Abstract:
A method is disclosed for carrying out microwave-assisted chemical reactions. The method includes the steps of adding reagents to a pressure-resistant microwave-transparent vessel, securing the vessel against pressure release with a penetrable septum made of a material that can be penetrated by a needle while surrounding and sealing against the needle even after penetration to thereby maintaining the pressure integrity of the vessel, thereafter inserting a needle through the penetrable septum and into the pressure-secured vessel to provide fluid communication to the interior of the scaled pressure-resistant vessel through the needle, applying mechanical pressure against the septum while the needle is inserted therethrough, and irradiating the vessel and its contents with microwaves with the needle inserted therein.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of Ser. No. 09/773,846 filed Jan. 31, 2001. 
    
    
     BACKGROUND OF INVENTION 
     The present invention relates to microwave-assisted chemistry, and in particular relates to a microwave instrument that offers particular advantages useful for chemical synthesis reactions. 
     The present invention relates to devices and methods for microwave-assisted chemistry. As generally recognized in the chemical arts, many chemical reactions can be initiated or accelerated by increasing the temperature—i.e. heating—the reactants. Accordingly, carrying out chemical reactions at elevated (i.e., above ambient) temperatures is a normal part of many chemical processes. 
     For many types of chemical compositions, microwave energy provides an advantageous method of heating the composition. As is well recognized in the art, microwaves are generally categorized as having frequencies within the electromagnetic spectrum of between about 1 gigahertz and 1 terahertz, and corresponding wavelengths of between about 1 millimeter and 1 meter. Microwaves tend to react well with polar molecules and cause them to rotate. This in turn tends to heat the material under the influence of the microwaves. In many circumstances, microwave heating is quite advantageous because microwave radiation tends to interact immediately with substances that are microwave-responsive, thus raising the temperature very quickly. Other heating methods, including conduction or convection heating, are advantageous in certain circumstances, but generally require longer lead times to heat any given material. 
     In a similar manner, the cessation of application of microwaves causes an immediate corresponding cessation of the molecular movement that they cause. Thus, using microwave radiation to heat chemicals and compositions can offer significant advantages for initiating, controlling, and accelerating certain chemical and physical processes. 
     In recent years, much interest in the fields of chemical synthesis and analysis has focused upon the use, synthesis or analysis of relatively small samples. For example, in those techniques that are generally referred to as “combinatorial” chemistry, large numbers of small samples are handled (e.g., synthesized, reacted, analyzed, etc.) concurrently for the purpose of gathering large amounts of information about related compounds and compositions. Those compounds or compositions meeting certain threshold criteria can then be studied in more detail using more conventional techniques. 
     Handling small samples, however, tends to present difficulties in conventional microwave-assisted instruments. In particular, small masses of material are generally harder to successfully affect with microwaves than are larger masses. As known to those of ordinary skill in this art, the interaction of microwaves with responsive materials is referred to as “coupling.” Thus, stated differently, coupling is more difficult with smaller samples than with larger samples. 
     Furthermore, because of the nature of microwaves, specifically including their particular wavelengths and frequencies, their interaction with particular samples depends upon the cavity into which they are transmitted, as well as the size and type of the sample being heated. 
     Accordingly, in order to moderate or eliminate coupling problems, conventional microwave techniques tend to incorporate a given cavity size, a given frequency, and similarly sized samples. Such techniques are useful in many circumstances and have achieved wide acceptance and use. Nevertheless, in other circumstances when one of these parameters—sample size, material, microwave frequency—is desirably or necessarily changed, the cavity typically has to be re-tuned in order to provide the appropriate coupling with the differing loads. Stated somewhat differently, and by way of illustration rather than limitation, in a conventional device a one gram load would require tuning different from a ten gram load, and both of which would require different tuning from a hundred gram load, and all of which would differ if the microwave frequency or type of material is changed. 
     As another issue, differently-sized samples are generally most conveniently handled in reaction vessels that are proportionally sized based on the size of the sample. Many instruments for microwave-assisted chemistry, however, are—for logical reasons in most cases—made to handle vessels of a single size; e.g. instruments such as described in U.S. Pat. No. 5,320,804 or open vessels as described in U.S. Pat. No. 5,796,080. Thus although such instruments are valuable for certain purposes, the are generally less convenient, and in some cases quite ineffective for samples, vessels, and reaction other than a certain size (volume) or type. 
     As yet another issue, many reactions proceed more favorably under increased (i.e. above atmospheric) pressure. Controlling and using increased pressures for small samples in microwave-assisted chemistry can, for the reasons stated above and others, be somewhat difficult. 
     Accordingly, the need exists for new and improved instruments for microwave assisted chemistry that can handle small samples, can conveniently handle a variety of sample sizes and vessel sizes and that can incorporate and handle higher pressure reactions when desired or necessary. 
     SUMMARY OF INVENTION 
     Therefore, it is an object of the invention to provide a microwave instrument suitable for chemical synthesis and related reaction and that can handle small samples, can conveniently handle a variety of sample sizes and vessel sizes and that can incorporate and handle higher pressure reactions when desired or necessary. 
