Abstract:
A reagent assembly for a combustion tube includes a reagent tube which is sealably and removably coupled to the open end of the combustion tube such that, when the reagent in the reagent tube is depleted, it can be easily removed without disassembly of the furnace or changing the combustion tube. The reagent tube includes a twist-lock cap to facilitate removal of the reagent tube.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to elemental analyzers and particularly an analyzer which employs a reagent assembly which is easily removable from the combustion chamber.  
         [0002]     A determination of concentration of elements, such as carbon, hydrogen, sulfur, and nitrogen, in an organic sample is desirable for a variety of reasons. In recent years, the food market in particular has become interested in determining the amount of protein in an organic sample, which can be determined by the nitrogen content. Further, the sulfur content, as well as the carbon-to-hydrogen ratio, is desirable in the characterization of coal and coke samples, as are the carbon, hydrogen, and nitrogen ratios in a variety of other organic materials.  
         [0003]     Elemental analyzers are commercially available from the Assignee of the present application, Leco Corporation of St. Joseph, Mich., which manufactures CHN analyzers, which are sold under the trademark TRUSPEC®. The analyzer may employ a variable volume ballast chamber of the type disclosed in U.S. Published Application 2004/0171165 A1 (now U.S. Pat. No. ______), the disclosure of which is incorporated herein by reference. The analyzer disclosed in this published application generally is used for the macro analysis of samples of from about 0.25 grams in size. The combustion system in such an analyzer uses a generally U-shaped quartz combustion tube of the type also disclosed in U.S. Pat. No. 4,622,009, the disclosure of which is incorporated herein by reference. The combustion tube includes a first vertically extending leg which receives a crucible for combustion of a sample and a second vertically extending leg downstream coupled to the first leg and which includes reagents that can serve several purposes. These include scrubbing undesirable products of combustion, enhancing the complete combustion of difficult samples, and/or the removal of excess reagents, such as oxygen. The selection of the reagent is dependent upon the characteristics of the application.  
         [0004]     Generally, the analysis of elemental carbon, hydrogen, sulfur, and nitrogen is well known and is discussed in several references, including  Methods in Microanalysis , Vol. 1, Mirra Osipovna Korshun, 1964,  Instrumental Organic Elemental Analysis , R. Belcher, 1977; and  Organic Elemental Analysis Ultramicro, Micro, and Trace Methods , Wolfgang J. Kirsten, 1983. U.S. Pat. No. 4,525,328 discloses an analyzer employing a fixed volume ballast chamber, which collects analytes in an approximately 4.5 L chamber for subsequent analysis. The amount of combustion oxygen used in filling the fixed ballast chamber is significant, and an analysis takes a significant amount of time for the combustion and ballast chamber filling. Also, the byproducts of combustion, i.e., the analyte gases, are somewhat diluted in the relatively large volume ballast chamber. The 2004/0171165 A1 application discloses a variable volume ballast chamber with a movable piston and a combustion detector, such that, during combustion of a sample, the chamber is only filled with byproducts of combustion until the completion of combustion is determined by the combustion detector. Typically, a significantly smaller volume than that of the fixed volume ballast chamber is captured in a more concentrated form of analyte which subsequently can be ejected from the variable volume ballast chamber by controlling a movable piston.  
         [0005]     With the variable volume ballast chamber system disclosed in the above-identified published patent application, a large or macro analysis sized sample of 0.10 grams or more are employed. It is desired to provide an analyzer which utilizes a smaller samples, if possible, and conduct an analysis on-the-fly (i.e., detection of the sample during the combustion event as opposed to storing a combustion sample and providing an aliquot sample from a ballast chamber). One difficulty with an on-the-fly analysis system is that, for such micro analysis utilizing a helium carrier gas, an influx plug of excess oxygen is employed to fully combust the sample, and the remaining oxygen must be eliminated prior to detection by flowing the gaseous byproducts of combustion through a reduction reagent, such as copper wire strips.  
