WO1997043619A1

WO1997043619A1 - Sampling device for spectrometric measurements

Info

Publication number: WO1997043619A1
Application number: PCT/US1997/003132
Authority: WO
Inventors: David G. Cameron; Hector L. Casal
Original assignee: Dhc Analysis, Inc.
Priority date: 1996-05-13
Filing date: 1997-02-28
Publication date: 1997-11-20
Also published as: AU2139597A

Abstract

A sample is deposited in a closable, flexible container (60). The container (60) has an expandable, sealable opening (64) permitting simple deposition of samples therein. The container (60) is placed between a pair of opposed windows (30) respectively mounted in a movable head (24) and another head (16). Alternatively, the container (60) is placed between a window (30) and a movable plate (84). The container (60) is suspended from an adjustable rod (38) having a clip (44). The movable head (24) or plate (84) moves to press the container (60). The thickness of the sample to be analyzed is controlled by movement of the head (24) or plate (84). The windows (30) are circular and each has a curved or beveled face (68) pressing against the sample. Movement of the head (24) or plate (84) is controlled by a programmable controller (52). Heating and temperature sensing devices (54) are provided to control sample temperature.

Description

SAMPLING DEVICE FOR SPECTROMETRIC MEASUREMENTS

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the measurement of absorption and reflection spectra of solids, liquids, and gases in the ultra-violet, visible, near infrared, infrared, and far-infrared regions of the electromagnetic spectrum. Specifically, the invention relates to a device for holding a sample to be analyzed by transmission or reflectance spectroscopy using a spectrometer.

