR sensors. When the rack  750  enters the heating chamber and the mini cavities are formed, each temperature sensor is positioned below a respective base plate opening for measuring the temperature of a respective sample contained in the corresponding sample tube  762 . 
       FIG. 50  illustrates one embodiment of a reflected power measuring device  800  for measuring the power reflected by a microwave cavity  802 . The microwave cavity  802  is connected to a microwave generator  804  by a coaxial cable  806 . The measuring device  800  comprises a directional coupler having a first RF waveguide  808  coupled to a second RF waveguide  810 . The first and second RF waveguides  808  and  810  are spaced apart by a distance corresponding to a quarter of the wavelength of the RF signal propagating between the microwave generator  804  and the microwave cavity  802 . The distance could also be ¾ of the wavelength, 5/4 of the wavelength, etc. The coaxial cable  806  comprises a central core  814  and a shield  816  separated by a dielectric  818 . Two holes  820  and  822  are made in the shield  816  of the coaxial cable  806 . The two holes  820  and  822  are spaced apart by a distance corresponding to a quarter of the wavelength of the RF signal propagating between the microwave generator  804  and the microwave cavity  802 . The first end  824  of the first waveguide  808  is inserted into the first hole  820  while the first end  826  of the second waveguide  810  is inserted into the second hole  822 . 
     When an RF signal propagates from the microwave generator  804  to the microwave cavity  802 , a part of the signal propagating in the core  814  of the coaxial cable  806  leaks via the first and second holes  820  and  822  and is coupled to the first end  824  of the first waveguide  808  and to the first end  826  of the second waveguide  810 . Because the length of a third waveguide  812  is equal to the quarter of the wavelength of the RF signal, no signal propagates at the output  828  of the first waveguide. As illustrated in  FIG. 51 , at the tee junction in port  4 , the signal coming from hole  820  is split in two parts, one going to port  3  and another going up to the output  828 . At the tee junction in port  3 , the signal coming from hole  822  is split in two parts, one going to port  4  and another going up to the output  830 . The signal coupled at port  3  is going in the opposite direction and will be subtracted at port  4 . Because the separation between the ports  3  and  4  is equal to a quarter wavelength, the signal coming from port  4  and going towards the output  828  is cancelled. As a result only one signal exits the directional coupler  800  by the output  830 . The signal collected at the output  830  may be used for determining the power of the RF signal propagating from the microwave generator towards the microwave cavity  802 . 
       FIG. 51  illustrates the propagation of an RF signal reflected by the cavity  802  and propagating from the cavity  802  towards the microwave generator  804 . Following the same reasoning as for a signal propagating from the generator  804  towards the cavity  802 , no signal is propagated towards the output  830  while the signal exiting the coupler  800  at the output  828  may be used for determining the power of the signal reflected by the cavity  802 . 
     In one embodiment, the waveguides  808 ,  810 , and  812  are microstrip lines. In another embodiment, the waveguides  808 ,  810 , and  812  are striplines. 
     In one embodiment, because the coaxial cable  806  is part of the directional coupler, the cable  806  is not sliced in multiple sections to build a coupler and a high decoupling factor is obtained, thereby rendering the coupler  800  adequate for high power applications. 
     In one embodiment in which the RF signal propagation speed in the waveguides  808 ,  810 , and  812  and in the coaxial line  806  are different, the coupler  800  comprises a dielectric substrate on which the waveguides  808 ,  810 , and  812  are deposited and the dielectric constant of the substrate is chosen to render the RF signal propagation speed in the waveguides  808 ,  810 , and  812  substantially equal to that in the coaxial cable  806 . In the case where the propagation speed of a signal in a coaxial line is larger than in microstrip or stripline, the separation of holes  820  and  822  is equal to ¼ of wavelength in the coaxial cable and the length of line  812  is equal to ¾ wavelength. In this case, the coupled signal when the propagation is coming from generator  804  to cavity  802  will be at  828  and canceled at  830 , and vice versa for reflecting signals. 
     In one embodiment, the hole is sized so that the coupling factor between the coaxial cable  806  and the coupler  800  is about −55 dB or less. This configuration is suitable for high power applications. In other embodiments, the coupling factor could be other than −55 dB for low power applications. 
       FIG. 52  illustrates one embodiment of a coupler  850  comprising a detecting diode  852  used for measuring the power of a signal propagating in the first waveguide  854  and therefore determining the microwave power reflected by the cavity. The coupler  850  further comprises a second detecting diode  856  used for measuring the power of a signal propagating in the second waveguide  858  and therefore determining the microwave power generated by the microwave generator. 
     In one embodiment, the isolation between the ports is substantially equal to −10 dB. 
     In one embodiment, a matching on output ports is achieved in order to maintain a good isolation between the cavity  802  and port  3 , and the microwave generator  804  and port  4 . 
       FIG. 53  illustrates one embodiment of a coupler provided with a detector for measuring the reflected power and another detector for measuring the incident power. Each detector comprises a Schottky zero bias diode that transforms the microwave signal into a DC voltage. In one embodiment, the detector is linear in the square law range means below −5 dBm. In one embodiment in which the coupling factor is about −55 dB, the detector is substantially linear for input powers as large as 50 dBm means 100 Watts, and can detect powers up to 60 dBm means 1000 Watts. In one embodiment, an input resistor is used to match the circuit and the result is about −15 dB which is lower than the directivity of coupler. 
     In one embodiment, the detected reflected power is used for determining cavity problems such as a missing sample tube, the complete evaporation of the sample contained into the cavity, the absence of a sample into a sample tube, the explosion of a sample tube, and the like. Upon detection of a problem, the generation of microwaves may be stopped and an alarm may be triggered. 
     In one embodiment, the detected incident power may be used for detecting microwave source problems. 
     While the present description refers to digestion of samples, it should be understood that the methods, apparatuses, devices, and system described above may be used for extraction. 
     It should be noted that the embodiments described above are intended to be exemplary only. Solely the scope of the appended claims is limitative.