Vanize C. Costa

Halogen determination in food and biological materials using plasma-based techniques: challenges and trends of sample preparation

Halogens are a group of reactive elements, which are capable of making stable bonds with metals and nonmetals. Due to their properties, they play an important biochemical role in living organisms; some, such as F, Cl and I, are essential for humans, whereas others, such as Br, are non-essential and can cause key toxicological effects. This is probably the main reason why this group of elements has been extensively studied in food and biological materials, especially in recent years. The determination of halogens at trace levels requires suitable instrumentation, and plasma-based techniques, such as inductively coupled plasma optical emission spectrometry (ICP-OES) and especially inductively coupled plasma mass spectrometry (ICP-MS), are particularly well suited for this purpose. Although they are very versatile for performing multielemental determination, halogens present high ionization potentials and low wavelength emission lines, and they are prone to interferences due to matrix effects. Such drawbacks are difficult to overcome without changing instrumental features, such as mass or optical resolution. On the other hand, some interferences due to the presence of dissolved carbon in solution, enhancement or suppression of ionization, and related matrix effects can be minimized by applying an appropriate sample preparation step. In this sense, the present review covers the main sample preparation methods applied for halogen determination in food and biological materials by using ICP-OES and ICP-MS. Methods based on dilution, solubilization, extraction or those involving matrix decomposition (e.g., acid digestion and combustion) are discussed based on key applications selected in the last 20 years.

Ultraviolet radiation combined with microwave-assisted wet digestion of Antarctic seaweeds for further determination of toxic elements by ICP-MS

In this work, the use of ultraviolet radiation combined with microwave-assisted wet digestion (MW-UV) was applied for digestion of seaweed samples and further determination of As, Cd and Pb by inductively coupled plasma mass spectrometry (ICP-MS). In the proposed method, UV radiation was generated in situ by electrodeless Cd discharge lamps inserted into digestion quartz vessels. This approach increased the digestion efficiency allowing lower consumption of acids. The feasibility of using diluted acid solution (0.5 to 7 mol L−1 HNO3) was evaluated for relatively higher sample masses (up to 800 mg). The efficiency of digestion was evaluated taking into account the residual carbon content and residual acidity in digests. Under the selected conditions, it was possible to digest up to 700 mg of sample using a nitric acid solution as diluted as 2 mol L−1 HNO3 allowing an efficiency of digestion higher than 77% (considering the total C content in the sample) and acidity as low as 0.19 mol L−1 that is convenient for ICP-MS measurements. The accuracy of the proposed MW-UV method was evaluated by the digestion of two certified reference materials of aquatic plant (BCR 060 and BCR 670) and by comparison with the results obtained after digestion by microwave-assisted wet digestion in a high pressure system using concentrated HNO3 (MW-AD). The results obtained by the proposed method did not present difference (t-test, 95% confidence level) with the certified values and with results obtained after seaweed sample digestion using the MW-AD method. Using 700 mg of sample, the limit of detection was 0.005, 0.001, and 0.012 μg g−1 for As, Cd, and Pb, respectively. A clear advantage of the proposed method over classical approaches is that only diluted solution was necessary and it is possible to digest a relatively high sample mass, that is important to minimize the generation of laboratory residues and to improve the limits of detection, respectively.

Feasibility of halogen determination in noncombustible inorganic matrices by ion chromatography after a novel volatilization method using microwave-induced combustion.

A microwave-induced combustion (MIC) system based on the volatilization process was applied for subsequent halogen determination from noncombustible inorganic matrices. Portland cement samples were selected to demonstrate the feasibility of the proposed method, allowing the subsequent determination of Cl and F by ion chromatography (IC). Samples were mixed with high-purity microcrystalline cellulose, wrapped with a polyethylene film and combusted in quartz closed vessels pressurized with oxygen (20bar). Water and NH4OH (10, 25 or 50m mol L(-1)) were evaluated for Cl and F absorption, but water was selected, using 5min of reflux after volatilization. Final solutions were also suitable for analysis by pontentiometry with ion-selective electrode (ISE) for both analytes, and no difference was found when comparing the results with IC. The accuracy of the proposed method for Cl was evaluated by analysis of certified reference materials (CRMs), and agreement with certified values ranged from 98% to 103%. Results were also compared to those using the procedure recommended by the American Society of Testing and Materials (ASTM) for the determination of total chlorides (C114-13), and no difference was found. Volatilization by MIC using a mixture of cement, cellulose and a biological CRM was carried out in order to evaluate the accuracy for F, and recovery was about 96%. The proposed method allowed suitable limits of detection for Cl and F by IC (99 and 18mg kg(-1), respectively) for routine analysis of cement. Using the proposed method, a relatively low standard deviation (<7%), high throughput (up to eight samples can be processed in less than 30min) and lower generation of laboratory effluents, when compared to the ASTM method, were obtained. Therefore, the method for volatilization of Cl and F by MIC and subsequent determination by IC can be proposed as a suitable alternative for cement analysis.

