Absorption cells, long path

Liquefied Xe and Kr have two features which make them particularly attractive as low-temperature solvents for examining the spectra of unstable organometallics. These solvents are inert, and this is important as the significant role of even innocuous solvents such as cyclohexane is more readily recognised in contrast to conventional solvents for IR spectroscopy they have no absorptions over a wide range of the spectrum, which also permits the use of special long path cells to overcome problems of low solubility. 

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). 

The nearly parallel laser beam furthermore allows absorption spectroscopy experiments which realize a long light path at low pressures in the absorption cell without loss in intensity due to beam spread effects it also eliminates disturbing reflections from cell walls. 

A system for cold vapour AAS is shown in Fig. 7.3. The evolved mercury vapour is passed to a long path-length absorption cell, usually constructed of Pyrex glass tubing with silica end windows. A transient absorption peak is observed. In some systems, a recirculating pump is used to cycle the mercury vapour around the system and achieve a steady reading. 

Atmospheric molecules such as 02, Os, NO and NOz are inherently reactive because of the free radical nature of their electronic structures. In addition, there are literally hundreds of free radical species produced in the atmosphere via either photochemical or dark reactions of various hydrocarbons [1,2,27]. Clearly, an important prerequisite to laboratory studies of atmospheric chemistry is the ability to generate key free radical species in a clean fashion. Some representative techniques for generating the major free radical reactants, i.e., HO, HOO, R, RO and ROO (R = alkyl or other organic group), in combination with a long path IR absorption cell-chemical reactor are described below. 

Mercury and those elements (antimony, arsenic, bismuth, germanium, lead, selenium, tellurium and tin) which form volatile covalent hydrides may be separated from the matrix by vapour generation. The use of tin(II) chloride to generate elemental mercury and its subsequent aeration into a long-path absorption cell with silica windows has been described elsewhere in this book, as has the use of sodium borohydride to produce hydrides which are swept to a flame or heated tube for atomisation. This approach is far more successful for mercury than for the other elements, as the hydride generation technique is subject to interference from a large number of transition metals and oxyanions. 

AlO. Ando, A., Fuwa, K., and Vallee, B. L., Physical basis of atomic absorption spectrometry, II. Influence of temperature gradients on spatial distribution of neutral atoms in long path cells. Anal. Chem. 42, 818-825 (1970). 

Other products observed in this first study with the long-path cell were aldehyde, alkyl nitrate, formic acid, carbon monoxide, carbon dioxide, and water. Several prominent absorptions vhich apparently all belong to one compound could not be identified. This interesting product, referred to as compound X, has been subjected to considerable study in an attempt to determine its structure and physical properties. Aside from ozone, it is probably the most important product of these reactions (37). 

Figure 17-32. Flow sensing in absorption with long optical path cells, (a) White cell [185] (b) CEA cell I187J.

For a single-pass FM spectrometer working at 150 GHz the best operating condition is to minimise background, by controlling power reflection within and at each end of a long-path absorption cell, and to apply a modulation depth of 240 kHz p-p at a sample pressure 8-13 Pa. By contrast in a cavity spectrometer, the optimum modulation depth is governed by the cavity width, rather than the sample linewidth (Section 2.4). 

New high resolution (0.01 nm) measurements of the absorption cross-sections of NO2 were performed in the 300-500 nm range at 293, 240 and 220 K using a coolable 5m long path absorption cell (Malicet). Experiments were carried out at very low NO2 pressures in order to reduce absorption contributions of the dimer, N2O4. A definite temperature effect (up to 6 %) was observed in the structured region of the spectrum. 

As the cold-vapor mercury sample is already in the atomic state, there is no need of an atomizer, per se. The vapor, transferred directly from the cell or desorbed as a plug from a heated amalgamation trap, is commonly swept into a moderately heated (resistance wound heating to 200°C) 10 cm quartz T-tube located within the optical beam of a conventional AA spectrometer. Attenuation of an intense electrodeless discharge lamp line source at 253.7nm is used as a measure of the absorption. Alternatively, dedicated continuum source AA-based spectrometers fitted with long path absorption cells (30 cm) are frequently used to increase sensitivity and detection limit. 

Various methods have been developed for remote gas sensing. These include differential optical-absorption spectroscopy (DOAS), differential absorption lidar (DIAL), and a number of methods that use spectroscopic methods with an atmospheric path in place of a laboratory long-path cell, for example tunable diode laser absorption spectroscopy (TDLAS) and Fourier transform infrared (FTIR) spectroscopy. 

In a variant of the long-path absorption technique, radiation from a diode laser or a light-emitting diode is transmitted fibre-optically to remotely located multi-pass absorption cells (Sects. 6.5.6, 9.2.1) and the partially absorbed beam is sent back to the measurement system also using fibre-optics. Using such a system, wliich operates at short IR wavelengths (1 to 2 pm) at which... 

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