     The invention meets this object with an instrument for microwave-assisted chemical processes that avoids tuning discrepancies that otherwise result based upon the materials being heated. The instrument comprises a source of microwave radiation a waveguide in communication with the source, with at least a portion of the waveguide forming a cylindrical arc, a cylindrical cavity immediately surrounded by the cylindrical arc portions of the waveguide, and at least 3 slotted openings in the circumference of the circular waveguide that provide microwave communication between the waveguide and the cavity. 
     In another aspect the invention is a method of conducting organic synthesis reactions comprising applying microwave radiation to a sample using a frequency to which the sample (solvent, etc) will thermally respond, and optimizing the coupling between the applied microwaves and the (load) sample without adjusting the physical dimensions of the cavity, without physical movement of the cavity (i.e. no tuning screws), without physical movement of the position of the sample and without adjusting the frequency of the applied microwaves as the sample heats and as the reaction proceeds. 
     In another aspect, the invention is a pressure-measuring vessel system for microwave assisted chemical processes. In this aspect, the invention comprises a pressure resistant vessel (i.e., it resists the expected pressure to which it is expected to be exposed) that is otherwise transparent to microwave radiation, a pressure-resistant closure for the mouth of the vessel, with portions of the closure including a pressure resistant synthetic membrane, a pressure transducer external to the vessel, and a tube extending from the transducer, through the membrane and into the vessel for permitting the pressure inside the vessel to be applied against the transducer while the closure and membrane otherwise maintain the pressure resistant characteristics of the vessel. 
     In another aspect, the invention is an instrument for microwave-assisted chemical processes that provides greater flexibility in carrying out microwave-assisted chemistry under varying conditions. In this aspect, the instrument comprises a source of microwave radiation, a cavity in communication with the source, with the cavity including at least one wall formed of two engaged portions that form a barrier to the transmission of microwaves when so engaged, with the engaged portions being disengagable from one another; and with one of the portions further including a microwave-attenuating opening for receiving a reaction vessel therethrough and into the cavity when the portions are engaged. 
     In yet another aspect, the invention is a method of increasing the efficiency of microwave-assisted chemical reactions. The method comprises carrying out a first chemical reaction in a reaction vessel in an attenuated cavity of a microwave instrument, removing the reaction vessel and the attenuator from the instrument, placing a different reaction vessel and a differently-sized attenuator in the same cavity of the instrument, and carrying out a second chemical reaction in the different vessel in the cavity of the instrument. 
     The foregoing and other objects and advantages of the invention and the manner in which the same are accomplished will become clearer based on the followed detailed description taken in conjunction with the accompanying drawings in which: 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a front perspective view of an instrument according to the present invention; 
     FIG. 2 is a rear perspective view of the instrument illustrated in Figure one; 
     FIG. 3 is a partially exploded interior view of the instrument illustrated in FIGS. 1 and 2; 
     FIG. 4 is a perspective view of a cavity and wave-guide according to the present invention; 
     FIG. 5 is an interior view of the waveguide and cavity illustrated in FIG.  4 . 
     FIG. 6 is a perspective exterior view of the wage guide, cavity and magnetron of the present invention; 
     FIG. 7 is a perspective view of the pressure-measuring assembly according to the present invention; 
     FIG. 8 is another perspective view of the pressure-measuring assembly; 
     FIG. 9 is a detailed exploded view of the pressure measuring assembly; 
     FIG. 10 is an exploded view of the cavity assembly of an instrument according to the present invention; 
     FIG. 11 is a cross-sectional view of a reaction vessel, pressure-measuring means and collet assembly of an instrument according to the present invention; 
     FIG. 12 is a cross sectional view of the cavity portion of the instrument according to the invention and including an exemplary reaction vessel; and 
     FIG. 13 is a cross-sectional view almost identical to FIG. 12, but illustrating the features of the invention in relation to a differently-sized reaction vessel. 
     FIG. 14 is a perspective view of the reaction vessel and 
     FIG. 15 is a cross-sectional view of the reaction vessel. 
    
    
     DETAILED DESCRIPTION 
     21An embodiment of the present invention is illustrated in perspective view in FIG. 1 with the instrument broadly designated at  20 . Most of the other details of the invention will be shown in other drawings, but FIG. 1 illustrates that the instrument  20  includes a housing  21 , a control panel  22 , and a display  23 . As will be discussed later herein, the control panel  22  can be used to provide the instrument with a variety of information that may relate to the chemical processes being carried out, or to set or define certain parameters, such as maximum pressure or temperature during the application of microwave energy to a particular reaction. The control panel  22  can be formed of any type of appropriate input devices, with buttons  24  being illustrated. It will be understood, however, that other types of input devices, including touch screens, keyboards, a computer “mouse” or other input connections from computers or personal digital assistants can also be used in any appropriate fashion known to those of skill in this art that does not otherwise interfere with the operation of the instrument. Similarly, the display  23  is most commonly formed of a controlled or addressable set of liquid crystal displays (LCDs) but can also comprise a cathode ray tube (CRT), light emitting diodes (LEDs), or any other appropriate display medium. 
     The housing  21  includes a removable upper portion  25 , attached by appropriate fasteners  26 (screws or Allen nuts are exemplary) to a lower housing portion  27  and a pedestal portion  30 , which in turn are supported by the pedestal feet  31 . 
