Patent Publication Number: US-2018052101-A1

Title: Apparatus and Method for Measuring Components in Fluidic Samples Sealed in a Bag

Description:
REFERENCES CITED 
     U.S. Patent Documents 
       
     
       
         
           
               
               
               
               
             
               
                   
               
             
            
               
                 5,510,621 
                 April 1996 
                 Goldman 
                 250/343 
               
               
                 7,582,869 
                 September 2009 
                 Sting et al. 
                     250/336.1 
               
               
                 6,280,690 
                 August 2001 
                 Tadion 
                 422/560 
               
               
                 4,872,868 
                 October 1989 
                 Chevallier 
                 604/327 
               
               
                 5,239,860 
                 August 1993 
                 Harris et al. 
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                 7,952,710 
                 May 2011 
                 Flank et al. 
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     OTHER PUBLICATIONS 
     
         
         EPA Method 418.1 Petroleum Hydrocarbons (Spectrophotometric, Infrared) 
         Rolene Bauer, Hilhne Nieuwoudt, Florian F. Bauer, Jens Kossmann, Klaus R. Koch, and Kim H. Esbensen, “FTIR Spectroscopy for Grape and Wine Analysis”, Analytical Chemistry 2008 80 (5), 1371-1379. 
         Shaw, R. A. and Mantsch, H. H. 2006 . Infrared Spectroscopy in Clinical and Diagnostic Analysis. Encyclopedia of Analytical Chemistry.    
         Duarte, I. F. et al  Journal of Agriculture and Food Chemistry,  2002, 50, 3104-3111. 
       