         [0006]     In the U-shaped combustion tubes used in analyzers, such reagents are packed in the downstream leg of the U-shaped combustion tube and it is necessary after several analyses, which can be anywhere from less than 100 to about 1000 samples, to remove the fused and contaminated reduction reagent from the combustion tube and replace it with new reagents. This requires complete disassembly of the furnace and frequently replacement of the combustion tube itself inasmuch as the reagent packed in the quartz tube tends to melt and stick as a plug in the combustion tube itself. Since combustion takes place at a temperature of nearly 1000° C., this requires considerable time, expense, and manpower, since the furnace must first be cooled, opened, the combustion tube disassembled, and frequently a new combustion tube with a new reagent installed.  
         [0007]     Thus, there exists a need for an improved system which allows for on-the-fly micro analysis, i.e. 2 mg to 10 mg samples, utilizing a combustion system which allows for the easy replacement of the reducing reagent.  
       SUMMARY OF THE INVENTION  
       [0008]     The system of the present invention satisfies this need by providing a reagent assembly for a combustion furnace having a combustion tube. The reagent assembly employs a reagent tube which is packed with a reagent and is concentrically positioned in the combustion tube. The reagent tube is sealably and removably coupled to an open end of the combustion tube such that, when the reagent is depleted, the reagent tube can be easily removed without disassembly of the furnace or changing the combustion tube.  
         [0009]     In one embodiment of the invention, the reagent tube is top loaded into one leg of a U-shaped combustion tube. In another embodiment, in order to reduce dead volume in the combustion flow path, the reagent tube is inserted into a second tube having an inner diameter greater than the outer diameter of the reagent tube such that a flow path exists between the annular space between the outer wall of the reagent tube and the inner wall of the second tube. The second tube is generally cylindrical and has a closed floor at one end and is inserted into the leg of the combustion tube with the outer diameter of the second tube having a diameter smaller than the inner diameter of the combustion tube. The reagent and second tubes are sealably mounted to the combustion tube such that byproducts of combustion are forced in a tortious flow path, which includes the concentric space between the combustion tube and the outer surface of the second tube then between the space between the inner surface of the second tube and the outer surface of the reagent tube and subsequently upwardly through the open end of the reagent tube to an exit port.  
         [0010]     In all embodiments, the reagent tube is packed with reagents and is coupled to a fitting which is easily removed from the combustion furnace and combustion tube and which allows the reagent tube containing an exhausted reagent to be removed from the top of the combustion tube, thereby eliminating the need to remove and/or replace the entire combustion tube from the furnace once the reagent has been expended. In a preferred embodiment of the invention, the fitting includes a twist-lock cap associated with the reagent tube to facilitate its removal. Such a system thereby allows for on-the-fly micro analysis of a sample which utilizes a reagent to remove the excess oxygen and allows the reagent to be replenished as necessary without the time and effort required by existing systems.  
         [0011]     These and other features, objects and advantages of the present invention will become apparent upon reading the following description thereof together with reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a fragmentary cross-sectional view of a resistance combustion furnace and combustion assembly including the reagent assembly of the present invention;  
         [0013]      FIG. 2  is an enlarged fragmentary exploded cross-sectional view, partly broken away, of the reagent assembly of the present invention;  
         [0014]      FIG. 3  is an exploded fragmentary perspective view of the combustion furnace and reagent assembly of the present invention;  
         [0015]      FIG. 4  is an exploded perspective view of the twist-off cap assembly of the reagent assembly of the present invention;  
         [0016]      FIG. 5  is a perspective view of the mounting block to which the cap is secured; and  
         [0017]      FIG. 6  is a flow diagram of an analyzer embodying the top-loading reagent assembly of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]     Referring initially to  FIG. 1 , there is shown an analytical furnace  10  embodying a reagent assembly  100  of the present invention. Furnace  10  is a resistance heating furnace including a generally U-shaped quartz combustion tube  20  having a generally cylindrical vertically extending first or combustion leg  22 , a transverse coupling conduit  24 , and a vertically upwardly extending second or reagent leg  26 . The combustion tube, thus, generally has cylindrical sections  22  and  26  which are joined by the transverse conduit  24 . The furnace  10  can generally be of the type disclosed in U.S. Pat. No. 4,622,009, the disclosure of which is incorporated herein by reference, which heats a sample  36  dropped by a sample load assembly  30  of the type disclosed in U.S. Pat. No. 6,291,802, the disclosure of which is incorporated herein by reference, through an oxygen lance and sample introduction tube  32  into a crucible  34 . Crucible  34  can be of the type disclosed in U.S. Pat. No. 6,270,727, the disclosure of which is incorporated herein by reference.  
