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Dispersive near-infrared spectrometer with automatic wavelength calibration

Dispersive near-infrared spectrometer with automatic wavelength calibration description/claims

This application is based on and claims priority from U.S. patent application Ser. No. 10/890,942 filed Jul. 14, 2004, which is a continuation of U.S. patent application Ser. No. 10/093,584 filed Mar. 8, 2002, which claims priority to Provisional patent Application Ser. No. 60/274,300 filed on Mar. 8, 2001.

The present invention relates to a Near-infrared (NIR) spectrometer with automatic wavelength calibration without the need of external calibrating. NIR spectroscopy is the measurement of the wavelength and intensity of the absorption of near-infrared light by a sample. Near-infrared light spans the 800 nm-2.5 micrometers (μm) range and is energetic enough to excite overtones and combinations of molecular vibrations to higher energy levels. NIR spectroscopy is typically used for quantitative measurement of organic functional groups, especially O—H, N—H, and C—H. Analyte detection limits are typically 0.11%.

NIR spectroscopy has been shown to be a powerful analytical tool for the analysis of agricultural products, food products, petroleum products, and pharmaceuticals products. Recently, NIR spectroscopy has been approved for the analysis of pharmaceutical products, a factor that is likely to dramatically extend the number of applications of the technique. In general, when NIR spectroscopy is combined with multivariate calibration procedures, the analytical methodology that results is rapid, accurate, and requires virtually no sample preparation.1

In conventional NIR spectroscopy, a multivariate statistical model is developed that attempts to correlate subtle changes in the NIR spectrum with known compositional changes determined by standard analytical technology. Once a robust model has been developed, NIR spectroscopic measurements can be substituted for the more time consuming, labor-intensive conventional analytical measurements.2 To be completely useful, however, a model developed on one spectrometer in the laboratory should be capable of being used on different spectrometers without having to go through the model development all over again with the new instrument. To transfer a model from one spectrometer to another successfully, both instruments must ideally be identical.3

Many NIR spectrometers in use today employ dispersive systems that use diffraction grating monochromators. For these instruments, accurate wavelength calibration is important if the calibration models developed in the laboratory are to be used successfully on other instruments in the production environment. If the wavelength scales of different spectrometers are miscalibrated (as they inevitably are), problems with calibration transfer will occur.4 Because of this, the standardization of NIR spectrometers has been pursued. The rational behind this being that if instruments are alike and remain stable enough, calibration transfer no longer becomes an analytical performance issue. Instrument standardization helps ensure that spectra produced from different instruments of the same design are essentially identical. In order to successfully carry out the various instrument standardization protocols, such as those suggested by Workman and Coates5 and Wang, et al.6, it is necessary to develop strategies that would accurately characterize all the instrumental variables of importance (i.e., wavelength and photometric accuracy, spectral bandwidth, and stray light). One way to avoid this problem is to use a wavelength standard to validate the wavelength scale of the spectrometer. Various wavelength standards exist.7-10

Recently, Busch and co-workers have proposed the use of trichloromethane as a substance with sharper, isolated absorption bands that are suitable for wavelength calibration of spectrometers in the NIR region.11 The study of the use of trichloromethane as a wavelength standard showed that calibration of the wavelength scale of NIR instruments is absolutely essential, and a typical dispersive NIR spectrometer may be off by as much as 12 nm in the NIR region. Busch and co-workers have also assembled a research-grade NIR spectrometer that has been designed to allow the effect of various instrumental parameters on spectrometer performance to be studied in a systematic fashion. This is the same NIR spectrometer used to study the role of trichloromethane as a wavelength standard for NIR spectroscopy and to evaluate the stray light level in dispersive NIR spectrometers that has been designed to allow the effect of various instrumental parameters on spectrometer performance to be studied in a systematic fashion.12 This disclosure describes a novel, dispersive, diffraction grating, NIR spectrometer that automatically calibrates the wavelength scale of the instrument without the need for external wavelength calibration materials.

In accordance with the above and related objects, the present invention is a dispersive, diffraction grating, NIR spectrometer that automatically calibrates the wavelength scale of the instrument without the need for external wavelength calibration materials. In a preferred embodiment, the present invention results from the novel combination of: 1) a low power He—Ne laser at right angles to the source beam of the spectrometer (FIGS. 2 and 3); 2) a folding mirror to redirect the collimated laser beam so that it is parallel to the source beam (see FIGS. 1 and 2); 3) the tendency of diffraction gratings to produce overlapping spectra of higher orders; 4) a “polka dot” beam splitter to redirect the majority of the laser beam toward the reference detector (FIGS. 3 and 4); 5) PbS detectors and 6) a software routine written in Lab VIEW that automatically corrects the wavelength scale of the instrument from the positions of the 632.8 nm laser line in the spectrum. Methods for making the aforesaid invention are included. In one particular embodiment, the claimed method includes obtaining an enhanced calibration set of NIR spectra by improving a dispersive, diffraction grating NIR spectrometer so that it automatically calibrates the wavelength scale of the spectrometer without the need for external wavelength calibration means. The improvement is further defined as obtaining and installing the novel parts as described above.

FIG. 1 is a block diagram of the present invention.

FIG. 2 is an enlarged view diagram of the low power He—Ne laser at a right angle to the source beam of the spectrometer.

FIG. 3 is a side view of the NIR spectrometer which shows a low power He—Ne laser at right angles to the source beam of the spectrometer.

FIG. 4A is a diagram showing a sample compartment.

FIG. 4B is a diagram showing a “Polka-dot” beam splitter which directs the majority of the laser beam toward the reference detector.

FIG. 5 is a side view of the “Polka-dot” beam splitter which directs the majority of the laser beam toward the reference detector.

FIG. 6 shows the mathematical basis for the LabVIEW program that automatically calibrates the spectrometer.

FIG. 7 is a graph showing the apparent locations of the 632.8 nm laser line that appears in different diffraction orders.

• Provisional Patent
• Utility Patent

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