Optical Absorption Spectrometry

Allen, H. C. Brauers, T. Finlayson-Pitts, B. J. Illustrating Deviations in the Beer-Lambert Law in an Instrumental Analysis Laboratory Measuring Atmospheric Pollutants by Differential Optical Absorption Spectrometry, /. Chem. 

The reaction with BrO, however, is potentially important under some conditions. As discussed in Chapter 11, UV-visible differential optical absorption spectrometry (DOAS) has been used to measure BrO under these conditions (e.g., see Hausmann et al., 1993 Platt and Hausmann, 1994 and Tuckermann et al., 1997). Concentrations up to about 30 ppt have been measured. The rate constant for the BrO-DMS reaction is 2.6 X 10 13 cm3 molecule-1 s-1 at 298 K (Bedjanian et al., 1996) and gives with essentially unit yield DMSO and a bromine atom (Barnes et al., 1993 Bedjanian et al., 1996) ... 

O, UV absorption Chemiluminescence Differential optical absorption spectrometry... 

Evidence for the contribution of the CIO + BrO interaction is found in the detection and measurement of OCIO that is formed as a major product of this reaction, reaction (31a). This species has a very characteristic banded absorption structure in the UV and visible regions, which makes it an ideal candidate for measurement using differential optical absorption spectrometry (see Chapter 11). With this technique, enhanced levels of OCIO have been measured in both the Antarctic and the Arctic (e.g., Solomon et al., 1987, 1988 Wahner and Schiller, 1992 Sanders et al., 1993). From such measurements, it was estimated that about 20-30% of the total ozone loss observed at McMurdo during September 1987 and 1991 was due to the CIO + BrO cycle, with the remainder primarily due to the formation and photolysis of the CIO dimer (Sanders et al., 1993). The formation of OCIO from the CIO + BrO reaction has also been observed outside the polar vortex and attributed to enhanced contributions from bromine chemistry due to the heterogeneous activation of BrONOz on aerosol particles (e.g., Erie et al., 1998). 

Pitts et al. (1985) first used differential optical absorption spectrometry (DOAS) to establish unequivocally that N02 injected into a mobile home forms HONO. Interestingly, the dependence of the rate of HONO generation on the N02 concentration was similar to that measured in laboratory systems, consistent with production in, or on, a thin film of water adsorbed on surfaces. A number of studies have confirmed that the behavior is similar to that in laboratory systems i.e., the rate of production of HONO increases with N02 and with relative humidity. Indoor levels of HONO as high as 8 ppb as a 24-h average and 40 ppb as a 6-h... 

The Frankfort LPA instrument (51-53) departs from both of these instruments in two principal ways it achieves the necessary path length within a 6-m folded-path cell, and it rapidly scans a narrow-band frequency-doubled dye laser across the spectral region of interest (the Qi(2) line group) in a process sometimes called differential optical absorption spectrometry (DOAS). The scanning rate is sufficient to ensure that the observed air volume is chemically and physically stationary during each scan (the baseline standard deviation is less than 2 x 10-4 for a 0.2-ms scan). The laser output is actively feedback-stabilized to provide a flat spectral baseline, and a detection limit better than 10"5 in optical density has been claimed. A summary of published LPA configurations is given in Table II. 

The purpose of this article is to study the viability of the copper chloride thermochemical cycle by studying the hydrolysis reaction of CuCl2 which is not favoured thermodynamically. To better understand the occurrence of possible side reactions, together with a good control of the kinetics of the hydrolysis reaction, the use of optical absorption spectrometries, UV visible spectrometry to detect molecular chlorine which may be formed in side reactions, FTIR spectrometry to follow the concentrations of H20 and HCl is proposed. 

To assess the viability of the copper-chloride cycle, a dedicated experimental programme is proposed the study of the occurrence of possible side reactions. In order not to change the speciation of the vapour phase, the use of optical absorption spectrometry is proposed UV visible spectrometry to detect the possible presence of molecular chlorine, product of side reactions. 

The hydrolysis reaction kinetics using optical absorption spectrometries... 

Fig. 7. Lower and Upper Detection Limits. When a measurement process has both minimum and maximum signal bounds, as in radioactive decay and optical absorption spectrometry, LLD and ULD must both be considered. Dashed line signal lower and upper detection limits map onto the age and concentration lower and upper limits (arrows) via the exponential function.

Beer s law shows mathematically, based on observed experimental facts, that there is a linear relationship between A and the concentration of an absorbing species if the path length and the wavelength of incident radiation are kept constant. This is an extremely important relationship in analytical spectroscopy. It forms the basis for the quantitative measurement of the concentration of an analyte in samples by quantitative measurement of the amount of absorbed radiation. The quantitative measurement of radiation intensity is called spectrometry. Beer s law is used in all quantitative optical absorption spectrometry—IR absorption spectrometry, AAS, UV/VIS absorption spectrometry, and so on. 

Conventional optical absorption spectrometry has detection limits of between 0.01 and 1 mM for the actinides. Highly sensitive spectroscopic methods have been developed, based on powerful laser light sources. Time resolved laser fluorescence spectroscopy (TRLFS), based on the combined measurement of relaxation time and fluorescence wavelength, is capable of speciating Cm(III) down to 10 mol/L but is restricted to fluorescent species like U(VI) and Cm(III). Spectroscopic methods based on the detection of nonradiative relaxation are the laser-induced photoacoustic spectroscopy (LPAS) and the laser-induced thermal leasing spectroscopy (LTLS). Like conventional absorption spectroscopic methods, these newly developed methods are capable of characterizing oxidation and complexation states of actinide ions but with higher sensitivity. 

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