Abstract:
The present invention refers to an apparatus for carrying out wet chemical reactions under pressure comprising a pressure sure vessel in which a reaction liquid can be provided and which is adapted to be sealed in a pressure-tight manner by closure means. When the reaction liquid is heated, the pressure in the interior of the pressure vessel will increase; the stressability limit of the pressure vessel must, however, not be exceeded. The total pressure in the interior is composed of the partial pressures of the reaction liquid and of the reaction products. The reaction liquid must be maintained at so low a temperature that the sum of the partial pressures of the reaction liquid and of the gaseous reaction products does not exceed the maximum load value of the pressure vessel. For achieving a wet chemical reaction which is as fast and as efficient as possible, the highest possible temperature of the reaction liquid is, however, required. In accordance with the present invention these problems are solved by a rinsing device for rinsing the pressure vessel while removing gaseous reaction products from the pressurized pressure vessel and while maintaining a predetermined pressure therein. This has the effect that the gaseous reaction products are removed from the pressure vessel so that said gaseous reaction products will neither displace the reaction equilibrium nor influence the pressure conditions.

Description:
TECHNICAL FIELD 
     The present invention refers to an apparatus for carrying out wet chemical reactions under pressure comprising a pressure vessel in which a reaction liquid can be provided and which is adapted to be sealed in a pressure-tight manner by closure means. 
     BACKGROUND ART 
     Such apparatuses for carrying out wet chemical reactions under pressure are known e.g. from U.S. Pat. No. 4,882,128 or from European patent applications EP 0 416 759 A1 and EP 0 461 383 A2. 
     Wet chemical reactions, such as disintegration and decomposition, hydrolysis or synthesis, are very important for analysing and synthesizing substanes. The higher the reaction temperature is, the faster and the more complete such wet chemical reactions will normally take place. Hence, closed reaction vessels are used in accordance with the above-mentioned prior art so that higher pressures and, consequently, higher boiling temperatures of the reaction liquid for the wet chemical reaction can be used. For this purpose, the reaction liquid is provided in a pressure vessel whose interior can be acted upon by pressure. When the reaction mixture is heated, e.g. by a microwave field generated by a microwave generator, the desired increase in the reaction speed is achieved, whereby new possibilities are provided for efficient methods that can be used in the analysis and synthesis of substances. 
     Closed reaction vessels have, however, various drawbacks. The pressure in the vessel results from the sum of the partial pressures of the substances contained in the pressure vessel. The total pressure is therefore composed of the vapour pressure of the reaction liquid and of the partial pressures of the gaseous reaction products. In view of the limited mechanical stressability of the closed pressure vessel, the sum of the partial pressures must not exceed a specific maximum pressure. It follows that, the higher the partial pressures of the gaseous reaction products formed in the course of the reaction, the lower the maximum applicable vapour pressure of the reaction liquid will be. This means that, in view of the fact that the vapour pressure increases continuously as the temperature increases, there is an upper limit to the temperature of the reaction liquid. 
     In the case of oxidative acid decomposition of organic samples, for example, gaseous carbon dioxide is produced. The larger the amount of organic substance that is decomposed in a specific pressure vessel at a constant pressure is, the lower the vapour pressure of the decomposing acid must be and therefore the lower the maximum possible boiling temperature will be. 
     A further drawback occurring when closed reaction vessels are used is to be seen in the fact that the gaseous reaction products cannot escape and that, consequently, the reaction equilibrium is displaced towards the starting product. 
     SUMMARY OF THE INVENTION 
     Taking into account the drawbacks of the prior art, it is therefore the object of the present invention to provide an apparatus for carrying out wet chemical reactions under pressure of the kind referred to at the beginning by means of which a faster and more efficient course of reaction can be achieved. 
     According to the present invention, this object is achieved by an apparatus of the type referred to at the beginning, which is characterized in that a rinsing device is provided for rinsing the pressure vessel while removing gaseous reaction products from the pressurized pressure vessel and while maintaining a predetermined pressure therein. The present invention therefore permits wet chemical reactions to be carried out under pressure while continuously removing gaseous reaction products, whereby an influence of the partial pressures of the gaseous reaction products on the total pressure in the interior of the pressure vessel is excluded. In addition, the displacement of the reaction equilibrium towards the starting products, which increasingly inhibits the progress of reaction, is prevented by the continuous removal of the gaseous reaction products from the. pressure vessel acted upon by pressure. 
