EFFECT OF EXTERNAL CONDITIONS (AIR TEMPERATURE AND PRESSURE) ON THE DETERMINATION OF MERCURY CONTENT IN AMBIENT AIR (Abstract)

Sholupov, S.¹;  Markelov, M.² ; Siperstein, J.²

¹ Lumex Ltd, St. Petersburg, Russia; ² OhioLumex Inc, Cleveland, OH, USA

 

Zeeman mercury atomic absorption spectrometer RA-915+ determines mercury concentration in the air “as is”, i.e., it directly measures the concentration of atomic mercury in the air without preliminary mercury collection in absorption traps. Results of the measurements depend on external conditions in two ways. First, it is well known that gas expands with increasing temperature and decreasing pressure (Boyle and Charles Law). Since the mercury atoms in the air are considered as trapped impurity, so a change in the air temperature and pressure gives rise to a change in the mercury concentration. Second, the profile of the mercury absorption line depends on the temperature (Doppler broadening of spectral lines) and pressure (Lorentz broadening of spectral lines) of the air. Therefore, a change in the external conditions leads to a change in the absorption line profile, differential cross section, and finally, in the sensitivity. To eliminate the effect of the external conditions, first, the data should be corrected to standard temperature and pressure (STP) and, second, the dependence of the sensitivity  (differential cross section) on the temperature and pressure should be found in the reasonable range (the pressure range is from 1.0 to 0.6 atm which corresponds to a change in altitude from sea level up to 4000 m and the temperature range is from 0⁰C up to 50⁰C).

Data correction to STP can be made using the simple formula:

C_STP=C_measured * (P_STP/P_measured) * (T_measured/T_STP)

In this paper, the dependence of the differential cross section is given for mercury absorption line l = 254 nm as a function of pressure. It follows from the calculations that at a certain magnetic field strength the sensitivity varies by no more than 20% with a decrease of pressure.

To verify the theory experimentally, the RA-915+ spectrometer was placed inside a chamber, the chamber was sealed and then evacuated to lower pressures using a vacuum pump. Experimental data concerning dependencies of the sensitivity on pressure are in good agreement with the theory.

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