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

Sep 01, 2011

Analytical techniques for measuring mercury include cold vapor atomic absorption spectroscopy, cold vapor atomic fluorescence spectroscopy, and direct analysis by thermal decomposition. David Pfeil discusses the advantages and disadvantages of the techniques and provides tips for choosing the right technique for various situations.

The United States Environmental Protection Agency (US EPA) 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 minimize 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 specifying their use. Table I provides 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 more detail and then we'll come back to the question of which technique is right for you.

Cold Vapor Atomic Absorption Spectroscopy

In many parts of the world, cold vapor atomic absorption spectroscopy (CVAAS) is still the most commonly used technique for the determination of mercury. Hallmarks of this approach include detection limits in the single-digit parts-per-trillion (ppt) range, a dynamic range of 2–3 orders of magnitude, and an abundance of analytical methods that allow for the measurement of mercury in almost any sample matrix.

The technique was introduced in 1968 by Hatch and Ott (2) soon after the first available atomic absorption spectrometer. Their work described a device for flame AA that enabled them to reduce mercuric ions in solution to ground state atoms and transport the mercury to the optical path of the spectrometer for measurement. Thus, cold vapor 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 method for monitoring drinking water per the Safe Drinking Water Act (3).


Figure 1: An overview of cold vapor atomic absorption.
While simple, manual systems like that described by Hatch and Ott are still available today, most modern CVAAS instruments are more sensitive, automated, smaller, faster, and less expensive than generic flame spectrometers with cold vapor devices attached. Today's CVAAS systems provide detection limits of just a few parts per trillion, analyze samples in about 1 min, 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 vapor atomic absorption system. 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 vapor. The mercury is then transported via carrier gas through a dryer and then into an atomic absorption cell. Mercury absorbs 254-nm light in proportion to its concentration in the sample.