Optical long path length

Gaseous samples require long path length cells to produce absorption bands of reasonable intensity up to several metres of optical path are obtainable from cells incorporating mirrors which produce multiple reflections. For GC-IR, light pipes provide the best sensitivity (p. 117). 

In the late 1970 s, a group of German researchers developed a novel instrument, the long-path length (e.g., 1-17 km) ultraviolet-visible differential optical absorption spectrometer, DOAS (Platt et al., 1979). Application of this instrument in field studies in remote... 

Optical cell grazing angle optical cell, - long path length optical cell... 

Figure 2 Optimal location of flow cell for maximum etendue showing the combination of long path length, small cell volume, and high optical throughput. The light paths are shown by the dotted lines. ...

TEFLON AF 2400 is highly permeable to gases. It has a low refractive index, lower than water. This allows total reflexion using water as core in long path-length capillary cells. It is UV-transparent and very expensive. It is commonly used in tubing, membranes, windows, and optical libers used as flow cells. 

Spectroelectrochemical experiments can be used to probe various adsorp-tion/desorption processes. In particular, changes in the absorbance accruing from such processes can be probed utilizing the large ratio of surface area to solution volume of OTEs with long optical path length (29). Additional information on such processes can be obtained from the Raman spectroelectrochemical experiments described later. 

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