Measuring Mercury in Ambient Waters by Cold Vapor Atomic Fluorescence: When Is Purge and Trap Preconcentration Really Necessary? - Mr. David Pfeil

Numerous studies have raised concerns about the physiological impact of exposure to low-level mercury; this is especially true when present as methylmercury, the most commonly found organic form of mercury.

Inorganic mercury, when introduced into the environment through natural or anthropogenic sources readily migrates into lakes, rivers and streams where bacteria and other micro-organisms convert it to methylmercury. This form of mercury is particularly insidious because it bioaccumulates in aquatic species such that waters low in mercury can produce fish containing dangerously high levels of the substance. In fact, the bioaccumulation factor, or the ratio of concentration in tissue (fish) to the concentration in the medium (water), can exceed 100,000:1. As a result of bioaccumulation, methylmercury levels in fish can often exceed ‘safe or acceptable’ levels (usually one part per million or less) even though the water the fish inhabited was relatively clean. Similarly for humans, methylmercury (consumed primarily from contaminated fish) is readily assimilated and efficiently stored in the body so it becomes a growing problem over a lifetime of exposure. Factors such as these make methylmercury especially dangerous for “at risk” groups such as pregnant women and young children.

Recognizing how pervasive mercury pollution has become, and in an attempt to minimize its harmful effects, environmental agencies around the world have published advisories which limit the recommended consumption of certain types of fish, or fish caught in specified bodies of water. For example, in the United States alone more than 2000 lakes and streams have fish advisories established.

The Water Quality Criteria for Mercury
As Europe pursues the establishment of a water quality criteria for mercury, it may be worth studying the path Canada and the United States have taken. As a result of the Great Lakes Initiative (a combined effort between Canada and the United States to ‘help restore, maintain, and protect’ the ecosystem of the Great Lakes Basin) a new water quality criteria for mercury in ambient waters was established. This included a 1.3 ppt limit for Hg in ambient waters to ensure the protection of wildlife. Monitoring progress toward achieving the new water quality criteria for mercury required much more sensitive analytical methods than were previously available; as a result, two new methods employing cold vapor atomic fluorescence (CVAF) were promulgated by the USEPA. Method 1631E “Sampling Mercury in Water by Oxidation, Purge and Trap, and Cold Vapor Atomic Fluorescence Spectrometry” is a low-level mercury test method which uses a gold amalgamation trap to preconcentrate mercury. This method is similar to the European Standard EN 12338. Method 245.7 “Mercury in Water by Cold Vapor Atomic Fluorescence Spectrometry” is a faster, simpler technique but somewhat less
sensitive than 1631E and is similar to EN 13506. Hereafter, this approach will be referred to as “simple fluorescence”. Along with the lower concentrations of mercury in samples comes a greater risk of
contamination so a new sample handling method was also developed, Method 1669: “Sampling Ambient Water for Trace Metals at EPA Water Quality Criteria Levels”.