Mercury Speciation Analysis by Gas Chromatography/electron Impact/mass Spectrometry

Fast and reliable analytical methods for mercury speciation would aid in understanding mercury offgas chemistry. Presently, the U.S. Environmental Protection Agency (EPA) regulates only total mercury; however, many studies indicate that mercury toxicity is highly dependent on the mercury speciation. For example, amalgams and mercury sulfides have relatively low toxicities, while organomercury and halogenated organomercury compounds, apparently through the action of the methylmercury ion, CH3Hg +, have irreversible deleterious effects on the central nervous system. In addition, the distribution of mercury emissions (local versus global) is highly dependent on initial mercury speciation in the offgas. Existing EPA methods for mercury analysis rely on capturing oxidized and elemental mercury in various solutions, and then analyzing the solutions for mercury by Cold Vapor Atomic Absorption (CVAA) or Cold Vapor Atomic Fluorescence (CVAF) techniques. Related procedures, for example the Ontario Hydro Method, give a measure of mercury speciation in offgases. Other methods based on carbon traps or direct AA or AF analysis of mercury vapor are applied as well, but give only limited information on mercury speciation. In addition to atomic adsorption and atomic fluorescence techniques, mass spectrometry (as part of inductively coupled plasma/mass spectrometry or ICP/MS) is often used to quantify mercury, but by itself does not provide information about mercury speciation. Gas chromatography/mass spectrometry (GC/MS) is a well established technique for separating, identifying, and quantifying individual volatile and semi-volatile organic compounds contained in complex gaseous mixtures. Because elemental mercury and many inorganic and organic mercury compounds are similarly volatile, it appeared possible that gas chromatography would lend itself to separation of offgas mercury compounds, which could then be individually identified and quantified by mass spectrometry. Accordingly, MSE Technology Applications, Inc., under contract to the U.S. Department of Energy, conducted preliminary experiments to determine whether standard GC/MS equipment can be adapted to the measurement of mercury species in combustion offgases. These developmental experiments focused on determining the behavior of organic and inorganic mercury species in a GC/MS system (1) by direct insertion of mercury compounds into an electron impact mass spectrometer (EI/MS), (2) by injection of liquid standards into a GC/EI/MS system, and (3) by head space sampling using solid phase microextraction (SPME). From these experiments, we concluded that SPME coupled with GC/MS is a promising technique for sampling and analyzing speciated mercury compounds, but that unbiased sampling/preconcentration of all mercury compounds, and “freezing” the species distribution of mercury compounds during sampling and subsequent analysis may be problematic. Future work will include will include determining the rate and extent of redistribution of mercury species during SPME sampling and GC analysis and the retention efficiency of individual mercury compounds by various sampling adsorbents.