For decades, selenium has been considered an essential element to human nutrition. It plays an important role in many biological processes, generally exerting its biological affect through selenoproteins. Selenium is incorporated specifically into proteins by a UGA codon that encodes for selenocysteine. There are over 30 known selenoproteins in mammalian systems, including a number of glutathionine peroxidases (GPx), iodothyronine 50-deiodinases (IDI), sperm capsule selenoprotein, and thioredoxin reductase (1, 2). Selenium also is required for the production of triiodothyronine, a thyroid hormone necessary for healthy brain and bone development, normal growth, and thermoregulation (3). Proper immune function also is selenium-dependent, which recently has prompted investigations into its ability to inhibit HIV and AIDS progression (4).

Much of the recent research on selenium is accomplished with inductively coupled plasma–mass spectrometry (ICP-MS). This method has proved to be sensitive, robust, and versatile enough to study selenium and selenium species in a multitude of situations. Although Ar–Ar dimers created by the plasma can act as an interference at m/z 80, the major selenium isotope, selenium still is detected easily and quantified by monitoring its other isotopes at m/z 77, 78, or 82. The use of collision cell technology, however, allows for the removal of the Ar–Ar interference, thus allowing for improved selenium detection. In this system, helium or hydrogen gas is introduced that bombards the dimers, thereby breaking them apart so that they no longer interfere at m/z 80. Many different liquid chromatography (LC) techniques are commonly coupled to ICP-MS for selenium speciation. These methods have allowed for the identification of specific selenium species of biological importance. Further confirmation of these species often is achieved through electrospray ionization MS (ESI).

As more research is conducted on selenium and selenium-containing compounds, the diversity of this element's various species becomes more apparent. Not only is selenium a known nutrient, it also can be a toxicant at even slightly elevated levels. Selenosis, or the poisoning caused by selenium, is characterized by discoloration of the skin and nails, gastrointestinal disturbances, fatigue, hair loss, nervous system abnormalities, and garlic-scented breath. Because high levels of selenium in the environment can have adverse effects on plant and wildlife, selenium remediation has become an issue that currently is being addressed in many areas of the world.

Ironically, even though selenium itself can be toxic, normal levels of selenium in blood plasma actually can reduce the toxicity of other heavy metals through the formation of equimolar complexes. These complexes follow different biological pathways than the free heavy metal, thereby reducing the potential damage caused by the metal as well as that of the selenium itself.

Despite the fact that extreme levels of selenium can prove toxic, it has been found that the consumption of slightly elevated levels of selenium actually can be quite beneficial. Certain selenium species, namely selenomethionine and methylselenocysteine, have exhibited chemopreventive properties, particularly towards prostate cancer. As a result, selenium speciation of many foods and nutritional supplements has become increasingly worthwhile.

This article will focus on three very distinct roles that selenium can play in biological processes: as a toxicant, a chemopreventive agent, and a heavy metal antagonist. By discussing the current research associated with each, it will become apparent how ICP-MS can be employed in a multitude of capacities to better understand and better utilize these selenium properties.

Selenium as a Toxicant

Although selenium is an essential nutrient, it can be toxic at relatively low levels. While the USDA recommends 70 mg Se per day for human health, roughly 800 mg daily can lead to selenosis. In 1983, the symptoms associated with selenosis were seen in many of the animals of the Kesterson National Wildlife Refuge in California. The oddly shaped beaks, missing wings, and twisted legs of newborn birds in the area prompted scientists to discover unusually high levels of selenium had accumulated in the Kesterson Reservoir water. It was determined that this had been caused by irrigation systems designed to support agriculture on saline soils. These systems flushed large amounts of water through the area, which drained into the Kesterson Reservoir, carrying with them naturally occurring levels of selenium salts. The accumulation of this selenium proved to have a toxic effect on many of animals as well as much of the plant life in the area.