Abstract:
A sample introduction system includes a drop chamber which can be sealed and outgassed and which includes a movable jaw selectively holding the sample in position above a conduit communicating with an open crucible for receiving a sample once the jaw has been moved to an open position releasing the sample. The jaw is actuated by a magnetic field which moves the jaw in an entirely enclosed system, thereby preventing the introduction of atmospheric contaminants during the operation of the sample dropping jaw from a closed, sample holding position to an open sample releasing position. By providing a magnetic actuator, such as a solenoid, for operation of the jaw, the sample chamber remains sealed during the sample dropping operation preventing contaminants from interfering with the analytical results.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a sample introduction assembly for loading small samples into analytical crucibles for subsequent analysis and particularly to a seal system which prevents the admission of contaminants. 
     In analytical furnaces for combusting relatively small (1 mg to 0.5 gram) samples of, for example, steel pins, chips, or the like, typically resistance or induction furnaces are employed. Graphite crucibles are employed for resistance heating of a crucible directly when placed between a pair of electrodes. Ceramic crucibles are employed in furnaces in which heating is by an induction field provided by an RF coil. In either furnace, it is necessary initially to outgas the crucible and assure no contaminant gases are mixed with the specimen gases during loading of the sample. 
     In several prior art systems, it is necessary to open the combustion chamber area after the outgassing to gain access to a crucible for insertion of a sample to be analyzed. In doing so, the crucible is exposed to atmospheric gases which can contaminate the crucible to an extent that the analytical results can be adversely effected. In order to prevent the introduction of contaminants, one solution has been to provide a sample loading mechanism which allows the introduction of a sample into a movable hopper which is subsequently sealed and the area purged with an inert gas. The jaws of the hopper are subsequently opened to allow admission of the sample into the crucible through an electrode assembly. U.S. Pat. No. 4,371,971 discloses such an apparatus which, although preventing a direct communication path with the atmosphere during admission of the sample, may allow a small amount of atmospheric gases to enter the combustion chamber during the sample loading operation through the dynamic seals on the movable jaw actuator. With analyzers designed to measure oxygen and nitrogen content of a specimen, even miniscule amounts of atmospheric oxygen and nitrogen adds inaccuracy to analytical results, particularly for low concentration samples. Even with sealed sample dropped mechanisms where linear acting pistons move through radial seals, gases trapped in imperfections on the shaft surface are introduced to the analytical specimen, degrading the precision and accuracy of the measured amount of oxygen and nitrogen. Also, with time, atmospheric leakage increases as dynamic seals wear due to high cyclical use. 
     Accordingly, there exists a need for an improved sample introduction system in which contamination from atmospheric contaminants can be eliminated. 
     SUMMARY OF THE INVENTION 
     The system of the present invention solves this need by providing a sample introduction system in which a sample is introduced into a drop chamber which can be sealed and purged and which includes a movable jaw selectively holding the sample in position above a conduit communicating with an open crucible for receiving a sample once the jaw has been moved to an open position releasing the sample. The jaw is actuated by a magnetic field which moves the jaw in an enclosed environment, thereby preventing the introduction of atmospheric contaminants during the operation of the sample dropping jaw from a closed, sample holding position to an open sample releasing position. By providing a magnetic actuator, such as a solenoid, for operation of the jaw, the sample chamber remains sealed during the sample dropping operation preventing contaminants from interfering with the analytical results. 
