Deployment of a sequential two-photon laser-induced fluorescence sensor for the detection of gaseous elemental mercury at ambient levels: fast, specific, ultrasensitive detection with parts-per-quadrillion sensitivity

摘要

The operation of a laser-based sensor for gas-phase elemental mercury,Hg(0), is described. It utilizes sequential two-photon laser excitation withdetection of blue-shifted laser-induced fluorescence (LIF) to provide ahighly specific detection scheme that precludes detection of anything otherthan atomic mercury. It has high sensitivity, fast temporal resolution, andcan be deployed for in situ measurements in the open atmosphere withessentially no perturbation of the environment. An ambient sample can alsobe pulled through a fluorescence cell, allowing for standard addition calibrations of the concentration. No type of preconcentration is requiredand there appears to be no significant interferences from other atmosphericconstituents, including gas-phase oxidized mercury species. As a consequence,it is not necessary to remove oxidized mercury, commonly referred to asreactive gaseous mercury (RGM), from the air sample. The instrument has beendeployed as part of an instrument intercomparison and compares well withconventional instrumentation that utilizes preconcentration on gold followedby analysis using cold-vapor atomic fluorescence spectroscopy (CVAFS).Currently, the achievable detection sensitivity is ~ 15 pg m (~ 5 × 10 atoms cm, ~ 2 ppq)at a sampling rate of 0.1 Hz, i.e., averaging 100 shots with a 10 Hzlaser system. Preliminary results are described for a 50 Hz instrument thatutilizes a modified excitation sequence and has monitored ambient elementalmercury with an effective sampling rate of 10 Hz. Additional work is requiredto produce the precision necessary to perform eddy correlation measurements.Addition of a pyrolysis channel should allow for the measurement of totalgaseous mercury (TGM) and hence RGM (by difference) with good sensitivityand time resolution.