Mercury vapor can have a toxic impact on human health. As a result, the Environmental Protection Agency (EPA) has set stringent limits on the amount of mercury vapor that can be present in air. In order to comply with these regulations, government agencies and industrial hygienists must use portable instruments to determine the levels of mercury vapor on site.
A number of technologies are available which can be used to detect mercury. This article shows the differences between atomic fluorescence spectroscopy, atomic absorption spectroscopy and gold film sensors, as well as exploring the functions of these technologies and their level of sensitivity to mercury vapor.
Gold Film Sensor Technology
Gold film sensors can reliably detect mercury, thanks to gold's affinity for elemental mercury. Arizona Instrument leveraged this affinity and combined it with gold's electrical conductivity to develop the Jerome 431 and J405 mercury vapor analyzers. When air sample rich in mercury is passed across a thin gold film, the mercury will accumulate on the gold and convert the foil’s electrical resistance. This change in resistance is directly related to the mass of mercury vapor obtained from a known volume of air, which can be quantified in mg/m3.
In case the gold becomes saturated after a while, the system provides a 'regeneration' feature that bakes the foil at an increased temperature and the mercury deposits are vaporized and gathered in the scrubber. The schematic diagram in Figure 1 shows how this mechanism works.
Figure 1. Schematic diagram of the gold film sensor set up.
The 'green' box is the Acid Gas Filter designed to remove hydrogen sulfide gas. This gas together with chlorine and ammonia gas will react with the gold film and create a false positive.
In addition to the internal gas filter, Arizona Instrument offers other external filters to remove chlorine and ammonia gas in case the internal gas filter presents a major interference sans reducing the concentration of mercury in the sample.
Arizona Instrument’s Gold Film Sensor mercury vapor analyzers can be used in a wide range of applications and detection limits. Based on the earlier version of the J431 model (Figure 2), the Jerome® J405 Gold Film mercury vapor analyzer has been updated with a number of features. However, both models are effective for different detection limits.
The J431 model has a detection range between 3µg/m3 to 999µg/m3 with a resolution of 1µg/m3. This detection limit is close to the EPA residential specification of 1µg/m3 and is within the industrial specification of 25µg/m3. This means, the J431 is suitable for industrial applications.
Figure 2. Jerome® J431 Gold Film Mercury Vapor Analyzer.
The Jerome® J405 (Figure 3) is integrated with an on-board data logging system and also features an optional USB data communication port. It has a detection range between 0.5µg/m3 to 999µg/m3 with a resolution of 0.01g/µm3. Since this latest version of gold film MVA can read below 1µg/m3, it enables users to conform to both commercial and industrial mercury regulations. Both models can be used in adverse environments and provide continuous modes for monitoring potential hot spots in the field.
Figure 3. Jerome® J405 Gold Film Mercury Vapor Analyzer.
Environments of Likely Interferences
Gold film is not suitable for environments rich in chlorine and ammonia. This is because only a single external filter can be utilized during analysis and will be difficult to determine mercury sans getting a signal from either chlorine or ammonia. Also, gold film is not suitable for oxygen-free environments because the sensor will not be effective when there is lack of oxygen.
Atomic Absorption Spectroscopy
Another technique used for mercury detection is cold vapor atomic absorption spectroscopy (CVAAS). In this method, a light source of known intensity and wavelength is radiated through an air sample and the light ultimately encounters a detector.
When mercury is present, electrons in the mercury atoms will take up some part of this energy from the light source. The variation between the energy determined by the detector and the initial energy of the light source provides an indirect measurement of the number of mercury atoms present.
The schematic diagram in Figure 4 shows the path of the radiated light. A number of photo multiplier tubes (PMTs) and mirrors were utilized to increase the signal difference.
Figure 4. Indirect method relies on how much energy was absorbed Interferences.
However, in addition to atomic mercury, other substances also absorb this wavelength and thus give false positive readings. The chart below shows some of the known interferences in the CVAAS method.
Table 1. CVAAS interferences
||Some Organic Solvents
The sensitivity range is often lower since atomic absorption spectroscopy quantifies on the atomic level. CVAAS analyzers such as the Lumex® 915 M and the Nippon® EMP-2 provide a low-end sensitivity of 0.002µg/m3 and 0.1µg/m3, respectively, and fall within the EPA regulations for residential specifications. Given that positive interferences are a problem for CVAAS, the instruments will be inspecting chemical species other than the specified mercury vapor.