         [0019]     Combustion crucible  34  is held in place by a suitable quartz porous plug  35  which allows the byproducts of combustion to flow downwardly through the leg  22  in the direction indicated by arrow A in  FIG. 1 . The tube  32 , in addition to providing a sample drop pathway, serves as an oxygen lance for the introduction of combustion oxygen to the open mouth of the cup-shaped crucible  34  during combustion. The furnace  10  is employed in a micro analysis system in which, after the sample  36  is introduced into the crucible and furnace, which has been heated to approximately 1000° C., a helium carrier gas flows through the combustion chamber  20  until a plug or aliquot of oxygen is introduced through lance  32 , for a period of about 5-10 seconds in one embodiment, to complete the combustion of the 1 to 50 mg sample  36  held within crucible  34  to completely combust the sample. The helium carrier gas then carries the byproducts of combustion through the transverse conduit  24  and upwardly, as indicated by arrow A, into the open mouth of the reagent tube  110  of the reagent assembly  100 .  
         [0020]     Reagent tube  110 , as seen in  FIGS. 1 and 2 , is also made of quartz and has an open lower end  112  with an inwardly tapered section  114  holding the reduction reagent  42  in place. Reagent tube  110  is generally cylindrical and includes an annular mounting flange  116  at its open upper end  117  ( FIG. 3 ). Flange  116  rests upon an annular surface  154  of mounting block  150  and is sealed to the mounting block  150  by an O-ring seal  152 , as best seen in  FIG. 2 .  
         [0021]     In a preferred embodiment of the invention, the open end  112  of quartz reagent tube  110  had a diameter of about 0.5 inches, while the inner diameter of tube  110  was approximately 0.75 inches, and had a wall thickness of about 3 mm. The outer diameter of tube  110  is approximately 1 inch. The tapered end  114  was tapered at an angle of approximately 20° over a length of approximately 0.70 inches while the overall length of tube  110  was approximately 9.25 inches. The flange  116  has a diameter of 1.2 inches, and tube  110  fits within the circular opening  153  of mounting block  150  with flange  116  engaging the annular surface  154  ( FIG. 5 ) of block  150 .  
         [0022]     Within the inner removable reagent tube  110 , there is packed the reagent comprising in a preferred embodiment, as seen in  FIG. 1 , copper wool  40  forming a plug at the tapered lower end  114  of the reagent tube  110 . Above the copper wool plug  40  there is placed the reduced copper reagent  42  itself comprising finely chopped copper wire sticks which are prepared by placing the sticks in a vacuum furnace with hydrogen to scavenge all the oxygen from the copper. The reagent is commercially available from Leco Corporation of St. Joseph, Mich.  
         [0023]     Tube  110  is concentrically and removably mounted within the second leg  26  of combustion tube  20  by a twist-off sealed locking cap  140  removably mounted to mounting block  150 , which is affixed to furnace wall  12  as described in greater detail below. Although the reagent tube can be dimensioned to reduce the dead space between its outer diameter and that of the inner diameter of the combustion tube leg  26 , in one embodiment, dead space is reduced by the use of an optional second concentric tube  120  as now described.  
         [0024]     The generally cylindrical quartz outer tube  120  has a closed lower end or floor  122  which rests on the bottom surface  23  of the leg  26  of combustion tube  20 , as best seen in  FIG. 2 . The quartz tube  120  has a length of about 9 inches and, when resting on the floor of the combustion tube, leg  26  does not extend fully to the top of the reagent tube but rather leaves an open annular space  124  ( FIG. 2 ) above the top edge  125  of tube  120  in the area between the inner wall  27  of combustion tube leg  26  and the outer wall  115  of reagent tube  110 .  