     FIG. 1 also illustrates that the housing  21  includes an opening  32 , which provides access to the microwave cavity in a manner that will be described with respect to other drawings. As FIG. 1 illustrates, the opening  32  provides much easier access for placing samples into the cavity than in many other types of microwave instruments. 
     FIG. 1 also illustrates the sample holder and microwave attenuator assembly  33 , and a collet assembly  91  which will likewise be described in more detail with respect to other of the drawings. 
     FIG. 2 is a rear perspective view of an instrument according to the present invention that illustrates some additional items. As in FIG. 1, FIG. 2 illustrates the upper housing portion  25 , the lower housing portion  27 , the fasteners  26 , the pedestal portion  30 , the feet  31 , the sample holder and attenuator assembly  33  and the opening  32  in the housing  25  that provides access to the cavity. 
     Additionally, FIG. 2 illustrates that the device includes at least one cooling fan  34  with a second being shown at  35 . The fans  34  and  35  serve to cool the electronics and the magnetron portions of the device, as well as helping to keep the cavity from becoming overheated in the presence of ongoing chemical reactions. Other than having the capacity to appropriately cool the instrument and the cavity, the nature or selection of the fans can be left to the individual discretion of those with skill in this art. 
     FIG. 2 also shows the power switch  36  and the power cord inlet  37 . In order to take advantage of the full capacity of the instrument, in preferred embodiments, the instrument includes the parallel port  41  and the serial port  40  for receiving input from or providing output to other electronic devices, particularly microprocessor based devices, such as personal computers, personal digital assistants or other appropriate devices. Similarly, FIG. 2 illustrates a connector  42  for the pressure transducer to be described later herein. 
     FIG. 3 is a partially exploded view of the interior of an instrument  20  according to the present invention. In common with FIGS. 1 and 2, the lower portion  27  of the housing and the pedestal portion  30  of the housing are both illustrated along with the pedestal feet  31 . FIG. 3 also illustrates several of the fasteners  26 , as well as the fan  34  along with its housing  42 . 
     FIG. 3 shows the display  23  in exploded fashion along with a first electronics board  43  and a second electronics board  44 . Basically, the electronics carried by the boards  43  and  44  are generally well understood in their nature and operation. With respect to the instrument of the present device, the electronics first control the power from a given source, usually a wall outlet carrying standard current. The electronics also control the operation of the device in terms of turning the magnetron on or off, and in processing information received from the ongoing chemical reaction, in particular temperature and pressure. In turn, the appropriate processor is used to control the application of microwaves, including starting them, stopping them, or moderating them, in response to the pressure and temperature information received from the sensors described later herein. The use of processors and related electronic circuits to control instruments based on selected measured parameters (e.g. temperature and pressure) is generally well understood in this and related arts. Exemplary (but not limiting) discussions include Dorf, The Electrical Engineering Handbook, Second Ed. (1997) CRC Press LLC. 
     In the embodiment illustrated in FIG. 3, the outer housing of the cavity is visible at  45 , along with the housing portions of the microwave source, illustrated as the magnetron  46 . FIG. 3 also illustrates the sample holder and attenuator assembly  33 , and a motor  47  for stirring reactants in a manner described later herein. FIG. 3 also illustrates the housing  50  for the second fan  35  present in the illustrated embodiment. Because the sample vessel (not shown) and the sample holder and attenuator assembly  33  are generally quite different in size than the cavity itself, FIG. 3 illustrates that the attenuator  33  according to the present invention further includes an upper rim  51  into which lower portions of the sample holder and attenuator assembly  33  can rest in a changeable receiving fashion. The features, advantages and details of the attenuator  33  are discussed in more detail with respect to FIGS. 11,  12 , and  13 . The attenuator  33  is in turn held in place by a pair of retaining rings  52  and  53  into which the attenuator  33  is received and which is also held in place by the interlock assembly broadly designated at  54 . 
     FIGS. 4 and 5 illustrate aspects of the waveguide and cavity portions of the instrument according to the present invention. In these illustrations, the waveguide is broadly designated at  55 , and includes both a parallelpiped rectangular portion  56 , and a cylindrical portion  57  that in preferred embodiments has a rectangular cross section. In the illustrated embodiment, the waveguide  55  is supported on a series of legs  60  which serve to position the cavity  61  and waveguide  55  in communication the magnetron  46  and the other elements within the particular housing  21 . One of the legs, designated at  96 , has a slightly different structure to support the motor  47  (not shown). It will be understood, of course, that such features as the leg  60  which merely positions the waveguide within a particular embodiment are not limiting of the present invention. In preferred embodiments the rectangular or parallelpiped portion  56  of the waveguide joins the cylindrical portion  57  perpendicularly to a tangent defined by the circumference of the cylindrical waveguide portion  57 . 
     FIGS. 4 and 5 also illustrate the cavity as broadly designated at  61 . In particular, the cavity is formed by an inner cylindrical wall  62  that forms a concentric cylinder inwardly of the cylindrical cavity housing  45 . An upper waveguide plate  63  and a lower waveguide plate  64  define the limits of the waveguide  55  in both its rectangular portion  56  and its cylindrical portion  57 . The waveguide  55  is constructed of a material that reflects microwaves inwardly and prevents them from escaping in any undesired manner. Typically, such material is an appropriate metal which, other than its function for confining microwaves, can be selected on the basis of its cost, strength, formability, corrosion resistance, or any other desired or appropriate criteria. In preferred embodiments of the invention, the metal portions of the waveguide and cavity are formed of stainless steel. 
