Gas- and Vapor-Phase Samples

To measure the spectrum of a gas or vapor, the sample must be contained in a gas cell of an appropriate pathlength. The standard length of a cell for the measurement of the spectra at relatively high partial pressures (0.1 to 0.01 atm) is 10 cm. Cells of pathlengths between 1 and 10 cm fit easily in the sample compartments of all 

Another type of multipass cell, known as the Herriott cell, was described by Herriott and Schulte in 1965 [8]. Although the Herriott cell is most commonly used with lasers, it has also been used for long-path FT-IR measurements. This cell consists of two spherical mirrors with a separation that is about the same as the radius of curvature of the mirrors. If a laser beam is projected in just off the edge of one of the spherical mirrors, so that it hits the center of the other mirror, multiple passing ensues. Because the beam is off-axis and the mirrors are not quite confocal, the beams do not come back on themselves as they do in a White cell, but 

Most White cells are designed to monitor static samples and do not have good flow characteristics. The ideal ceU in this respect has a high pathlength/volume ratio, so that the contents are readily flushed out. An example of such a cell is an internally gold-coated light-pipe of the type used to monitor the effluent firom a capillary gas chromatograph by FT-IR spectrometry (GC/FT-IR). As described 

Vidrine, Optical materials, http //www.vidrine.coin/iropmat.htm, 2002. 

Vidrine, Liquid chromatography detection using FT-IR, in Fourier Transform Infrared Spectroscopy Applications to Chemical Systems, J. R. Ferraro and L. J. Basile, Eds., Academic Press, New York, 1979, Vol. 2, p. 129. 

---