WUILLOUD Rodolfo German
congresos y reuniones científicas
Molecular size distribution patterns of several elements of toxicological and nutritional interest in nuts by SEC-ICP-MS
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Nuts are complex plant foods that are not only rich sources of unsaturated fat but also contain several nonfat constituents such as plant protein, fiber, micronutrients, plant sterols, and phytochemicals 1. Among foods with favorable fatty acid profiles, nuts have received particular attention because of the epidemiological association of their frequent intake with protection from coronary heart disease. Despite their high calorie content, consumption of nuts and nut products is highly popular. They are used extensively by manufacturers in fruit and nut cereal combinations such as granola bars, chewy fruit bars, breakfast cereals, and desserts. Furthermore, nuts are the third most heavily used ingredient in confectionery manufacture.The elemental composition of plant foods has always been important due to the varied information proved such as, nutritional value, toxicity, pollution, geographic origin of plants, etc. Essential trace elements are vitally important for various metabolic processes, and toxic elements if present in relatively high amounts, adversely affect these processes. On the other hand, the presence of metals in living organisms, and more specifically in plants, is of particular concern for an adequate nutrition of others organisms in the food chain. Therefore, information about trace element distribution is extremely needed in order to estimate the intake of essential elements and in evaluating the potential health risks caused from exposure to toxic elements in foodstuffs. Despite the importance of the presence of metals in food, the number of studies developed to determine total elemental concentrations in nuts is scarce. Moreover, with the only exception of selenium, studies about elemental speciation analysis in nuts samples have not been developed. In fact, the only two studies about speciation analysis of selenium in nuts have been developed in our research group.2, 3 Selenium was analyzed in several nuts investigating its distribution among molecular weight fractions by using size-exclusion chromatography (SEC) coupled to inductively coupled plasma mass spectrometry (ICP-MS). The results showed that selenium was mainly associated to high molecular weight fractions, which in the particular case of nuts corresponded to proteins. After hydrolysis of Se-proteins, the selenoaminoacids were analyzed by high-performance liquid chromatography (HPLC) with on-line ICP-MS detection.3 In a different study, the selenium species were separated and determined by using ion-pairing HPLC-ICP-MS previous to enzymatic extraction with proteinase K. The identification of the selenium species was performed with electrospray-mass spectrometry (ES-MS) and selenite, selenocystine, selenomethionine, and selenoethionine were found to be present in the medium after enzymatic extraction of Brazil nuts samples.2Because the chemical composition of nuts is complex (lipids 50-70%, proteins 10-20%, and carbohydrates 10-20%) trace elemental analysis is difficult and a suitable sample preparation step has to be developed previous to the analysis. Several wet-digestion procedures and detection systems have been used for analysis of total elemental analysis in nuts. However, these experiments showed that incomplete digestion of the sample was the main problem producing poor reproducibility and accuracy. Moreover, extremely cares have to be taken in the sample preparation step when speciation studies are performed in order to preserve the natural composition of the elemental species.SEC is a very useful technique to study the distribution of elements in different molecular weight fractions giving information about the association of elements with the different compounds of the sample. In the case of nuts and as mentioned before, SEC-ICP-MS has only been applied for distribution studies of Se. However, there are other elements of nutritional and toxicological interest in nuts, which have not been studied. Although the application of SEC-ICP-MS coupling for metal speciation has suffered from a number of limitations and difficulties, it has been extensively accepted as a very useful hyphenated technique in speciation studies to estimate the association of elements to the compounds present in the sample, and it is considered as the starting point for a deeper evaluation of the nature of elemental species and subsequent studies. In the present work, the distribution patterns of several elements among the different molecular weight fractions corresponding to the compounds present in most consumed nuts was studied. The studies were developed by coupling SEC technique to ICP-MS for the separation and on-line detection of the elements in nuts. Additionally, different procedures for the extraction of the organometallic species from the nuts samples were evaluated and optimized. The performance of the separation was studied by modifying the different SEC variables involved in the system.Soluble species were extracted from nuts samples by using different solutions, including 0.1 mol/L NaOH, 0.1 mol/L HCl, and hot distilled water. The extractability values of elements expressed as percentages of total element content are given in this paper. The Superdex Peptide column was selected in order to differentiate the compounds between high and low molecular weight fractions. Element quantities eluted by SEC were compared with known amounts of elements present in injected samples extract and chromatographic recoveries were calculated. Incomplete recoveries of some studied elements were caused by low stability of some element species. These extracted chelates of metals were dissociated during chromatography or even sample preparation. A broad elemental distribution among different compounds was observed in the nuts samples. Most of the elemental species were found to be associated to high molecular weight fractions, and more specifically some of them follow a similar distribution pattern than sulfur. This observation and the known fact that nuts contain high amounts of protein (10-20%), are suggesting that most of the elements present in nuts are associated with proteins. (1) Kris-Etherton, P. M.; Zhao, G. X.; Binkoski, A. E.; Coval, S. M.; Etherton, T. D. Nutrition Reviews 2001, 59, 103-111.(2) Vonderheide, A. P.; Wrobel, K.; Kannamkumarath, S. S.; B'Hymer, C.; Montes-Bayon, M.; De Leon, C. P.; Caruso, J. A. Journal of Agricultural and Food Chemistry 2002, 50, 5722-5728.(3) Kannamkumarath, S. S.; Wrobel, K.; Vonderheide, A.; Caruso, J. A. Analytical and Bioanalytical Chemistry 2002, 373, 454-460.