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Mercury is a toxic element, harmful to the environment and poisonous to humans, even at very low levels. It is generated by industry, by burning fossil fuels and as a by-product of some bacteria. Mercury builds up through the food chain, and levels can be particularly high in marine life. The monitoring of mercury levels is a regulatory requirement for the U.S. EPA and for other governmental agencies around the world. The EPA regulation for mercury content in water is particularly stringent; levels are not permitted to exceed 2 parts per billion (ppb).
Mercury analyzers are used to detect mercury levels in solids, liquids, or gases for environmental and safety reasons. Analysis may be carried out on a per-sample basis or for continuous monitoring, for example, checking mercury levels in river water or the atmospheric levels of mercury on an industrial site.
This guide focuses on mercury analyzers for lab work, analyzing solids and liquid samples. However, the detection technology described is also applicable to continuous monitoring.
Applications for mercury analysis
The monitoring of mercury levels is required for a wide range of industries:
- Food—mercury levels in fish, vegetables, and drinking water
- Mining and oil—mercury is used in the extraction of gold; mercury traces are found in natural gas and oil
- Chemical—mercury levels in effluents; monitoring mercury levels in by-products such as hydrogen
- Environmental—mercury in soil, mercury in water, and volcanic mercury emissions
- Biological—mercury in hair, urine, blood, and saliva.
Typical samples include water and aqueous samples; biological samples; foods; coal, ash, and minerals; and soil.
Regulatory compliance: determining mercury levels
U.S. EPA methods and EN standards call for specific analysis techniques for determining mercury levels. There are different methods for analyzing mercury in samples, and your choice of method may be determined by regulatory compliance. When choosing an analyzer, be sure to check which regulatory methods you need to comply with, and whether your chosen equipment complies with that method.
For example, if your lab is required to analyze samples using U.S. EPA Method 245.1, then you will need to use cold vapor atomic absorption (CVAA). U.S. EPA Method 1631 requires cold vapor atomic fluorescence spectroscopy (CVAFS) with gold amalgamation. EN 13806 for the determination of mercury levels in foodstuffs requires mercury analysis by cold vapor atomic absorption spectrometry (CVAAS) after pressure digestion.
Analytical methods
There are different methods of separating mercury from the sample and for measuring the levels of mercury. All methods rely on extracting elemental mercury into a vapor.
Methods of separating atomic mercury from the sample matrix are cold vapor (CV), thermal decomposition, and inductively coupled plasma (ICP).
Mercury vapor is elemental at room temperature and can be detected using a number of different spectroscopy methods, i.e., atomic absorption spectroscopy (AAS), atomic fluorescence spectroscopy (AFS), atomic emission spectroscopy (AES), or mass spectrometry (MS).
The main mercury analysis techniques are:
- Cold vapor atomic absorption spectroscopy
- Cold vapor atomic fluorescence spectroscopy
- Direct analysis by thermal decomposition
- Inductively coupled plasma-atomic emission spectroscopy
- Inductively coupled plasma-mass spectrometry.
Cold vapor atomic absorption methods: CVAAS and CVAFS
Cold vapor methods are suitable for liquid samples. If you are handling solid samples and wish to use a cold vapor method, the sample must first be digested to turn it into a solution.
The first step is the chemical reduction of the liquid sample, typically carried out with stannous chloride, to convert the mercury to its elemental form. The liquid sample and reductant are pumped into a gas liquid separator where a gas is bubbled through the liquid mixture to release the mercury vapor. A typical analysis time is 1–3 min.
Figure 1 – MERX-1500 Mercury Analyzer. (Image courtesy of Brooks Rand Instruments.)The amount of mercury in the vapor is detected using atomic absorption or atomic fluorescence. Atomic fluorescence is the more sensitive detection method. CVAAS typically has a detection limit of 1 ppt, compared to 0.1 ppt for CVAFS.
In CVAFS, a gold amalgamation step may be added to concentrate the mercury levels before measurement. This increases the detection limit to 0.5 ppt.
The MERX-1500 from Brooks Rand Instruments (Seattle, WA; www.brooksrandinc.com) (see Figure 1) is suitable for low-level mercury detection by U.S. EPA Method 245.7 through oxidation of mercury with bromine monochloride, neutralization of excess bromine with hydroxylamine hydrochloride, and reduction. Detection is by CVAFS.
Direct analysis by thermal decomposition
This method is ideal for solid samples because it removes the requirement for predigestion. It can also be used for liquid samples.