     According to an advantageous further development, the rinsing device comprises a source for a pressurized gas as well as a first gas conduit extending from said source into the interior of the pressure vessel, and, in addition, a second gas conduit extending from the interior of said pressure vessel to the outside thereof and including a first restrictor for producing a first predetermined flow rate. Preferably, also the first gas conduit includes a restrictor for producing a predetermined flow rate. Said first and second gas conduits including said first and second restrictors constitute a simple, but efficient embodiment of the rinsing device; by means of the restrictors in said first and second gas conduits, the flow rates for the supply and discharge of a purge gas can be preselected such that a pressure, which is predetermined by means of a pressure regulator valve, can be maintained in the reaction vessel. This predetermined pressure determines the boiling temperature of the reaction liquid. 
     According to a further advantageous embodiment, said first and second restrictors each comprise means for adjusting a flow rate. It follows that the conditions for carrying out wet chemical reactions can be adjusted individually, e.g. in dependence upon the reaction starting and end products. 
     According to a further advantageous embodiment, the apparatus for carrying out wet chemical reactions under pressure comprises a recondensation device provided in the interior of the pressure vessel and used for condensing evaporated reaction liquid. This recondensation device prevents losses of reagents and of important constituents of the sample. Hence, the efficiency of the wet chemical reaction under pressure is increased still further. 
     In accordance with advantageous further developments, this recondensation device can be implemented either as a cooling body which is adapted to be cooled such that the evaporated reaction liquid condenses, or as a means for applying a cooling gas stream to an area of the reaction vessel constituting an upper area with regard to the direction of the force of gravity, said reaction vessel being arranged in the pressure vessel and being adapted to hold the reaction liquid in a lower area thereof. 
     In accordance with a further advantageous development of the present invention, a heating device is provided for increasing the temperature of the reaction liquid. In accordance with an advantageous embodiment, this heating device consists especially of a microwave generator for generating a microwave field passing through the reaction liquid. By means of the microwave field produced by said microwave generator, a predetermined temperature-time characteristic can be controlled precisely through the power of the microwave field. In the case of a predetermined constant microwave power, a freely selectable time-temperature characteristic of the reaction liquid can also be achieved by controlling the pressure in the interior of the pressure vessel by the pressurized purge gas supplied, provided that a controllable pressure regulator valve is used in the gas conduit leading into the pressure vessel. 
     According to a further advantageous embodiment, a device is provided for feeding gaseous or liquid reaction substances into the pressure vessel. This feed device preferably comprises a multiway valve with a sample loop, said multiway valve being adapted to be switched at least to a first and to a second position for receiving in said sample loop a substance to be fed under a preselectable pressure at said first position and for feeding the received substance into the pressurized pressure vessel at said second position. With the aid of the multiway valve, a substance to be fed can be introduced e.g. without pressure into the sample loop of the multiway valve so that the amount of the substance to be fed can be determined precisely. When the multiway valve is switched to the second position, the substance introduced into the sample loop is fed under pressure into the pressure vessel. 
     According a further advantageous embodiment, a temperature sensor is provided for determining the temperature of the reaction liquid. This temperature sensor is especially arranged in a channellike indentation provided in the area of the lower end of a reaction vessel, whereby a particularly precise temperature measurement is possible. 
     Further advantageous embodiments are disclosed by the subclaims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following, the present invention will be explained and described in detail on the basis of preferred embodiments which serve as examples and with reference to the drawings enclosed, in which: 
     FIG. 1 shows a schematic side view of a first embodiment of an apparatus for carrying out wet chemical reactions under pressure according to the present invention; 
     FIG. 2 shows a schematic side view of a second embodiment of the apparatus for carrying out wet chemical reactions under pressure according to the present invention; 
     FIG. 3 shows a schematic side view of a third embodiment of the apparatus for carrying out wet chemical reactions under pressure according to the present invention; 
     FIG. 4 shows a schematic representation of a multiway valve for the third embodiment shown in FIG. 3, at a first position; 
     FIG. 5 shows the multiway valve shown in FIG. 4, at a second position; and 
     FIG. 6 shows a schematic view of a reaction vessel provided with a temperature sensor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A first embodiment of the apparatus for carrying out wet chemical reactions under pressure according to the present invention is described hereinbelow making reference to FIG.  1 . The apparatus comprises a pressure vessel  1  closed by a cover  2  via a gastight seal  3 . Within said pressure vessel there is a removable reaction vessel  4  in the lower area of which (with regard to the force of gravity) the reaction liquid  5  can be provided, said reaction liquid  5  being shown in the drawing by way of example. The lower area of the pressure vessel and of the reaction vessel, in which the reaction liquid  5  is preferably provided, is located in the interior of a microwave resonator  33  communicating with a microwave generator  32  via a waveguide  43 . Within said microwave resonator  33  a microwave field  15  can be generated. A first gas conduit  12  extends through the cover  2  which is attached to the pressure vessel  1  such that no gas can escape; said gas conduit  12  extends in a space between the reaction vessel  4  and the pressure vessel  1  down into the area of the bottom of the reaction vessel  4 . In the first gas conduit  12 , a restrictor  17  is provided for limiting the gas flow rate through said first gas conduit  12 . In addition, said first gas conduit  12  is connected via a pressure regulator valve  42  to a source  35  of pressurized purge gas. 
     Furthermore, a second gas conduit  36  extends through the cover  2 , said second gas conduit  36  leading via a second restrictor  14 , which is used for limiting the flow rate through the second gas conduit, to an outlet conduit  13 . A gas conduit  70  branches off at a point between the end of the second gas conduit  36 , which is located in the interior of the pressure vessel, and the restrictor  14 , said gas conduit  70  being adapted to be connected via a valve  16  to a vacuum source which is not shown. 
     In the interior of the pressure vessel  1 , a cooling body  6  is provided, which is attached to the cover  2  and implemented as a cold finger. The cold finger  6  projects into the upper area of the reaction vessel  4 . The cold finger  6  is hollow in the interior thereof and it is connected to a source of cooling liquid via a conduit  8  extending through the cover  2  and a valve  7 . The conduit  8  for the cooling liquid extends down to a point close to the bottom of the cold finger  6 . Between the valve  7  and the point where said cooling liquid conduit  8  passes through the cover  2 , a vacuum conduit  10  is connected to said conduit  8  via a valve  9 . A discharge conduit  11 , which extends through the cover  2 , ends in the upper area of the hollow cold finger  6 . 
     In the following, the operation of the first embodiment according to the present invention will be described. 
     When a wet chemical reaction is carried out under pressure, the temperature of the reaction liquid  5  in the interior of the reaction vessel  4  can be increased by generating the microwave field  15 . The pressure in the interior of the pressure vessel  1  always corresponds to the predetermined pressure of the pressure regulator valve  42 . The temperature of the reaction liquid  5  can be increased until the vapour pressure of said liquid  5  corresponds to the pressure predetermined by means of the pressure regulator valve  42 . The liquid then starts boiling and the temperature remains constant. 
     The gaseous reaction products produced during the wet chemical reaction are continuously removed from the pressure vessel by the purge gas via the second gas conduit  36  and: the restrictor  14  leading to the outlet, said purge gas being fed from the source  35  into the interior of the pressure vessel  1 . The pressure of the purge gas can be adjusted freely by means of the pressure regulator valve  42 . The temperature of the reaction mixture can be increased until the vapour pressure of the reaction mixture corresponds to the gas pressure adjusted. The flow rate of the purge gas is adjusted with the aid of the restrictor  14 . The valve  16  leading to the vacuum conduit  70  permits the generation of a vacuum in the pressure vessel  1 . In this case, the gas stream through the pressure vessel  1  is adjusted with the aid of the restrictor  17 . The flow resistance of said restrictor  17  is from 10 to 100 times lower than that of the restrictor  14 . 
     In the case of a specific predetermined pressure, a specific amount of gas is discharged via the restrictor  14  into the conduit  13 . Since the flow resistance of the restrictor  17  is much lower than that of restrictor  14 , a sufficient amount of gas is always resupplied via the conduit  12  so that the pressure in the interior of the pressure vessel  1  will always correspond to the pressure predetermined by.the pressure regulator valve  42 . If gases are set free during the reaction, they will flow off via the restrictor  14 ; in this case, a correspondingly smaller amount of purge gas is resupplied via the conduit  12  and the restrictor  17 , since the amount of gas flowing off via the restrictor  14  remains approximately constant when the pressure is constant. 
