Grantee Research Project Results
Final Report: Real-Time Trace Detection of Elemental Mercury and its Compounds
EPA Grant Number: R825380Title: Real-Time Trace Detection of Elemental Mercury and its Compounds
Investigators: Barat, Robert
Institution: New Jersey Institute of Technology
EPA Project Officer: Hahn, Intaek
Project Period: November 25, 1996 through November 24, 1998
Project Amount: $199,121
RFA: Analytical and Monitoring Methods (1996) RFA Text | Recipients Lists
Research Category: Environmental Statistics , Water , Land and Waste Management , Air , Ecological Indicators/Assessment/Restoration
Objective:
The two overall technical objectives of this research program were to: (1) test the effectiveness of photo-fragmentation fluorescence (PFF) spectroscopy for measuring airborne mercury compound vapors at low concentration; and (2) fabricate a supersonic jet spectrometer using PFF and atomic resonant fluorescence spectroscopy (AFS) for the detection of airborne mercury species vapors.Summary/Accomplishments (Outputs/Outcomes):
The research addressed the need for real-time stack monitoring technologies for emissions of elemental mercury (Hg) and its vapor compounds, which are considered a public health threat. The detection concept for Hg vapor uses AFS excited by an ultraviolet (UV) light source. Time gating is used to extract the useful AFS signal from the elastic scattering background. The detection concept for mercury compound vapors uses PFF spectroscopy excited by UV light. Examination of the fluorescence spectrum permits the identification of the original compound. Both techniques were employed in static and flow cells; both techniques are potentially enhanced by optical interrogation of the mercury species-doped air sample stream after its expansion in a supersonic jet. This technique initially was tested with Hg vapor, with PFF studies to follow.Detection limits were estimated for the PFF experiments in an atmospheric pressure flow cell with mercuric bromide in argon. Comparisons were made for PFF optical signal detection between interference filter + photomultiplier and monochromator + charge-coupled device (CCD) systems.
The vacuum chamber/supersonic jet system with optical access was completed. A laser beam (253.7 nm) or other UV source was directed into the chamber to intersect an expanding gas jet containing Hg or compound vapor. Collection optics and light detection devices were assembled. The atomic resonance fluorescence of Hg (argon carrier gas) was detected, with the useful signal distinguished from the background by time gating.
In addition to the vapor detection studies described above, which were the primary activity of this project, a complementary need was recognized. Unfortunately, a significant portion of the heavy metals emissions from combustion sources (e.g., incineration) is in the form of low vapor pressure particulates such as oxides. Species such as mercuric oxide would not be detectable via PFF. A chemical transformation of a metal oxide particle into a volatile form, such as a metal chloride or bromide, however, would render the metal compound detectable by PFF. A bench-scale flow apparatus was constructed to test the reaction of antimony oxide particles flowing in a nitrogen gas stream containing hydrogen chloride. Significant conversions were observed. A conceptual overall design of a multi-metals continuous emissions monitor (CEM) based on PFF analysis for metal compound vapors, some produced via chemical volatilization of particles, was developed. This design could be modified to include AFS for elemental metal vapors such as Hg.
The vacuum chamber with expansion jet was assembled. Vacuum is provided by a large mechanical roughing pump. A controlled temperature reservoir was designed and constructed to allow air to pass over elemental mercury and become saturated with its vapor. A heated source for mercury compound vapor-in-air also has been constructed and tested. Each setup serves as the sources for the jet. Mercury species concentration in air (or other gas) will be controlled by adjusting the source temperature.
The optical system was assembled. Vacuum chamber optical access through quartz windows has been achieved. An optical rail has been mounted in the chamber to hold components. Steering optics direct a 253.7 nm UV laser beam into the chamber through one window, perpendicular to the expanding jet of argon + mercury vapor. Emitted light is collected by an off-axis parabolic reflector, and directed out through another window. The fluorescence is turned and focused onto a detector. An interference filter (253.7 nm) + photomultiplier tube combination detector is used, interfaced to a boxcar/gated integrator. Careful optical baffling considerably reduced resonant scattering background. Preliminary experiments have identified the atomic Hg fluorescence, which occurs several nanoseconds after the nearly instantaneous background scattering. Judicious placement of the boxcar time gate allows preferential measurement of the fluorescence signal?this simple approach offers a relatively simple way to extract the desired signal from the elastic background. The PFF of mercury compounds is nonresonant with the excitation UV wavelength; hence, it does not suffer from this background problem if an appropriate UV excitation source is used.
