Analytica Chimica Acta 565 (2006) 81–88

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82 A.A. Momen et al. / Analytica Chimica Acta 565 (2006) 81–88 temperature [16,17]. In addition, the lower blank levels and the capability in handling large sample, up to 10 g, sometimes make dry ashing methods desirable for toxic and nutrient elements determination. On the other hand, wet digestion methods are widely used for the digestion of food materials, because of their simplicity. They are fairly rapid and flexible in term of being able to change sample weights and digestion conditions, less prone to either volatilization or retention losses, and inexpensive. The main drawbacks are the co-precipitation of sparingly soluble compounds, the incomplete digestion of organic material and the formation of insoluble compounds [18,19]. Correspondingly, the use of oxidizing acids such as HNO 3 and/or H 2 SO 4 is important especially for wet digestion of food samples with high carbohydrate and/or fat content, to reduce calcinations and to ensure completeness of the digestion. Also, acid mixtures of HNO 3 /HClO 4 or HNO 3 /HCl or combination of more than two acids are usually employed in wet digestions, while the use of H 2 O 2 may be needed when the sample material is difficult to digest [20,21]. It important that, the analytical procedure which is used for sample digestion should involve minimal sample handling to decrease the levels of contamination during sample preparation. Moreover, the analytical technique should be rapid, sensitive, with low quantification limits and wide dynamic range, etc. [13,16,22]. Considering these requirements, ICP-OES is a convenient technique for this task (analysis), because it allows a multi-element determination in a single solution, with sufficiently low quantification limits, sensitive, wide dynamic range, etc. [14,17,23]. In the present work, different digestion procedures that have been used for food samples digestion were carefully investigated and accurately evaluated with respect to their affects on the analysis of legume samples. The analytical performances, such as limits of quantification, precision of the overall procedures and accuracy were assessed statistically to optimize the ICP- OES parameters and to evaluate the investigated procedures. Moreover, the present study is focused on the mineral content in legumes, analyzed in digested samples with a validated ICP- OES method under optimized conditions. We also evaluated different digestion methods to develop and recommend an analytical procedure for the digestion of legume samples, followed by the determination of selected toxic and nutrient elements. 2. Experimental 2.1. Instrumentation and apparatus All experiments were carried out using a Perkin-Elmer Optima 3100XL axial viewing ICP-OES, according to operating conditions given in Table 1. The analytical wavelengths (nm) were set at the following two different spectral atomic (I) and ionic (II) lines for each analyte: Al I (308.215), Al I (237.313), Cd II (214.440), Cd II (226.502), Cr II (283.563), Cr II (284.325), Cu I (324.752), Cu II (224.700), Fe II (238.204), Fe II (239.562), Mg II (279.077), Mg II (280.271), Mn II (257.610), Mn II (259.372), Pb II (220.353), Pb I (217.000), Zn I (213.857), Zn II (202.548). Table 1 Operating conditions and description of ICP-OES instrument RF generator RF incident power a Torch alumina injector, i.d. Nebulizer argon gas flow rate a Argon gas flow rates Signal measurement mode Spray chamber Nebulizer type Pump Sample uptake flow rate a Polychromator Resolution Detector a Optimized value. 40 MHz (free running) 1300 W 2.0 mm 0.85 l min −1 1.5 l min −1 (auxiliary); 15 l min −1 (plasma) Peak area (three points per peak) Scott double-pass Gem tips cross-flow Peristaltic, three channel 1.0 ml min −1 Echelle grating 0.006 nm at 200 nm Segmented-array charge-coupled (SCD) 235 sub-arrays Muffle furnace (Stuart Scientific Co. Ltd., England) has been used for ashing. Hot plate and drying oven (Thermolyne, Sybron Corporation) were also used. A peristaltic pump was used to introduce the sample solutions into the ICP and to discard the wastes. To avoid contamination before use, all glassware, digestion vessels and storage bottles (which are used for samples, reagents and reference solutions preparations and storage) were soaked in freshly prepared 10% (v/v) HNO 3 for at least 48 h, and finally washed three times with double de-ionized water (DDW). Crucibles were immersed in diluted HCl for about 2 days, and rinsed with DDW several times to be used for the next experiment. 2.2. Reagents and reference solutions All chemicals were of analytical-reagent grade and were purchased from Merck (Darmstadt, Germany), unless stated otherwise. Multi-element working standard solutions were prepared by appropriate dilution of stock ICP multielement standard IV (23 elements) containing 1000 mg l −1 for each element in 0.8 M HNO 3 . Mineral acids, chemical reagents, and oxidizing agents [65% (m/m) HNO 3 (d = 1.40 kg l −1 ), 37% (m/m) HCl (d = 1.19 kg l −1 ), 30% (m/m) H 2 O 2 (d = 1.11 kg l −1 ), 97% (m/m) H 2 SO 4 (d = 1.84 kg l −1 ), absolute ethanol (d = 0.79 kg l −1 ), (Mg(NO 3 ) 2·6H 2 O)] were used. Magnesium nitrate ethanolic solution [(5% (m/v) Mg(NO 3 ) 2·6H 2 O) in ethanol] was also used. All solutions were prepared with highly purity double de-ionized water (DDW), which was prepared by successfully passing tap water through two ion exchange columns (resin). Also, DDW has been used for washing and rinsing of all apparatus and glassware. IAEA-331 (spinach leaves) and IAEA-359 (cabbage) supplied by IAEA (International Atomic Energy Agency, Monaco) were used for optimizing, setting up and validation of the whole analytical procedures (i.e., the wet digestion and the dry ashing procedures). The certified reference (or information) values are available for most of analytes under investigation for assessment of the methods accuracy.