         [0025]     The outer diameter of the second or outer tube  120  is about 1.18 inches as compared to the inner diameter of 1.25 inches of the combustion tube leg  26 , thereby leaving an annular space for the flow of combustion gases in the direction of arrow A around the outer surface of inner tube  120  and the inner surface  27  of combustion tube leg  26  into the annular space  124 , which is sealed by an O-ring seal  28  sealing the combustion tube section  26  to the furnace wall  12 , as best seen in  FIG. 2 . The gases, therefore, are forced to flow downwardly in a second annular space  128  between the outer diameter of reagent tube  110  and the inner diameter of outer tube  120 . The inner diameter of outer tube  120  is approximately 1.063 inches, such that a gap of approximately 0.0315 inches is formed between the outer wall of the reagent tube  110  and the inner wall of the outer tube  120 , allowing the gaseous byproducts of combustion to flow downwardly, as indicated by arrow A, into the open area  130  below open end  112  of tube  110  and above floor  122  of outer tube  120 . The gas then flows upwardly as indicated by arrow A through reagents  42  into the exit aperture  142  of removable cap  140 .  
         [0026]     Cap  140  is shown in detail also in the exploded view of  FIG. 4  and includes a first cylindrical section  141 , which extends downwardly within the open mouth of inner tube  110 , as best seen in  FIG. 2 , and is sealed to the inner surface of tube  110  by sealing O-ring  143 . The cap  140  also includes a second, larger diameter annular section  144  which sealably fits within the aperture  153  ( FIG. 5 ) of mounting block  150  and is sealed thereto by an O-ring  145 . Cap  140  includes a mounting flange  146  having a diameter greater than that of section  144 . Flange  146  includes a pair of keyhole-shaped arcuate slots  147 . The outer edge  148  of flange  146  is knurled to allow the cap to twist off from the mounting block  150 , which includes a pair of cap bolts  160  over which the cap  140  can be extended and rotated while pressing downwardly to sealably engage the combustion tube section  26  as well as reagent tube  110 , which is coupled to cap  140  by the interference fit with O-ring  143  during assembly of the unit. A gas elbow  149  of conventional configuration is threadably coupled to the opposite end of aperture  142  to provide an exit flow path  149 ′ for the byproducts of combustion into the remaining components of the analyzer, as shown in  FIG. 6 .  
         [0027]     Mounting block  150 , as seen in  FIG. 5 , includes blind threaded apertures  151  for receiving the cap bolts  160 , which extend upwardly a distance sufficient for extending through slots  147  in flange  146  of the cap  140 . Mounting block  150  also includes a plurality of apertures  155  for securing the cap to the furnace wall  12  in a conventional manner in sealed engagement by the use of O-ring  28 , as seen in  FIG. 2 . The details of this mounting arrangement are not shown in the flow path cross-sectional views of  FIGS. 1 and 2 , however, the furnace wall  12 , as seen in  FIG. 3 , includes threaded apertures  156  for receiving conventional fasteners, such as cap head screws, which extend through apertures  155  in the mounting block  150  for securing mounting block  150  to the furnace wall  12 . The cap  140  and mounting block  150  are machined of aluminum or other suitable metal to withstand the pressure and temperature of the byproducts of combustion flowing therethrough. Cap  140  includes an annular recess  145 ′, as seen in  FIG. 4 , for receiving the sealing O-ring  145 , which forms a double seal with the cap and the seal  152  in cap-receiving recess  153  of mounting block  150 .  
         [0028]     As can be seen in reviewing  FIGS. 1-5 , the removable reagent tube  110  of assembly  100  allows the furnace  10  to be employed for combusting several samples until the reagent is exhausted. The furnace can be opened to expose the combustion tube and the cap  140  of the removable reagent assembly. Cap  140  is rotated and lifted to gain access to the reagent holding inner tube  110 , which can be lifted from the combustion tube reagent section  26  while retaining the outer tube  120  in place to allow the easy replacement of the reagent inner tube  110  either by inserting a freshly made and repacked reagent tube or by cleaning out the existing tube external to the furnace and repacking it with reagent materials  40  and  42 . By providing a reagent tube with a flanged upper end and a tapered lower end and having a diameter in cooperation with either the combustion tube leg  26  or the outer tube  120 , the flow of byproducts of combustion through the reagent tube is assured, and an easily replaceable reagent section of the combustion system is provided. This greatly reduces the time, effort, and expense required of the prior art systems, where frequently combustion tube  20  itself had to be replaced.  