Although the actual analysis takes longer than CV methods, removing the need for predigestion increases the overall throughput time for solid samples. The typical analysis time for solid samples using the SMS 100 from PerkinElmer® (Waltham, MA; www.perkinelmer.com) is 5 min.
Thermal decomposition methods remove the need for reductants and therefore reduce hazardous waste and the possibility of contamination. Thermal decomposition is suitable for small sample volumes, typically less than one gram. The combustion chamber is flooded with oxygen and the sample is heated, first to dry it and then for combustion. The released gases are taken out of the combustion chamber and passed over a gold amalgamation trap to separate the atomic mercury. The trap is flooded with oxygen to eliminate other decomposition products. The amalgamation cell is heated to release the mercury, which is measured by atomic absorption spectroscopy.
The DMA-80 direct analysis system from Milestone Inc. (Shelton, CT; www.milestonesci.com) enables the operator to analyze for mercury in liquid, solid, or gas sample matrices without the need for any sample preparation. The company claims that this not only saves laboratory time, but can result in substantial cost savings of up to 70% when compared to CVAAS.
Inductively coupled plasma (ICP) methods
Inductively coupled plasma methods heat the sample in an argon plasma. The thermal excitation frees atomic and ionic species so that they can be measured by atomic emission spectroscopy or mass spectrometry. ICP-AES is sometimes referred to as ICP-OES (optical emission spectroscopy).
The plasma temperature is on the order of 6000–10,000K. The detection limits for mercury using ICP-AES are comparatively poor (1–10 ppb), but with ICP-MS they increase to 1 ppt.
ICP-AES analyzers are often used to detect just one emission frequency at a time. In contrast, ICP-MS is particularly useful if you need to detect a range of metals. ICP-MS is used for very fine metal detection and speciation for elements from lithium through uranium.
Gold amalgamation
Gold amalgamation is a method of preconcentrating mercury to minimize background interference. It is commonly used to detect very low levels of mercury, often in combination with CVAFS or thermal decomposition.
Choosing the right mercury analysis technique
Figure 2 – 10.025 Millennium Merlin Mercury Analyzer. (Image courtesy of PS Analytical.)
Your chosen method will depend on a number of factors. The principal factors are:
- Is there a regulatory procedure with which you must comply? Most regulatory guidelines state the method that must be used.
- Is the sample matrix a solid or a liquid? For a liquid sample, a cold vapor method will be more appropriate. For a solid, you need to consider your required throughput time combined with predigestion requirements to decide whether to choose a predigestion followed by a cold vapor method or direct analysis by thermal decomposition.
- What detection limit is required? See Table 1 for a comparison of detection limits for the different analytical techniques.
- What speed of analysis and throughput are required? CV methods are quicker than thermal decomposition, but require predigestion.
- Will you need to detect other species of mercury or other metals? It is worth bearing in mind that some analyzers will detect other metals or species of mercury. ICP-MS detects from lithium through uranium. Mercury analyzers are expensive, so it is important to plan your purchase with future alternative use in mind.
Detection limits and usable ranges
The detection limits and usable ranges given by Teledyne Leeman Labs (Hudson, NH; www.teledyneleemanlabs.com) for Hydra analyzers are given in Table 1, along with detection limits for ICP methods. Note that the detection limits for thermal decomposition are measured differently than those by other methods; they are measured in absolute grams, rather than relative concentrations.
The 10.025 Millennium Merlin Analyzer from PS Analytical (PSA) (Deerfield Beach, FL; www.psanalytical.com) (see Figure 2) allows sub-ppt detection by CVAFS for liquid samples.
Table 1 – Detection limits and usable ranges for different analysis methods
Speed of analysis and sample throughput
CVAAS and CVAFS typically take short analysis times of 1–3 min. The RA-3000A Series from Nippon Instruments North America (College Station, TX; www.hg-nic. us) has an ultrahigh sensitivity of 0.5 ppt and wide range up to 200 ppb, and only requires a small sample size of 5 mL. The speed of analysis is 10–180 sec.
Thermal decomposition takes 5–8 min. However, when handling solid samples, thermal decomposition may be the quicker method, since for CVAAS and CVAFS the sample would need to be digested before analysis.
Mercury analyzer manufacturers
Table 2 – Manufacturers of mercury analyzers
The list in Table 2 represents a selection of leading mercury analyzer manufacturers, but is not comprehensive.
Katriona Scoffin, B.Sc., is a freelance science writer; e-mail: [email protected].
Please check out our Mercury Analyzer / Hg Analyzer section to find manufacturers that sell these products