Sequential monitoring of elemental mercury in stack gas by dielectric barrier discharge micro-plasma emission spectrometry

Abstract

A simple device was constructed for the on-site sequential monitoring of emitted elemental mercury (Hg0) in coal-fired stack gas. The device integrates on-line gold amalgam preconcentration, external thermal desorption and cold excitation in an enclosed atmospheric-pressure dielectric-barrier discharge (DBD) micro-plasma chamber. The emission was monitored by using a charge coupled device spectrometer. The Hg0 emission at 253.7 nm was well separated from the emission spectra of the Ar-DBD-plasma which eliminates the spectral interferences of ambient air and gaseous inorganic small molecular pollutants in the stack gas, e.g., NOx, SO2, H2S and HCl. Quantification was based on first derivative spectra. Among the three mercury species, Hg0 is efficiently and selectively trapped on the gold amalgam at <220 °C, the threshold temperature for thermal desorption of the trapped Hg0 is ca. 420 °C above which the gold amalgam releases the enriched Hg0 rapidly and gives rise to a high concentration of Hg0 in the flowing Ar stream in a very short period of time which significantly improves the detection sensitivity. A linear calibration graph was achieved within 8–256 ng (the mass of enriched Hg0 on the gold trapping micro-column) with a detection limit of 2.3 ng and an RSD of 4.0% at 32 ng. A spiking recovery of 96.1% was achieved at a spiking mass level of 50 ng Hg0. For real sample analysis (stack gas from a honeycomb briquette stove), a calibration range of 2.7–60.8 ppt was achieved after 5 min preconcentration with a 0.5 L min−1 sampling flow rate.