         [0029]     The reagent assembly  100  is initially installed by placing the outer tube  120  within the leg  26  of combustion tube  20 , which need not be critically centered in view of the existence of a gap between the outer diameter of reagent tube  110  (or tube  120 ) and the inner diameter of leg  26  of combustion tube  20 , allowing a flow pass of gas therebetween regardless of the precise centering. Similarly, the insertion of reagent tube  110  within the outer tube  120  always allows a generally annular gap therebetween such that the byproducts of combustion will be forced downwardly around the space between the outer or second tube and the reagent tube and then upwardly through the open end of the reagent tube and through the reagent. The overall analyzer, including the unique top-loaded removable reagent assembly  100  of the present invention, is shown in  FIG. 6 , which is now briefly described.  
         [0030]     Inlet  31  ( FIG. 6 ) of furnace  10  receives combustion gas (O 2 ) from a source  15  of oxygen which has a flow rate adjusted between 0.5, 1, 3, 5, or 6 L per minute by the selective activation of parallel flow control valves  11 ,  13 , and  17  in conduit  16  leading from the supply of oxygen to the inlet  31  of the combustion furnace. The O 2  pressure is monitored by a pressure sensor  18 . The oxygen is jetted into the open mouth of a sample-holding crucible  34  through an oxygen lance  32  to combust the sample. As described above, the byproducts of combustion (i.e., analytes) flow through reagent  42  in reagent tube  110  positioned in leg  26  and from combustion chamber  20  through exit port  149 ′. Conduit  41  transfers the byproducts of combustion through a heater  43 . The byproducts of combustion flowing in conduit  41  then pass through a combustion detector  45  comprising an H 2 O IR cell, which detects the hydrogen content in the gas stream as a result of the combustion of the sample  36  in crucible  34 . The combustion detector  45  is coupled to a CPU, as described in the above identified ′ 165  publication, for storing the detected hydrogen level.  
         [0031]     As seen in  FIG. 6 , the byproducts of combustion are forced through a flow path including an anhydrone scrubber  47  and an SO 2  determining IR cell  49  and a CO 2  determining IR cell  50 , all contained within a heated chamber  52 .  
         [0032]     The He carrier gas in conduit  14  then carries the byproducts of combustion through pinch valve  19  in conduit  51  to valve  76 . The sample gas then passes through valve  74  into catalytic reduction heater  78  and through anhydrone scrubber  80 . Conduit  82  carries the sample gas through a 300 cc/minute flow controller  84  into the nitrogen sample inlet port  86  of thermal conductivity module  60  and through the thermal conductivity measurement device  88 , which is coupled to a CPU to provide data relative to the nitrogen concentration detected. After measurement, the gas is then exhausted through an exhaust outlet valve  90 . During the measurement of nitrogen concentration by cell  88 , He carrier gas at T-junction  68  also flows through a flow restrictor  91  to a thermal conductivity reference cell  92 .  
         [0033]     He carrier gas from source  54  flows through filter  56  in conduit  58  to inlet port  59  of thermal conductivity module  60  via the actuation of He valve  62 . The He gas exits module  60  via port  63 , travels through a 12 psi pressure regulator  64  and scrubber  65  into port  66  of thermal conductivity module  60 . Heater  78  is filled with copper (Cu) heated to about 750° C. to remove any remaining oxygen and convert NO to free nitrogen (N 2 ), which subsequently flows through the scrubber  80 , which includes sodium hydrate silicate for removing CO 2  and an anhydrone, which removes water from the gas flow stream.  
         [0034]     The control of the valves and the combustion furnace, as well as the measurement and detection of the concentration of gases, is conventionally controlled by a CPU(not shown). The CPU receives an input signal as to the size of the sample from balance  12  and controls the loading head  30  ( FIG. 1 ) to drop the sample within the combustion chamber. The CPU also controls the application of power to furnace  10  through a suitable power control module. The CPU may be coupled to a printer to print the results of the gas concentrations detected. The CPU is programmed in a conventional manner, to analyze the sample based upon standard ASTM standards utilizing data from the infrared detectors and thermal conductivity detectors shown in  FIG. 6 . As is well known after an analysis cycle, the analyzer is purged to condition it for a subsequent analysis.  
         [0035]     It will become apparent to those skilled in the art that various modifications to the preferred embodiment of the invention as described herein can be made without departing from the spirit or scope of the invention as defined by the appended claims.