2. Description of the Related Art

Optical spectrometers are used for qualitative and quantitative analyses. They can be used to identify chemical compounds contained in samples through recognition of the characteristic absorptions at various wavelengths. Such spectrometers are also used for determination of the amounts of compounds in samples through calibration equations and for determination of physical properties of samples through calibration with samples of known properties. An optical spectrometer typically includes a light source. There are several modes of operation of spectrometers. In some, radiation from the source is collimated and directed to an optical device, such as a monochromator, where the radiation is separated according to the wavelength, the radiation exiting the monochromator is directed to a detector. There is provision for inserting a sample held in an appropriate device in the path of the radiation. In other spectrometers the radiation from the source is directed to an optical device, such as an interferometer, where constructive and destructive interference occurs. The modulated radiation exiting this device is directed to an appropriate detector. In these cases there is also provision for inserting a sample held in an appropriate sampling device. A spectrometer measures the radiation energy transmitted, reflected, or emitted by a sample as a function of wavelength. It is the practice in the art to measure transmission, reflection, and emission spectra. In a transmission measurement the amount of radiation energy transmitted through the sample is measured, and usually ratioed against the energy .incident on the sample. The measurement of the transmitted energy and the incident energy are made as a function of the wavelength of the radiation emitted by the source. The wavelength is expressed in the units commonly used for the particular spectral region. All units used are equivalent and can be converted to each other with well known conversion factors. The level of energy reflected by the sample is measured in a reflection measurement, and usually ratioed against that incident on the sample. As in the case of transmission measurement, the level of energy is measured as a function of wavelength which is expressed in the units commonly used for the particular spectral region. If the sample is optically smooth at the measurement wavelengths, the measurement is commonly referred to as specular reflection, and the angle of reflection is equal to the angle of incidence. If the surface is optically rough, there occurs a combination of reflection and absorption phenomena; the incident radiation is reflected at many ang'les, and the measurement is commonly referred to as a diffuse reflectance measurement. These measurements are also made as a function of the wavelength of the incident radiation, expressed in the units commonly used for the particular spectral region. The level of energy emitted by the sample is measured in an emission experiment. In this case, the radiation source of the spectrometer is turned off or disconnected and the sample is located such that the radiation emitted can enter the optical device in the same manner as the radiation from the source. The level of emitted radiation is measured as a function of wavelength and usually ratioed against the radiation emitted by a material of known emissivity at different temperatures. In transmission and reflection measurements, the samples to be analyzed for qualitative or quantitative purposes must be located in the path of the radiation from the source, or in lieu of the source for emission measurements. There are several experimental considerations to be taken into account when placing samples to be analyzed in spectrometers. In transmission measurements the amount of sample is usually controlled by placing the samples in special cells. The purpose of the cells is to hold the samples. The thickness of material held in the cell must be such that it is possible to control the amount of radiation absorbed by the sample. Depending on the spectral region and characteristics of the samples, the thicknesses vary from a few microns to several meters. For example, for organic liquids in the infrared spectral region typical sample thicknesses used are 20 microns while thicknesses of several meters are used for the measurement of the spectra of gases. The thickness of the cell must be known for quantitative analyses, and further, it must be possible to measure different samples at known and reproducible thicknesses. There are many cells well known in the prior art for achieving these measurements. ' Optical components of spectrometers, such as mirrors, lenses, monochromators, interferometers, are built of materials having the appropriate properties of transmission or reflection for the optical range being used. The cells must consist of materials which are not opaque in the spectral region of interest to the spectrometric measurement. Window materials for the different regions are well known in the prior art. In some cases, for example for the visible, ultraviolet, and near- infrared ranges, cells made of different types of glasses may be used. A liquid to be analyzed must be introduced in the cell. After the measurement is completed, the liquid is removed from the cell and the cell is cleaned with appropriate solvents. In some cases, it is difficult to introduce viscous liquids in cells of short pathlength and cells are very difficult to clean after measuring viscous samples. The problems encountered are related to the difficulty of pushing a viscous liquid through a small opening. In general, cells for spectrometric measurement are expensive and made of delicate materials. Cells for use in the infrared spectral region are commonly made of hygroscopic materials which require the use of dry solvents for cleaning. Solids are measured in different forms as large, flat, single particles, such as flat surfaces and large single crystals and as multiple particles such as grains, powders, etc. In both cases, the particles must be constrained relative to the beam of radiation used for the spectrometric measurement. For diffuse reflection measurements of particulate solids, it is customary to "smooth" the surface to be measured. There exists a large group of samples which cannot be conveniently sampled in the cells used commonly for transmission and reflection measurements. This group of samples includes suspensions (solids in liquids) and emulsions (liquids in liquids) . Examples of these include milk (suspension of oil in water) , paints, polymer pastes composed of fillers and the polymerization mixture before the polymerization reaction has taken place. There is no convenient sampling strategy for these types of samples. It is difficult to introduce them in cells or tubes and a major drawback is that cleaning of the cells is generally difficult or impossible. Control of sample thickness in spectrometric measurement is of critical importance for several reasons. When the sample thickness is too large, most of the incident energy will be absorbed by the sample and the resulting signal measured at the detector is very weak and, therefore, the measurements are imprecise and inaccurate. The extreme case is that of total absorption by the sample. In such cases, it is not possible to measure a transmission spectrum. If the sample thickness is too small, the amount of energy transmitted by the sample is practically the same as the energy incident on the sample. In these cases, the amount of energy absorbed is of the order of magnitude of the noise in the spectrum and therefore the measurement will also be imprecise and inaccurate. It is also important that the sample thickness be uniform such that all the incident radiation travels through the same amount of sample. Otherwise, the resultant measurement is equivalent to the summation of transmission of the sample through different pathlengths, this results in non- reproducible measurements. The requirements of reproducible, uniform pathlengths are critical for obtaining reproducible spectrometric measurements. These requirements can be met with the commonly used practices in the prior art for some samples. For some other types of samples, it is difficult and sometimes impossible to obtain transmission spectra in a reproducible manner. Another drawback of most of the sample cells used commonly in the prior art is that these must be cleaned, generally using chemical solvents. Cleaning with chemical solvents increases the overall costs of 'spectrometric measurements, operator costs increase as time is used for cleaning, and disposal of chemical solvents also adds to the overall cost. SUMMARY OF THE INVENTION

