Cold vapor atomic absorption spectrometer

These early GC-element-specific detection systems are described in several review articles [18-21]. An excellent summary of the applications of these and other sterns, to be discussed later, to the determination of mercury, lead, selenium, tin and arsenic compounds is provided in the review of this field by C.J. Cappon [9]. The detection limits obtained with these systems reach the low picogram levels under ideal conditions however, detection limits in the nanogram range, are more common. With elemental mercury or organic mercury compounds in a sample, 100 picograms of mercury were detected in a system consisting of a fused silica capillary gas chromatograph and a cold vapor atomic absorption spectrometer [22]. 

Measurement was by cold-vapor atomic absorption (CVAA), which is based on the reduction of ionic mercury to the elemental form by divalent tin. The resultant elemental mercury is swept by a gas stream through a quartz-windowed absorption cell which is placed in the beam of a mercury lamp of an atomic absorption spectrometer. Hatch and Ott (11) optimized this measurement system in their study of the mercury content of geological samples. Since their work, this method of mercury measurement has gained widespread popularity because of its sensitivity and relative simplicity. Two recent reviews which cover the Hatch and Ott technique and its subsequent modifications have appeared (17,18). 

FIGURE 4.5 MCFIA system for mercury determination in fish by cold vapor absorption spectrophotometry. AAS atomic absorption spectrometer quartz cell Ar argon carrier gas C carrier (HCl) GLS gas-liquid separator PP peristaltic pump R reducing reagent (NaBFL, in NaOH) RC reaction coil V three-way solenoid valve and W waste. 

Since mercury is present already in the atomic state in the cold vapor technique, there is no need for an atomiser as such. The sample vapor is swept directly from the reduction cell or the amalgamation trap in the carrier gas stream to a 10 cm length T-shaped quartz tube that is moderately heated (to ca. 200 °C to prevent condensation of mercury). This quartz cell is located in the light path of a conventional AA spectrometer where the attenuation of a characteristic Hg line source is measured. Dedicated AA spectrometers (which, in this case, often have a continuum light source) may also be used with longer absorption cells (300 mm pathlength) to increase the sensitivity. 

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. 

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