Considerable effort has been made to employ UV interferometry as an alternative to time gating. The UV light (i.e., background scattering + fluorescence) was collected and focused into a Michelson interferometer. Modulation of one arm of the interferometer produced an oscillation fringe pattern (i.e., interferogram). A Fourier transform of the interferogram should allow a spectral extraction of the narrow atomic mercury fluorescence line from the spectrally broader excitation light. This task proved to be formidable. A difficult, but obtainable, goal of the demonstration of UV fringes is near, and once shown, this effort will end, with a return to gating.
Work is complete on the complementary experiment, which is providing insight for the jet experiment. An atmospheric pressure flow cell with optical access has been used to gather considerable data on PFF of mercuric bromide in argon. Limits of detection were estimated for both photomultiplier tube and CCD detectors.
In this work, low concentrations (6 ppb to 30 ppm) of mercuric bromide (HgBr2) vapor were introduced into an atmospheric pressure flow cell. The PFF technique used 222 nm laser radiation to photolyze HgBr2 and excite fluorescence from the resulting Hg atoms at 253.7 nm. The PFF was collected and focused through dual interference filters, centered at 253.7 nm, into a photomultiplier tube. The fluorescence intensity was linear with laser fluence over the range of 45 to 180 mJ/cm2. Extrapolated detection limits by this method below 1 ppb of HgBr2 in the absence of air were estimated. The dynamic detection range is linear up to 0.7 ppm (11 mg/m3).
The use of a CCD camera for the detection of HgBr2 vapor at low concentrations by laser PFF spectroscopy was investigated. The CCD detection system (camera + monochromator) offers reasonable sensitivity plus spectral information, enhancing PFF as a technique for the environmental monitoring of airborne mercury compounds. The experiment used laser radiation at 222 nm to photolyze HgBr2 and produce excited atomic mercury (Hg*). The PFF was monitored at 253.7 nm. The unenhanced CCD detection limit was about 30 ppb HgBr2 in the absence of air. The CCD response remained linear up to 20 ppm. Observed nonlinearity of the PFF signal at higher concentrations is discussed. With the same collection optics and under the same experimental conditions, the sensitivity of a photomultiplier tube (PMT) with interference filters (253.7 nm) also was investigated for comparison. The detection limit for the PMT system was 10 ppb without signal averaging, but the linear dynamic range ended at 0.7 ppm. It is expected that the CCD detection system would be more versatile for measuring metal compound species by PFF spectroscopy in any future metals CEM.
A series of experiments was performed reacting antimony oxide particles flowing in nitrogen with gaseous hydrogen chloride to produce antimony chloride and water vapor. This reaction is exothermic, with significant conversions occurring at temperatures of at least 150 C. The particle stream was generated with a Wright dust feeder. The co-flow fed a nonisothermal plug flow reactor in a heated furnace. Conversions were based on measured hydrogen chloride concentrations in the outlet gas. The nonisothermal temperature profile complicated data analysis. Lumped kinetic parameters were obtained, however, assuming a Langmuir-Hinshelwood model for gas/solid systems.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 4 publications | 2 publications in selected types | All 2 journal articles |
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Tong X, Barat RB, Poulos AT. A charge-coupled device-based laser photofragment fluorescence spectrometer for detection of mercury compounds. Review of Scientific Instruments 1999;70(11):4180-4184. |
R825380 (1998) R825380 (Final) |
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Tong X, Barat RB, Poulos AT. Detection of mercuric bromide in a gas phase flow cell by laser photofragment fluorescence spectroscopy. Environmental Science & Technology 1999;33(18):3260-3263. |
R825380 (1998) R825380 (Final) |
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Supplemental Keywords:
air, heavy metals, mercury, incineration, physics, laser, monitoring, optics, fluorescence., Scientific Discipline, Water, Ecosystem Protection/Environmental Exposure & Risk, Environmental Chemistry, Monitoring/Modeling, Environmental Monitoring, Engineering, Mercury, resonant flourescence, ambient particle properties, industrial waste, doppler shifting, air pollution, elemental mercury, high vapor pressure, waste combustion, aerosol analyzers, air quality, metalsProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.