Near Infrared Process Measurements

How do NIR process measurements work?

Near Infrared (NIR) Process Measurements are based on organic chemical group absorption bands such as O-H, C-H, N-H and C=O.  The NIR region is above the visible and below the IR region between 800 nm and 2.5 microns or 12,500 – 4000 cm-1  in wave numbers.  Quality Control and Process Monitoring demands have driven the vast expansion of applying near infrared process measurements in the chemical, pharmaceutical, paper, wood, tobacco, food and other industries.

NIR is faster, less expensive, and easier to use than gas and liquid chromatography, mass spectrometry, atomic absorption, and nuclear magnetic resonance.  It’s also faster than wet chemistry and titration methods and easily adaptable to on-line applications. Laboratory full spectrum Near Infrared analyzers help identify where specific components of interest absorb light energy, and chemometric mathematical treatments such as Partial Least Squares and Principle Component Analysis greatly aid selection of appropriate non-absorbing wavelengths.

Near Infrared Process Analyzers typically operate between 1200 and 2400 nm with lead sulphide detectors and narrow band pass interference filters.  These near infrared photometers measure “slices” of the near infrared region where the component of interest absorbs and compare those measurements with non absorbing wavelengths to provide a measurement with analog and digital outputs.  The wide spread acceptance of these Near Infrared Process Measurements is largely due to their accuracy, reliability, and low cost.  Of the three main vibrational spectroscopic techniques (Mid-IR/FTIR, Raman, NIR), near infrared process analyzers are the least expensive to manufacture.

Moisture which is important in the drying and baking processes is also a major component in goods sold by weight.  Customers want high quality goods and don’t want to purchase raw materials with artificially high water weight.  Moisture levels also impact performance characteristics in many products, as well as shelf life in food products.  Moisture strongly absorbs at 1.94 and 1.42 microns.  Similarly, hot melt coatings and adhesives absorb at 2.34 microns, and oil at 1.72 microns.  Water based coatings are calibrated in dry end engineering units by measuring the water and maintaining a consistent liquid/solid ratio.  Oil and water spray additions are based on the same principle.

Near Infrared Process Measurements and Laboratory Measurements are improving product quality every day, and will continue to expand their presence in all industries.

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