A.A. Momen et al. / Analytica Chimica Acta 565 (2006) 81–88 83 2.3. Preparation of sample solutions Five types of legumes (white bean, faba bean, lentil, green pea and chickpea) were purchased from the market. Samples were selected carefully, to represent the major categories that available in the market. The green pea was refrigerated in the laboratory until sample preparation. Since the grinding of samples help the digestion process, samples were first shelled, when necessary, grounded in a homemade polyethylene mortar and pestle and sieved into fine powders. Ten sub-samples (i.e., different independent samples) from each type were dried to constant mass, cooled and weighed as soon as they reached room temperature for further sample pre-treatment. The frozen green pea was first dried, then grounded and sieved using the same process that applied for other samples. IAEA-331 and IAEA-359 were used as bottled, without further grinding and sieving, but before digestion or ashing, they were dried as described in the certificates of analysis and recommended by the supplier. 2.3.1. Wet digestion procedures About 0.5 g sample was weighed into dry, clean PTFE digestion vessel. One milliliter of DDW was added first and then supplied with appropriate digestion mixture: HNO 3 /H 2 SO 4 , 2 + 1 (v/v) for wet digestion method 1 (WD1), and HNO 3 /H 2 SO 4 /H 2 O 2 , 4+1+1 (v/v) for WD2. When the initial reaction has subsided, vessel contents were gently mixed and heated, first at low temperature (about 125 ◦ C), then the temperature was gradually increased until heavy evolution of fumes ceases. If carbonization appeared, the vessel was removed from the hot plate, cooled and about 1 ml of HNO 3 was added. The mixture was then heated again until complete digestion. The final residue (sulfuric acid and inorganic constituent) was dissolved in 2.5 ml of HCl and diluted to 50 ml with DDW. The same digestion procedures were carried out for blank solution, and CRM’s (IAEA-331, IAEA-359) to validate the quality of the analytical procedures. WD1 was partly modified from that of Tinggi et al. [24] and the procedure that recommended by AOAC [25] while, WD2 was based on that of D’llio et al. [26] which applied for determination of trace elements in food samples, with major modifications. Matrix effect studies were carried out by spiking some of the original un-digested samples (accurately weighed different amounts) with variable amounts of standard solution of the metals. The spiked samples were then mineralized using the same digestion procedures as were applied to the non-spiked samples. All digestions were performed in triplicate. 2.3.2. Dry ashing procedures About 3 g sample was weighed into a porcelain crucible. Two milliliters of DDW were added first, and then supplied with appropriate ashing aid: ethanolic solution of Mg(NO 3 ) 2 for dry ashing method 1 (DA1), and (5%, v/v) HNO 3 /ethanolic solution of Mg(NO 3 ) 2 , 2 + 1 (v/v) for DA2. When the initial reaction has subsided, the crucible was heated at low temperature (100 ◦ C) to evaporate moisture and excess reagents and calcination of the sample. Then, the crucible was transferred into the muffle furnace at 450 ± 20 ◦ C for about 2–3 h for sample ashing. After ashing was completed (white or a semi-gray ash), the crucible was left to cool. If carbon particles remained the crucible was moistened with a several drops of HNO 3 and the suspension was again re-ashed for another 30 min to the same temperature. This procedure yielded a white or a semi-gray ash. The ash was first moistened with a few drops of DDW, then dissolved by subsequently adding 2.5 and 1.5 ml of HCl and HNO 3 to the crucible, and finally, quantitatively transferred to 50 ml with DDW. The same digestion procedures were carried out for blank solution, and CRM’s to ensure the quality of the analytical procedures. DA1 was based on the dry ashing procedure which described by Coco et al. [27] with minor changes, while, DA2 is the dry ashing procedure that described by Adeloju et al. [28] with major modifications. Matrix effect studies were carried out by spiking some of the original un-digested samples with variable amounts of standard solution of the metals. The spiked samples were digested using the same ashing procedures as were applied to the non-spiked samples. All digestions were performed in triplicate. 2.4. Statistical analysis Differences between mean values were evaluated by Student’s paired t-test. Linear regression statistical test and correlation analysis were performed for calculation of slope (b), intercept (a) and correlation coefficient (r) of the regression lines. Statistical analysis was based in triplicate measurements of all samples. 3. Results and discussion 3.1. Selection of spectral emission lines All analytes were measured in two different emission lines, and the main criteria to select between them are the possible spectral interferences and the concentration range of the analyte. In any case the sensitivities were calculated at spectral lines of no spectral interferences (with high sensitivity). The results of regression analysis of aqueous standard and standard addition solutions of lentil sample using WD2 are presented in Table 2, including slope (b), the 95% confidence intervals of b and r-values. The slope of the regression line was used as a measure of the sensitivity for each analyte and the r as a measure of the correlation between data. Therefore, in the present study the following spectral lines (in nm) were selected: Al I (237.313), Cd II (226.502), Cr II (284.325), Cu I (324.752), Fe II (238.204), Mg II (279.077), Mn II (259.372), Pb I (217.000), Zn I (213.857). The dynamic range of all analytes is extended at least up to 1 mg l −1 , and is considered as sufficient for the accurate quantification of analytes in various legume samples, which commonly contain lower concentrations of the examined analytes. In Table 2, the b-values (for the two emission lines) of all analytes that obtained for aqueous solutions were 1–18% higher than those obtained for standard addition solutions, with some exceptional cases for Cd, Cr, Fe, Cu and Mn. These differences may results from spectral and/or chemical interferences.
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Analytica Chimica Acta 565 (2006) 81–88

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