The present invention provides an apparatus for locating a fixed and uniform thickness of sample in the measurement beam of a spectrometer such that precise, accurate spectra can be measured and used for both qualitative and quantitative analysis. The apparatus includes a source of a radiation beam, a first rigid window disposed in the path of the beam, and a second rigid window disposed in the path of the beam and opposing the first window. A sample container has flexible walls and an expandable opening, and the container is adapted for containing a sample therein. A support is adapted for holding the sample container between the windows in the path of the beam. A detector is provided for receiving the beam transmitted through the sample. The opening of the container is sealable. The windows are adapted for pressing the container so as to maintain a selected sample thickness during a spectroscopic analysis. The windows each have a face adapted for contacting the sample container, said faces being convex. The convex faces have radii such that the pathlength for the light beam through the windows, container, and sample is constant over the radius of the light beam. Alternatively, the windows each have a face adapted for contacting the sample container, the faces having substantially flat, parallel portions and beveled portions sloping away from the parallel portions. The flat portions each have a diameter at least as great as the diameter of the light beam. First and second opposed heads have the respective fir'st and second windows mounted therein. A passage through each of the heads is adapted for passing the light beam therethrough and the windows are disposed at respective passages. One of the heads is movable and the other of the heads is stationary so as to press the container between the windows. A first cover connects the source to the stationary head so as to enclose a space between the stationary head and the light source and a second cover connects the detector to the movable head so as to close a space between the movable head and the detector. The second cover is extendable so as to extend with movement of the movable head. A driver is connected for moving the movable head toward the other head so as to press the sample container between the first and second windows so as to provide a selected transmission pathlength of the light beam through the sample. A driver controller is connected for operating the driver. The support is a vertically movable, horizontally extending rod and a clip for holding the container, the clip being disposed on the rod and located above a space between the windows. A heater/cooler is disposed adjacent the container and adapted for adjusting temperature of the sample. An adjustable attenuator is disposed between the source and the detector. According to another embodiment of the invention, the spectroscopy apparatus includes a source of a radiation beam, a window disposed in the path of the beam, and a plate disposed opposite the window. A sample container has flexible walls and an expandable opening, the container being adapted for containing a sample therein. A support is adapted for holding the sample container between the window and the plate in the path of the beam. A detector is provided for receiving the beam reflected by the sample or the plate. The plate can have a reflective face opposing the window.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a partially schematic top plan view of a transmission spectroscopy apparatus according to the invention; Fig. 2 shows a front view of the apparatus of Fig. 1; Fig. 3 shows a perspective view of a sample container according to the invention; Figs. 4, 5 and 7 show side views of different embodiments of windows according to the invention; and Fig. 6 shows a partially schematic top plan view of a reflectance spectroscopy apparatus according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to Figs. 1 and 2, a spectroscopy apparatus 10 includes a base 12 having a source of electromagnetic radiation, such as a light source 14, mounted thereon. The light source 14 is preferably adapted for directing a beam of light at a frequency in the ultraviolet, visible, and/or infrared regions suitable for performing spectroscopic analysis of a sample. A stationary head 16 is mounted on the base 12 adjacent the light source 14. The stationary head is provided with a central passage 18 adapted for passing a light beam from the source 14 that is directed through the passage. A cover 20 is disposed between the light source 14 and the stationary head 16 and permits passage of the light beam from the source to the passage 18. The cover 20 blocks stray light and to maintain instrument purge. A light detector 22 is disposed opposite the light source 14. A movable head 24 is provided adjacent the detector 22 and opposite the stationary head 24. The movable head 24 is provided with a central passage 26 and a cover is disposed between the movable head and the detector 22. Opposed faces of the heads 16, 24 are provided with windows 30, described in more detail below, covering the respective passages 18, 26. A light attenuator 32 is disposed between the light source 14 and light detector 22, preferably in the passage 18 of stationary head 16. The attenuator 32 can be manually or automatically adjustable in place or changeable by removal and replacement with a different attenuator. An optical device, such as a monochrometor or interferometer, is included in the path of the beam, preferably as a component of the source 14 or detector 22. Other components, such as mirrors or lenses, can also be provided in the beam path. A container support 34 is mounted on the base 12 adjacent the heads 16, 24 and includes a post 36 extending vertically from the base. A horizontally extending rod 38 is mounted to the post 36 by a vertically movable bracket 40 held in place by a thumb screw 42. The rod 38 extends above and between the opposed faces of the heads 16, 24. A fastener, such as a spring clip 44, is mounted on the rod 38 above the center of the passages 18, 26. The movable head 24 is mounted to a mounting plate 45. The mounting plate 45 is movably mounted on the base 12 by a support 46 having a track or guide along which the mounting plate slides. A driver 48 mounted on the base 12 is connected to move the movable head 24 by moving a- rod 50 connected to the mounting plate 45. The driver includes a motor or manual crank, for example, connected to operate the rod 50 by gears, a linkage or a belt, for example. A controller 52 is connected to operate the driver 48. For example, the controller can automatically operate a servo or stepping motor to precisely control movement of the movable head 24 according to user input. Mechanical stops can also be provided to control the movable head position. The cover 28 is extendable so as to cover the space between the movable head and the detector when the head is moved. According to one embodiment of the invention, both heads 16, 24, and therefore both windows 30, are movable. In such an apparatus, a driving mechanism similar to the driver 48, mounting plate 45, support 46, and linkage 50 is provided for each head 16, 24. The windows can thus be simultaneously moved toward each other. Corresponding controls are also provided. The heads 16, 24 are provided with pressure sensors 53 and/or temperature sensors 54 adjacent the windows 30. Heater/coolers 55, such as Pelletier junctions or separate heating and cooling devices, in the heads 16, 24 are connected to a temperature and pressure control and display panel 56. The pressure control interfaces with the driver controller 52. The temperature and driver controllers can be adapted for deenergizing the heater/coolers and retracting the movable head on sensing overtemperature or