     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 
     FIG. 1 is a vertical cross-sectional schematic view of an analytical furnace showing the environment of the present invention; 
     FIG. 2 is an exploded perspective view of a sample drop assembly of the present invention which can be used with the furnace shown in FIG. 1; 
     FIG. 3 is an assembled perspective view of the structure shown in FIG. 2; 
     FIG. 4 is an enlarged vertical cross-sectional view of one of the elements of the sample drop assembly shown in FIG. 2; 
     FIG. 5 is an enlarged vertical cross-sectional view of another one of the elements of the assembly shown in FIG. 2; 
     FIG. 6 is an enlarged perspective view of the sample drop jaw employed in the system of the present invention; 
     FIG. 7 is an exploded perspective view of the sample drop slide and seal assembly shown also in FIG. 2; 
     FIG. 8 is a vertical cross-sectional view of the sample drop assembly shown in a first position for loading a sample into the sample drop jaw assembly; 
     FIG. 9 is a vertical cross-sectional view of the sample drop assembly shown in a second position in which the sample drop jaw assembly is in a sealed position; and 
     FIG. 10 is a vertical cross-sectional view of the system of the present invention showing the sample drop jaw assembly in a sample drop position for admitting a sample into a crucible of the analytical furnace shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1, there is shown an analytical resistance furnace  10 , which includes an upper electrode assembly  12  and a lower electrode assembly  14  for supporting a graphite crucible  16  having a pedestal base sitting upon the electrode post  15  of the lower electrode assembly  14 . The upper electrode assembly includes a conduit  18  for admission of a sample from the sample drop assembly shown in FIG. 3, which rests on and is attached to the upper surface  19  of the upper electrode assembly  12  in a conventional manner by fasteners or the like. The electrode assemblies  12  and  14  can be of the type disclosed in U.S. Pat. No. 4,056,677 or the type employed in commercially available instruments such as the TC 500  manufactured by Leco Corporation of St. Joseph, Mich. During combustion of a sample, electrode assemblies  12  and  14  come together with O-ring seals  17  enclosing the combustion area and byproducts of combustion exit through a discharge tube  13  into an analyzer for analysis of byproducts of combustion. A carrier gas, such as helium, is introduced through conduit  18 , as described in greater detail below, through the sample drop jaw assembly. The upper edge of crucible  16  engages the annular electrode  11  of the upper electrode assembly  12  and an electrical current is passed through the graphite crucible  16  to heat and combust samples positioned therein through the unique sample jaw drop assembly of the present invention. Crucible  16  may, for example, be of the type disclosed in U.S. Pat. No. 3,899,627. Although this invention is described in the environment of a resistance heating furnace  10 , the invention can be used in induction and other types of furnaces where it is necessary to admit a sample into an analytical crucible for combustion. 
     Suitably mounted on top of surface  19  of the furnace  10  shown in FIG. 1 is a sample drop assembly  20  of the present invention, which is shown in FIGS. 2 and 3. The sample drop assembly  20  includes a fixed sample drop block  30 , a sample drop jaw assembly  40 , and a sample drop slide assembly  90  positioned, as seen in FIG. 3, with block  30  positioned on surface  19  with a conical aperture  32  aligned with the open tapered mouth of conduit  18 . Aperture  32 , as seen in FIGS. 8-10, is generally conical or funnel shaped, having a relatively wide open mouth narrowing to a size conforming to that of conduit  18 . Block  30  is positioned with aperture  32  aligned with conduit  18  such that samples dropped, as described in greater detail below, will fall into the open mouth of crucible  16  during the sample loading operation. Sample drop block  30  includes a pair of toggle bolts  34  and  36  which are pivotally mounted to the undersurface of the sample block and rotate upwardly within slots  35  to allow the sample drop jaw assembly  40  to be removably attached thereon. The sample drop jaw assembly is mounted to the upper surface  31  of block  30 , as seen in FIG. 3, with the toggle bolts  34  and  36  including socket heads  38 , which seat in configured sockets  44 ,  45  of assembly  40  when tightened into a threaded aperture in rotatable dowels  35 ′ in apertures  35 ″ to seal and secure the sample drop jaw assembly  40  to the upper surface of block  30 . For such purpose, assembly  40  includes an O-ring seal  42  (FIGS. 8-10) which is mounted in an annular recess  41  in the lower surface  43  of jaw assembly  40  to seal the interface between block  30  and drop jaw assembly  40 . Blocks  30 ,  40  and  90  are all machined of suitable nonferrous material, such as aluminum. Both jaw assembly  40  and block  30  are fixedly mounted to the top surface  19  of furnace  10 , and slide assembly  90  is slidably mounted to the drop jaw assembly  40  as described below. 
     Block  40  includes semicylindrical configured sockets  44  and  45  on opposite corners thereof for receiving the toggle bolts  34  and  36 , respectively, for securing block  40  to block  30 . Block  40  includes, at its upper opposed edges, a pair of outwardly projecting flanges  55  (FIGS. 2 and 3) for captively and slidably receiving the sample drop slide  90  as described below in greater detail. Block  40  includes a central, vertically extending opening  46  (FIGS.  2  and  8 - 10 ), which has a side wall  47  tapered to define one side of a sample drop hopper together with a movable jaw  50  (FIG. 6) having a semi-conically tapered side wall  57  mating with side wall  47  and joined together when the jaws are in the closed position as shown in FIGS. 8 and 9 to enclose the lower end of the conical sample dropping chamber  52  so defined. Block  40  includes a semicylindrical surface  48  spaced from and opposed to conical surface  47 . 
