Near infrared spectroscopy
Near infrared spectroscopy , NIR spectroscopy or NIRS for short, is a physical analysis technique based on spectroscopy in the range of short-wave infrared light. It essentially corresponds to infrared spectroscopy , which is used in the medium and far infrared range (MIR and FIR), but allows other materials and radiation sources to be used. However, it usually offers easier access and other forms of analysis.
principle
Near-infrared spectroscopy is based, like other vibration spectroscopy, on the excitation of molecular vibrations by electromagnetic radiation in the (near) infrared range. With NIRS, detection takes place in the near infrared (760–2500 nm or approx. 13,000–4,000 cm −1 ). In this area there are overtone or combination oscillations of the fundamental molecular oscillation from the middle infrared.
The overtone and combination bands are not interpreted directly when analyzing samples, but are evaluated with the help of statistical methods. For quantitative determinations, as is generally the case with infrared spectroscopy, data sets with a known content or known concentration of the substance of interest are created beforehand.
Advantages and disadvantages compared to IR spectroscopy in the medium and far infrared range
Advantages:
- Due to the higher energy of the near-infrared light compared to the mid-infrared and the lower absorption coefficient, there is a greater penetration depth and thus easier handling (greater layer thicknesses: millimeters instead of micrometers)
- Simpler devices through the use of quartz glass or sapphire (single crystalline Al 2 O 3 )
- Measurements in the reactor through the use of fiber optic technology, which can only be used to a limited extent with special materials in the MIR and FIR range.
- Simplified sample preparation (measurement on native samples, no upstream extraction)
- In the case of transmission or absorbance measurements in the reactor, pipeline or tank, less susceptible to contamination
Disadvantage:
- The absorption bands in the near infrared are much broader than in the middle infrared and therefore overlap more strongly.
- Water is a strong near infrared absorber and a very strong far infrared absorber.
- The use of chemometric methods is necessary ( chemometrics ), except for simple measurements.
application
NIRS is an almost ideal method for determining the water content in all kinds of products.
The method is used in quality analyzes of agricultural products ( grain , flour , milk , oil fruits ) and animal feed to determine moisture ( OH group ), proteins (proteins, amino groups , etc.), raw fibers (fibers, CH bonds and others), carboxy groups (COOH) in plastics and fat content (CH bond).
Today it is widely used in process control in the food industry , e.g. B. in potato chips , also in chemical and pharmaceutical products and petrochemicals . In the chemical industry, FT-NIR spectroscopy is widely used in process control, for example for online analysis of intermediate and end products, especially in esterification reactions. Chemical companies often use robust and sensitive FT-NIR spectroscopy to inspect incoming raw materials, as well as in process development.
Another application is waste separation : beverage cartons , composite materials and the various types of plastic are recognized and separated from the product flow using compressed air nozzles.
The use of infrared spectroscopy in medicine was first investigated in 1958 at the Max Planck Institute for Physics (Werner Heisenberg Institute) in Munich. The aim was to be able to measure metabolic processes in biological tissues without blood. The breakthrough came in 1969 when the first successful animal experiment with an infrared laser showed the measurability of carbon dioxide and - indirectly - of oxyhemoglobin in the blood using this method. This experiment laid the foundation for non-invasive, infrared spectroscopic oximetry in medicine.
For 30 years, near-infrared spectroscopy has been used in medicine and neuroscience as an imaging method for measuring the activity of the brain or for determining the oxygen content, blood volume and blood flow of various tissues such as B. brain, muscles or chest applied. When measuring brain activity, dynamic changes in the oxygen content of the blood are measured through the skull. From this, based on the principle of neurovascular coupling, conclusions can be drawn about circumscribed activations in the cerebral cortex . This process can also be used to create an optical brain-computer interface . The near-infrared spectrum of the light is used because between 650 nm and 1000 nm the light passes through tissue particularly well and thus an analysis of deeper tissue layers is made possible. Measurements of the oxygen content, blood volume and blood flow of tissue are based on the fact that the red blood pigment hemoglobin , which is the main carrier of oxygen in the body, changes its color with the oxygen content. Thus, the hemoglobin or blood concentration can be determined based on the light permeability of the tissue (the more blood in the tissue, the less light passes through) and the oxygen content can be determined based on the color. Since the oxygen supply is medically very important, because a lack of oxygen can lead to serious damage within a few minutes, near-infrared spectroscopy is increasingly used clinically today. The field of application is wide, e.g. B. monitoring the oxygen supply in the intensive care unit, during operations, in emergency situations, in circulatory disorders, in sports medicine (blood flow to muscles, training optimization), etc. Near infrared spectroscopy is very much appreciated by patients because the measurements are non-invasive and painless, and near infrared light is harmless in the intensities used. The technology has developed significantly in recent years, so that reliable, quantitative measurements and imaging are possible today. Thanks to miniaturization, wireless systems are already available.
literature
- FF Jobsis: Noninvasive, Infrared Monitoring of Cerebral and Myocardial Oxygen Sufficiency and Circulatory Parameters . In: Science . tape 198 , no. 4323 , December 23, 1977, p. 1264-1267 , doi : 10.1126 / science.929199 .
- M. Kouli: Experimental investigations for the non-invasive determination of the cerebral blood flow in adults with the help of near-infrared spectroscopy . Dissertation . Technical University of Munich, 2001.
- Martin Wolf, Marco Ferrari, Valentina Quaresima: Progress of near-infrared spectroscopy and topography for brain and muscle clinical applications . In: Journal of Biomedical Optics . tape 12 , no. 6 , November 15, 2007, p. 062104-062104-14 , doi : 10.1117 / 1.2804899 .
- Thomas Muehlemann, Daniel Haensse, Martin Wolf: Wireless miniaturized in-vivo near infrared imaging . In: Optics Express . tape 16 , no. 14 , July 7, 2008, p. 10323-10330 , doi : 10.1364 / OE.16.010323 .
swell
- ↑ Hannes Ebding: Process analysis in the chemical industry . In: GIT Labor-Fachzeitschrift . No. 8 , 2011, ISSN 0016-3538 , p. 2 ( hellma-analytics.com [PDF]).
- ↑ zollernalbkreis.de: sorting and recycling of lightweight packaging from the yellow bag ( Memento of 6 July 2007 at the Internet Archive ), accessed on 22 December of 2009.
- ↑ Henry H. Mantsch: The road to medical vibrational spectroscopy - a history . In: The Analyst . tape 138 , no. 14 , 2013, p. 3863 , doi : 10.1039 / c3an90035e .
- ↑ Dr. Nils Kaiser - the forefather of infrared spectroscopy in medicine. In: dr-nils-kaiser.de. Retrieved April 16, 2016 .
- ^ W. Von Casimir, N. Kaiser, F. Keilmann, A. Mayer, H. Vogel: Dielectric properties of oxyhemoglobin and deoxyhemoglobin in aqueous solution at microwave frequencies . In: Biopolymers . tape 6 , no. 12 , December 1, 1968, ISSN 1097-0282 , p. 1705-1715 , doi : 10.1002 / gdp . 1968.360061205 .
- ↑ Patent US4169676 : Method for determining the contents of metabolic products in the blood. Registered on February 17, 1977 , published October 2, 1979 , inventor: Nils Kaiser.