overpressure conditions. Referring to Fig. 3, a sample container 60 is provided for a sample to be analyzed. Preferably, the container comprises a bag having flexible walls 62, a sealable opening 64, and a sealing member 66. The container walls 62 are such that the opening 64 is expandable so as to permit insertion of a viscous sample, for example. The sealing member 66 is adapted for sealing the opening 64 temporarily or permanently. The container is made of a durable material that is chemically inert to the components of the sample and transparent or adequately transmits light at the frequency or frequency ranges used for analysis. Polyethylene having a thickness of 25 to 50 μm, Teflon®, and other materials are generally suitable, depending on the light frequency and sample components. Referring to Figs. 4 and 5, each window 30 is a generally circular disk provided with a face 68 adapted for contacting and pressing one wall of the sample container. As shown in Fig. 4, the face 68 is convex so as to remove wrinkles in the container wall at a region through which the light travels. As shown in Fig. 5, the face 68 is bevelled or otherwise provided with a flat portion 70 at a region through which the light travels. The flat portion preferably has a diameter at least as great as the diameter of the light beam. The face 68 slopes or curves away from the flat portion 70 toward edges of the window. The beveled configuration removes wrinkles in the container wall at a region through which the light travels. The windows are preferably configured so that the pathlength for the light beam through the windows, container, and sample is constant over the radius of the light beam. The windows 30 are preferably rigid for pressing the container and sample to a selected thickness. The windows are preferably transparent or adequately transmissive at the frequency used for analysis. The windows are preferably inert to sample components and the atmosphere. For example, polyethylene and silicon are used for far infrared; zinc selenide is used for mid and near infrared; calcium fluoride and barium fluoride is used for mid and near infrared and visible; quartz is used for near infrared, visible, and ultraviolet; sapphire is used for near infrared and visible; and glass is used for near infrared to ultraviolet ranges of the spectrum. A protective coating, such as Teflon® or diamond can also be used. In another embodiment, the window 30 has a diameter substantially equal to the beam diameter. The face 68 of the window is substantially flat. The window is countersunk in the head 14 or 26 so that a face 72 of the head is flush with and slopes away from the window face 68. In operation, a sample is placed in the container 62 through the opening 64, which can be adequately opened to permit automatic or manual insertion of samples having any of many different properties. Depending on the state and viscosity of the sample, the sample is sprayed, poured, dropped, scraped or otherwise appropriately placed in the container. For example, a highly viscous sample can be selected with a sterile applicator and scraped into the container. The container is hung from the rod 38 by the clip 44. The height of the sample is adjusted by vertically moving the rod 38 and engaging the thumb screw 42 against the post 36. ' The driver 48 moves the movable head 24 toward the sample and the stationary head 16 until the opposed faces of the heads 16, 24 and windows 30 press the container to provide a desired sample thickness as determined manually or input to the driver controller 52. Movement of the movable head 24 can be controlled to achieve a selected pressure in the sample as determined by the pressure sensors 53. The container 60 can be sealed prior to or subsequent to being pressed by the windows. The sample can be heated or cooled to a selected temperature by the heater/coolers 55 as determined by the temperature sensors 54 and controller 56. The light source 14 directs a light beam at a selected frequency through the sample to the detector 22. The attenuator 32 is adjusted or replaced to achieve a desired light magnitude. The detector analyzes or is connected to analyze the sample based on the characteristics of the light be transmitted through the sample from the source 14 to the detector 22. Measurements can be performed at different pressures, pathlengths, and temperatures by appropriately moving the movable head 24 or adjusting the heater/coolers 55. Pathlength calibration is performed by physical measurements or by spectral analysis of a sample having known characteristic absorption indices. The light beam is discontinued and the movable head 24 is retracted. The sample container 60 is released from the clip and appropriately discarded. Referring to Fig. 6, many of the components previously discussed are utilized in a different embodiment of the invention and are referred to with the same reference numbers. The light source 14 and light detector 22 are interconnected by an enclosure 80 defining an angled light path having two intersecting legs. A stationary window mount 82 is located at the corner or intersection of the enclosure 80 legs. One of the windows 30 is disposed in the window mount 82. A movable plate 84 is disposed opposite the window 30 and provided with a driver 48 for moving the plate. The plate 84 is a rigid material having a face 88 parallel with and opposed to the window face 68. The plate face 88 is flat or configured similarly to the window face 68 previously described. Where a substantial amount of light is to be reflected by the plate, a reflective material, such as polished stainless steel, surface coated metal mirror, or a non-absorbing ceramic diffuse reflector, can be used. Where substantial reflection from the plate is not required or desired, for example, when only diffuse reflectance spectra are to be measured, innumerable non-reflective materials are suitable. The pressure sensors 53, temperature sensors 54, and heater/coolers 55 are mounted adjacent the window and at the plate. The sensors and heater/coolers are connected to the temperature and pressure controller 56 and the driver 48 is connected to the driver controller 52. The container support 34 is disposed adjacent and between the plate 84 and window 30. The attenuator 32 is located in the enclosure 80 between the light source 14 and the window 30. In operation, the sample is placed in the container and hung from the support as discussed above. The driver 48 moves the plate against the container to press the sample between the plate and window 30. Sample thickness, temperature, and pressure are controlled as previously described. The light source directs a light beam at a selected frequency through the window to the sample. Light is diffused by the sample and reflected to the detector 22 for analysis. For some samples light is transmitted by the sample and reflected by the plate to the detector for analysis. When a diffuse reflectance measurement is required from a solid, emulsion, or solid-liquid suspension, then the thickness of the sample must be such that all of the energy measured is reflected from the sample. This is done by locating the sample in the apparatus according to the invention and selecting the thickness such that this criterion is met. Depending on the viscosity and the fluidity of the sample, the movement of the window or plate is controlled by either sensing the pressure or fixing the pathlength. The invention is appropriate for samples comprising gases, liquids, emulsions and solid-liquid suspensions and particulate solids. The present disclosure describes several embodiments of the invention, however, the invention is not limited to these embodiments. Other variations are contemplated to be within the spirit and scope of the invention and appended claims.

