Element Speciation

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Elements can exist in various chemical valence states, forms, and compounds. For example, is the mercury present as some mercurous salt [Hg⁺], mercuric salt [Hg⁺²], elemental mercury [Hg⁰], methylmercury salt, dimethylmercury, or some other covalent mercury compound such as thimerosal, a preservative? Complete speciation of all the forms of an element can be a daunting task. While difficult, the answer is extremely important due to the tremendous difference in biological activity of the various forms. This article summarizes some speciation techniques available here. If your favorite element is not covered, we would like to hear about it because we are always exploring new methods of analysis.

Tin has been used in organic forms as a marine antifouling paint. Butyltin forms (mono-, di-, and tributyltin salts) are extracted and converted to hexyl butyl derivatives and determined by gas chromatography with a flame photometric detector (FPD).

Tributyltin Analysis by GC-FPD

The main interest in chromium is determining how much is present in the hexavalent state (Cr⁺⁶, normally as chromate or dichromate salts). Traditional methods typically use a spectrophotometric analysis of the intense colored complex formed between DPC and Cr⁺⁶. A newer, more specific, and dramatically more sensitive method couples the separation of Cr⁺⁶ from interferences by ion chromatography (IC) with the colorimetric detection with DPC post column.

In addition to inorganic forms of lead (Pb) used in paints and other products, tetramethyl- and tetraethyl lead were used for many years in gasoline. Typically these organic forms are determined as total organic lead. A sample is extracted with an organic solvent which extracts the organic forms but not the inorganic forms. The extract is evaporated, digested to convert it to an acid soluble form, and analyzed by ICPMS. Tetramethyl- and tertraethyllead can be determined specifically using GC-ICPMS. We no longer perform this analyses at this facility.

Arsenic has been found to exist in the environment and the body as arsenite (As⁺³), arsenate (As⁺⁵), methylarsonate (MMA), dimethylarsinate (DMA), as well as arsenosugars and amino acids. Currently we use IC-ICPMS to measure As⁺³, As⁺⁵, MMA, DMA, and arsenobetaine.

Selenium normally exists in the enviroment as selenite (Se⁺⁴) or selenate (Se⁺⁶). Selenocyante (SeCN^-) is a major species found in petroleum refinery wastewater. Selenomethionine and other seleno aminoacids are important forms in selenized yeast supplements. All these forms can be determined by LC-ICPMS.

Chlorine, bromine, and iodine can exist in several ionic forms (e.g.. chloride, chlorite, chlorate, and perchlorate). These are generally determined by various ion chromatographic analyses. Similarly phosphates and other complex phosphorus oxyanions are determined by IC.

Anions by Ion Chromatography

While silicon is quite common, the organic polymer polydimethylsiloxane (silicone) form has received considerable attention due to its use in implants. Silicone can be hydrolyzed, derivatized, and determined by gas chromatography. This method does not have any interference from inorganic forms of silicon; it is specific for dimethylsiloxane polymers.

This is just a sample of the speciation techniques available at Exova.

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