Near Infrared (NIR) Spectroscopy

NIR spectroscopy is an optically based measurement technique that uses radiation from the near infrared region of the electromagnetic spectrum as shown in the table. NIR radiation from a broadband source (quartz tungsten halogen - QTH bulb) interacts with the material under test. A device known as a spectrometer is used to measure the results of this interaction at a number of discrete wavelengths. Materials that can be tested include gases, liquids and solids. For gases and liquids the interaction usually takes the form of transmission/absorption of the NIR radiation through/by the material. For solids the most common method is diffuse reflectance. The following figure illustrates the most common optical arrangement for gases and liquids. A transmission cell allows the sample to flow between two optically transparent windows. NIR radiation is introduced on one side of the cell. The radiation interacts with the sample in the gap between the windows. Typical gap widths (referred to as pathlength) range from 1mm - 100mm for liquids and 0.2m - 2m for gases. For materials with very high radiation absorption properties, the attenuated total reflection (ATR) method can be used to reduce the effective pathlength to the micron range. The radiation is collected by a focusing lense and projected through a fiber optic cable to the spectrometer. The spectrometer measures the radiation intensity at a number of wavelengths. The vector of generated measurements is called the spectral signature of the sample. This spectral signature contains all of the chemical and physical properties of the sample that can be measured in the NIR spectral range. The spectral signature is transmission data and is usually converted to absorbance data to establish a “more” linear relation between the spectral data and chemical/physical properties of the sample. Transmission data is converted to absorbance data using the Beer-Lambert or similar relation. Typically other preprocessing steps are also applied to the spectral signature to maximize the correlation between the spectral data and the desired measurements. Analysis of solids is usually implemented using a diffuse reflectance arrangement. The following figure illustrates one possible optical arrangement for diffuse reflectance. NIR radiation is directed toward the sample. A portion of the radiation penetrates the sample and interacts with the sample through multiple internal reflection and absorption events. A portion of this radiation exits the sample and is collected by a large diameter lense. This radiation is then directed through a fiber optic cable to the spectrometer to generate the sample’s spectral signature. The spectral data is linearized using the Kubelka-Munk relation in a similar fashion that the Beer-Lambert relation is used. Preprocessing of the spectral signature also applies in the case of solids the same as gases and liquids. A regression relation, that has been previously established during the calibration phase, is applied to the spectral signature to compute the desired chemical/physical properties of the sample.

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