Claims

CLAIMSWHAT IS CLAIMED IS:

1. A spectroscopy apparatus comprising: a source of a radiation beam; a first window disposed in the path of the beam; a second window disposed in the path of the beam and opposing the first window; a sample container having flexible walls and an expandable opening, the container being adapted for containing a sample therein; a support adapted for holding the sample container between the windows in the path of the beam; and a detector for receiving the beam transmitted through the sample.

2. An apparatus according to claim 1, wherein the opening of the container is sealable.

3. An apparatus according to claim 1, wherein the windows are rigid.

4. An apparatus according to claim 1, wherein the windows are adapted for pressing the container so as to maintain a selected sample thickness during a spectroscopic analysis.

5. An apparatus according to claim 1, wherein the windows each have a face adapted for contacting the sample container, said faces being convex.

6. An apparatus according to claim 1, wherein the windows each have a face adapted for contacting the sample container, the faces having a substantially flat, parallel portions and beveled portions sloping away from the parallel portions.

7. An apparatus according to claim 6, wherein the flat portions each have a diameter at least as great as the diameter of the beam.

8. An apparatus according to claim 1, further comprising first and second opposed heads having the respective first and second windows mounted therein.

9. An apparatus according to claim 8, further comprising a passage through each of the heads, the passages being adapted for passing the beam therethrough and the windows are disposed at respective passages.

10. An apparatus according to claim 8, wherein one of the heads is movable so as to press the container between the windows.

11. An apparatus according to claim 10, wherein the other of the heads is stationary.

12. An apparatus according to claim 11, further comprising a first cover connecting the source to the stationary head so as to enclose a space between the stationary head and the source and a second cover connecting the detector to the movable head so as to close a space between the movable head and the detector, the second cover being extendable so as to extend with movement of the movable head.

