Near infrared spectroscopy
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Near infrared spectroscopy (NIRS) is a spectroscopic method utilising the near infra-red region of the electromagnetic spectrum (from about 800 nm to 2500 nm). Typical applications include pharmaceutical, medical diagnostics (including blood sugar and oximetry), food and agrochemical quality control, as well as combustion research.
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[edit] Theory
Near-infrared spectroscopy is based on molecular overtone and combination vibrations. Such transitions are forbidden by the selection rules of quantum mechanics. As a result, the molar absorptivity in the near IR region is typically quite small. One advantage is that NIR can typically penetrate much farther into a sample than mid infrared radiation. Near infrared spectroscopy is therefore not a particularly sensitive technique, but it can be very useful in probing bulk material with little or no sample preparation.
The molecular overtone and combination bands seen in the near-IR are typically very broad, leading to complex spectra; it can be difficult to assign specific features to specific chemical components. Multivariate (multiple wavelength) calibration techniques (e.g., principal components analysis or partial least squares) are often employed to extract the desired chemical information. Careful development of a set of calibration samples and application of multivariate calibration techniques is essential for near infrared analytical methods.
[edit] History
The discovery of near-infrared energy is credited to William Herschel in the early 19th century. However, systematic exploration of near-infrared spectroscopy did not begin until the 1950s.
Initially, NIR spectroscopy was treated as an extension of techniques at other wavelengths, such as UV/Vis or mid-IR methods. The first commercial standalone NIR system was introduced in the 1980s.
Academic and industrial interest (and commercial instrumentation) rapidly grew in the 1990s, due in part to the introduction of systems based on optical fibers and more sensitive detectors based on modern semiconductors such as GaAs.
[edit] Instrumentation
Instrumentation for near-IR spectroscopy is similar to instruments for the visible and mid-IR ranges. There is a source, a detector, and a dispersive element (such as a prism, or more commonly a diffraction grating) to allow the intensity at different wavelengths to be recorded. Fourier transform instruments using an interferometer are also common, especially for wavelengths above ~1000 nm. Depending on the sample, the spectrum can be measured in transmission or in reflection.
Common incandescent or quartz halogen light bulbs are most often used as broadband sources of near infrared radiation for analytical applications. Light-emitting diodes (LEDs) are also used; they offer greater lifetime and spectral stability and reduced power requirements.<ref>Alper Bozkurt et al., Biomedical Engineering Online 2005, 4:29 (DOI:10.1186/1475-925X-4-29)</ref>.
The type of detector used depends primarily on the range of wavelengths to be measured. Silicon-based CCDs are suitable for the shorter end of the NIR range, but are not sufficiently sensitive over most of the range. InGaAs and PbS devices are more suitable.
Many commercial instruments for UV/vis spectroscopy are capable of recording spectra in the NIR range (to perhaps ~900 nm). In the same way, the range of some mid-IR instruments may extend into the NIR. In these instruments the detector used for the NIR wavelengths is often the same detector used for the instrument's "main" range of interest.
[edit] Applications
Typical applications of NIR spectroscopy include the analysis of foodstuffs, pharmaceuticals, combustion products and a major branch of astronomical spectroscopy.
[edit] Astronomical Spectroscopy
Near-infrared spectroscopy is used in astronomy for studying the atmospheres of cool stars where molecules can form. The vibrational and rotational signatures of molecules such as titanium oxide, cyanide and carbon monoxide can be seen in this wavelength range and can give a clue towards the star's spectral type.
[edit] Remote monitoring
Techniques have been developed for NIR spectroscopic imaging. These have been used for a wide range of uses, including the remote investigation of plants and soils. Data can be collected from instruments on airplanes or satellites to assess ground cover and soil chemistry.
[edit] Medical uses
It is commonly used for medical diagnostics, in particular for oximetry (the measurement of oxygen levels in the blood) and for blood sugar determination. NIR spectroscopy is not typically the most sensitive technique; however, it is non-invasive, as measurements can be obtained directly through the skin.
NIRS can be accompanied by other modalities such as magnetic resonance imaging (MRI) or computerized tomography (CT). For example, NIRS can be used for non-invasive assessment of the brain function through an intact skull in human subjects, by detecting changes in blood hemoglobin concentrations associated with neural activity. This is sometimes known as fNIR (functional near-infrared imaging) or NIRSI (near-infrared spectroscopic imaging). ("NIRSI" techniques are not unique to medical applications.)
Nirs is also currently being used in branches of Cognitive psychology as a partial replacement for fMRI techniques. NIRS can be used on infants, where fMRI cannot, and NIRS is much more portable than fMRI machines. However, NIRS cannot fully replace fMRI because it can only be used to scan cortical tissue, where fMRI can be used to measure activation throughout the brain.
[edit] References
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