SI-TRACEABLE MERCURY MEAUREMENT RESULTS WITH THE VSL PRIMARY MERCURY GAS STANDARD

摘要

The working principle of the primary mercury vapour generator is based on diffusion according to ISO 6145-8:2005. Specially developed stainless-steel diffusion cells are filled with approximately 2 mL of elemental mercury. The mercury vapour diffuses from the cell through a capillary. To obtain several diffusion ranges, cells with different capillary diameters are used. By weighing the diffusion cells at regular time intervals on a high-resolution analytical balance the mercury mass flow rate (diffusion) is determined gravimetrically. In the dynamic mercury gas generator, the cells are housed in a diffusion chamber. The diffusion chamber is temperature (20.0 °C± 0.1 °C) and pressure (105.0 kPa ± 0.1 kPa) controlled, At the bottom a nitrogen flow of 500 mL min~1 enters the diffusion chamber. All of the flow, also enriched by mercury vapour, is then guided to the outlet of the diffusion chamber through an aperture at the top. Mercury concentrations are prepared by mixing the mercury vapours in nitrogen in a second step with flow rates, between 1 L min~1 and 20 L min~1, of matrix gas, e.g., purified air. Using diffusion cells with different capillaries a range of mercury concentrations can be obtained.