     The top plate  63  (as well as the bottom plate  64 ) is also held in place by a series of connectors  65  which can be rivets, screws or nuts, provided that their size and shape avoids undesired interference with the microwaves in the cylindrical or other portions of the waveguide  55 . 
     Perhaps most importantly, FIG. 4 illustrates that a plurality of slotted openings  66  are present in the inner cavity wall  62  for facilitating the transmission of microwaves from the waveguide  55  into the cavity  61 . It will be understood that because the inner wall  62  defines the border of the waveguide  55  and the cavity  61 , the slotted openings  66  can also be described as being in the inner circumference of the cylindrical portion  57  of the waveguide. 
     herehereIn particular, it has been discovered in accordance with the present invention that a plurality of such slots in a circular orientation in a static structure in the cavity  61  provides an appropriate amount of coupling with a wide variety of sample sizes or types that may be present in the cavity. Although the inventors do not wish to be bound by any particular theory, it appears that the plurality of slots  66 , permit a variety of microwave patterns (modes) to be established in the cavity  61 , depending upon the load to which the microwaves are coupled. The cavity includes at least three slots, preferably at least five, and in the presently most preferred embodiment includes seven slots spaced at least about 40 degrees from each other. Preferably, the slots  66  are oriented parallel to the axis of the cavity  61 . 
     As other details, FIG. 4 illustrates a connector plate  67  and connecting pins  70  are at one end of the waveguide  55  for connecting the waveguide  55  to the magnetron  46  or other microwave source, which can, depending upon choice and circumstances, also comprise klystron, a solid state device, or any other appropriate device that produces the desired or necessary frequencies of electromagnetic radiation within the microwave range. FIG. 4 also shows a gas inlet fitting  58  that is part of a system for cooling the cavity that is discussed in more detail with respect to FIGS. 10,  12  and  13 . 
     As some additional details, in the preferred embodiments, the cylindrical waveguide completes an arc of more than 180°, and preferably between 270° and 360°, and the cylindrical cavity  61  completes a full 360°. 
     FIG. 5 shows the same details as FIG. 4, but in a broken line interior view. Accordingly, FIG. 5 likewise illustrates the overall structure of the waveguide  55 , its rectangular and cylindrical portions  56  and  57  respectively, the cavity  61 , the slots  66  in the inner wall  62 , and the supporting legs  60 . FIG. 5 also illustrates that the fasteners  65  have a relatively low profile within the waveguide  55  to avoid interfering with microwave propagation therethrough. 
     In particular detail, FIG. 5 shows that the waveguide  55  is connected to the magnetron  46  (not shown) through the launching opening  71  in the plate  67 . The microwaves can then propagate through the rectangular portion of the waveguide  56  into the circular portion  57  of the waveguide  55 . The structure also includes two walls  72  and  73  that are positioned in the cylindrical portion  57  of the waveguide just adjacent one of the places where it intersects with the rectangular portion  56 . Accordingly, to the extent that standing waves or modes are in the waveguide  55  and cavity  61 , they will be confined to the illustrated geometry by the reflecting wall  73 . In the absence of the walls  72  or  73 , the modes in the waveguide and the cavity  61  would be quite different because they would interact through a full 360° of the waveguide housing rather than in the somewhat lesser portion than they do in the illustrated embodiment. 
     FIG. 5 also shows that in the preferred embodiment of the present invention there are seven slots  66  in the inner cavity wall  62 , with each of the slots being at least about 40 degrees apart from each of the next adjacent slots. Furthermore, none of the slots  66  are directly at the end of the rectangular portion  56  of the waveguide  55  so that the modes that set themselves up in the waveguide  55  and cavity  61  must enter the cavity  61  after having entered at least a portion of the cylindrical portion  57  of the waveguide  55 . 
     FIG. 5 also illustrates that in preferred embodiments, the cavity floor  74  includes a plurality of small openings  75  for ventilation and fluid drainage purposes, with ventilation being expected and liquid drainage being less frequent, typically in the case of spills. FIG. 5 also illustrates a circular shaft  76  that depends from the floor  74  of the cavity  61  for permitting optical access to the cavity in a manner that will be described later herein. 
     Alternatively, FIG. 5 also illustrates the optional use of a cavity liner  59  for containing spills, splashes or other incidents in the cavity  61 . The cavity liner  59  optionally includes a small opening  68  to facilitate optical temperature measurement through the opening  76  in the cavity floor  74  and the window  69 . If the cavity liner  59  is formed of a material that is transparent to the optical measurement (typically IR-transparent for IR temperature measurements), the window  69  may be unnecessary. The liner  59  is preferably formed of a chemically-resistant polymer, and can (depending on the user”s cost and benefits) provide a disposable alternative to physically cleaning reagents or by-products from the cavity  61 . 
     FIG. 5 also illustrates the dielectric insert  95  that is described in more detail with respect to FIG.  10 . 