     Communicating with the chamber  52  defined by the volume between the semi-conical tapered surface  47  and block  50  and the opposed semicylindrical wall  48  is an inclined passageway  49  communicating with an axially extending cylindrical aperture  51  (FIGS. 8-10) terminating in a threaded cylindrical aperture  53  into which a plunger assembly comprising an actuator rod  60  and plunger  70 . Aperture  53  is threaded at  56 , as best seen in FIG. 2, to receive the threaded end  76  of plunger  70  as shown in the assembled view of FIGS. 8-10. The drop jaw assembly block  40  includes an end wall  54  (FIGS. 8-10) with an aperture  58  therethrough for allowing coupling between the actuator rod  60  and movable jaw  50 . Movable jaw  50  is shown in FIG.  6  and is a generally semicylindrical machined aluminum block which slidably moves within the chamber  52  with tapered surface  57  facing mating surface  47  to define an enclosed hopper which can be opened, as seen in FIG. 10, for dropping a sample therefrom into the analytical furnace  10 . 
     The side wall  59  (FIG. 6) of movable jaw  50  includes a threaded stud  59 ′ extending therefrom. The actuator rod  60  includes a cylindrical end  62  having an internally threaded socket  63  that threads onto stud  59 ′ for coupling the actuator plunger rod  60  to the movable jaw  50 , as seen in FIGS. 8-10. The end  62  of actuator rod  60  thus extends through aperture  58  in wall  54  of block  50  to communicate with and engage movable jaw  50 . Rod  60  is machined of a ferro-magnetic material such as steel, and includes an annular flange  64  (FIGS.  2  and  8 - 10 ) near end  62  for receiving a compression spring  65  which, as seen in FIGS. 8 and 9, urges the movable jaw  50  coupled thereto to a closed sample holding position. The rod  60  includes a post  66  at an opposite end for receiving an O-ring  67  which engages an end wall  77 ′ of plunger  70  (FIG. 10) to prevent a metallic interface upon retraction of the rod actuator  60  within plunger  70  as described in greater detail below. 
     As best seen in FIG. 5, plunger  70  comprises a thin non-ferrous cylindrical tube  72  which has an annular collar  74  at one end with external threads  75  and an annular shoulder  76  for receiving an O-ring  77  which seats and seals against surface  57 ′ (FIGS.  2  and  8 - 10 ) of block  40  for sealing the interface between plunger  70  and block  40 . 
     Plunger  70  further includes a nipple  78  at an end opposite O-ring seal  77  for the admission of an inert gas through an axial opening  79  therein which communicates with a transversely extending aperture  80  to allow an inert gas, such as helium, to flood into the space surrounding the outer diameter of the movable actuator rod  60  and the interior wall  82  of plunger  70 . Wall  77 ′ is formed of a cylindrical block dimensioned to allow the helium gas to extend around the periphery thereof and is secured to the nipple  78  by a solder joint  84  (FIG.  5 ). An O-ring  85  surrounds nipple  78  to allow an airtight coupling of a helium source to nipple  78 , which may be threaded to receive a coupling nut or the like for the introduction of the inert gas. 
     A solenoid actuating coil  86  (FIGS. 2,  3 , and  8 - 10 ) surrounds the outer cylindrical surface  72  of plunger  70  and includes a pair of conductors  87  coupled to a suitable electrical control circuit for inducing a magnetic field within plunger  70 , drawing the ferro-magnetic actuator rod  60  into the plunger cylinder to a position shown in FIG. 10 when actuated for sliding jaw  50  to the open position as shown in FIG.  10 . The jaw can move relatively freely within the chamber  52  defined within block  40  and yet is completely sealed by the utilization of the O-ring seal  77  from the outside atmosphere. Thus, there are no dynamic seals associated with the movable jaw assembly as it moves from a closed to an open position. Instead, the jaw is freely movable under the influence of a magnetic field which couples the plunger to the actuating solenoid  86 . 
