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., NO_x, 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.