Long-path absorption, direct measurement

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. 

The principle of long-path absorption techniques is illustrated in Fig. 10.17. A laser beam is transmitted continuously into the atmosphere against a corner-cube retro-reflector (Fig. 6.21) that is placed at a distance of up to 10 km. The reflected beam is received by an optical telescope that is placed at the site of the laser and is directed towards the retro-reflector. The received light intensity is measured photo-electrically as a function of the laser wavelength. The absorption spectrum of the atmosphere between the laser and the retro-reflector is then recorded and the mean concentrations Nj of pollutant molecules can be determined using the Beer-Lambert relation... 

The observed mixing ratios of active halogen species in the troposphere is in general 1-100 ppt, and the direct measurements of these species are usually made by using the methods, Long-Path Differential Absorption Spectroscopy (LP-DOAS), Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS), Chemical Ionization Mass Spectrometer (CIMS) and Laser Induced Fluorescence (LIF). As for BrO and lO, satellite observation gives spatial distribution of its tropospheric column. 

UV-absorption detection at a chosen wavelength is most commonly used. Peptides are usually measured at k = 210 nm, proteins and DNA at A. = 260 nm or A = 280 nm (see section 1.3.3). The absorbance is measured directly through a detection window in the capillary approximately 1 mm long. For this, the poly-imide coating of the capillary has to be removed. The small capillary diameter, less than 100 p,m, results in a short detection path length and thus in low sensitivity. This problem may be partly overcome by use of a Z-cell (Fig. 3.13), which effectively increases optical path length 10 to 15 times. However, due to band broadening within the cell, the separation efficiency is decreased. 

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