The principal monitoring technique for carbon monoxide is IR absorption spectroscopy Most commercial IR absorption instrument use nondispersive gas filter approach. The Instrument consists of an IR light source, (through which ambient air is passed), and a detector The IR source produces continuous radiation at approximately 46 um, which is passed through a rotating gas filter unit that is divided into nitrogen and a CO filled section, The IR beam then passes through the absorption cell, and is directed onto the detector via a narrow band-pass filter The rotating filter unit alternately removes those wave- lengths that are absorbed by CO, or allows the full IR beam to pass through, depending on its position, in the former case, the “reference" beam will not undergo any further attenuation by CO in the ambient sample ,in the latter case, the “measurement” beam will be attenuated by CO according to the Beer-Lambert law. Comparison of the signal derected in the two modes, which is rapidly modulated by the spinning gas filter at 300-400 Hz determines the Co concentration other gases do not give rise to response, as they equally attenuate the reference and measurement beams (assuming that their features do not overlap with those of co). A multipass arrangement is commonly employed in absorption cell, using white-type optics to achieve greater path length and hence improved instrument sensitivity. As with UV absorption measurements of ozone, the temperature and pressure within the absorption cell are required to determine the ambient co mixing ratio. Typical commercial instruments report detection limits of approximately 50 ppb with a time response better than 1 min