    
     1. BACKGROUND OF THE INVENTION 
     The present invention is an apparatus and a method for non-invasive, fast and economic analysis of fluid samples using Infrared (IR) transmission spectroscopy. 
     IR transmission spectroscopy has been widely employed to qualitatively identify and quantitatively assay the constituents in fluid media. For instance, EPA Method 418.1 specifies that the petroleum hydrocarbon contaminants in soil and water shall be extracted by fluorocarbons and then the extract fluid shall be analyzed by the IR absorption in transmission mode. In food industries, IR transmission spectroscopy has been routinely used for quality control. For instance, the article titled “FTIR Spectroscopy for Grape and Wine Analysis” by Bauer et al published on  Analytical Chemistry,  1371, 2008 summarizes the applications of infrared transmission spectroscopy in wine analysis. In clinical analysis, Infrared transmission has been accepted as a powerful approach to analyze body fluids and medicinal fluids. (Shaw, R. A. and Mantsch, H. H. 2006.  Infrared Spectroscopy in Clinical and Diagnostic Analysis. Encyclopedia of Analytical Chemistry .) U.S. Pat. No. 5,510,621 demonstrates a health-care industry application of IR transmission spectroscopy for analysis the total parenteral nutrients (TPN), which are eventually the source of intravenous feeding. TPN solution is a clear liquid, which has the components of saline water, amino acid and dextrose. The concentrations of TPN constituents are vital for patients&#39; health. In hospitals, samples of TPN solution can be analyzed using the IR transmission spectroscopy method to verify the quality of the TPN solution to be fed to the patients. 
     In a standard procedure of IR transmission analysis, a certain amount of liquid is injected into an IR-transparent liquid cell, which is placed between an IR source and an IR detector. The IR beam passes the IR transparent cell windows perpendicularly. By measuring the absorption of light at the IR band, an IR spectrum is obtained, with peaks at frequencies corresponding to the specific chemical bond vibrations. The concentration of the sample can be computed by using the IR absorbance of the sample at a given wavelength. To achieve valid measurements, the liquid cell must contain a pair of optical grade flat windows, which is made of IR transparent materials such as ZnSe, CaF 2 , NaCl, or KBr. The optical path is the distance between the two windows. Commercially available liquid cells for IR transmission spectroscopy are sold as demountable cells with adjustable optical path by changing spacers or as fixed optical path such as the SL2 sandwich cell from the International Crystal Laboratory, or as variable optical path cells such as the TumblIR®/Dialpath® from Agilent Technology Inc. 
     Major inconvenience and technical difficulty in IR analysis involving liquid samples arises from the direct liquid-window contact. 
     Due to the cost of the IR cell windows, disposable windows are not a practical solution, and the windows have to be reused. Therefore, in all current liquid cells on the market, the fluid samples directly contact the IR windows. As the result, the windows have to be cleaned thoroughly after each test to avoid cross-contamination for the subsequent testing using the same liquid cell. This recovering process is usually cumbersome and time-consuming. 
     Also, due to the sample-window direct contact, the selection of the window material is significantly restricted to those that do not react with or dissolve in the contacting liquid. For example, KBr windows, an economic broad-band IR-transparent window, are NOT compatible with aqueous solutions, because these solutions dissolve the KBr window. As the result, high-cost ZnSe or CaF 2  windows must be used, which has inferior performances (i.e. narrower-band) than KBr. 
     2. SUMMARY OF PRIOR ARTS 
     Established approaches for obtaining transmission spectra of liquids require direct liquid-sample holding cell contact. U.S. Pat. No. 7,582,869 describes a design a cell for obtaining transmission spectroscopy for liquids. The cell has two movable windows, between which is the chamber for holding liquids. Liquids can be introduced into the cell, and the optical path can be adjusted before measurement. 
     U.S. Pat. No. 6,280,690 describes methods and apparatus for obtaining transmission spectra of liquid and solid samples. In this disclosed method, liquid contacts a wire mesh at first. The liquid-soaked wire mesh is inserted into the sample holder of a spectrometer so that a beam of radiation passes the liquid remaining on the wire mesh and generates a transmission spectroscopy. 
     An approach that avoids direct liquid-cell contact is to use a flexible enclosure such as a bag or a tube to hold the sampling liquid and analyze the signal after the incident electromagnetic beam passes the enclosure. In this approach, a means is employed to ensure the pre-determined optical path is fixed during a test. The enclosure material must have a low absorption in the particular band of the electromagnetic wave of which the analytical method employs. With this approach, the liquid in the enclosure can be directly loaded/unloaded with the enclosure together without the need of rinsing since the liquid samples do not directly contact the optical components made of KBr, ZnSe or NaCl. Therefore, the analysis time for each sample can be reduced, and various liquid samples can be analyzed. 
     U.S. Pat. No. 4,872,868 shows an analyzer for collection bags which provides an envelope that permits the insertion of reagent&#39;s test strips and the like. 
     U.S. Pat. No. 5,239,860 describes a sensor for continuously measuring alcohol and gasoline fuel mixtures in a clear Teflon tube using a pre-determined optical path and electromagnetic radiation at a pair of wavelengths which are generated by rapidly switching currents through a light-source. Thermopile detectors are used to detect an increase in temperature due to light transmitted through the flowing gasoline/alcohol mixture. 
     U.S. Pat. No. 7,952,710 describes an apparatus and a method for detecting and quantifying constituents in solutions that are held in a bag by spectrometric methods. 
     U.S. Pat. No. 5,510,621 describes an apparatus for measuring components in liquid media, in particular, parenteral nutrients, within a flexible transparent bag. A threaded rod is utilized to adjust the optical path across the bag chamber and includes a passage for electromagnetic radiation of selected wavelengths. The source of electromagnetic radiation is capable of sending radiation into the bag chamber and to detector means which analyzes the radiation passed through or reflected from the components in the bag chamber. 
     This invention employs bags made of thin, low IR-absorbance film. The liquid sample is introduced into the bag and sealed. The bag is first fixed on a supporting bed then the bag-supporting bed assembly is placed into an apparatus in which two parallel IR-transparent windows are pushed against the bag from opposite positions until the pre-determined optical path is reached. The apparatus is then inserted into an IR spectrometer to analyze the liquid sample held between the windows. After the analysis, the bag is removed from the apparatus, and a new bag containing the next liquid sample will be loaded into the apparatus for the next analysis. 
     The invented apparatus and method enable the rapid analysis of multiple liquid samples, and reduction of the cost for analyzing solutions using IR transmission spectroscopy. The structure layout, the mechanism for setting the optical path the design of the bag, as well as the method of conducting sample preparation, loading, unloading are different from all prior arts. 
     This invention has fundamental differences with U.S. Pat. No. 7,952,710 in the following aspects:
         1. The mechanism of setting optical path is different. This invention employs a pair of spacer sheet. In contrast, U.S. Pat. No. 7,952,710 uses a spring-loaded caliper to set optical path;   2. U.S. Pat. No. 7,952,710 is specifically designed for instruments that employ fiber optics as means for introducing incident beam of electromagnetic wave, which is indicated by the design of two ports in the apparatus for incident and outgoing optic fibers. The apparatus disclosed in this invention can be used with spectrometers with fiber optics or regular spectrometers that do not use optic fibers;   3. This invention employs a supporting bed to fix the flexible bag so that the deformation, slippage and wrinkling of the flexible bag can be prevented. U.S. Pat. No. 7,952,710 does not disclose such mechanism.       

