Abstract:
A sample cup assembly for use with an analytical instrument having a sample cup holder with an inner diameter which is greater than the outer diameter of the sample cup assembly. The sample cup assembly can include a main cell for retaining a specimen to be analyzed; a collar for locating a thin film over the open end of the main cell such that the film sealingly closes the open end of the main cell; and two or more projections extending outwardly from either the main cell or collar. The projections engage the interior surface of the sample cup holder when the sample cup assembly is installed therein, thereby centering the sample cup assembly within the sample cup holder.

Description:
FIELD OF THE INVENTION 
     This invention relates to sample cup assemblies for use in spectroscopic analysis, and more particularly, to a sample cup assembly having a sample cup centering system that enables the assembly to be utilized without an adapter in a conventionally sized sample cup holder of a spectroscopic instrument. 
     BACKGROUND OF THE INVENTION 
     Spectroscopic techniques are commonly used for analyzing various materials. Such techniques rely on the changes that take place in the atoms and molecules of a material when electromagnetic radiation is absorbed or emitted thereby. Technological advancements in both wavelength-dispersive (WD-XRF) and energy-dispersive (ED-XRF) X-ray fluorescence instrumentation enable spectroscopic analysis of virtually all types of materials. 
     Sample cups of various well known designs are used in spectroscopic techniques to hold or contain liquid, solid and powdered specimens. Many of these sample cups consist of at least three components: a cup-shaped main cell; a thin film of material covering the open end of the cell; and a collar that pulls the thin film of material taut over the open end of the cell. The thin film of material operates as closure for the cup to retain the specimen to be spectroscopically analyzed within the cell, and as a sample surface plane that becomes exposed to an excitation source during the spectroscopic analysis. Such a sample cup is described in U.S. Pat. 5,451,375 entitled APPARATUS FOR TRIMLESS SAMPLE CUP USED IN X-RAY SPECTROSCOPY issued on Sep. 19, 1995 to Monte J. Solazzi. 
     Spectroscopic instruments typically come equipped with a non-removable sample cup holder of a fixed inner diameter. The bottom of holder includes an aperture for exposing the sample cup assembly to the instrument&#39;s excitation source. Only sample cups assemblies with outer diameters complimentary to the inner diameter of the sample cup holder can be mounted therein such that the sample cup assembly is centered within the aperture of the holder. Sample cups assemblies with outer diameters that are significantly less than the inner diameter of the sample cup holder require adapters to center them with respect to the holder&#39;s aperture. Improper centering of the sample cup assembly in the sample cup holder of the instrument can detrimentally alter the intensity of radiation impinging upon the specimen from the excitation source, thus causing the spectroscopic analysis to produce erroneous quantitative data. 
     Accordingly, there is a need for an inexpensive sample cup assembly which can be used in a conventionally sized sample cup holder of a spectroscopic instrument without the use of an adapter. 
     SUMMARY OF THE INVENTION 
     A sample cup assembly for use in retaining a specimen, comprising: a main cell for retaining the specimen, the main cell having an open end; a collar for locating a thin film over the open end of the main cell such that the film sealingly closes the open end of the main cell; and at least two projections extending from one of the main cell and collar for centering the sample cup assembly within a sample cup holder of a spectroscopic instrument. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The advantages, nature, and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments now to be described in detail in connection with accompanying drawings where like numerals are used to identify like elements and wherein: 
     FIG. 1 is an exploded perspective view in partial section of a typical sample cup assembly utilizing an embodiment of the centering system of the present invention; 
     FIG. 2 is a cross-sectional view of the sample cup assembly of FIG. 1; 
     FIG. 3 is an end view (sample film end) of the sample cup assembly of FIG. 1; 
     FIGS. 4A-4D are end views of sample cup assemblies with additional embodiments of the centering system; 
     FIG. 5 is a cross-sectional view of the sample cup assembly of FIG. 1 installed in a sample cup holder of a spectroscopic instrument; 
     FIG. 6 is a cross-sectional view illustrating the assembly of the sample cup assembly of FIG. 1; 
     FIGS. 7 and 8 are cross-sectional views showing other types of sample cup assemblies which can utilize the centering system of the present invention; and 
     FIG. 9 is a cross-sectional view showing the centering system adapted to the main cell of the sample cup assembly. 
