Cold Vapour Atomic Absorption (CVAA)
OPSIS HG200 mercury analyser is a double beam photometer, which measures mercury (Hg0) by cold vapour atomic absorption technology (CVAA). High sensitivity (0.1 ng/Nm3) is achieved by pre-concentrating a sample on a gold trap.
The mercury analyser performs its measurements in batches. A measurement cycle consists of three activities: sampling, analysis and reconditioning.
During sampling, the flow control valve is open, and a specified volume (1-1.5 LPM) is pumped through the system. Elementary mercury in the sample stream will then be amalgamated on the gold trap.
After the sampling, the instrument enters the analysis stage. Then, the valve is shut and a critical orifice reduces the flow rate. The gold trap is heated and the mercury will be released and transported through the measurement cell where the mercury is measured. This analysis takes one minute to perform. The measurement yields an absorption spectrum from which the mass in pico-grammes of mercury in the sample is derived.
A mass flow meter registers the flow rates during the different stages. When the analysis is complete, the third stage is entered during which the gold trap is cooled in order to prepare the analyser for the next measurement cycle.
Before entering the gold trap section, the sample gas is passed through a sample-conditioning scrubber containing soda lime.