Optics: measuring and testing – By light interference – Spectroscopy
Reexamination Certificate
2001-02-20
2003-12-23
Glick, Edward J. (Department: 2882)
Optics: measuring and testing
By light interference
Spectroscopy
C356S452000, C250S339080
Reexamination Certificate
active
06667808
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains generally to Fourier transform infrared spectrometers and to sample holders for such spectrometers.
BACKGROUND OF THE INVENTION
Fourier transform infrared (FTIR) spectrometers are utilized to perform accurate and efficient identification of the chemical composition of a sample. Such spectrometers typically incorporate a Michelson interferometer having a moving mirror. The interferometer modulates the infrared beam from an infrared source to provide an output beam in which the intensity of the infrared radiation at various wavelengths is periodically varied. The output beam is focused and passed through or reflected from a sample, after which the beam is collected and focused onto a detector. The detector provides a time varying output signal which contains information concerning the wavelengths of infrared absorbance or reflectance of the sample. Fourier analysis is then performed on the output signal data to yield usable information on the chemical composition of the sample.
Conventional FTIR spectrometers include a sample chamber in which a sample is held in a position to be exposed to the infrared beam from the spectrometer. The sample which is to be analyzed may take various physical states, i.e., a liquid, solid or gas, and solid samples may have various physical characteristics. For example, a solid material to be analyzed may be in the form of a block or sheet of material (e.g., polymer plastics), in the form of powders or granulates, or in specific formed shapes (e.g., pharmaceutical tablets, pills and capsules).
The conventional manner of analyzing these various materials has been to prepare the sample so that it is in a form that can be accepted by the sample holder in the sample chamber of the FTIR spectrometer. For example, for a bulk liquid a small sample of the liquid may be transferred to a small cuvette or other container which is then mounted in the sample holder in the sample chamber. For bulky solid materials, small shavings or particles may be removed from the bulk sample, appropriately prepared (e.g., ground, pulverized, etc.) and placed in a sample holder which can then be inserted into the sample chamber. Other materials may be reduced to a powder which can be held in the sample holder or may be dissolved in a solvent which is then transferred to a cuvette or tube of an appropriate size to be mounted in the sample holder. Such conventional sample preparation techniques may not always be feasible or desirable, and specialized spectrometry equipment has been developed for specialized applications. These include probes, connected by fiber optic cables to a spectrometer, that can be inserted into a liquid, solid powder, or gas to be sampled (e.g., a flowing material where composition information is to be gathered for process control).
Another particular specialized use of spectroscopy equipment is in the pharmaceutical industry. The finished pharmaceuticals are usually in a specific shape, e.g., as pills, tablets, or caplets, some of which may be coated or printed with markings, as well as powder filled hard gel capsules and soft gel capsules having active ingredients suspended in water-free media surrounded by a soft gelatin shell. Classical wet chemistry methods and liquid and gas chromatographic techniques were traditionally used in the pharmaceutical industry to analyze the chemical composition of the finished pharmaceuticals. These methods require chemicals such as solvents, indicators, derivitizing agents, and chromatographic mobile phase solvent mixtures. The use of such chemicals requires specialized facilities and trained personnel, and involves fire and toxicity hazards. Such procedures involve not only the expense of the materials themselves but also the expense of their safe disposal after the analysis is done. For these reasons, nondestructive analysis techniques are increasingly being used for analysis of pharmaceuticals, as well as other compounds. One of the most widely used nondestructive techniques is near-infrared spectrographic analysis. The near-infrared region, generally in wavelengths from about 666 nm to 3333 nm, has been found to be particularly suitable for such nondestructive analysis because of its penetration depth into a pharmaceutical sample. Using near-infrared light, the sample can be analyzed in a reflectance mode or a transmittance mode.
The reflectance mode obtains information from the illuminated surface of the sample. The infrared light reflects from the surface of the sample and from shallow layers beneath the surface. Due to absorption and scattering, most of the information in the reflected light received by the detector is dominated by the composition of the surface layers, such as the coatings of pharmaceutical tablets. Some coating films are made with near-infrared transparent (e.g., modified cellulosic) materials such that the active substances in the tablet are readily detected in reflectance mode without much distortion. Other pharmaceutical formulations have coatings that have color additives or scattering materials, such as TiO
2
, talc, CaCO
3
, etc., that hinder the light from adequately reaching the interior of the tablets. In any event, the reflectance mode is sensitive to variations of the coating thickness and of the composition of the coating material. Further, if a tablet being analyzed is imprinted with ink, the spectral signature of the ink will be detected (which can be shown by comparing analyses of the printed and unprinted side of the tablet). A particular disadvantage of the reflectance sampling mode is that because the interior of the tablet is not readily analyzed, the overall dosage of the tablet cannot be directly quantified. Further, the repeatability of the optical reflectance measurement is affected by the angular position of any imprint pattern on the sample, which may vary from tablet to tablet. Thus, it is often desirable to transmit the near-infrared light through the tablet and analyze the transmitted light in addition to or as an alternative to reflectance measurements. Specialized spectrometry equipment, including specialized sample holders, have been developed for the analysis of pharmaceutical samples in the reflectance mode and in the transmission mode, but generally such equipment is not well suited to carry out both reflectance and transmission measurements on the same sample.
SUMMARY OF THE INVENTION
In accordance with the invention, a multifunctional infrared spectrometer system is capable of performing transmission or reflection measurements, or both, on a variety of samples, including liquids and powders as well as shaped solid samples such as pharmaceutical pills and tablets. The various samples can be tested utilizing the same spectrometer system without modification of the spectrometer and without the addition or rearrangement of sample compartments and sample holders. Preferably, the spectrometer system includes a sample position at which a sample may be mounted in a sample holder for transmission of a modulated infrared beam through the sample, while a sample may be analyzed at a second sample position using a probe connected by fiber optic cables to the spectrometer to analyze samples remote from the spectrometer, while at a third sample position a shaped solid sample such as a pharmaceutical tablet may be analyzed in reflection, transmission, or both. The spectrometer system is adapted to easily and quickly switch between sample positions under the command of the operator by simple commands without requiring the attachment or removal of auxiliary sample compartments or holders.
The multifunctional infrared spectrometer system of the invention includes a source of infrared radiation that provides a beam of infrared, an interferometer which receives the beam from the source and produces a modulated output beam, at least two spatially separated infrared detectors, optical elements transmitting the modulated output beam from the interferometer on a main beam path to a junction position, and optical elements defining a firs
Clermont Todd R.
Deck Francis Jerome
Delaware Louie
Hyatt James Ronald
Jones George Douglas
Barber Therese
Glick Edward J.
Thermo Electron Scientific Instruments Corporation
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