     The restrictor  17  is especially also used for rapidly bleeding the liquid  5  at a low temperature. For this purpose, the valve  16  is opened and a vacuum is applied to conduit  36 . This has the effect that, on the one hand, an insignificant amount of gas is sucked in the direction of  70  via the restrictor  14  and that, on the other hand, a vacuum is created in the interior of the pressure vessel  1 , whereby a gas stream which is much too big would now suddenly flow through the pressure vessel  1  via the pressure regulator valve  42  and the conduit  12 . The restrictor  17  restricts this gas stream to a usable amount. This reduced gas stream serves to discharge the vapours of the liquid from the pressure vessel  1 . 
     When valve  7  is open, the cold finger  6  can be filled with a cooling liquid via the conduit  8 , said cooling liquid flowing off via the discharge conduit  11  positioned at the upper end of said cold finger. Evaporated reaction liquid condenses on the cold finger  6  provided in the upper area of the reaction vessel  4 , whereas the gaseous products, such as carbon dioxide in cases in which an organic sample is decomposed, can escape. The recondensed reaction liquid drops back into the reaction liquid  5  provided in the lower area of the reaction vessel  4 . In a condition in which valve  7  is open and valve  9  is closed, cooling liquid circulates via the cooling liquid conduit  8  and the discharge conduit  11  through the cold finger  6  for cooling the same. For bleeding the reaction liquid  5  from the reaction vessel  4 , the cooling liquid must be removed from the cold finger  6 . For this purpose, valve  7  is closed and valve  9  is opened so that the cooling liquid is sucked from the cold finger  6  through the vacuum conduit  10 . 
     It follows that the embodiment shown in FIG. 1 permits wet chemical reactions to be carried out while continuously removing gaseous reaction products and while selectively recirculating evaporated reaction liquid into the reaction vessel  4 . 
     The reaction vessel  4  can preferably be removed from the pressure vessel  1  so that the reaction liquid can be filled into the reaction vessel  4  simply and without any problems prior to carrying out the wet chemical reaction. Subsequently, the reaction vessel  4  is introduced in the pressure vessel  1 . The pressure vessel  1  is closed by the cover  2  and by means of the seal  3  such that not gas can escape. For attaching the cover  2  to the pressure vessel  1  and for removing it therefrom, it is possible to provide a mechanism for automatically charging the pressure vessel  1  with a reaction liquid. 
     When the reaction liquid has been introduced in the pressure vessel  1 , a temperature-time program which is suitable for the intended reaction is started, the temperature of the reaction liquid  5  being controlled through the power of the microwave field  15 . The maximum temperature results from the vapour pressure curve of the reaction liquid  5  in accordance with the pressure in the interior of the pressure vessel  1 , said pressure being applied via the valve  42  and the restrictor  14 . A predetermined temperature-time program can also be realized by using a constant power of the microwave field  15 , which suffices to make the reaction liquid  5  boil also at high boiling temperatures. The boiling temperature of the reaction liquid  5  is adjusted as desired by suitably adjusting the purge gas pressure in the pressure vessel  1  by means oft he pressure regulator valve  42 . With the aid of the cold finger  6 , the evaporated liquid is condensed and continuously recirculated into the reaction vessel  4 . 
     By means of the apparatus according to the present invention, which is shown in FIG. 1, the reaction liquid  5  can be evaporated partly or fully. For this purpose, the cooling liquid is sucked out of the cold finger  6  by means of the vacuum conduit  10  in the open condition of valve  9 . Valve  7  is closed at this time. For reducing the boiling temperature during the evaporation process, the interior of the pressure vessel  1  is connected to the vacuum conduit  70  by opening the valve  16 , and evacuated. By means of the purge gas, the evaporated liquid is discharged from the first gas conduit  12  via the valve  16  and the vacuum conduit  70 . The flow rate of the purge gas is adjusted with the aid of the restrictor  17 . 
     FIG. 2 shows a schematic side view of a second embodiment of the apparatus for carrying out wet chemical reactions under pressure according to the present invention. The elements that are identical with or similar to those of the first embodiment are designated by corresponding reference numerals in FIG. 2; said reference numerals have, however, been augmented by 100. A,detailed description of the elements and of their functions, which have already been described for the first embodiment with reference to FIG. 1, is dispensed with in the case of the second embodiment. In this connection, the relevant description concerning FIG. 1 is referred to. 
