Milking AAS for iodine content

A simple, precise and accurate automatic method for determining total iodine concentration in milk products based on indirect atomic absorption spectrometry could improve studies of this essential micronutrient.

Carmen Yebra and Minuca Herminia Bollaín of the Department of Analytical Chemistry, Nutrition and Bromatology, at the University of Santiago de Compostela, Spain, explain how their technique allows them to determine iodine concentration in the range 0.011 to 0.35 micrograms per millilitre with a relative standard deviation of just 1.3 to 6.8% and with test rate of about 17 samples per hour.

Iodine is an essential part of a healthy diet as it is needed by the thyroid gland for the biosynthesis of thyroid hormones; an excessive intake iodine can lead to thyroid disorders, however. "Seafood, iodized table salt, milk and dairy products are common sources of iodine," explains the team. They point out that the iodine concentration of cow's milk is influenced most by the iodine content of cattle feed and by the season, but this assumes that iodide is the only form in which iodine might be present in milk. The precise and reliable determination of low levels of iodine present in all forms in dairy products is important nutritionally for ensuring people obtain adequate but not excessive amounts of this micronutrient.

There are numerous approaches to total iodine determination available, the researchers explain, among them, amperometric detection, ion chromatography with photodiode array, series bulk acoustic wave or electrochemical detection, gas chromatography with electron capture, mass spectrometry, ion selective electrodes, optical sensors, neutron activation analysis, catalytic spectrophotometry, and inductively coupled plasma optical emission spectrometry. "However," the team says, "most of these approaches, although sensitive, either require expensive instrumentation or suffer from the need for an extensive sample pre-treatment."

Others have suggested that indirect methods, such as atomic absorption spectrometry (AAS) might be useful for determine of non-metallic elements and organic species. Many laboratories have inexpensive AAS equipment and a complexation and precipitation procedure using mercury or silver would seem to be the obvious answer to the problem of iodine detection. Unfrotunately, the methods that have so far been proposed operate in batch mode, the team points out, and require laborious and time-consuming separation procedures using toxic and expensive organic solvents.

The researchers have now circumvented these and various other problems by turning to flame AAS, that uses thiosulfate as the dissolving solution (so avoiding the need for toxic cyanide) and precludes interference from other precipitating anions by virtue of the fact that silver iodide is insoluble in dilute ammonia.

Their FAAS experiment thus simply involves three steps. First, sample introduction and analyte precipitation, secondly, washing of the precipitate with dilute ammonia, and finally, dissolution of the precipitate in dilute thiosulfate solution, and tag element (Ag) for AAS detection. The value obtained for metal content is then directly proportional to the iodine content of the original sample; detected silver some only from precipitated silver iodide as all other silver salts a dissolved away by the ammonia.

"This method showed enough sensitivity for the determination of total iodine in infant formulas and powder milk samples, and it may be adapted to the determination of total iodine in other food types," the team says. The technique not only improves the quality of analytical methodology but also adds an environmentally greener hue to analytical chemistry by avoiding toxic cyanide and volatile organic solvents, the researchers add.