Environments of Likely Interferences
It is difficult to determine low levels of mercury vapor over smoke and dust if the environment being sampled is contaminated with these particulates.
Crude oil aromatic hydrocarbon vapors plus xylene and toluene co-solvents are some of the main sources of interferences for atomic absorption spectroscopy.
Due to these interferences, atomic absorption spectroscopy is not a viable method for mercury vapor analysis in the petroleum processing industry.
Atomic Fluorescence Spectroscopy
When compared to the conventional CVAAS, cold vapor atomic fluorescence spectroscopy (CVAFS) is relatively better. When energy from the UV wavelength is absorbed by a mercury atom, an electron changes from a stable to an unstable state; this excitation event explains the atomic absorption.
On the other hand, when the source of energy is removed, the excited electron comes back to its ground state. As this occurs, a photon of light is produced during the loss of potential energy. Mercury can absorb both light and fluoresces at 254nm wavelength.
Figure 5. Jerome® J505 Atomic Fluorescence Spectroscopy Mercury Vapor Analyzer.
Arizona Instrument leveraged this unique resonance fluorescence of mercury to trace extremely low concentrations of mercury vapor and at the same time reduced the interferences associated with atomic absorption.
The Jerome® J505 is an atomic fluorescence spectroscopy mercury vapor analyzer (Figure 5) that detects only the wavelength that is fluoresced from the air sample. The concentration of mercury is shown by the amount of light fluoresced at a 90° degree angle.
This is a direct method of analysis because the instrument measures individual photons of excited mercury atoms in the air sample. Figure 6 shows how this is performed without increasing the signal via a range of mirrors.
Figure 6. Direct method relies on how much energy was fluoresced Interferences.
The J505 instrument determines only the radial resonance fluorescence at 254nm wavelength, and so chemical species agitated at 254nm and fluoresces at 254nm alone can be quantified. This prevents most sources of interferences and provides precise and repeatable results.
The J505 analyzer has a detection range between 0.05µg/m3 and 500µg with a resolution of 0.01µg/m3, which is below the NIOSH, EPA, and OSHA standards for low mercury vapor specifications.
Environments of Likely Interferences
Atomic fluorescence spectroscopy selects for a narrow standards of chemical species, which can absorb and fluoresce at 254 nm. When large amounts of acetone are present in industries, it would be difficult to detect mercury vapor with the J505 analyzer.
This article has described three different methods of mercury vapor analysis. However, these methods have certain advantages and disadvantages. The Jerome® J505 atomic fluorescence spectroscopy mercury vapor analyzer is designed to determine mercury atoms during their excitation phase. Although each method has its own drawback, a better understanding of these technologies will help in choosing the right method for specific applications.
About Arizona Instrument LLC
Initially known as the Quintel Corporation, Arizona Instrument LLC was founded in 1981 by a group of engineers breaking away from The Motorola Corporation who were dedicated to the idea of providing precision moisture analysis instruments that were accurate, reliable, and easy to use.
The first instrument released was the MA Moisture Analyzer, but the company quickly expanded its Computrac® moisture analysis line and became an accepted leader in moisture analysis, setting a standard that has been adopted by many Fortune 500 companies. Today the Computrac® line is comprised of three technologies: rapid loss-on-drying, high temperature loss-on-ignition, and moisture specific analysis using polymer capacitance sensor, GREEN alternative to Karl Fischer.
In 1986, Arizona Instrument acquired Jerome Instrument Corporation the manufacturers of the Jerome® toxic gas analyzers. At the time of purchase the corporation had an established reputation for accuracy and durability, which complemented and added depth to the Arizona Instrument’s offerings; and these traditions continue today. The Jerome® line is comprised of instruments used for detecting low-level mercury and hydrogen sulfide gases. Both portable detection and fixed position monitoring solutions are available, using gold film sensor and atomic fluorescence spectroscopy technologies as the method of detection.
Through the years Arizona Instrument has pursued and maintained a total quality management system, being certified initially as ISO 9001:1994 then ISO 9001:2000 and most recently ISO 9001:2008. Though the company is located in Chandler, Arizona, its distributors and service centers located around the world provide consistent, dependable service to its many customers worldwide.
This information has been sourced, reviewed and adapted from materials provided by Arizona Instrument.
For more information on this source, please visit Arizona Instrument.