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What is Near-Infrared Spectroscopy (NIR) ?
Near infrared spectroscopy (NIR) is a vibrational spectroscopy technique which employs photon energy in the range of 2.65x10-19 to 7.96x10-20 J. This energy range corresponds to the wavelength range between 750-2500 nm (wavenumbers: 13300-4000 cm-1). Observed absorption bands in the NIR region correspond to overtones and combinations of carbon-hydrogen (CH), oxygen-hydrogen (OH) and nitrogen-hydrogen (NH) vibrations. NIR is a fast and nondestructive technique that provides multi-constituent analysis of virtually any matrix. Its objective is to probe a sample in order to acquire qualitative and/or quantitative information coming from the interaction of near-infrared electromagnetic waves with its constituents.
The analytical methods resulting from the use of the NIR spectroscopic region reflect its most significant characteristics, such as:
• fast (one minute or less per sample),
• high penetration of the probing radiation beam,
• suitable for in-line use,
• nearly universal application (any molecule containing C-H, N-H, S-H or O-H bonds),
• minimum sample preparation demands.
Near infrared spectroscopy (NIR) is routine application in agriculture, for the analysis of pharmaceuticals, combustion products, astronomy, food stuffs and in research. The method has been universally accepted and is widely used in quality assessment of foods, beverages and in food and beverage research.
Major constituents/properties in foods determined with NIR;
• Fats (oils)
• Cereals: dietary fiber
• Scattering properties
• Condiments sucrose, starch, flour
• Meats: beef, poultry, fish
• Fresh foods: fruits, nuts
• Honey, Corn syrup, molasses
• Candy, chocolate, caramel
• Sweets: sucrose, saccharin, honey, corn syrup, molasses
• Beverages: milk, soft and hard
There are three measurement modes used in NIR spectroscopy: transmission, reflection and trans-reflection. The appropriate NIR measurement mode is determined by the optical properties of the samples.
Transmittance (T) measurements are generally used with liquids and solids in liquid suspensions, if the liquid medium is transparent.
Reflection (R) spectroscopy measures the intensity of the light reflected by a sample. Since the spectral reflectance contains little information about composition, diffuse reflectance gives a lot of information about the structure of the sample. Therefore, it stands out as the most used measurement mode in NIR spectroscopy.
Trans-reflectance (T) associates the transmittance and reflectance measurements. In trans-reflectance mode, a mirror or a diffuse-reflectance surface are placed at the end of the probe. Light is sent to the sample and reflected unabsorbed radiation is measured, thus doubling the path length. Spectra of turbid or transparent liquids can be obtained by using trans-reflectance.