     The second embodiment according to FIG. 2 differs from the first embodiment insofar as, instead of or in addition to the cold finger, a recondensation device is provided in the form of a means for applying a cooling gas stream to an area of the reaction vessel constituting an upper area with regard to the direction of the force of gravity. The means for applying a cooling gas stream comprises a source of cooling gas  137  connected via a cooling gas conduit  131  to the interior of the pressure vessel  101  in an upper area of the reaction vessel. The gas conduit  131  extends from the interior of the pressure vessel to the outside thereof at a second point of said pressure vessel. The lower area of the pressure vessel  101 , in which the reaction liquid  105  can be provided in the reaction vessel  104 , is preferably, but not necessarily, separated through a seal  139  in a gastight manner from the area acted upon by the cooling gas from the source of cooling gas  137 . For preventing the cooling gas coming from source  137  from mixing with the purge gas coming from a source  112 , further sealing means  140  can be provided between a projection  141  of the cover  102  and the reaction vessel  104 . The first gas conduit  112  used for feeding the purge gas via the restrictor  117  and the second gas conduit  136  used for removing the purge gas and the gaseous reaction products via the second restrictor  114  to the outlet  113  extend either through the additionally provided cold finger  106  or parallel thereto. 
     The second embodiment of the apparatus for carrying out wet chemical reactions under pressure according to the present invention is operated in a manner corresponding to the mode of operation of the first embodiment that has been described with reference to FIG.  1 . However, according to said second embodiment, the cooling for recondensing evaporated reaction liquid can be carried out alternatively or additionally by cooling the upper area of the reaction vessel by means of the cooling gas circulated from the source  137  via conduit  131 . 
     To make things easier, the conduits for filling and emptying the cold finger  106  have been omitted in FIG.  2 . The cold finger according to the second embodiment can, however, be provided with the same connection and discharge conduits as the cold finger of the first embodiment. 
     In FIG. 3, a third embodiment of the apparatus for carrying out wet chemical reactions under pressure according to the present invention is shown. Also in this case, elements that are similar to or identical with those of the first embodiment are designated by corresponding reference numerals, but now said reference numerals have been augmented by 200. A detailed description of the elements of the third embodiment is dispensed with at this point and reference is made to the corresponding description of these elements and of their function in connection with the first embodiment. 
     The third embodiment differs from the first embodiment insofar as there are provided an additional system for feeding gaseous or liquid reaction substances into the reaction vessel as well as a discharge conduit  227  including a valve  226  for discharging reaction liquid  205  from the reaction vessel  204 . 
     The device for feeding gaseous or liquid reaction substances comprises a multiway valve  220  with a sample loop  221 . The multiway valve has 8 connections a, b, c, d, e, f, g and h. The connections a and e communicate with said sample loop  221 . Connection b communicates with conduit  222 , connection c with valve  218 , connection d with conduit  225 , connection f with conduit  223  and connection h with conduit  224 . Connection g is tightly sealed. Conduit  222  is e.g. connected to a supply of reaction liquid. Conduit  223  can in this case be connected to an outlet or to a waste collecting tank. Conduit  224  is connected to the first gas conduit  212  and the source  235  for the pressurized purge gas. Conduit  225  is connected to a conduit  251  which extends through the cover  202  into the interior and down to the bottom of the reaction vessel  204 . 
     FIGS. 4 and 5 show a first and a second position of the multiway valve  220  shown in FIG.  3 . At the first position shown in FIG. 4, one end of the sample loop  221  communicates with connection a, and the second end of the sample loop  221  communicates with connection e. It is thus possible to fill the sample loop  221  via line  222  with a liquid reagent independently of the pressure within the pressure vessel  1 . 
     When the multiway valve  220  is switched over from the first position to the second position shown in FIG. 5, one end of the sample loop  221  is connected to conduit  224  and the other end of said sample loop  221  is connected to conduit  225 . The sample loop  221  is connected to the pressurized purge gas conduit  212  in this way. By means of the pressure of the purge gas, the reagent contained in the sample loop  221  is fed into the reaction vessel  204  via the conduit  225  and via the conduit  251  extending through the cover  202 . Also in the case of the third embodiment, the boiling temperature is determined by the purge gas pressure selected. 
