Non-dispersive Infrared spectroscopy (NDIR)

Non-dispersive infrared sensors (NDIR) are simple spectroscopic devices often used as gas detectors for air pollution monitoring in the field. The non-dispersive instrument does not scan the spectrum, but instead is designed to look at that portion of the IR spectrum where the pollutant shows peak absorption. Gas concentration is measured electro-optically by its absorption of a specific wavelength in the infrared region. The NDIR detector has an optical filter in front of it that eliminates all light except the wavelength that the selected gas molecules can absorb. A number of simple, rugged non-dispersive instruments have been designed for quantitative infrared analysis. They can be simple filter- or non-dispersive photometers, or instruments that employs filter wedges in lieu of a dispersing element. Some other instruments do not have wavelength selection device at all. Another type of IR analyser that is popular for analysing carbon monoxide in ambient air uses the gas-filter correlation (GFC) method. With this technique, the IR spectrum from the sample gas is compared with a spectrum engraved on a grating. Two filter chambers are fixed to a rotating filter wheel, one containing pure sample gas and the other containing pure nitrogen. The two filter chambers are alternately brought into the light path from the sample cell. This arrangement is designed to minimise the effects of interfering gases. Disadvantages of non-dispersive instruments are that interference from highly absorbing compounds like water and CO2 other compounds can reduce their usefulness. In addition, there is limited speciation beyond general classes of hydrocarbons. Therefore, the technique is suited best to single compounds or simple mixtures where there are no interferences.