IR spectroscopy detects the molecular vibrations during which the polarity of the molecule changes. Traditional IR spectroscopy, 2.5 to 16 microns,(2500 to 16000 nm) is used primarily for the identification of functional groups in organic compounds. This is exemplified by CO2. Carbon Dioxide
IR Spectroscopy U. Colorado Tutorial Tutorial IR Absorbance frequencies
With computer data handling near infrared (NIR), 800 to 2500 nm, has become an analytical technique in quality control laboratories. Many compounds that are colorless in the visible exhibit absorptions in this region. In a quality control situation where the mixture is similar from batch to batch, methods are developed which monitor the NIR at several wavelengths. The levels and differences between absorbances is calibrated with wet chemistry tests. Computers calculate and analyze the absorbances instantaneously and sound alarms whenever a system is out of specifications. The technique is like monitoring several different colors in a mixture and signaling when any one is out of specifications. Many things absorb in this region so most mixtures have overlapping absorbances.
The instruments are rugged and equipped with many different cell types which can be connected to the instrument with fiber optics. Cells may be reflectance types for solids and powders, or flow cells with paths of different lengths for liquids.
Methods are not developed for a desired constituent like alcohol or iron, but for the mixture required at this point in the process. Wet chemistry calibrations may allow the determination of each component in the mixture from the NIR measurements. However, if an additional component were added to the mixture or one of the components were added in a much larger amount, new calibrations would be required to determine concentrations of each component. Near infrared Tutorial