13. An apparatus according to claim 10, further comprising a driver connected for moving the movable head toward the other head so as to press the sample container between the first and second windows so as to provide a selected transmission pathlength of the beam through the sample.

14. An apparatus according to claim 13, further comprising a driver controller connected for operating the driver.

15. An apparatus according to claim 1, wherein the support comprises a movably mounted fastener for holding the container.

16. An apparatus according to claim 1, wherein the support comprises a vertically movable, horizontally extending rod and a clip for holding the container, the clip being disposed on the rod and located above a space between the windows.

17. An apparatus according to claim 1, further comprising a heater/cooler disposed adjacent the container and adapted for adjusting temperature of the sample.

18. An apparatus according to claim 1, further comprising an attenuator disposed between the source and the detector.

19. A spectroscopy apparatus comprising: a source of a light beam; a first head having a passage therethrough adapted for passing the light beam from the source; a first rigid window disposed at the passage of the first head; a second head having a passage therethrough adapted for passing the light beam from the source, the second head being movable; a second rigid window disposed at the passage of the second head; a sample container having flexible walls and an expandable, sealable opening, the container being adapted for containing a sample therein; a rod and fastener for supporting the sample container between the heads; a driver connected for moving the second head toward the first head so as to press the sample container between the first and second windows so as to provide a selected transmission pathlength of the light beam through the sample; and a detector for receiving light transmitted through the sample.

20. An apparatus according to claim 19, wherein the windows each have a face adapted for contacting the sample container, said faces being convex.

21. An apparatus according to claim 19, wherein the windows each have a face adapted for contacting the sample container, the faces having a substantially flat, parallel portions and beveled portions sloping away from the parallel portions.

22. A spectroscopy apparatus comprising: a source of a light beam; a stationary head having a passage therethrough adapted for passing the light beam from the source; a first cover connecting the source to the stationary head so as to enclose a space between the stationary head and the light source; a first window disposed at the passage of the stationary head, the first window being rigid and transparent to the light beam; a first heater/cooler disposed on the stationary head; a movable head having a passage therethrough adapted for passing the light beam from the source; a second window disposed at the passage of the movable head, the second window being rigid and transparent to the light beam; a second heater/cooler disposed on the movable head; a sample container having flexible walls and an expandable, sealable opening, the container being adapted for containing a sample therein and made of a material that is transparent to the light beam; a rod and fastener for supporting the sample container between the movable and stationary heads; a driver connected for moving the movable head toward the stationary head so as to press the sample container between the first and second windows so as to provide a selected transmission pathlength of the light beam through the sample; a driver controller connected for operating the driver; a detector for receiving light transmitted through the sample; a second cover connecting the detector to the movable head so as to close a space between the movable head and the detector, the second cover being extendable so as to extend with movement of the movable head; and an adjustable attenuator disposed between the sample container and the light source.

23. An apparatus according to claim 22, wherein the windows each have a face adapted for contacting the sample container, said faces being convex.

24. An apparatus according to claim 22, wherein the windows each have a face adapted for contacting the sample container, the faces having a substantially flat, parallel portions and beveled portions sloping away from the parallel portions.

25. A spectroscopy apparatus comprising: a source of a radiation beam; a window disposed in the path of the beam; a plate disposed opposite the window; a sample container having flexible walls and an expandable opening, the container being adapted for containing a sample therein; a support adapted for holding the sample container between the window and the plate in the path of the beam; and a detector for receiving the beam reflected by the sample or the plate.

26. An apparatus according to claim 25, wherein the window is rigid.

27. An apparatus according to claim 25, wherein the plate has a reflective face opposing the window.

28. An apparatus according to claim 25, wherein the plate is movable and adapted for pressing the container between the plate and window so as to maintain a selected sample thickness during a spectroscopic analysis.

29. An apparatus according to claim 28, wherein the window is stationary.

30. An apparatus according to claim 25, wherein the source is disposed so as to transmit the beam through the window and the detector is disposed so as to receive the beam reflected at an angle with respect to the transmitted beam.

31. An apparatus according to claim 8, wherein each head has a face adapted for contacting the container and each window has a substantially flat face adapted for contacting the container, wherein the head faces slope away from the respective window faces.

32. An apparatus according to claim 19, wherein each head has a face adapted for contacting the container and each window has a substantially flat face adapted for contacting the container, wherein the head faces slope away from the respective window faces.