     This invention has several differences with U.S. Pat. No. 5,510,621. 
     1. This invention differs from U.S. Pat. No. 5,510,621 in the designs and mechanism in setting the optical path in the following aspects:
         1a) The field of application of U.S. Pat. No. 5,510,621 is the Near Infrared (NIR) and the apparatus is optimized for the typical NIR use. Specifically, the optical path for NIR transmission is 1-15 mm, as the U.S. Pat. No. 5,510,621 mentioned. This invention is optimized for the application in Mid-IR, which has a typical optical path of 0.01-1 mm. U.S. Pat. No. 5,510,621 sets the optical path by turning a threaded rod. In this invention, the optical path is set up using a series of spacer sheet, which have standard thickness from 0.01 to 1 mm.   1b) U.S. Pat. No. 5,510,621 changes the optical path from 0 to 25 mm by turning a threaded rod. There is no mechanism that prevents the rod from overtighten when the optical path is close to 0 mm, which may exert excessive force on the window and damage them. This invention uses the sheet spacer mounted in the elastic window holder to set optical path, which avoids this potential hazard.   1c) In the design of U.S. Pat. No. 5,510,621, by turning the threaded rod, the optical path can be increased or decreased. U.S. Pat. No. 5,510,621 offers no design to set the exact the optical path. To obtain the exact optical path, a caliper or experimental calibration work has to be employed to obtain the distance between the windows. In contrast, in this invention, the spacer sheet with known thickness is used to set the optical path.       

     This invention differs from U.S. Pat. No. 5,510,621 in the design of the bag. 
     In U.S. Pat. No. 5,510,621, the bag is directly placed inside the liquid cell, and the bag is open to the air. To load the bag into the apparatus, the bag has to rely on external suspension devices until the inner surfaces of  42  and  44  of fences  22  and  24  are pushed against the bag and provide sufficient friction to prevent bag slippage. 
     In this invention, two distinct mechanisms are employed to prevent the bag slippage and deformation.
         2a) The bag is placed on a supporting bed and physically fixed on said bed. The weights of the liquid and the bag are supported by said spacer bed. Hence, during analysis, the bag stays vertically, and its surface remains fully stretched. No bag slippage occurs.   2b) The bag is sealed, the liquid sealed in the bag has a positive internal pressure, which renders the both bag walls to maintain a positive curvature. Said positive curvature ensures that when the two windows are pushed against the bag from opposite directions, bag wall that contacts the flat windows remains flat.   These designs avoid bag slippage and wrinkle formation on the bag surface. Said bag seals the liquid sample from the windows, the cell parts and the operators.       

     3. OBJECTIVES AND ADVANTAGES 
     The objectives of this invention are 
     1) to achieve rapid loading and unloading of liquid samples into IR spectrometer for fast analysis by loading and unloading bags sealed with sample liquid as a whole;
 
2) to reduce the analysis costs by using disposable sealed bags for holding liquid sample;
 
3) to remove the restriction on window material selection so that the low-cost and the boarder-band window material can be used for previously non-compatible liquid samples;
 
4) to protect the windows from damages associated with sample contacting;
 
5) to ensure measurements on reactive, unstable, corrosive, hazardous, contagious and filthy samples which were not suitable for the current IR transmission apparatus;
 
6) to set the optical path to a pre-determined value in the range of sub-millimeters (e.g. 0.01-1 mm).
 
     This invention has the following advantages compared to prior arts. 
     Commercial available IR liquid cells such as the SL2 sandwich cell from the International Crystal Laboratory, or Omni-Cell from Specac Inc., Cranston, R.I., use the spacer to set the optical path. These cells are designed for directly inject liquid into said cell, where said liquid contacts the window and the ports. Therefore rinsing is required after each test. Flexible bags that holding sample liquid cannot be used in said liquid cell.
         (a) This invention employs a design that uses the spacer sheet as a means to set the optical path.   (b) This invention employs a design that uses a flexible bag to hold the sample liquid. During IR transmission measurement, said bag is squeezed by two movable windows to reach said optical path set by the spacer.       

