Near-infrared spectroscopy

Near-infrared spectroscopy, NIR spectroscopy or NIRS abbreviated, is a physical analysis technique based on spectroscopy in the field of short-wave infrared light. It corresponds substantially to the infrared spectroscopy, which is used in the medium and far infrared range (MIR and FIR), but allows the use of other materials, and radiation sources. But it offers easier access and other forms of analysis generally.

Principle

Near-infrared spectroscopy based spectroscopies as other vibration on the excitation of molecular vibrations caused by electromagnetic radiation in the (near) infrared region. NIRS in the detection takes place in the near infrared (760-2500 nm, or about 13000 to 4000 cm -1). In this area there are overtone and combination vibrations of the basic molecular vibration from the mid-infrared.

The overtone and combination bands are not directly interpreted in the analysis of samples, but is evaluated using statistical procedures. Previously for quantitative determinations, as generally, created in infrared spectroscopy datasets with known content or known concentrations of the substance of interest.

Advantages and disadvantages compared to the IR spectroscopy in the mid-and far-infrared range

Advantages:

• Due to the higher energy of the near-infrared light in comparison to the mid-infrared and the lower absorption coefficient, there is a greater penetration depth and thus easier handling ( larger layer thicknesses: millimeters instead of micrometers)
• Simpler devices by the use of quartz glass or sapphire ( single crystal Al2O3)
• Measurements in the reactor by use of optical fiber technology that can be applied in the MIR and FIR region limited with special materials.
• Simplified sample preparation ( measurement on native samples, no upstream extraction)
• In transmission or absorbance measurements in the reactor, pipeline or tank less susceptible to pollution

Cons:

• The absorption bands in the near infrared are much wider than in the mid-infrared and overlap thereby stronger.
• The use of chemometric methods is necessary ( chemometrics ), except for simple measurements.

Application

NIRS is a nearly ideal method for determining the water content in all kinds of products.

The method finds application in quality analysis of agricultural products (cereals, flour, milk, oil, fruits) and feed for the determination of moisture ( OH ), proteins (proteins, amino group, etc. ), crude fiber ( fiber, CH bond, and others) carboxyl groups (COOH ) in plastics and fat content ( CH bond ).

Today it is used in process control in the food industry, eg in potato chips, also in chemical and pharmaceutical products as well as the petrochemical industry. In the chemical industry, the FT -NIR spectroscopy is widely used in process control, for example, for online analysis of intermediate and end products, particularly in esterification reactions. For incoming inspection of raw materials are often put chemical company, robust, and sensitive FT -NIR spectroscopy, as well as in the development process.

A further application is the separation of waste: beverage cartons, composite materials and the different types of plastics are detected and sorted out with compressed air nozzle from the product stream.

For 30 years, the near-infrared spectroscopy is applied in medicine and neuroscience as an imaging method for measuring the activity of the brain or for the determination of the oxygen content, blood volume and blood flow of various tissues such as brain, muscle, or chest. For measurements of brain activity dynamic changes in the oxygen content of the blood through the skull to be measured throughout. Hence conclusions about circumscribed activations in the cerebral cortex can be derived based on the principle of neurovascular coupling. This procedure can also be an optical brain-computer interface realized. The near-infrared spectrum of the light is used because of between 650 nm and 1000 nm, particularly the light passing through tissue, and thus, an analysis is made ​​possible by the lower tissue layers. Measurements of the oxygen content, blood volume and blood flow of tissue based on the fact that the red blood pigment hemoglobin, which is the main transporter of oxygen in the body that changes color with the oxygen content. Thus, based on the light permeability of the fabric (the more blood in the tissue, the less light passes through ) the blood or hemoglobin concentration can be determined and on the basis of the color of the oxygen content. Since the oxygen supply is medically very important because a lack of oxygen can result within a few minutes to serious damage, the near-infrared spectroscopy is now used increasingly clinically. The field of application is broad, such as the monitoring of oxygenation in the ICU, during operations, in emergency situations, circulatory disorders, sports medicine ( blood circulation in muscles, training optimization ), etc. The near-infrared spectroscopy is highly valued by patients because the measurements are not are invasive and painless and near-infrared light in the intensities used is harmless. The technology has developed considerably in recent years, so that reliable, quantitative measurements and imaging are possible today. Thanks to the miniaturization of wireless systems are already available.