    
    
     It should be understood that the drawings are for purposes of illustrating the concepts of the invention and are not to scale. 
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1-3 collectively show a typical sample cup assembly  10  which utilizes an embodiment of the centering system of the present invention. The sample cup assembly  10  includes three components: an open cup-shape main cell  12  for holding a specimen to be analyzed, a sheet  16  of thin film material for covering and sealing the cup-shape main cell  12 , and a collar  14  for securing the sheet  16  over the main cell  12 . Such sample cup assemblies are described in U.S. Pat. No. 5,451,375 to Solazzi. The disclosure of U.S. Pat. No. 5,451,375 is incorporated herein by reference. 
     The main cell  12  includes a generally cylindrical side wall  18  having a first end  20  closed by a generally circular end wall  22  and an open second end  24 . The side and end walls  18 ,  22  define an open cup-shape enclosure for accommodating a specimen to be analyzed (FIG.  5 ). A continuous groove  28  is defined in the outer surface  26  of the side wall  18  proximate the open second end  24 . A generally annular reservoir side wall  32  depends from the outer periphery  30  of the end wall  22 . The end wall  22  includes opening  33  (FIG. 2) closed by a thin membrane  34 . The membrane  34  can be easily punctured to vent interior  36  of the main cell  12 . 
     The sheet  16  of thin film material is flexible and transparent to the radiant energy used in the spectrochemical analysis. Such thin film materials are well known in the art. 
     The collar  14  is a generally cylindrical member having a first end  40 , a second end  42  opposite the first end  40 , an outer surface  44  and inner surface  46 . The outer surface  44  of the collar  14  immediately adjacent the first end  40  thereof includes a circular peripheral flange  48 . The inner surface  46  of the collar  14  includes a continuous (as shown) or segmented protrusion  50  adjacent the first end  40  thereof. When the collar  14  is assembled to the main cell  12 , the protrusion  50  of the collar  14  snaps into the groove  28  on the outer surface  26  of the main cell side wall  18  thereby locking the collar  14  and main cell  12  together. Since the collar  14  of the sample cup assembly is virtually the same height as the side wall  18  of the main cell  12 , the second end  42  of the collar abuts a flange-like edge  35  of the reservoir side wall  32 . 
     Other embodiments of the sample cup assembly can use different methods for locking the collar  14  and the main cell  12  together. For example, the protrusion and groove arrangement can be omitted by tapering the collar  14  and the side wall  18  of the main cell  12  such that during assembly of the sample cup assembly  10 , as the second end  42  of the collar  14  is moved toward the flange-like edge  35  of the reservoir side wall  32  of the main cell  12 , a friction fit gradually develops therebetween which locks the collar  14  and main cell  12  together. 
     In accordance with the present invention, the collar  14  further includes a centering system comprised of at least two and preferably three unitarily formed tabs  52   a,    52   b,    52   c  that extend outwardly from the circular peripheral flange  48 . The tabs  52   a,    52   b,    52   c  of the collar  14  shown in the embodiment of FIGS. 1-3 are equi-spaced and of equal length. The tabs  52   a,    52   b,    52   c  extend radially outward from the central axis A-A′ of the assembly  10 . The tabs of the centering system of the invention can also be provided on the main cell as shown in FIG.  9 . 
     The main cell  12  and the collar  14  are preferably fabricated from unrecycled natural polyethylene. This eliminates the potential possibility of introducing metallic contamination that may adversely affect the analysis of a sample substance. Polyethylene is one of a number of thermoplastic materials that can be utilized in this application because of its excellent mass attenuation properties encompassing the 1 to 12 Angstrom analytical wavelength range. In addition, polyethylene is resistant to chemical attack, temperature softening, and degradation from excitation energy sources, as well as exhibiting excellent tensile strength for adequate sample retention. 