     FIG. 6 is a complementary view of a number of the elements of the invention and illustrates the cavity  61  from the perspective of its housing  45  in conjunction with the rectangular portion  56  of the waveguide  55  and the magnetron  46 . In particular, FIG. 6 offers a larger view of the retaining rings  52  and  53  along with the removable attenuator  33 . The attenuator  33  includes an axial opening that will be described in more detail with respect to FIGS. 12 and 13. As described with respect to FIG. 3, the retaining rings and the attenuator  33  are held in place by the interlock assemblies  54 . One of the particular advantages of the invention is that with the use of the retaining rings  52  and  53 , along with the interlock assembly  54  to retain the attenuator  33  in place, the interlock assembly  54  can be relatively easily released, and the attenuator  33  replaced with one that contains a different sized opening that in turn supports a different size reaction vessel while still preventing microwaves from propagating past the attenuator  33 . 
     Thus, the retaining rings  52  and  53 , along with the engaged attenuator  33  form the upper horizontal wall of the cavity and a barrier to the transmission of microwaves when so engaged. The retaining rings  52  and  53  are fixed to the cavity (i.e., removable only by disassembling the instrument with tools), while the attenuator  33  is easily removable from the rings  52  and  53  with a simple turning and lifting movement. The removable attenuator  33  includes the microwave attenuating opening  118  (FIGS. 12 and 13) for receiving a reaction vessel therethrough, and into the cavity  61 . It will thus be understood that in preferred embodiments, the instrument comprises two or more of the removable and engagable attenuators  33  that have differently-sized (from one another) microwave-attenuating openings for receiving differently-sized reaction vessels. 
     FIGS. 7,  8 , and  9  illustrate detailed aspects of the pressure measuring means of the instrument including the transducer assembly  38 . FIG. 7 shows the assembly  38  in assembled fashion with a series of retaining screws  82 , a collet adjustment slot  83 , and a collet tension screw  84  all of which are perhaps best understood with respect to FIG.  9 . 
     FIG. 8 shows the backshell of the assembly  38 , apart from the collet housing  86  which includes the retaining screws  82  that are also illustrated in FIG. 7. A pressure transducer  116  is positioned inside a transducer holder  123  which in turn is surrounded by the adjustable collet assembly  91 , the details of which are best illustrated in FIG.  9 . 
     FIG. 9 is an exploded view of the transducer assembly  38 . As in FIGS. 7 and 8, the collet backshell is illustrated at  85 , and the collet housing at  86 . The setscrews  82  illustrated in FIGS. 7 and 8 are also illustrated in FIG.  9 . 
     FIG. 9 is perhaps best understood with respect to its relation to a vessel (not shown in FIG. 9) that is in the cavity  61  undergoing a microwave-assisted chemical reaction. Such a vessel, and its cap, are schematically illustrated in somewhat more detail in FIG. 11, but for the purposes of FIG. 9, it will be understood that a vessel would be positioned under and in engagement with a vessel receptor  106  that is illustrated in FIG.  9 . In order to engage the entire transducer assembly  38 , and in turn the pressure measuring transducer, with a vessel, the transducer assembly  38  forms an adjustable device that can move in linear relationship to its own housing  86 , and with respect to a vessel in the cavity. Accordingly, and in order to accomplish this, FIG. 9 shows that the transducer assembly  38  includes a plurality (four are preferred) of collet leaves  107 . The leaves  107  are held in flexible relationship to the collet trunk  110  by the garter spring  111 . Among other features, the collet trunk  110  includes a plurality of pins  112 . As a result, when the leaves  107  are attached to the collet trunk  110  by the garter spring  111 , the leaves  107  can flex inwardly and outwardly with respect to the overall axis of the assembly  38 . Each leaf  107  further includes a gripping edge  113  that engages a cap on a vessel in a manner that is illustrated in FIG.  11 . FIG. 9 also shows that the retaining screws  84  are received into the threaded bolts  114 . In use, the threaded bolts  114  are received into the openings  119  in the collet trunk  110  and the screws  84  are received into the threaded bolts  114 . The screws  84  can move parallel to the axis of the assembly  38  in the collet adjustment slots  83  that are also illustrated in FIGS. 7 and 8. The two-part nature of the screws  84  and  114  permit the collet  86  housing and the collet leaves  107  to be tightened in place in an appropriate relationship to a vessel as may be desired or necessary in given circumstances. 
     The present invention measures the pressure inside of a vessel by transmitting the pressure through a needle that extends through a septum and into the vessel to the transducer  116  that converts the pressure into an appropriate electrical signal for the processor or the display. FIG. 9 also illustrates these features in more detail as does FIG.  11 . First, the needle  115  extends into the reaction vessel  105  (FIG.  11 ). In turn, the needle  115  transmits the pressure, in the well-understood fashion of fluid mechanics, to the transducer  116 . In turn, the transducer  116  transmits its signals through the wires  117 . In a typical arrangement (and although not specifically illustrated in FIG.  9 ), the transducer  116  includes four wires: power and its ground, and signal and its ground. 
