Trace Element Speciation in Food

BERTON, P.; MARTINIS, E.M.; WUILLOUD, R.G.

Advances in Food Sciences and Nutrition

Wiley & Sons

Año: 2013; p. 1 - 40

Life sciences obtain more and more benefits from the development of novel analytical methods. At present, and on the subject of trace element determination, this refers to identify, separate and quantify the different chemical forms (species) under which a chemical element can occurs in biological systems, environment and food. Thus, the word "Speciation" has become very popular and it is commonly utilized in those studies related to trace elements.1-6 This concept results extremely relevant within nutritional and contamination studies as it generates crucial information regarding the bioavailability and toxicity of an element. Moreover, speciation studies have demonstrated that these two properties for a specific element depend not only on its chemical nature, but also on its chemical forms and combinations with other more complex molecular structures.5, 7-9 Some classical examples mentioned in the literature are as follows: ·       The higher toxicity of Cr(VI) as compared to Cr(III) due to its high bioavailability.10 ·       The easiness of organic Hg species to be transported through cell membranes with the consequent bioaccumulation in living organisms, such as fish, thus leading to magnification of Hg in human food chain.11 ·       Although Se has been widely recognized as an essential nutrient for humans, its toxicity is definitively linked to the type of inorganic or organic species occurring in food.6 Widely accepted definitions of elemental speciation along with its related terminology have been already reported by the International Union for Pure and Applied Chemistry (IUPAC) as follows:12 Chemical species. Chemical element: specific form of an element defined as to isotopic composition, electronic or oxidation state, and/or complex or molecular structure. Speciation analysis. Analytical chemistry: analytical activities of identifying and/or measuring the quantitities of one or more individual chemical species in a sample. Speciation of an element. Speciation: Distribution of an element amongst defined chemical species in a system. Despite these definitions, many times it is not possible to specifically evaluate the concentration of individual elemental species in a given sample due to insufficient chemical stability, conversion to a different chemical form, changing of oxidation state, etc. Therefore, when total elemental speciation analysis is not achieved, the term "Fractionation" is employed and defined as follows: Fractionation. Process of classification of an analyte or a group of analytes from a certain sample according to physical (e.g., size, solubility) or chemical (e.g. bonding, reactivity) properties. Trace elements speciation, and particularly for toxic trace elements, is nowadays considered to be the main interest to ensure food safety and it is becoming an important need for industrial quality control and regulatory implementation by many countries around the world. Therefore, the evaluation of total level of trace elements does not guarantee the safety of foods, since species often differ considerably in their adverse effects, which can range from beneficial to highly toxic. The effects on living organisms by essential trace elements, such as Fe, Se and Zn, and traditional toxic elements, for example As and Hg, are defined by their total levels and species distribution in food. In fact, food authorities have drawn attention to the need of establishing proper dietary requirements and upper safe intake limits for essential elements and tolerable dietary intake levels for potentially toxic trace elements. On the other hand, and since trace element bioavailability is highly variable among chemical forms as well as total levels in food, speciation analysis is essential to ensure food safety and to evaluate nutritional quality. Therefore, the role of speciation analysis is becoming an increasingly important tool in defining and assuring food safety and nutritional requirements. In order to accomplish these aims, speciation in foods requires robust analytical methods with well-established procedures for food sampling and sample treatment which assure the identity of the different chemical species. However, the availability of standard procedures for trace element speciation is still an important issue within speciation studies, which is illustrated considering the limited number of existing certified reference materials as compared to the broad spectrum of food matrixes to be currently analyzed by most routine laboratories. In this chapter, an introductory discussion to the issue of trace elemental speciation in food is given. General aspects demonstrating the great relevance that elemental speciation is having for different fields, such as toxicology and nutrition, are critically commented and discussed. Furthermore, a summary of mostly relevant and applied analytical methods and techniques to develop speciation studies in different foods is exposed.