     U.S. Pat. No. 5,510,621 discloses an apparatus that use a bag to hold sample liquid, and the bag is placed between two windows, the distance between the windows can be adjusted by turning a threaded rod. 
     This invention combines said two designs, which are not simultaneously disclosed in U.S. Pat. No. 5,510,621 or demonstrated by any commercial available liquid cell on the market. The combination enables the rapid loading and unloading liquid samples by eliminating the time-consuming and high cost rinsing process, and to use the sealed bag with sample in a disposable way using Mid-IR analysis. The combination also enables the use of all IR window materials for analysis of various liquid samples, which reduces the total hardware cost for IR analysis. The combination also enables the use of reactive, unstable, corrosive, hazardous, contagious and filthy samples by isolating the samples from the other parts of the transmission cell and the operators. The low-cost and rapid analysis of multiple liquid samples using Mid-IR transmission spectroscopy, which is a long existed technical difficulty, can be overcome with this invention. Furthermore, liquid samples are sealed in bags before analysis. The sample loading process and analysis process can be separated and completed by different personnel. For analyst who operates the instrument, the training for liquid sample handling and personal protection equipment for that liquid may not be required. Another advantage is that the test is non-invasive and thus the sample can be recovered easily for further test, which is critical when the amount of sample is limited. 
     An apparatus and method for identifying solutions in a translucent transparent or semi-transparent bag, such as sugar in beverages, non-invasively, qualitatively and quantitatively would be a notable advance in the chemical analysis field. 
     4. SUMMARY OF THE INVENTION 
     This invention describes an apparatus and a method for non-invasive, fast analysis of fluid samples using IR transmission spectroscopy. The approach is to analyze liquid samples in flexible sealed bags. Said apparatus loads said bag in a fixed position and sets the pre-determined optical path for subsequent analysis. Said apparatus is composed of three parts, the front plate, the loading bed, and the back plate. The three parts are stack against each other in the order of the front plate, the loading bed, and the back plate and by means of fasteners such as bolts/nuts, springs, or magnets. Two windows are mounted on the center of the front and back plate. Said windows are made of material transparent to said electromagnetic radiation said instrument is employed. The sealed bag is physically attached to the loading bed. Next, said loading bed with said bag is mounted on the back plate. Holes on said loading bed and corresponding posts on said back plate are designed to align said loading bed to be placed exactly in the pre-determined position on said back plate. After the front plate is fastened to loading bed and the back plate, said apparatus is mounted between the source and the detector of an instrument that uses the transmittance of electromagnetic radiation as means of analysis. The sample is ready for analysis. After the measurement, said apparatus can be disassembled and the loading bed can be replaced by a new loading bed with the next sample mounted on it. 
    
    
     
       5. BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is the exploded view of the apparatus. 
         FIG. 2 a   . The exploded view of the back plate, from right side 
         FIG. 2 b   . The exploded view of the back plate, from left side 
         FIG. 3  The exploded view of the supporting bed 
         FIG. 4 . The exploded view of the front plate, from left side 
         FIG. 5  The layout of the sample bag 
         FIG. 6  The scheme for loading and sealing liquid sample into the bag
         a) Inject fluid sample into the bag via the port section; b) After fluid is accumulated in the cell section, squeeze the bag; c) The bag is squeezed until the liquid reaches the neck section; d) and e) The bag is then sealed.       

         FIG. 7  The scheme illustrating the process of installing a sample bag onto the apparatus
         a. Select the correct pair of spacer sheet to set the optical path, and then mount the supporting bed to the back plate through the guiding posts.   b. Mount the bag onto the supporting bed through the guiding posts   c. Install the front plate   d. The apparatus with the sample installed, ready to be measured.       

         FIG. 8  The process of inserting the apparatus into a generic sample holder inside a spectrometer
         a. Slide the apparatus down   b. The apparatus in position for measurement inside the spectrometer       

         FIG. 9  IR peak area of a series of sucrose solutions and their linear relationship with concentration
         (a) IR transmission spectra of sucrose solutions with different concentrations (spectral range 1300-1000 cm −1 ), the samples were in polyethylene bags with a constant OP of 25 μm. (b) Sucrose concentration vs. Absorbance for the band area at 1050 cm −1 .       

