A differential photoacoustic mercury detector

A sensitive instrument for the detection of mercury is described. The difference in mercury concentration between two identical flow-through silica cells is measured by means of the photoacoustic effect. The cells are illuminated laterally by a 20-W germicidal mercury discharge lamp. A microphone in each cell detects pressure changes produced when mercury vapour absorbs resonance radiation from the lamp and releases the energy to nitrogen flowing in the partially closed cells, as heat derived from quenched fluorescence. A differential microphone connection discriminates against ambient noise and contributes to a detection limit of less than 0.02 ng of mercury with a linear response up to 70 ng. Variables such as gold collector design and heating rate, inlet filters and traps, changes in carrier gas flow, carrier gas composition, lamp operating current and modulation frequency which all affect instrument performance are discussed. Photochemical loss of mercury can be avoided if dry nitrogen, carbon dioxide, or air is used as the carrier gas provided contamination by avid vapours and water is prevented by a calcium oxide trap at the inlet.