Infrared beam, reference cell

When measuring CO concentration, the reference signal is obtained when the beam is passed through the sample chamber and the CO cell. The absorption is then saturated due to the high CO concentration in the cell. Consequently, the reference signal is practically nondependent on the CO concentration in the sample gas. When the beam passes through the sample chamber and the N2 filter, the absorption is dependent on the CO concentration in the sample chamber, as the N2 filter does not absorb energy from the infrared beam. 

Air drawn through a vacuum pump into the gas cuvette of a nondispersive infrared spectrophotometer IR absorption by CO is measured using two parallel IR beams through sample and reference cell and a selective detector, detector signal amplified concentration of analyte determined from a calibration curve prepared from standard calibration gases (ASTM Method D 3162-91, 1993). 

An infrared spectrometer measures the frequencies of infrared light absorbed by a compound. In a simple infrared spectrometer (Figure 12-4), two beams of light are used. The sample beam passes through the sample cell, while the reference beam passes through a reference cell that contains only the solvent. A rotating mirror alternately allows light from each of the two beams to enter the monochromator. 

An important and widely encountered noise source, one that is proportional to transmittance, results from failure to position sample and reference cells repro-ducibly with respect to the beam during replicate transmittance measurements. All cells have minor imperfections. As a consequence, reflection and. scattering losses vary as different sections of the cell window arc exposed to the beam small variations in transmittance result. Rothman, Crouch, and Ingle have shown that this uncertainly often is the most common limitation to the accuracy of high-quality ultraviolet-visible spectrophotometers. It is also a serious source of uncertainty in infrared instruments. 

The operation of FTIR spectrometers has been described in detail elsewhere, (see, for example. Refs. [4, 64]), as have the advantages of these over dispersive instruments [4, 5, 64]. Briefly, the heart of an FTIR spectrometer is the Michaelson interferometer (MI) (Fig. 6). The infrared beam leaves the source, S, and is incident on a beam-splitter, B. Fifty percent of the light is transmitted to a moving mirror, MM, and 50% to a fixed mirror, FM. On reflection, these rays recombine and interfere at the beam-splitter before reaching the detector, D, via the window, W, and reflective working electrode, WE, of the spectroelectrochemical cell. The system also includes a reference laser, RL, which follows the same path through the interferometer, after which it is intercepted and directed at the laser detector, LD. 

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