     Whereas at the first position shown at FIG. 4, the sample loop is filled e.g. with a liquid reagent via conduit  222 , conduit  225 , which communicates with the interior of the reaction vessel via the conduit  251  extending through the cover  202 , is connected to a gas conduit  219  via a valve  218 . Said gas conduit  219  can be used for introducing at a first position, e.g. in the period of time in which the sample loop  221  is filled, a gaseous reagent, e.g. halogen or ozone, into the reaction liquid  205  from said conduit  219  and via conduit  225 . A continuous supply of gaseous reagents is thus possible when valve  218  is open at the first position of the multiway valve  220 . 
     For a discontinuous feed of reagents, valve  218  is closed and the reagent filled into the sample loop  221  at the first position is fed by switching the multiway valve over to the second position shown in FIG.  5 . When the feeding of the reagent contained in the sample loop  221  has been finished, the multiway valve  220  is switched back from said second position to the first position so that the filling and feeding process can be started again. 
     Although the third embodiment shown in FIG. 3 is only shown with the cold finger  206  for recondensing evaporated reaction liquid, it is, of course, also possible to provide the kind of recondensation according to the second embodiment shown in FIG.  2 . 
     The discharge of the reaction liquid  205  from the reaction vessel  204  can also take place under pressure with the aid of the pressure of the purge gas in the pressure vessel  201 . For this purpose, the valve  226  included in the discharge conduit  227  is opened; this has the effect that, due to the pressure prevailing in the interior, the reaction liquid  205  is forced into the discharge conduit  227  extending from the bottom of the reaction vessel  204 . For rinsing the reaction vessel  204  subsequently, specific volumes of a suitable rinsing liquid are successively fed via the multiway valve  220 . 
     The apparatus for carrying out wet chemical reactions under pressure according to the present invention is preferably provided with a temperature sensor which is shown in FIG.  6 . The temperature sensor, which can be a thermocouple, a resistance sensor, an infrared sensor or a fibre optics with fluorescence measurement, extends through the pressure vessel in a gastight arrangement, and it is arranged in a channel indentation  529  provided in the bottom  541  of the reaction vessel  504 . By means of the embodiment of the reaction vessel  504  shown in FIG.  6  and on the basis of the arrangement of the temperature sensor  530 , the temperature of the reaction liquid can be measured precisely. 
     For a simple oxidation of organic samples, a simple version of the apparatus without any means for bleeding the reaction liquid will suffice. It is then not necessary to suck off the cooling liquid. The elements designated by reference numerals  9  and  10  are therefore not provided in the case of this simple embodiment. In view of the fact that no vacuum is applied, the elements designated by reference numerals  16 ,  70  and  17  are not provided either. 
     In all the embodiments described hereinbefore, the reaction vessel consists preferably of PTFE or of quartz. Various fluoridized polymers, such as e.g. PTFE, PFA or PTFE-TFM, can be used for the reaction vessel. When vessels consisting of PTFE are used, temperatures up to 250° C. can be applied in the case of microwave heating. When quartz vessels are used, temperatures up to and above 300° C. can be be applied in the case of microwave heating. 
     In the following, the apparatus according to the present invention is compared with a commercially available apparatus used for carrying out wet chemical reactions under pressure and provided with microwave heating, said last-mentioned apparatus being known from the prior art. The commercially available system is a system produced by the firm of Anton Paar in Graz/Austria under the name of PDM. In the comparison tests, reaction vessels holding 50 ml are used; in said respective reaction vessels 100 mg, 130 mg and 160 mg sorbite monolaurate are oxidized with 2 ml nitric acid. For this purpose, the respective sample amount is introduced into the 50 ml reaction vessel consisting of quartz glass and the 2 ml nitric acid (65%) are added thereto. The vessel is closed and treated at a reaction pressure of 30 bar in the commercially available PDM apparatus for a period of 10 minutes. When the weighed portion is 100 mg, 88% of the sample are oxidized, when it is 130 mg, 80% are oxidized, and when it is 160 mg, only 68% of the sample are oxidized. In the apparatus according to the present invention, the same sample amounts are oxidized again in 50 ml quartz vessels in the dynamic pressure system, each of said sample amounts being again oxidized with 2 ml nitric acid (65%) at a pressure of 30 bar. The purge gas used is nitrogen. The flow rate for the purge gas is 100 ml per minute. The reaction takes place for a period of 10 minutes, the microwave power being 600 W. This method has the effect that, independently of the weighed sample amounts, 95% of the sorbite monolaurate are oxidized whereby CO 2  and H 2 O are formed.