     A sample is admitted to the sample drop jaw assembly  40  through the sample drop slide assembly  90  now briefly described in conjunction with FIGS. 7-10. Sample drop slide  90  is a machined aluminum block which includes a conically tapered aperture  92  which aligns with the chamber  52  when in the sample drop position shown in FIG.  8 . Adjacent aperture  92  is a sealing piston assembly comprising a disk-shaped piston  94  having a piston seal  95  mounted to the outer cylindrical periphery thereof and an annular groove  96  on its face facing the upper surface  41 ′ of block  40  for receiving an O-ring seal  98 . Seal  98  effectively seals the open mouth  46  of the sample drop jaw assembly when in a position shown in FIG.  9  and described below. The piston  94  and its seal  95  is received in a piston cylinder  100  formed in block  90  which includes a pair of inwardly facing slots  102  which slidably fit over and captively hold sample drop slide  90  to block  40  by engaging flanges  55 . A source of pressurized air communicates with cylinder  100  through aperture  104  and a sealed coupling  106  coupled to threaded aperture  104  by an O-ring seal  105  to pressurize the piston  94 , pushing it downwardly against the sealing surface  41 ′ of block  40  during dropping of a sample and subsequent combustion of the sample by furnace  10 . The sliding block  90  may include a sealed window  108  allowing an operator to view downwardly into the analytical furnace during a cycle of combustion. For such purpose, a quartz window  110  (FIGS. 8-10) suitably sealed to block  40  can be employed for providing viewing of the combustion operation. An actuator arm  120  is coupled to slide  90  on a pneumatic actuator (not shown) for moving slide  90  between sample loading and sample dropping positions during operation of the sample loading assembly  20  as now described in connection with FIGS. 8-10. 
     Sample loading is accomplished by positioning sample drop slide  90  with open mouth  92  above the chamber  52  of sample drop jaw assembly  40  as seen in FIG.  8 . In this position, a sample, such as a pin, chip, or rod sample  112 , can be dropped by an operator downwardly in the direction indicated by arrow A through the funnel-shaped opening  92  into the hopper defined by fixed side wall  47  of block  40  and the movable side wall  57  of movable jaw  50 . The sample is retained in the bottom of the hopper so defined and slide  90  is then moved in a direction indicated by arrow B, as shown in FIG. 9, such that the piston sealing O-ring  98  surrounds the upper circular opening of hopper  52  and pressure is applied to the piston through fitting  106  to pressurize the piston, thereby forming a sealing engagement with drop jaw assembly  40 . 
     At this time, an inert gas, such as helium, is introduced through fitting  78  with the flow of gas entering opening  79 , extending through transverse opening  80  into the annular space between the outer surface of actuator rod  60  and the inner surface  82  of cylinder  72  through upwardly extending passageway  49  into the volume of hopper  52  including the area surrounding cylindrical wall  48 . The gas advances downwardly through the jaws into channel  18  of the now enclosed electrodes of the furnace, outwardly through tube  13  and into the analyzer. After a suitable purge time, solenoid  86  is actuated by a control signal on conductors  87  to retract jaw  50  to the right, as indicated by arrow C in FIG. 10, allowing the sample  112  to drop by gravity through the funnel-shaped opening  32  aligned with conduit  18  in upper electrode assembly  12 . It is noted that by elimination of separate jaw members and by machining surface  47  into block  40 , the amount of trapped air space needing to be purged is greatly reduced, allowing the purging time to be less. The helium gas continues to flow through the opening  79  and passageway  49  into the area provided by the loosely fitted movable jaw  50  downwardly, as indicated by arrow D in FIG. 10, to continuously sweep byproducts of combustion out of the furnace  10  through conduit  13  into an analyzer (not shown) during a cycle of analysis. 
     Actuator arm  120  can be coupled to a suitable pneumatic cylinder with a throw length sufficient for moving slide  90  between the sample admission position shown in FIG. 8 to a sealing position shown in FIGS. 9 and 10. It is noted also that the slide  90  may be moved from left to right as opposed to right to left, such that a sample can be admitted to opening  92  and rest on the upper surface  41 ′ of block  40  until such time as it is desired to be dropped into the hopper  52  by moving the slide to the position shown in FIG. 8 from a position to the left of that shown in FIG.  8 . Subsequently, the slide will be moved again to a position as shown in FIG. 9 for the operation of the piston seal enclosing the hopper  52 . 
     It is seen, therefore, with the sample drop assembly  20  of the present invention, a sample can be admitted to a sample drop jaw assembly which is subsequently sealed from the atmosphere and the jaw can be moved without the use of dynamic seals on the moving parts of the jaw, thereby preventing any minute amount of contaminant gas which may otherwise be present in a dynamic seal construction from entering the combustion zone during an analysis. The result is that very small levels of oxygen and nitrogen can be detected by an analyzer without interference from atmospheric oxygen and nitrogen which otherwise may leak into the system through sample assemblies. By providing a single movable jaw element also, the volume which must be purged using an inert gas is reduced, and, by providing a spring loaded jaw assembly which holds a sample in a closed sample holding position, only momentary actuation of the solenoid  86  is required to drop a sample into the furnace for analysis. The jaw  50  can be retracted as desired, however, for viewing the sample through the quartz window  110  during an analysis, if desired. 
     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.