Multi-pass cell

Spectrophotometer cell, n - an apparatus which allows a liquid sample or gas to flow between two optical surfaces which are separated by a fixed distance, referred to as the sample pathlength, while simultaneously allowing light to pass through the liquid. There are variations of this including variable-pathlength cells, and multi-pass cells, and so on. 

TG-FT-IR, Pyrolysis analyses were performed on the preliquefaction solids using thermogravimetric (TG) analysis with on-line analysis of the evolved products (including an infrared spectrum of the condensables) by FT-IR. The TG-FTIR method has been described previously (23-25). The Bomem TG/plus instrument was employed. A sample is continuously weighed while it is heated. A flow of helium sweeps the products into a multi-pass cell for FT-IR analysis. Quantitative analysis of up to 20 gas species is performed on line. Quantitation of the tar species is performed by comparison with the balance reading. 

The only exception is the Irina sensor from DrSger, which can be placed in the mainstream. Owing to its bulky sensor, location in the inspiration or expiration limb is preferred to locations between the Y-piece and tubus. Figure 23-27 presents a sectional view with the filter-wheel and the multi-pass cell. 

Figure 23-30. Multi-gas sensor based on Raman scattering, external multi-pass cell.

Vapor phase The vapor or gas is introduced into gas cells which are either about 10 cm long or shorter with a series of internal mirrors (multi-pass cells) which reflect the Infrared beam back and forth lengthening the path-length. The gas cell has NaCl windows at the ends (NaCl is transparent to infrared). 

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