         FIG. 10  IR spectra of toluene with different optical path and the relationship between the peak area and corresponding optical path 
       (a) IR transmission spectra of Toluene (spectral range 2200-1650 cm −1 )), the samples were in polyethylene bags and tested with different OP (i.e. various thick spacers) (b) Absorbance vs. Optical path curve for the band area at 1952 cm −1 . A least-square fitting line is plotted, and the coefficient of determination (R 2 =0.996) is displayed as well. 
     
    
    
     For a better understanding of the invention, reference is made to the following detailed description of the preferred embodiments which should be referenced to the herein before described drawings. 
     6. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Various aspects of the present invention will evolve from the following detailed description of the preferred embodiments thereof which should be taken in conjunction with the hereinbefore described drawings. 
     The invention as a whole is depicted in the drawings by reference character  10 . The invention is composed of an apparatus  20  and a bag  12 , which can be mounted and dismounted from  20 . The apparatus  20  is shown in the drawings as including three embodiments, a front plate  22 , a supporting bed  24 , and a back plate  26 . Referring to  FIG. 1 , during measurement, the flexible bag  12  is mounted on the supporting bed  24 . The back plate  26 , the supporting bed  24  with the mounted bag  12  and the front plate  22  are held against each other by means of fasteners, spring-loaded hinges, spring-loaded clips, clamps, magnets or friction. 
       FIG. 2 a    shows the layout of the back plate  26 , which is viewed from right side. The back plate  26  is composed of a back chassis  28 , a window holder  30 , and a window  42 . The drawing in  FIG. 2 b    shows that the back chassis has a rectangular shape from the left viewpoint. The back chassis has two wings  34 ,  36  in the left and right sides of the back chassis. Wings  34 ,  36  are used to guide the apparatus  20  to slide into the alignment slots of standard sample holders of a spectrometer. A round hole  38  in the center of the back chassis  28  is designed for mounting windows on the back chassis  28 . The window holder  30  is made of elastomer. The shape of the window holder  30  can be depicted as three fused co-axial tubes  40 ,  41 ,  44 . The outside diameter of the smallest tube  40  is the same as the diameter of hole  38 . Tube  40  is inserted into hole  38 . Thereby the window holder  30  is mounted on the back chassis  28 . Tubes  40  and  41  have the same inside diameter. The outside diameter of tube  41  and  44  are the same, which is larger than the diameter of the window  42 . The inside diameter of tube  44  is same as the diameter of the window  42 , but it is larger than the inside diameter of the smaller tube  40 . Window  42  is held by tube  44  when it is mounted on window holder  30 . Window  42  is made of materials that have a low Mid-IR absorbance such as KBr, NaCl, ZnSe, Si, CaF 2  or Ge. On the back chassis  28 , four posts  46 ,  48 ,  50 ,  52  are located around hole  30 . Posts  46 ,  48 ,  50 ,  52  are symmetric with respect to the center of hole  30  and they are arranged in a rectangle shape. The border lines of this rectangle formed by posts  46 ,  48 ,  50 ,  52  are parallel to the borderlines of the back chassis  28 . Posts  46 ,  48 ,  50 ,  52  are perpendicular to the flat surface of window  42 . Posts  46 ,  48 ,  50 ,  52  function as the alignment guiding rods for hooking up supporting bed  24 , which has four holes in the corresponding positions. Two spacer bars  54 ,  56  are located along the borders of the back chassis  28 . Spacer bars  54  and  56  are perpendicular to the wings  34 ,  36 . Spacer bar washers  55 ,  57  are on top of spacer bar  54 ,  56 , respectively. Spacer bar washers  55 ,  57  are made of elastomer. There are four tapped holes  58 ,  60 ,  62 ,  64  on the spacer bars  54 ,  56 . Each bar has two holes. There are four through holes  59 ,  61 ,  63 ,  65  on the spacer bar washer  55 ,  57 . Each bar washer has two holes. Bolts are inserted into holes  58 ,  60 ,  62 ,  64  to assemble back plate  26 , supporting bed  24  and front plate  22 . 
