Near Infra-Red (NIR) Spectroscopy
Spectroscopy is a scientific discipline studying interactions of light with matter. Light can be of different wavelengths, which is represented by the electromagnetic spectrum. The IR region is roughly divided into three intervals: near, mid and far-IR. The near-IR (NIR) region covers the wavelength range 780 - 2500 nm (12500 - 4000 cm-1).
Absorption of light in the IR region causes molecules to vibrate and rotate. Absorption of light in the matter is usually not uniform and depends on molecular structure. At certain intervals the absorption is more intense, which is represented in the form of absorption bands. In NIR, the absorption bands are related to the combination vibrations and overtones of C-H, O-H and N-H chemical moieties in the material. Plots of absorbance vs. wavelength are called absorption spectra. Almost each absorption spectrum is unique and spectra of slightly different materials are only slightly different.
NIR spectra have only a few significant peaks, but they are exceptionally information-rich due to the number of overlapping absorption bands. Thus, interpretation of NIR spectra is usually combined with mathematical and statistical methods (i.e. chemometric methods) in order to extract the necessary information.
Spectrometers can be divided into non-dispersive IR spectrometers (no wavelength selection possible), dispersive ones, and Fourier transform (FT) spectrometers. Briefly, a dispersive spectrophotometer contains a polychromatic radiation source, a monochromator, a sample holder and a detector. Dispersive systems represent an attractive compromise between instrumental size (portability) and spectral quality.
The quality of IR spectra depends on sample properties and instrumentation. When the incident IR light illuminates the sample, the beam is reflected, absorbed, transmitted or scattered. Thin samples are ideal for transmission spectroscopy, whereas rough, porous or powdery samples are ideal for diffuse reflection spectroscopy. Small samples require microscopic methods, for samples with low transmission internal reflection spectroscopy is used.
Below, a portable and a bench-top instrument are shown.
In the frame of the SurveNIR project, our goal was to construct an instrument, which would allow rapid handling of a number of documents (books, archival documents, drawings) during collection surveys. Thus, we designed a flat-surface instrument to allow for safe manuulation of flat paper-like objects. To know more about our instrument, see SurveNIR Instrument.