Near-infrared spectroscopy (NIRS) is a spectroscopic method that uses the near-infrared region of the electromagnetic spectrum (from 780 nm to 2500 nm)
Where Infra red (IR) range spectroscopy measures thermal vibrations of molecular bonds, the Near-infrared spectroscopy range is measuring overtones and combination vibrations of molecular bonds.
This means NIR has the potential to reveal how molecular bonds interacts, where IR mostly reveals single molecule bond absorption.
Spectroscopic range. Source: Shimadzu.com
Applications for NIR
NIR well suited to analyze intra molecular bonds in water and other hydrocarbon liquids.
Liquid water has less light absorption in the 780nm to 2500 nm region. This gives better penetration through the whole sample, but lower resolution.
When Using the latest ultra sensitive CCD chip set and multivariate analysis, it is however possible to obtain detailed information about the hydrogen bond formation and other intra molecular bonds in the solution.
The method has been extensively used in food, agrochemical and pharmaceutical industry for content analysis of liquid samples. The main advantage is the deep penetration capability of several cm in the sample. FT-NIR can be used for a wide variety applications, such as for acceptance testing when receiving raw materials or before shipping products, or for monitoring in reaction or mixing processes.
Below is seen examples of samples suitable for NIR measurements.
| Petroleum / Fuel |
Octane number, aromatic components, or olefin number |
| Foods / Beverages |
Moisture, protein, fat, sugar, or alcohol content |
| Fibers |
Fiber type, moisture content, or oil content |
| Chemistry / Polymers |
Polymerization level, residual monomer content, hydroxyl group content, blend ratio, or moisture content |
| Pharmaceuticals |
Main ingredients, additives, moisture content, or raw material acceptance testing |
| Pulp / Paper |
Pulp / Paper |
Study of water using NIR
A relatively new field for spectroscopic analysis of water solutes is called Aquaphotomics. It uses near-infrared (NIR) spectroscopy but other wavelengths in the visual range can be used.
The field studies the hydrogen bond formation of water solutes to obtain information about hydration and potential cluster formation of water molecules around substances, mainly bio molecules. Below is seen the absorption of water in near infrared region.
Source: Tsenkova, R.. “AquaPhotomics: Water Absorbance Pattern as a Biological Marker for Disease Diagnosis and Disease Understanding.” NIR News 18 (2007): 14 - 16.