Inorganic Trace Analytics by Henryk Matusiewicz Ewa Bulska

Author:Henryk Matusiewicz,Ewa Bulska
Language: eng
Format: epub
Publisher: De Gruyter
Published: 2018-03-10T16:00:00+00:00

4.3.3Selenium

Dietary sources of selenium include seafood, animal liver and kidney, brazil nuts, milk, eggs and grains; however, the daily recommended intake of this element (40–55 μg/day) is not always assured, and supplementation has been a common practice [232, 233]. For this purpose, biofortification of yeast, vegetables or mushrooms has been extensively studied and the obtained products are the main targets of speciation analysis. A variety of selenium compounds have been found in food products; among them inorganic oxoanions selenite Se(IV) and selenate Se(VI), low molecular organic species such as selenomethionine (SeMet), selenocysteine (SeCys), selenocystine (SeCys2), Se-methylselenocysteine (MeSeCys), Se-methylselenomethionine (MeSe-Met), γ-glutamyl-Se-methylselenocysteine (γ-Glu-MeSeCys), and high molecular mass compounds such as Se-containing proteins, selenoenzymes or selenosugars [28, 29, 114, 204]. According to their abundance in food and importance to a good health, inorganic Se, SeMet, SeCys/SeCys2 and MeSeCys are the most commonly determined compounds and the related recent studies carried out in edible oils, seafood and enriched food are presented in Table 4.3 [30, 124–132, 140, 162, 200, 234].

Since the important part of organic Se is protein-bound, enzymatic or acid hydrolysis is often required, whereas for nonprotein-bound species water extraction has usually been used. In recent studies on Se speciation in seafood, KOH/MeOH or enzymatic treatment with protease XIV has been reported [162, 200, 234]. For enriched rice, yeast, wheat flour, sprouts and vegetables, pretreatment procedures consisted of water, water/methanol, methanesulfonic acid, protease XIV treatment or simulated gastrointestinal digestion [30, 126, 127, 130–132]. In explorative studies, centered at Se biotransformation by lactic bacteria during elaboration of selenized yogurt [124] or selenized edible mushrooms [129], proteins were extracted, fractionated, enzymatically digested and the obtained extracts submitted to speciation analysis. More detailed description of sample pretreatment for Se speciation analysis in food-related products can be found in several reviews and book chapters [23, 28, 29, 114, 116, 204, 215].

The most explored hyphenated technique in Se speciation has been LC with ICP-MS detection. Specifically, Se(IV), Se(VI), SeMet, SeCys2, MeSeCys have been separated on anion-exchange columns with mobile phases at pH 5–7 [126, 127, 162, 200, 234]. Furthermore, different perfluorinated and alkylsulfonic acids have been extensively used as ion-pair reagents for separation of Se compounds on reversed-phase columns; among them, heptafluorobutyric acid, 1-butanesulfonic acid and 1-pentanesulfonic acid have been recently applied [124, 127–129, 131]. As to ICP-MS detection, ionization efficiency of Se in Ar plasma is relatively low (about 30%), the most abundant 80Se isotope suffers from polyatomic interference caused by argon dimer (40Ar40Ar+) and mobile phase composition may have deteriorating influence on the detection sensitivity. Due to these limitations, the use of CRC pressurized with hydrogen is strongly recommended as well as the addition of methane gas for increased sensitivity and careful selection of chromatographic conditions giving preference to isocratic elution with diluted mobile phases and low concentration of organic modifier [29, 113, 116, 235]. In some applications, postcolumn HG and AFS detection has been used; however, all Se compounds eluting from the column have to be converted to Se(VI) and then reduced to Se(IV) to assure efficient H2Se generation [162, 164, 236].

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