     The other elements in the left-hand portion of FIG. 9 help maintain the transducer  116  and the needle  115  in proper relationship with each other and with the vessel. Thus, FIG. 9 shows a needle holder  120 , which is fixed on the collet adjustment housing  86  using the screws  121  which are respectively received in the screw holes  122  in the housing  86 . The transducer  116  is received in a transducer holder  123  that also encloses a needle receptor  124  that receives the upper (cap) portion  125  of the needle  115 . The transducer  116  includes a small bushing  126  that receives the needle receptor  124 , with the O-ring  127  providing an additional pressure seal. The A clip ring  130  helps hold these elements together in the transducer holder  123 . FIG. 9 thus illustrates that when the collet assembly and transducer assembly are properly assembled, the needle  115  passes axially through the needle holder  120 , the housing  86 , the collet trunk  110 , and the vessel receptor  106 , and into the vessel itself, thus permitting the transducer to read the pressure in the vessel as desired. 
     FIG. 10 illustrates additional features of the instrument of the present invention in exploded fashion. A number of the elements illustrated in FIG. 10 have already been described with respect to the other figures. These include the magnetron  46 , the rectangular portion  56  of the waveguide  55 , the circular portion  57 , the retaining rings  52  and  53 , and the interlock assembly  54 . FIG. 10 illustrates the attenuator in a resting, but not fully engaged position with respect to the retaining ring  52 . A polymer bushing  51  is positioned between the retaining rings  52  and  53  and helps provide a better physical and microwave seal for the cavity  45 . 
     FIG. 10 also illustrates a dielectric insert  95  that fits in the cavity  61  immediately adjacent the inner wall  62  of the cavity  61 . The dielectric insert  95  at least two purposes: first, the dielectric insert  95  is preferably formed from a chemically inert material to help protect the interior of the cavity  61  from reagents. Preferred materials include polymeric fluorinated hydrocarbons such as polytetrafluoroethylene (PTFE). 
     Second, the insert  95  forms a portion of a preferred system for cooling the interior of the cavity  61  during or after chemical reactions have been carried out therein and in response to the elevated temperatures generated by the reactions. In particular, in preferred embodiments, the waveguide  55  includes a gas inlet fitting ( 58  in FIGS. 4 and 6) through which a cooling gas can be circulated into and throughout the waveguide. In order to take advantage of this, the insert  95  includes the circumferential channel  98  through which the cooling gas can flow. A series of small, radially-oriented openings (too small to be illustrated in the scale of FIG. 10) permit the gas to flow into the center of the cavity  61  and cool it and any vessels and reagents inside. Although the insert  95  changes the tuning characteristics of the cavity, the tuning can be adjusted as desired to compensate for the insert  95 . Such tuning is familiar to those of ordinary skill in this art and can be carried out without undue experimentation. 
     FIG. 10 also illustrates the stirring mechanics of the instrument of the present invention. As illustrated therein, the stirrer motor  47  is positioned on a motor platform leg  96  from which it drives a pulley  97 . In turn, the drive pulley  97  drives a belt  100  to thereby drive the driven pulley  101 . The driven pulley  101  contains one or two magnets  102 , which, because of their position on the driven pulley  101 , orbit the center of the bottom floor  64  of the cavity  61 . When a magnetic stirrer bar is placed in a vessel in the cavity  61  and the motor  47  drives the pulleys  97  and  101 , the motion of the magnets  102  will in turn drive the stirrer bar in the reaction vessel. 
     FIG. 10 also illustrates a liquid drain  103 . The liquid (fluid) drain  103  works in conjunction with the floor openings  75  that are best illustrated in FIG. 5 to allow any fluids that may collect in the cavity  61  to drain through the openings  75  and then through the drain  103  to a collection point (not shown) which in a presently preferred embodiment comprises a small removable trough located at the floor of the instrument  20 . 
     FIG. 10 further illustrates means for measuring the temperature of items (vessels and reagents) in the cavity, shown as the temperature measuring device  104 , which is positioned immediately below and coaxially with the depending shaft  76  (FIG. 5) to thus have an optically clear view of the interior of the cavity  61 . Accordingly, when the temperature measuring device is an optical device, with an infrared sensor being preferred, it can accurately measure the temperature of vessels or contents of vessels within the cavity and provide the appropriate feedback to the processor of the instrument. As known to those familiar with such measurements, the infrared sensor  104  must be appropriately positioned and focused to record the proper temperature of the intended objects, but doing so is generally well understood by those of skill in this art and will not be otherwise described in detail. Indeed, particular and appropriate adjustments can be made on an instrument-by-instrument basis without undue experimentation. 
     In preferred embodiments, the temperature measuring device  104  is an infrared sensor, of which appropriate types and sources are well known by those of skill in this art. Additionally, and although not illustrated in detail in FIG. 10, the driver pulley  101  also carries an infra-red transparent window through which the sensor  104  can read the infrared transmissions from the cavity  61 . In preferred embodiments, the window is formed of an amorphous composition of germanium (Ge), arsenic (As) and selenium (Se), which provides the greatest accuracy, but at a relatively high cost. Thus, in other embodiments the window can be formed of infrared-transparent polymers such as polytetrafluoroethylene (PTFE) or polypropylene which provide accurate transmission at a generally lower cost. 
     With respect to both pressure and temperature measurement, and the processors referred to earlier, the instrument includes the capability for moderating the application of microwave power in response to the measured temperature or pressure. The method of moderating can be selected from among several methods or apparatus. A simple well-understood technique is to carry out a simple “on-off” cycle or series of cycles (i.e., a duty cycle). Another technique can incorporate a variable or “switching” power supply such as disclosed in commonly assigned U.S. Pat. No. 6,084,226; or techniques and devices that physically adjust the transmission of microwaves, such as disclosed in commonly assigned U.S. Pat. Nos. 5,796,080 and 5,840,583. 
     FIG. 11 is a cross-sectional view of the relationship between the removable attenuator  33 , a reaction vessel  105 , and the collet assembly  91 . In a broad sense, FIG. 11 illustrates the relationship between the pressure transducer  116 , the needle  115 , and the closure for the vessel, which is formed of the deformable metal portion  133  and the septum  134 . The relationship is such that the collet assembly  91  urges the transducer  116  and needle  115  towards the vessel  105  while concurrently bearing against the septum  134  and while urging the vessel and collet towards one another to provide the appropriate pressure seal. 
     By urging the various elements together in such fashion, the invention prevents the puncturable septum from becoming a weak point in the pressure integrity of the vessel  105  and the transducer  116 . As well recognized in this art, many chemical reactions will generate gases and in a closed system these generated gases will cause a corresponding increase in gas pressure. 
     Many of the items illustrated in FIG. 11 are also illustrated in FIG. 9 and, thus, corresponding numerals will be used in each case. In more detail, the vessel  105  rests in the central opening  118  defined by the removable attenuator  33 . As illustrated in FIG. 11, the vessel  105  includes an annular lip portion  109  that rests upon the inner opening  118 . In order to maintain the vessel in place while measuring the temperature, the leaves  107  of the collet assembly are brought to bear against the removable attenuator  33  and, because of the threaded relationships between the vessel receptor  106 , the collet trunk  110 , and the collet housing  86 , the collet can be brought to an appropriate position and tightened there to maintain the leaves  107  in forced contact against the removable attenuator, while at the same time urging the vessel receptor  106  downwardly against the vessel  105 . In turn, the position of the collet trunk  110  with respect to the collet housing  86  can be adjusted using the collet adjustment slot  83  and the threaded nut and bolt portions  84  and  114 . 
     Accordingly, FIG. 11 shows that when the vessel is in place in the removable attenuator  33 , the collet assembly  91  can clamp it in place and at the same time maintain an appropriate pressure against the septum  134 , while at the same time seating the needle  115  and its upper needle portion (cap) against the transducer in a manner which permits the pressure to be accurately measured, while at the same time maintaining the integrity of the vessel and preventing it from becoming dislodged when gases generated by the reaction increase the pressure in the vessel  105 . 
     FIG. 11 illustrates that the reaction vessel  105  includes a closure shown as the cap assembly  132 . The cap assembly  132  is, in preferred embodiments, formed of a deformable metal ring  133  and a penetrable septum  134 . The septum  134  is made of a material, preferably an appropriate polymer or silicone related material, that can be penetrated by the needle  115 , but which will surround and seal against the needle  115  even after penetration, thus maintaining the pressure integrity of the vessel  105 . The ring  132  is formed of a metal thick enough to have appropriate pressure resistant properties, but which can be deformed relatively easily, preferably with an ordinary clamping tool, to engage the lip portions  135  of the reaction vessel  105  and thereby seal the vessel. With the vessel so sealed by the cap assembly  132 , the leaves  107  of the collet assembly  91 , are brought into engagement with the attenuator  33  and the vessel  105 , with the ledges or gripping edges  113  engaging the attenuator  33  in a horizontal fashion and the cap assembly  132  in a vertical fashion to help maintain the sealed integrity of the entire assembly when in use. 
     In this fashion, the needle  115  extends from the transducer, through the cap  132  and into the vessel  105  to provide pressure communication between the interior of the vessel  105  and the transducer  116 . The collet assembly  91  engages the transducer, the needle  115 , the cap  132  and the vessel  105  in linear relationship so that the pressure in the vessel  105  is transmitted to the transducer  116  while the vessel is in use (i.e., a reaction taking place while microwaves are being applied). 
     FIGS. 12 and 13 illustrate some of the additional advantages of the removable attenuator system of the present invention. Many of the items illustrated in FIGS. 12 and 13 have also been previously described with respect to the other Figures, and in such cases the same reference numerals will again refer to the same items. Both FIG.  12  and FIG. 13 are cross-sectional views with FIG. 12 being taken directly through the center of the cavity  45  and FIG. 13 being taken from a point at which an entire vessel is illustrated. 
     FIG. 12 shows the cavity housing  45 , the inner cavity wall  62 , the dielectric insert  95 , and the removable attenuator  33 . As illustrated in FIGS. 12 and 13, in the preferred embodiments of the invention the removable attenuator  33 , which comprises the second portion of the two engaged portions that together form the upper horizontal wall of the cavity (the other being retaining ring  52 ), the attenuator  33  comprises an outer cylindrical wall  39  and an inner cylindrical wall  49 , the inner and outer walls being separated by and perpendicular to an annular floor  48 . The inner wall  49  thus provides a receptacle for receiving the vessel  105  therein, and likewise provides the attenuating function required to prevent microwaves generated by the source and propagated into the cavity from propagating outside the cavity when the vessel  105  is in place. 
     FIG. 13 is almost identical to FIG. 12 with the exception that the first attenuator  33  has been replaced a second attenuator  33 ′ and the vessel  105  has been replaced with the round bottom flask  105 ′ illustrated in FIG.  13 . It will be immediately seen that the removable attenuators  33  and  33 ′ provide a quick and easy method of exchanging reaction vessels without otherwise changing the size, capability, function or operation of the overall instrument  20 . Thus, for a larger vessel such as  105 ′ illustrated in FIG. 13, the outer wall  39  of the attenuator  33 ′ is essentially the same as the outer wall  39  of the attenuator  33  in FIG.  12 . The inner cylindrical wall  49 ′, however, is somewhat taller (in the orientation of FIG.  13 ), defines a larger diameter opening and provides for an attenuating function even though the flask  105 ′ is larger than flask  105 . By way of brief comparison, prior devices (e.g., U.S. Pat. No. 5,796,080) have attempted to customize the attenuator in a permanent sense for one particular sized vessel. Accordingly, an instrument that was capable of handling a somewhat smaller vessel such as  105  illustrated in FIG. 12 could not handle the larger vessel  105 ′ illustrated in FIG.  13 . Furthermore, because the attenuator had to be sized to accommodate the largest possible reaction vessels being used, the attenuator had to be permanently large, rather than just large enough for the particular vessel being used. 
     As one further advantage of the removable attenuators  33  and  33 ′, in prior devices the diameter of the attenuator opening was kept large enough to receive the largest portion of the vessel. With respect to FIG. 13, this required the opening to be large enough to receive the bulb portion of the round bottom flash  105 ′. In turn, a larger diameter opening requires a taller (longer) attenuator to prevent microwaves from propagating beyond the attenuator. 
     In contrast, and as FIG. 13 illustrates, in the present invention, the attenuator need only be large enough to accommodate the nearby portions of the vessel  105 ′ rather than the largest portions thereof. It will thus be understood as a further advantage that in some circumstances (e.g., FIG. 12) the attenuator  33  is put in place first, after which the vessel  105  is placed in the attenuator  33  and the cavity  61 . In other circumstances (e.g., FIG.  13 ), the vessel  105 ′ is placed in the cavity  61  first, after which the attenuator  33 ′ is put into position. 
     Accordingly, in another aspect the invention comprises a method of carrying out chemical reactions using microwave assisted chemistry by carrying out a first reaction in a first vessel of a particular size; removing the vessel and the attenuator  33  from the cavity; replacing the vessel with a new, differently sized vessel, and then replacing the attenuator with a new differently sized attenuator that nevertheless fits into the same opening. 
     FIGS. 14 and 15 illustrate some details of the reaction vessel  105 . FIG. 14 is perspective view of the reaction vessel  105  alone, and illustrates that in certain (but not all) embodiments, it superficially represents a test tube in its cylindrical shape. As illustrated by the vessel  105 ′ in FIG. 13, the reaction vessel can be one of any number of shapes and types while still incorporating the pressure-resistant aspects of the invention. FIG. 14 also illustrates the deformable metal portion  133  of the cap, along with an opening for the septum  134  (not shown) through which the needle  115  (not shown) can penetrate in a manner described with respect to the other drawings. 
     As stated previously, the vessel  105  is preferably pressure resistant; i.e., it can withstand pressures above atmospheric. This capability enables reactions to be out at elevated pressures, which can offer certain advantages in some circumstances. For example, particular reaction mechanisms can change in a favorable manner at above-ambient pressures, and in other circumstances, more efficient or even different (and better) mechanisms will take place at above ambient pressures. Additionally, under most circumstances, an increased pressure will produce or maintain an increased temperature, in accordance with the ideal gas law and its several related expressions. In turn, higher temperatures generally favorably initiate or accelerate most chemical reactions. 
     FIG. 15 illustrates some additional details of the vessel  105 . As shown therein, the vessel  105  has at least a cylindrical portion, and as illustrated in FIG. 15, may be entirely cylindrical, with the cylindrical portion being defined by the concentric inner and outer walls  136  and  137  that terminate in a cylindrical opening  135 . As illustrated in FIG. 15, the cylinder includes an annular rim  140  that extends outwardly from the circumference of the cylindrical opening  135  and defines a rim circumference  141  that is concentric with the cylindrical portion of the vessel  105  and the cylindrical opening  135 . 
     The vessel  105  further includes a curved outer wall portion  142  between the concentric outer wall  137  and the rim circumference  141 . In this regard, it has been discovered that under higher pressures, a perpendicular relationship between the outer wall  137  and the rim  140  tends to be the weakest point under stress applied from the interior of the vessel  105 . It has been discovered according to the present invention, however, that by providing the curved outer wall portion  142 , the pressure resistance of the vessel can be significantly increased. Specifically, in current embodiments, a reaction vessel with a 90-degree relationship at the portion described will withstand pressures up to about 200 pounds per square inch (psi) before failing. The curved outer wall portion  142  of the present invention, however, can withstand pressures of up to about 1000 psi. 
     The invention has been described in detail, with reference to certain preferred embodiments, in order to enable the reader to practice the invention without undue experimentation. A person having ordinary skill in the art will readily recognize that many of the components and parameters may be varied or modified to a certain extent without departing from the scope and spirit of the invention. Furthermore, titles, headings, or the like are provided to enhance the reader”s comprehension of this document and should not be read as limiting the scope of the present invention.