Technical and Physical Limitations in the Determination of Background Mercury Concentration

The optical schemes and characteristics of atomic absorption Hg
⁰
analyzers based on the Zeeman effect and a low-pressure capillary mercury lamp, as well as on 190 fm linewidth UV dye laser systems have been studied in detail. The method of evaluating the limit of detection (LOD) of atomic absorption analyzers is proposed. The characteristics of modern ultra-narrowband sensors and sources of UV radiation are analyzed. The efficiency of using new thermal ultrasonic sensors as optical power detectors in the atomic absorption analyzer is discussed. Technical characteristics and reliability of such sensors are compared with all known thermal sensors. The main physical and technical limitations that have a negative effect on the sensitivity and miniaturization of thermal ultrasound sensors are presented. The study shows that such thermal sensors have low sensitivity and accuracy, which makes their use in portable express analyzers of background mercury impractical. The evaluation results prove that the analyzers which use a transverse magnetic field for splitting the lamp emission line also cannot register background mercury concentration. This fact is explained by the low signal-to-noise ratio (SNR) of the discharge low-pressure UV lamp operating in a continuous mode and by the absorption of forbidden oxygen lines by the reference optical signal. The estimation results shown that the portable Hg
⁰
analyzers based on UV dye laser systems can reach the LOD <; 0.1 ng/m
³
. Thus, the main stage in the design of the portable express background mercury analyzers is a measurement of the SNR of UV sources and sensors.