Measurement Techniques for Mercury: Which Approach is Right for You?


David Pfeil, Teledyne Leeman Labs 6 Wentworth Drive, Hudson NH 03051 USA Tel: 603-886-8400 • Email: dpfeil@teledyne.com • Web: www.TeledyneLeemanLabs.com


The United States Environmental Protection Agency classifies mercury as a persistent, bio-accumulative toxin(1)


, indicating that its toxicity does not diminish through decomposition or


chemical reaction, and that it is absorbed faster than it can be excreted. Recently, efforts to minimise the release of mercury, and to track its migration when released, have demanded more sensitive analytical techniques for its measurement. As these techniques have become available, regulatory agencies around the world have written new analytical methods for their use. Table 1 shows a listing of many of the regulatory methods that are available for use with today’s technologies.


Let’s take a look at the analytical techniques in a bit more detail and then we’ll come back to the question of which technique is right for you.


Cold Vapour Atomic Absorption Spectroscopy (CVAAS)


In many parts of the world, CVAAS is still the most commonly used technique for the determination of mercury. Hallmarks of this approach include: detection limits in the single digit part per trillion (ppt) range, a dynamic range of 2 to 3 orders of magnitude and an abundance of analytical methods which allow for the measurement of Mercury in almost any sample matrix.


The technique was introduced in 1968 by Hatch and Ott(2) on


the heels of the first commercially available atomic absorption spectrometer. In their work they described an attachment for flame AA that enabled them to reduce mercuric ions in solution to ground state atoms and transport that mercury to the optical path of the spectrometer for measurement. Thus, cold vapour atomic absorption was born. Very quickly CVAAS became the reference technique for mercury determinations. Within a few years, the US EPA adopted the technique for the determination of mercury in water, soil, and fish. Now, almost 40 years later, CVAAS remains one of the primary techniques for mercury analysis and is the reference technique for drinking water monitoring per the Safe Drinking Water Act(3)


.


In contrast to those early systems, most modern CVAAS instruments are: more sensitive, more automated, smaller, faster and less expensive than generic flame spectrometers with cold vapour attachments. Today’s CVAAS systems provide detection limits of just a few ppt, analyse samples in about one minute, require very little operator interaction and take up just a couple of square feet of bench space. Figure 1 provides an overview of a cold vapour atomic absorption system along with a photograph of a typical instrument. With CVAAS instruments a peristaltic pump is typically used to introduce sample and stannous chloride into a gas liquid separator where a stream of pure, dry gas is bubbled through the mixture to release mercury vapour. The mercury is then transported in the carrier gas through a dryer and then into an atomic absorption cell. Once in the absorption cell mercury absorbs 254 nm light in proportion to the concentration of mercury in the sample.


Cold Vapour Atomic Fluorescence Spectroscopy (CVAFS)


Hallmarks of CVAFS based mercury analysers include sub ppt detection limits and a much wider dynamic range than can be achieved by CVAAS; typically 5 orders of magnitude for CVAFS versus 2 to 3 for CVAAS. CVAFS instruments are available in two configurations; one which employs simple atomic fluorescence and one which employs a gold amalgamation system to preconcentrate mercury prior to measurement by atomic fluorescence. The detection limit for the simple fluorescence approach is about 0.2 ppt whereas using the preconcentration approach with fluorescence detection can be as low as 0.02 ppt. The US EPA has promulgated methods for each of these approaches; Method 245.7(4) without preconcentration and 1631(5)


is for use is with preconcentration.


These methods were developed to satisfy the need for quantitation at the National Recommended Water Quality Criteria for Mercury(6)


EN13806 . These criteria are published pursuant to


Section 304(a) of the Clean Water Act (CWA) and provide guidance for states to use in adopting water quality standards which ensure that ambient waters are safe to fish, and subsequently, that fish are safe for consumption. Additional information on this subject is available at http://water.epa.gov/scitech/swguidance/standards/current/index.cfm


Figure 2 provides an overview of a cold vapour atomic fluorescence instrument; in this case both with and without a gold amalgamation system for preconcentration. With CVAFS instruments a peristaltic pump is typically used to introduce sample and stannous chloride into a gas liquid separator where a stream of pure, dry gas (typically argon) is bubbled through the mixture to release mercury vapour. The mercury is then transported in the carrier gas through a dryer and then to a valve which selects between simple fluorescence or the preconcentration approach. With fluorescence the drying stage is quite important as water vapour and other molecular species can interfere with the fluorescence measurement. Once in the detector, mercury vapour absorbs 254 nm light and fluoresces at the same wavelength. Measurement of the fluorescence signal is usually made at 90 degrees to the incident beam to minimise scatter from the excitation source. The intensity of the fluoresced light is directly proportional to the concentration of mercury.


The concentration of standards and samples with this technique Table 1: Commonly Used Regulatory Methods


are typically 100–1000× lower than those used with CVAAS, demanding much cleaner reagents. To ensure reagents are low in mercury, methods such as EPA Method 1631 describe techniques to remove mercury from salts and some solutions.


Direct Analysis by Thermal Decomposition


Hallmarks of the direct analysis approach include: elimination of the sample digestion step, fast analysis times and a detection limit of about 0.005 ng. Elimination of sample digestions means solid samples can typically be run in their native form. For laboratories that analyse large numbers of solid samples, or that would simply rather not perform the digestion typically associated with CVAAS and CVAFS, direct analysis may be ideal. It is noteworthy that this approach also carries with it the benefit of generating less acid waste than the solution-based techniques. However direct analysis is not well suited for the laboratory whose primary need is to run large numbers of samples that are already in aqueous solution. For liquid sample analysis, the detection limit available using direct analysis is not typically comparable with those of CVAAS or CVAFS. This is primarily due to the relatively small liquid volumes that can be processed using direct analysis; which are


typically well under 1 ml per sample. Consider, for example,


Analytical Techniques


Cold Vapour Atomic


Absorption (CVAAS)


245.1 245.5


Analytical Methods


7470 7471


3111B EN1483


Cold Vapour Atomic


Fluorescence (CVAFS)


245.7 1631


Direct Analysis or Thermal Decomposition


7473 6722-01


EN13506 EN12338 ISO 17852:2008


AET October/November 2011 www.envirotech-online.com


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