     FIG. 5 shows the assembled sample cup assembly  10  installed in a sample cup holder  60  of a spectroscopic instrument. As can be seen, the tabs ( 52   b,    52   c  visible) of the collar  14  slidingly abut against the inner surface  62  of the sample cup holder  60  thereby centering the sample cup assembly  10  with respect to the aperture  64  of the holder  60 . The centering system of the invention thus enables the sample cup assembly  10  to be used in the sample cup holder of the spectroscopic instrument without an adapter. 
     FIGS. 4A-4D show various other embodiments of the centering system of the present invention. In FIG. 4A, the collar  14  of the assembly  70  is provided with a centering system comprised of two equi-spaced, equal length tabs  72   a,    72   b  that extend radially outward from the central axis A of the assembly  70 . In FIG. 4B, the collar  14  of the assembly  80  is provided with a centering system comprised of three equal length tabs  82   a,    82   b,    82   c  that extend radially outward from the central axis A of the assembly  80 . The first and third tabs  82   a,    82   c  are spaced 180 degrees apart from one another while the second tab  82   b  is spaced 90 degrees apart from each of the first and third tabs  82   a,    82   c.  In FIG. 4C, the collar  14  of the assembly  90  includes a centering system comprised of four equi-space, equal length tabs  92   a,    92   b,    92   c,    92   d  that extend radially outward from the central axis A of the assembly  90 . In FIG. 4D, the collar  14  of the assembly  100  is provided with a centering system comprised of three equi-spaced, flexible or resilient angled tabs  102   a,    102   b,    102   c.  Flexible tabs  102   a,    102   b,    102   c  allow the assembly  100  to be used in variously sized sample cup holders as the tabs bend to accommodate the inner diameter of the holder. 
     In FIG. 6, the components of the sample cup assembly  10  are conventionally assembled as described in U.S. Pat. No. 5,451,375. The collar  14  is advanced over the main cell  12  (which contains the specimen to be analyzed) such that the sheet of thin film material  16  becomes pinched between the outer surface of the main cell side wall and the inner surface of the collar  14 . Consequently, the sheet of thin film material  16  is immediately pulled taut across the open end  24  of main cell  12 . Assembly is completed once the collar  14  is advanced far enough over the main cell  12  so that the protrusion  50  on the inner surface  46  of collar  14  fits into the groove  28  on the outer surface  26  of the main cell side wall  18 . 
     Once the sample cup  10  is assembled, the sheet of thin film material  16  is drawn tightly over the open end  24  of the cell body  12  creating a wrinkle-free sample surface plane for the spectroscopic analysis. The thin film material  16  creates a seal over the edge of the main cell  12  which is impermeable to the specimen contained therein. 
     Any time after the sample cup  10  is assembled, the specimen can be subjected to spectrochemical analysis. This normally requires inverting the sample cup assembly  10  so that the open end  24  of the main cell body  12  is facing downward and placing it into the sample cup holder of an associated spectroscopic instrument as shown for example in FIG.  5 . At that time, any substance contained within the sample cup  10  will come in contact with the portion of thin film material  16  covering the open end  24  of the main cell  12 . The specimen is then ready for analysis. 
     If venting is required during analysis of the specimen contained within the sample cup assembly  10 , the membrane  34  can be ruptured with a blunt device. 
     The centering system of the present invention can be used on other types of sample cup assemblies. For example, FIG. 7 shows the centering system of the invention comprised of tabs  116  adapted to a sample cup assembly  110  that utilizes a collar  112  with an integrally molded thin film  114  as disclosed in U.S. Pat. No. 4,643,033 to Solazzi. In another example, FIG. 8 shows the centering system of the invention comprised of tabs  128  adapted to a sample cup assembly  120  that utilizes a double open ended main cell  122  closed by two collar and thin film sheet assemblies  124 ,  126  as disclosed in U.S. Pat. No. 5,454,020 to Solazzi. The disclosures of U.S. Pat. No. 4,643,033 and 5,454,020 are incorporated herein by reference. 
     While the foregoing invention has been described with reference to the above embodiments, various modifications and changes can be made without departing from the spirit of the invention. Accordingly, such modifications and changes are considered to be within the scope of the appended claims.