       FIG. 3  shows the layout of supporting bed  24 . Supporting bed  24  is a thin sheet with a large round cavity  66  in the center. The diameter of cavity  66  is larger than the diameter of window  42 ,  44 . The center of cavity  66  and center of window  42  are on the same axis as tubes  40 ,  41 ,  44 . Four holes  68 ,  70 ,  72 ,  74  are located on supporting bed  24 . The distances from the centers of holes  68 ,  70 ,  72 ,  74  to the center of cavity  66  are the same. Lines connecting the centers of holes  68 ,  70 ,  72 ,  74  form a rectangle. The diameter of holes  68 ,  70 ,  72 ,  74  is the same as the diameter of posts  46 ,  48 ,  50 ,  52 . The distance between the centers of holes  68 ,  70 ,  72 ,  74  are the same as the distance between the centers of posts  46 ,  48 ,  50 ,  52 . Two sheet spacers  76 ,  78  are attached along the border lines of supporting bed  24 . Sheet spacers  76 ,  78  adopt the same shape as spacer bars  54 ,  56  and are aligned in the same directions as spacer bars  54 ,  56 . The width of sheet spacers  76 ,  78  determines the gap between sheet spacer  76 ,  78 . This gap is designed to be smaller than the diameter of window  42 . Sheet spacers  76 ,  78  are designed to set the optical path, which have exactly the same thickness and their thickness determines the distance between windows  42 ,  84  when they are pushed against each other from opposite directions. Sheet spacers  76 ,  78  are made of ultra-flat sheet with a series of standard thickness such as 10 μm, 20 μm, 30 μm, 50 μm, 100 μm, 200 μm and so on. By choosing a pair of sheet spacers  76 ,  78  with designated thickness, the optical path is set accordingly. 
     The front plate  22  is composed of a front chassis  80 , a window holder  82 , and a window  84 . The drawing of  80  in  FIG. 4  shows that the front chassis has a rectangular shape. Window holder  82  is the same as window holder  30 . Window  84  is identical to window  42 . Front chassis  80 , window holder  82 , and window  84  are mounted in the same way as the assembly of back chassis  28 , window holder  30 , and window  42 . Four through holes  86 ,  88 ,  90 ,  92  are located at the four corners of front chassis  80 . The diameter of holes  86 ,  88 ,  90 ,  92  is the same as the diameter of holes  58 ,  60 ,  62 ,  64 . Four bolts are inserted into holes  86 ,  88 ,  90 ,  92 , and are fastened in holes  58 ,  60 ,  62 ,  64 , respectively. 
     The shape of bag  12  is illustrated in  FIG. 5 . The bag is formed by fusing two pieces of thin film together. The means of fusing includes thermos-fusing, press-fusing, adhesive glue, or stitching by wires. Four holes  94 ,  96 ,  98 ,  100  are located on the four corners of the bag. The diameter of holes  94 ,  96 ,  98 ,  100  is the same as the diameter of posts  46 ,  48 ,  50 ,  52 . The liquid-holding section of bag  12  is composed of a port  102 , a neck  104  and a cell  106 . 
     7. DESCRIPTION OF OPERATION 
     Loading Liquid into the Bag 
     The loading process is illustrated in  FIG. 6 . Bag  12  is placed in the direction that port  102  is up, and cell  106  is down. A pre-determined amount of liquid sample is injected into bag  12  through the opening port  102 . Next, cell  106  is squeezed gently from the two window areas so that the liquid meniscus inside bag  12  reaches to neck  104 . Then a means of sealing, such as thin plastic film thermal sealer, clamps, clip, magnate, glue or stitch, is used to close the bag at port  102  section. The sealing can be conducted once or multiple times at different positions in port  102  section so that no air is remained in the sealed bag. The volume of bag  12  is zero before liquid is injected and the flexible thin film is fully extended with no tension. After liquid injection, because of the volume of liquid sealed in bag  12 , bag  12  forms a positive curvature over cell  106 . 
     Mounting the Sealed Bag on Back Plate  26   
     Four posts  46 ,  48 ,  50 ,  52  on the back chassis  28 , four holes  68 ,  70 ,  74 ,  72  on supporting bed  24 , and holes  96 ,  100 ,  98 ,  94  on bag  12  are aligned according to  FIG. 7 . Next, supporting bed  24  is pushed to let posts  46 ,  48 ,  50 ,  52  inserted into corresponding holes  68 ,  70 ,  74 ,  72  so that supporting bed  24  is fixed on back chassis  28 . Next, bag  12  is pushed to let posts  46 ,  48 ,  50 ,  52  inserted into corresponding holes so that bag  12  is fixed to avoid bag slippage and bag collapse. 
