The high penetration depth and easy handling in particular make near infrared spectroscopy (NIRS) relevant for many application areas. Whether it is the quality inspection of chemical, pharmaceutical or agricultural products, process inspections in the food industry or the identification of composite materials: various analyses can be performed in many industries using NIRS.
Light is a type of electromagnetic radiation with a spectrum that ranges from short wave, energy-rich gamma rays to long wave, low-energy radio waves. The spectral range between a wave length of 760 and 2500 nm in particular plays a role in near infrared spectroscopy, because it is above the light that can be seen by humans. When this radiation hits substances where a moving electrical charge is available to their molecules, they are stimulated to vibrate. The mechanical vibrations are measured in the form of deflections and are characteristic for every NIR active bond. This allows conclusions to be made regarding the sample material.
The most common model to illustrate NIR-spectroscopy describes these physical processes as four fundamental vibration types. In addition to normal vibrations along the bond axis of a molecule (stretching vibration) there are also deformation vibrations. The latter are sub-divided further, depending on the direction of vibration, into rocking, wagging and twisting vibration. In this way, different functional groups and molecule bond types exhibit characteristic vibrations in the near infrared range. For instance, these include C-O (alcohols, ether, acids, ester), N-H (amine, amide) NO2 (nitrogroup), C=C (aromatics, alkene) and C=O (aldehyde, ketones, acids, ester).
NIR-spectroscopy makes use of a special effect: harmonic and combination vibration of the basic vibration can be observed in the wave length range of NIR spectroscopy. Unlike radiation in the medium infrared range (MIR), the short wave, energy-rich radiation has the crucial advantage that only the basic vibrations can be observed here. Harmonic vibrations can result from the simultaneous intake of several light quanta. Combination vibrations occur when a light quanta stimulates two different vibrations at the same time.
In comparison, only a little energy is used when higher order vibrations are stimulated. This results in the much greater penetration depth of the near infrared light compared to the medium infrared range where only a few micrometers are possible. This means that a sample pre-treatment usually isn't required. The additional vibration modes lead to additional bands that increase the information content of the spectra. NIR-spectra contain overlays of many complex bands with a higher order and are much more complicated in evaluation than MIR spectra for instance. The molecule absorbed radiation portion is characteristic for its structure. The corresponding absorption bands are reflected in very particular areas of the NIR-spectrum and can be used to identify the molecules.
In an analysis using NIRS, the samples are not interpreted directly instead the harmonic and combination bands are evaluated with the help of static procedures. For quantitative determinations data sets with known content or concentrations of the relevant substance must first be created. The requirements for an NIR-spectroscopic analysis are satisfied by many substances. However, there are also materials that are not NIR-active. These include inorganic salt for instance, which cannot be made to vibrate by energy in the wave range of NIR-radiation due to its rigid, crystalline grate structure.
During practical application NIR-radiation is applied to the sample substance and split according to wave length after reflection or transmission by the sample using a dispersive element or monochromator. The light components are guided onto a photosensitive detector, which measures the photon density using the photoelectric effect depending on the wave length interval. The light intensities recorded in this way are transferred to the wave length axis and identified as the sample's intensity spectrum. The absorption spectra of the sample is obtained from the ratio of the incident radiation to the intensity spectrum. All absorption spectra present a complex interplay of absorption, reflection and scatter effects, which is why extensive structuring of data is required in order to make the chemical information accessible.
The extracted absorption spectra, which ideally only includes the chemical information, can now be consulted to identify the measured substance. Due to the nature of the absorption spectra at hand with bands that are often broad, chemometric analysis methods are used. These enable the identification of the substance using sophisticated mathematical and statistical methods.
Dr. Alexander Wolter is founder and Managing Director of HiperScan.