Determinação de bromo e iodo em alga marinha comestível por ICP-MS após decomposição por combustão iniciada por micro-ondas

A method based on microwave-induced combustion (MIC) was applied for the decomposition of different types of edible seaweed (Nori, Hijiki and Wakame) for subsequent determination of bromine and iodine by inductively coupled plasma mass spectrometry (ICP-MS). Decomposition of 500 mg of each sample was achieved in less than 30 min. A single and diluted solution (150 mmol L-1 (NH4)2CO3) was used for the absorption of both analytes and a reflux step of 5 min was applied to improve analyte recoveries. Accuracy was evaluated using certified reference materials and agreement was between 103 and 108% for both elements.

A feasible method for indirect quantification of L-T4 in drugs by iodine determination

In this work, a method combining microwave-induced combustion (MIC) for sample preparation of commercial levothyroxine sodium (L-T4) drugs (L-T4: 25-200µg/tablet), and potentiometry with ion selective electrode (ISE) for iodine determination and subsequent indirect quantification of L-T4 was proposed. The type and concentration of the absorbing solution were evaluated to select the most suitable conditions for this study. Using the MIC method, it was possible to use solutions as diluted as 150mmolL-1 (NH4)2CO3 (for samples containing 25-200µg of L-T4/tablet) for I absorption. In these conditions, recoveries for L-T4 were between 99% and 101%, and relative standard deviations were lower than 10%. The limit of detection for L-T4 was 11.2µg/tablet, which is almost two times lower than the minimum concentration of L-T4 in commercially available drugs. Thus, the MIC was suitable for the digestion of several L-T4 drugs for subsequent I determination by ISE and indirect quantification of L-T4. Furthermore, the proposed method presents high throughput with low reagent consumption and consequently lower waste generation, making it suitable for routine determination of L-T4 in drugs. From the obtained results, it was possible to observe that one of the analyzed samples is not in agreement with the limits established by the United States Pharmacopeia, indicating the importance of the drug quality control. The United States Pharmacopeia establishes that each tablet must contain between 90% and 110% of the amount of active substance declared by the manufacturer.

A Novel and Eco-Friendly Analytical Method for Phosphorus and Sulfur Determination in Animal Feed

An eco-friendly method for indirect determining phosphorus and sulfur in animal feed by ion chromatography was proposed. Using this method, it was possible to digest 500 mg of animal feed in a microwave system under oxygen pressure (20 bar) using only a diluted acid solution (2 mol L-1 HNO3). The accuracy of the proposed method was evaluated by recovery tests, by analysis of reference material (RM) and by comparison of the results with those obtained using conventional microwave-assisted digestion. Moreover, P results were compared with those obtained from the method recommended by AOAC International for animal feed (Method nr. 965.17) and no significant differences were found between the results. Recoveries for P and S were between 94 and 97%, and agreements with the reference values of RM were better than 94%. Phosphorus and S concentrations in animal feeds ranged from 10,026 to 28,357 mg kg-1 and 2259 to 4601 mg kg-1, respectively.

Determinação de Cl e S em Algas Marinhas Comestíveis por Cromatografia de Íons após Decomposição por Combustão Iniciada por Micro-ondas

A determinacao de Cl e S nas algas marinhas comestiveis (Hidaka-Kombu, Hijiki, Kombu, Nori e Wakame) foi realizada por cromatografia de ions (IC) apos a decomposicao por combustao iniciada por micro-ondas (MIC). A concentracao de Cl nas algas foi mais elevada (6 a 10 vezes) que a de S, exceto para as algas Hijiki e Nori. O lixiviamento dos analitos apos lavagem das algas foi avaliado, sendo observada uma reducao nas concentracoes de Cl (de ate 75%) e um aumento da concentracao de S (de ate 58%). Usando a MIC-IC foi possivel obter boa exatidao (> 95%) para ambos os analitos e adequados limites de deteccao (Cl: 11 mg kg -1 e S: 9,1 mg kg -1 ).