     Assembling the Apparatus  20   
     Front plate  22  is pushed against back plate  26 . A typical means of pushing is to use four long bolts to fasten front plate  22  and back plate  26 . Four threaded bolts are inserted into through holes  86 ,  88 ,  90 ,  92  on front plate  22 , then into four through holes  61 ,  59 ,  65 ,  63  on spacer bar washers  55 ,  57  and ended in tapped holes  60 ,  58 ,  64 ,  62  on back plate  26 . 
     Because front plate  22  is pushed against back plate  26 , window  42  and window  84  are moved against each other until window  42  and window  84  contact sheet spacers  76 ,  78 . Sheet spacers  76 ,  78  are sandwiched between windows  42  and  84 . A further pushing of windows  42  and  84  causes the elastomer window holders  30 ,  82  to deform. The elastic deformation of windows holders  30 ,  82  maintains the distance between windows  42  and  84  and prevents the window from cracking due to excessive pushing forces. 
     Installing the Apparatus into Spectrometer 
       FIG. 8  shows that apparatus  20  is inserted into the standard sample holder in a generic spectrometer by aligning wings  34 ,  36  of apparatus  20  with the slots in the sample holder and then slide the apparatus into the sample holder. A beam of electromagnetic radiation then passes through the sample loaded inside bag  12  and reaches the detector. 
     It will be understood by those skilled in the art that while an embodiment of the invention was disclosed in considerable detail for purposes of illustration, many of these details may be varied without departing from the spirit and scope of the invention. 
     8. EXAMPLES OF APPLICATION 
     Example 1 
     Determine the Sucrose Concentration of Regular Coca-Cola. 
     Six standard solutions containing 0%, 2%, 5%, 10%, 15% and 20% w/w sucrose in distilled water are prepared. These solutions are sealed in the bag and loaded on the apparatus, respectively. The optical path is set to 25 μm and the Mid-IR transmission spectra are acquired for each sample, which are shown in  FIG. 9 a   . The band near 1050 cm −1  reflects the O-C stretching in sucrose molecules, and its peak area is used to assay the concentration of sucrose. [ref. Duarte, I. F. et al  Journal of Agriculture and Food Chemistry,  2002, 50, 3104-3111.] The peak area versus concentration is plotted in  FIG. 9 b   . The fitting shows that the concentration and the corresponding peak area demonstrate a linear relationship, which follows the Lambert-Beer&#39;s law. The least square fitting of the data points in  FIG. 9 b    yields that the coefficient of determination (R 2 ) is 0.991, the slope is 2.23±0.02 and the intercept is 0.12±0.06. Next, a sample coca-cola with unknown sucrose content is also analyzed in the IR spectrometer using the same setup. Its peak area at 1050 cm −1  is 29.5. Using the standard calibration curve plotted in  FIG. 9 b   , we determine that the sucrose concentration in the sample coca-cola is 13.17±0.12% w/w. 
     Example 2 
     Demonstrate that the Absorbance of Toluene is Linearly Dependent on the Set Optical Path of the Apparatus 
     The Lambert-Beer&#39;s law states that the absorbance is a linear function of the optical path. In this experiment,
         1. We set the optical path of the apparatus to 13 μm, 75 μm, 125 μm, 200 μm, 300 μm, and 400 μm;   2. We loaded toluene in sealed bag into the apparatus;   3. We measured the corresponding absorbance of toluene sealed in a bag.       

     The obtained toluene mid-IR spectra in the 1650 cm −1 -2200 cm −1  range are plotted in  FIG. 10 a   . The area of the peak at 1952 cm −1  is used to represent the absorbance of toluene. We then plotted the peak area as a function of the set optical paths in  FIG. 10 b   . The least square fitting of data points in  FIG. 10 b    yields the coefficient of determination (R 2 ) to be 0.996, which indicates the good linearity. Such excellent linearity of the plot demonstrates the accuracy and precision of the optical path setting mechanism of this invention.