Mercury Emission Monitoring

The Importance of Monitoring Mercury

Mercury is naturally occurring and exists in several forms. High mercury exposure results in permanent nervous system and kidney damage. Exposure is most likely to occur during mining, production, and transportation of mercury, as well as mining and refining of gold and silver ores. Mercury is commonly found in thermometers, manometers, barometers, gauges, valves, switches, batteries, and high-intensity discharge (HID) lamps. It is also used in amalgams for dentistry, preservatives, heat transfer technology, pigments, catalysts, and lubricating oils.

The Health Effects of Mercury Exposure

Mercury and its compounds exist in three general forms:

• Elemental (or metallic).
• Inorganic. Mercury can combine with other elements (mainly chlorine, sulfur, and oxygen) to form inorganic mercury compounds.
• Organic. Mercury may combine with carbon or carbon-containing substances to make organic mercury compounds. These organic compounds are further divided between alkyl (carbon-chain) and aryl (aromatic ring) groups.

Although all mercury compounds are toxic, the small-chain alkyl compounds are the most hazardous. Mercury compounds vary in toxicity, so OSHA provides standards for each. It is important to clarify which category a compound belongs to before comparing it with a standard or determining its relative toxicity.

How to Monitor Mercury

Gold Film Sensor Technology

Gold Film Sensors were the first reliable forms of mercury detectors due to gold’s affinity for elemental mercury. If a mercury rich air sample is swept over a thin gold film, the mercury will deposit on the gold and change the electrical resistance of the foil. This change in resistance is directly proportional to the mass of mercury vapor taken from a known volume of air, which can be calculated in mg/m3. If the gold becomes saturated over time, the instrument offers a ‘regeneration’ feature that bakes the foil at an elevated temperature where the mercury deposits are vaporized and collected in the scrubber.

Atomic Absorption Spectroscopy

Cold Vapor Atomic Absorption Spectroscopy (CVAAS) is another method used for mercury detection. In mercury CVAAS, a light source of known wavelength and intensity (~254nm, middle ultraviolet spectrum) is radiated through a sample of air where the light eventually encounters a detector. If mercury is present, electrons from within the mercury atoms will absorb some of this energy from the light source. The difference between the initial energy of the light source and the energy. Unfortunately, atomic mercury in CVAAS is not the only chemical species to absorb this wavelength. Many other substances can also absorb this wavelength and produce false positive readings.

Atomic Fluorescence Spectroscopy

Atomic fluorescence and absorption are two terms that are related but have two different meanings. Cold Vapor Atomic Fluorescence Spectroscopy (CVAFS) is an improvement upon the traditional CVAAS. When a mercury atom absorbs the energy from the UV wavelength, an electron transitions from a stable ground state to an unstable ‘excited’ state. This excitation event describes the atomic absorption as discussed in the previous section.

However, when the energy source is removed the excited electron returns to its ground state. In doing so, a photon of light is emitted during the loss of potential energy. This fluorescence of light is often unique for various chemical species. Mercury in particular absorbs light at 254nm and fluoresces light at the same wavelength. Because the light absorbed and emitted are at the same wavelength, this form of fluorescence is referred to as resonance fluorescence. Other chemicals such as chlorides, sulfides and hydrocarbons absorb light at 254nm but either do not fluoresce or fluoresce at a different wavelength. One manufacturer took advantage of the unique resonance fluorescence of mercury to detect ultra-low concentrations of mercury vapor while minimizing the interferences involved with atomic absorption alone. The Jerome® J505 Atomic Fluorescence Spectroscopy Mercury Vapor Analyzer is the first hand-held instrument of its kind. Although other chemical species may still absorb the energy from the light source, the J505 only detects the specific wavelength that is fluoresced radially from an air sample. The amount absorbed is inconsequential because the mercury concentration is revealed by the amount of light fluoresced at a 90° degree angle. This technology is a more direct method of analysis since the instrument is quantifying individual photons of excited mercury atoms in a sample.