Microwave-assisted diluted acid digestion for trace elements analysis of edible soybean products José Tiago P. Barbosa a , Clarissa M.M. Santos b , Vanessa N. Peralva c , Erico M.M. Flores b , Mauro Korn c , Joaquim A. Nóbrega d , Maria Graças A. Korn a,⇑ a Instituto de Química, Universidade Federal da Bahia, Campus Universitário de Ondina, Salvador 40170-280, Bahia, Brazil b Departamento de Química, Universidade Federal de Santa Maria, 97105-900 Santa Maria, RS, Brazil c Departamento de Ciências Exatas e da Terra, Universidade do Estado da Bahia, Salvador, BA, Brazil d Departamento de Química, Universidade Federal de São Carlos, São Carlos, SP, Brazil article info Article history: Received 15 January 2014 Received in revised form 12 November 2014 Accepted 16 November 2014 Available online 25 November 2014 Keywords: Soybeans Sample preparation Microwave radiation Trace elements Nitric acid ICP OES ICP-MS abstract A new method for the decomposition of soybean based edible products (soy extract, textured soy protein, transgenic soybeans, and whole soy flour) was developed to essential (Co, Cr, Cu, Fe, Mn, Ni, Se, V, and Zn) and non-essential (As, Ba, Cd, Pb, and Sr) trace elements determination by ICP OES and ICP-MS respec- tively. Effects related to the concentration of HNO 3 (2.1–14.5 mol L À1 ) and the use of hydrogen peroxide on the efficiency of decomposition was evaluated based on the residual carbon content (RCC). It was dem- onstrated that 2.1 mol L À1 HNO 3 plus 1.0 mL H 2 O 2 was suitable for an efficient digestion, since RCC was lower than 18% and the agreement with certified values and spike recoveries were higher than 90% for all analytes. The concentrations of analytes in the samples (minimum–maximum in mg kg À1 ) were: The concentrations of analytes in the samples (minimum–maximum in mg kg À1 ) were: As (<0.007–0.040), Ba (0.064–10.6), Cd (<0.006–0.028), Co (0.012–102), Cr (0.56–5.88), Cu (6.53–13.9), Fe (24.9–126), Mn (16.4–35.2), Ni (0.74–4.78), Se (<2.90–25), Sr (2.48–20.1), Pb (<0.029–0.11), V (<0.027–20), and Zn (30.1–47.3). Soy-based foods investigated in this study presented variable composition in terms of essen- tial and potentially toxic elements, which can be attributed to different methods of processing. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Soybeans are grown for their seeds that are rich in nutritional ingredients, such as proteins (40%), oils (20%), carbohydrates (35%) and essential elements. Due to the increased production of soy-based foods, the determination of trace elements is important in view of toxicity and nutritional assessments (Llorent-Martinez, Fernandez de Cordova, Ruiz-Medina, & Ortega-Barrales, 2012; Yip, Chan, Cheung, Poon, & Sham, 2009). There are previous studies related to the contents of essential and non-essential elements in different species of soybeans and related products in some coun- tries (Fernandez et al., 2002; Hseu, 2004; Karr-Lilienthal, Griesshop, Merchen, Mahan, & Fahey, 2004; Kazi, 2009; Liu, Wang, Liu, Zhao, & Zhan, 2004; Sola-Larranaga & Navarro-Blasco, 2009; Souza, 2005; Wei, Shen, Rui, & Jiao, 2008; Yip et al., 2009). However, despite the large consumption, there are few literature reports regarding the trace elements contents in soybeans pro- duced in Brazil (Carvalho et al., 2011; Gonzalez et al., 2009; Gris et al., 2008; Mataveli, Pohl, Mounicou, & Arruda, 2010; Sussulini, Souza, Eberlin, & Arruda, 2007; Zhou et al., 2011). The determination of trace elements in food requires sensitive analytical techniques such as inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma optical emission spectrometry (ICP OES). However, results obtained employing these techniques may be affected by matrix and/or spectral interferences when working with solutions containing ele- vated concentrations of dissolved organic compounds. Further- more, usually before performing measurements by ICP-MS or ICP OES, food samples must be decomposed with suitable methods leading to digests containing reduced amounts of residual carbon compounds and low residual acidity (Korn et al., 2008; Nardi et al., 2009). Microwave-assisted digestion procedures using closed vessels and various acid mixtures have been employed to decompose organic matrices of biological samples at elevated temperatures and/or pressure. It was already demonstrated that microwave- assisted sample digestion accelerates and minimises contamina- tion and losses of volatile elements (Bakkali, Ramos Martos, Souhail, & Ballesteros, 2009; Cindric ´ et al., 2012; Guo et al., 2012; http://dx.doi.org/10.1016/j.foodchem.2014.11.092 0308-8146/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel./fax: +55 71 3283 6830. E-mail address: korn@ufba.br (M.G.A. Korn). Food Chemistry 175 (2015) 212–217 Contents lists available at ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem