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Evaluation of sample preparation methods for the determination of As, Cd, Pb, and Se in rice samples by GF AAS
Microchemical Journal 124 (2016) 402–409
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Evaluation of sample preparation methods for the determination of As, Cd, Pb, and Se in rice samples by GF AAS Richard Macedo de Oliveira, Ana Clara Nascimento Antunes, Mariana Antunes Vieira, Aline Lisbôa Medina, Anderson Schwingel Ribeiro ⁎ Universidade Federal de Pelotas (UFPel), Laboratório de Metrologia Química, Programa de Pós-Graduação em Química, 96160-000 Capão do Leão, RS, Brazil
a r t i c l e
i n f o
Article history: Received 16 September 2015 Accepted 22 September 2015 Available online xxxx Editor: J. Sneddon Keywords: Rice Inorganic contaminants Sample preparation GF AAS
a b s t r a c t This study presents a comparative evaluation of three different methods for sample preparation in rice for As, Cd, Pb, and Se determination by graphite furnace atomic absorption spectrometry (GF AAS). Raw and cooked white rice, parboiled rice, and brown rice samples were studied with the following procedures: i) HNO3 decomposition in open system with cold finger, ii) HNO3 decomposition in ultrasonic bath, and iii) alkali solubilization with TMAH. The sample preparation using reflux system was optimized and the best conditions were: 750 mg of sample, 10 mL of HNO3, 120 °C (heating), and 90 min in the digester block. In rice samples, medium concentrations of 12.0 ± 4.0 and 177.0 ± 64.0 ng g−1 for Cd and Pb were found, respectively, while for As and Se, the concentrations were below the limit of detection of the studied methods. After cooking, samples presented a maximum decrease of 61% in Cd concentration. The method validations were assured by analysis of certified reference material (NIST 1568a), and results were in accordance with the certified values. The recovery test was carried out and results ranged between 80 and 115% for all analytes in the rice samples, before and after cooking. All evaluated procedures presented accurate and precise results; however, the decomposition with reflux system proved to be more effective, simple and secure. Such characteristics are necessary for the application in routine analysis with medium detection limits of 77.2; 1.0; 35.5 and 192.1 ng g−1 for As, Cd, Pb and Se, respectively. © 2015 Published by Elsevier B.V.
1. Introduction Rice is a commodity of utmost importance to world food security because it feeds approximately 50% of the world population [1]. It is considered a complex matrix because it presents carbohydrates, mainly starch, water, proteins (18 essential amino acids), fiber, vitamins (B1, B2, B3), and minerals [2]. Asia countries are the main producers and consumers of this cereal, with about 90% of all world production. Brazil is the only non-Asian country among the main ten producers of rice, with approximately 1.6% of total world production. As a consequence of this large production, rice is daily consumed by Brazilians [3]. Due to extensive contamination of soil and irrigation water, mainly from the indiscriminate use of pesticides and fertilizers [4], rice intake may also be dangerous to humans, since inorganic contaminants such as Cd, Pb, and As may be absorbed into the cereal composition, being exposed to human body by ingestion [5–7]. Attention is needed to Se because although it is an essential element, it may cause risks to human health if ingested in large amounts [8]. All these elements are associated with different diseases in the human body, such as tumor development in various regions, continuous effects in central nervous system, besides effects on skin, tachycardia, as well as hair, nail and teeth losses [9–12]. ⁎ Corresponding author. Tel.: +55 53 3275 7179. E-mail address: [email protected] (A.S. Ribeiro).
http://dx.doi.org/10.1016/j.microc.2015.09.018 0026-265X/© 2015 Published by Elsevier B.V.
The determination of these elements is usually carried out in the trace level and the technique of graphite furnace atomic absorption spectrometry (GF AAS) is used, since it has the ability to reconcile high sensitivity, low limits of detection, and a small amount of sample for analysis (~20–50 μL) [13–15]. However, extensive sample preparation is required, especially when it is a complex matrix, such as rice [16]. Regarding rice crops, literature have reported wet decomposition methods in open systems, such as open system with heating plates [17, 18], heating in water bath [19] in digesters blocks [20–22], or even dry decomposition through sample calcination and ash production [21]. However, these methods may provide incomplete decomposition, low recovery of some elements [23–26], as well as analyte or acid volatilization [25,27,28]. Moreover, analyzing rice samples by wet decomposition in closed systems, especially in microwave oven with only concentrated HNO3 [29,30] or mixture of H2O2 as complement for organic elimination, appears to be the most appropriate [5,31], since these methods are fast, safe and reduce the risk of contamination and volatilization [32]. Despite this fact, these methods are not simple because they frequently have a low frequency rate and require sophisticated and expensive equipment thus a great number of laboratories do not use this technology, hindering its use in routine analysis. An alternative for sample preparation is the use of cold finger as reflux system for biological decomposition of bovine meat samples for the determination of Cd, Pb, and Sn [33], fish for determining Hg [34] and
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rice samples for quantification of Cu, Fe, Mn, and Zn [35]. However, the efficiency of this system has not been evaluated in the decomposition of rice samples for quantification of elements easily volatilized and in lower concentrations, such as Cd, Pb, Se and As. In addition, the evaluation of dissolution assisted by ultrasound with HNO3 and in alkaline medium with TMAH needs to be better explored for sample rice, since only extraction methods have been applied to this type of sample being not efficient for all elements [36–39]. Therefore, this work aims to carry out a comparative study of three alternative methods of sample preparation of polished white rice (WR), parboiled rice (PR) and brown rice (BR) for the quantification of Cd, Pb, Se and As by GF AAS using: i) decomposition in open system with cold finger, ii) dissolution assisted by ultrasound in HNO3, and iii) solubilization with TMAH. Furthermore, the behavior of these elements after cooking the rice was also evaluated.
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Table 2 Temperature program of graphite furnace for the determination of Cda, Pbb, Sec and Asd in rice samples treated with HNO3 and TMAH. ⁎Treated with TMAH. Step
Temperature (°C)
Ramp (°C s−1)
Hold (s)
Gas flow rate (mL min−1)
Drying Drying Pyrolysis Atomization Cleaning
110 130 400a; 680a⁎; 900b; 1000c,d 1200a,a⁎; 1700b; 1800c; 2200d 2450
5 15 10 0 1
30 30 20 5 3
250 250 250 0 250
and 3 μg into graphite furnace for each measurement, respectively. Before using, all glass apparatus were conventionally washed, soaked in 10% (v/v) HNO3 for at least 48 h, and then rinsed with ultrapure water prior to use. 2.3. Samples
2. Experimental 2.1. Instrumentation Integrated absorbance signals were measured in an atomic absorption spectrometer PinAAcle 900z (Perkin Elmer, Walthan, EUA) equipped with longitudinal Zeeman-effect based background correction system, autosampler (AS-900 model), color furnace camera (TubeView™), and transverse-heated graphite tube atomizer (THGA). The operational conditions recommended by the manufacturer were employed. Hollow cathode lamps (Perkin Elmer, Walthan, EUA) were used. Argon, 99.996% (Linde, Brazil), was used as protected and purge gas. The instrumental parameters used for each analyte are presented in Table 1 and the temperature program optimized is shown in Table 2. Samples were weighed using an analytical balance model 2140 from Ohaus Adventurer, USA, with a resolution of 0.1 mg and tare maximum of 210 g. For the sample-acid digestion, a heated digester block was used (MA-4025 model, Marconi, Brazil). In each digester tube, a cold finger with continuous water recirculation (~15 °C) was introduced to avoid losses by volatilization of analytes and reagents. Acid dissolution was also performed by the use of ultrasound energy, with an ultrasound bath (Elmasonic S 40 (H) model from Elma, Singen, Germany). This equipment was also used to enhance sample solubilization effect in an alkaline medium. 2.2. Reagents Analytical reagent grade materials were used for all the experiments. All the solutions were prepared using ultrapure water, which was obtained employing a Direct-Q 3 Water Purification System (Millipore Corporation, USA) with a resistivity of 18.3 MΩcm. The following reagents were used for different sample preparation methods: nitric acid (Synth, Brazil) and tetramethylammonium hydroxide 25% m/v in water medium (Sigma Aldrich, Germany). The acid was purified by doubly sub-boiling distillation in a quartz system MA-075 (Marconi, Brazil). Working reference solutions of As and Se (Fluka, Buchs, Switzerland), Cd and Pb (Merck, Darmstadt, Germany) were prepared daily by appropriate dilutions of a stock solution containing 1000 mg L−1 in ultrapure water from a standard concentrate solutions. Palladium and Mg (Sigma Aldrich, Germany) were used as a chemical modifier with addition of 5
Samples of WR, PR and BR were used for method development and analyte quantification. All samples were acquired in the local market. First, for the determination of analytes, the samples were submitted to the miniaturization step, using a commercial processor in order to provide better handling and homogeneity. Then, samples were stored at room temperature (~ 25 °C) in plastic recipients previously cleaned and decontaminated. Second, analytes were prepared and measured in the cooked rice samples. Sample of ~ 50 g of WR, BR and PB were cooked using 115 mL of deionized water (as recommended on the label of the product). After cooking, samples were submitted to miniaturization step, using a commercial processor in order to provide better homogeneity and ease of handling. Finally, samples were stored at 16 °C in plastic recipients previously cleaned and decontaminated. Certified reference materials (CRMs) 1568a (Rice flour) from National Institute of Standards and Technology (NIST, Gaithersburg, EUA) were used to evaluate accuracy. 2.4. Sample preparation procedures Initially, studies to determine sample humidity were carried out. Approximately 1 g of sample was weighed in triplicate and dried at 103 °C in oven until constant weight. Humidity levels of 11.7 and 69.4%; 8.8 and 68.3%; and 8.7 and 67.3% were found in the samples of WR, PR and BR, raw and cooked, respectively. The amount of dry matter of each sample was taken into account in calculating the addition of TMAH during the preparation procedures. 2.5. HNO3 decomposition in open system with cold finger (Procedure 1) Different parameters may affect the efficiency of decomposition of samples with a high content of organic compounds: acid volume, amount of sample used, time, and temperature of decomposition using the digester block. This efficiency in the decomposition is an important factor to let the analytes free in the solution, without interfering or losing analyte and reagent by volatilization. Thus, HNO3 decomposition in open system with cold finger has been explored for the determination of volatile elements (Cd, Pb, As and Se) in WR samples using surface response methodology (SRM). A central composite design Table 3 Factors and their respective levels in the central composite design.
Table 1 Instrumental parameters for Cd, Pb, Se and As determinations.
Wavelength (nm) Lamp current (mA) Spectral band pass (nm)
Cd
Pb
Se
As
228.8 4 0.7
283.3 10 0.7
196.0 16 2.0
193.7 18 0.7
Variables
Level −2
−1
0
+1
+2
Temperature (°C) Time (min) Sample mass (g) Acid volume (mL)
120 30 0.25 5.0
145 60 0.50 7.5
170 90 0.75 10.0
195 120 1.0 12.5
220 150 1.5 15.0
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(CCD) was carried out, 24 with 3 central points and 8 axial points, totaling 27 experiments (supplementary material, Table S1). Parameters and values used in the design are shown in Table 3. Statistical analysis was performed using Statistica version 8.0 for Windows software (Tulsa, OK, USA).
decomposition in ultrasonic bath with heating at 80 °C for 2 h with half open for pressure relief to avoid explosion risk during the procedure. After cooling to room temperature, the flask was filled up to a volume of 30 mL with deionized water for subsequent analysis. 2.8. Alkaline solubilization with TMAH (Procedure 3)
2.6. Method After the optimization of HNO3 decomposition using statistical design, the decomposition of the samples was performed as described below. Approximately 750 mg of WR, PR and BR samples, raw and cooked, or 250 mg of CRM was weighed directly in digester tubes, followed by the addition of 10.0 mL of HNO3. After coupling the cold finger in each tube, it was placed in the digester block and subjected to a heating step at 120 ° C for 90 min, with cold finger system with water recirculation at 15 °C. The digested samples were cooled to room temperature, and the resulting solutions of the samples and CRM were transferred to volumetric flasks, adjusting the volume with deionized water to 30 and 14 mL, respectively. The same sample amount and adjusted volume used in this study was applied in the other procedures. The adjustment needed for the CRM was necessary due to the available CRM amount, following the recommendation of the minimum amount reported by the certificate. 2.7. HNO3 dissolution in ultrasonic bath (Procedure 2) All samples were directly weighed in 50.0 mL polyethylene vials, followed by the addition of 5.0 mL of HNO3 and submitted to
All samples were directly weighed in 15 or 50 mL polyethylene flasks, followed by the addition of 2.3 and 1.2 mL of tetramethylammonium hydroxide solution (25% w/v in H2O) for raw and cooked samples, respectively, while for CRM 1.25 mL was added. The TMAH volume was added following the ratio stipulated by Torres et al. [40]. Subsequently, all flasks were placed in ultrasonic bath with heating at 60 °C for 2 h in order to improve the solubilization rate of the samples. After cooling to room temperature, the flask was filled up to a volume final with deionized water for subsequent analysis. 3. Results and discussion 3.1. HNO3 decomposition in open system with cold finger optimization According to the results of the absorbance signal for As, Cd, Pb, and Se generated by CCD, it was possible to evaluate the parameters that were statistically significant in the method of decomposition with HNO3, enabling the generation of the Pareto diagram for each analyte (Fig. 1). Only the sample mass for Cd and As was statistically significant at 95% confidence level, whereas the acid volume and the sample weight were statistically significant for Pb. At 90% confidence level, the block
A
B
C
D
Fig. 1. Pareto diagram for Cd (A), As (B), Pb (D) at 95% confidence level, and Se (C) at 90% confidence level.
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Fig. 2. Surface response graphics: (A) As, mass (mg) × time (min), (B) Cd, temperature (°C) × mass (mg), (C) Se, mass (mg) × temperature (°C) and (D) Pb, HNO 3 volume (mL) × mass (mg).
temperature was statistically significant for Se. Significant values were considered for the analysis of variance calculation (ANOVA). Therefore, the models are significant and suitable to describe the results from the surface response graphics (Fig. 2), since the F-values calculated for As, Cd, Pb and Se were 7.310, 125.170, 6.269, and 5.283, respectively. As presented in Fig. 2A and B, Cd absorbance response was proportional to the mass (the greater the mass, the grater the response), whereas the best results for As were found in intermediated rice mass (500.0 and 750.0 mg). For Se (Fig. 2C), the surface response showed that the lower the temperature applied in the digestion block, the greater the absorbance signal, independently of the other variables.
Conversely, for Pb (Fig. 2D), central values (750.0 mg of sample and 10.0 mL of HNO3) presented the highest absorbance signal. The other combinations of the parameters follow the same trend for all analytes, and for this reason, surface responses are not shown. In order to standardize the method in the digester block with cold finger as a reflux system, the following parameters for the simultaneous quantification of the four analytes were used: 120 °C in the digester block, 10.0 mL of HNO3, 90 min of sample exposure and 750.0 mg sample. These parameters were selected because they were appropriate for the mineralization and determination for all investigated analytes. For method validation, the optimized condition was repeated and the
Table 4 Figures of merit for Cd, Pb, Se and As. Range (μg L−1)
Cd Pb Se As
0.25–1.0 5.0–20.0 25.0–100.0 10.0–40.0
HNO3 (reflux)
HNO3 (ultrasound)
TMAH
a (L μg−1)
LOD (ng g−1)
LOQ (ng g−1)
a (L μg−1)
LOD (ng g−1)
LOQ (ng g−1)
a (L μg−1)
LOD (ng g−1)
LOQ (ng g−1)
0.0505 0.0008 0.0012 0.0018
1.20 26.6 193.2 39.6
3.40 88.6 644.4 132.0
0.0538 0.0006 0.0012 0.0019
2.70 38.5 127.0 89.8
8.20 128.5 423.2 299.2
0.0304 0.0006 0.0007 0.0015
1.94 41.4 256.0 102.3
5.90 138.0 852.8 364.3
a: slope; LOD: limit of detection; LOQ: limit of quantification.
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Table 5 Concentrations before and after the additions of Cd, Pb, Se and As in white, parboiled and brown rice raw samples (n = 3). Samples
Found value, ng g−1 ± SD HNO3 (cold finger)
WR PR BR
WR PR BR
WR PR BR
WR PR BR
AV (ng g−1) 0.0 20.0 0.0 20.0 0.0 20.0 0.0 400.0 0.0 400.0 0.0 400.0 0.0 2000.0 0.0 2000.0 0.0 2000.0 0.0 1000.0 0.0 1000.0 0.0 1000.0
Cd 7.4 ± 0.12 29.2 ± 0.38 11.8 ± 0.45 34.7 ± 0.72 17.0 ± 0.21 39.5 ± 0.51 Pb 148.3 ± 17.50 488.8 ± 13.19 120.1 ± 7.21 524.32 ± 1.05 262.5 ± 12.64 631.2 ± 17.04 Se bLOD 1983.2 ± 1.98 bLOD 2067.8 ± 45.49 bLOD 1681.2 ± 7.6 As bLOD 979.6 ± 59.76 bLD 960.0 ± 34.56 bLOD 1109.6 ± 90.69
HNO3(ultrasound)
TMAH
7.8 ± 0.50 27.5 ± 0.79 10.9 ± 0.11 29.9 ± 0.27 16.0 ± 0.53 35.2 ± 0.81
7.7 ± 0.08 27.2 ± 1.13 11.3 ± 1.14 30.5 ± 1.10 18.7 ± 1.40 37.6 ± 0.04
149.0 ± 11.3 506.5 ± 44.57 120.0 ± 10.00 502.8 ± 3.02 260.0 ± 5.72 678.4 ± 2.03
153.3 ± 18.86 493.3 ± 9.37 122.5 ± 10.65 557.1 ± 17.82 257.7 ± 10.05 625.6 ± 26.90
bLOD 1818.0 ± 45.45 bLOD 1617.2 ± 4.51 bLOD 1703.2 ± 105.60
bLOD 2008.8 ± 4.96 bLOD 1885.2 ± 56.56 bLOD 2254.8 ± 1.10
bLOD 844.0 ± 5.90 bLD 982.4 ± 23.57 bLOD 820.0 ± 12.3
bLOD 836.0 ± 88.61 bLD 806.4 ± 34.67 bLOD 800.0 ± 40.00
AV = added value.
recovery test was carried out in WR sample to verify the reliability of these optimized parameters. Recoveries were 106.0; 91.1; 105.7 and 90.3% after the addition of 20, 400, 2000, and 1000 ng g−1 of Cd, Pb, Se, and As, respectively (Table 5). In addition, in order to evaluate the accuracy of the results obtained, one certified reference material of
rice was analyzed and results are presented in Table 7. The application of the Student t-test for a confidence level of 95% showed good agreement between the measured values with the certified values and no statically significant differences were found, proving the accuracy of the results for all analytes studied.
Table 6 Concentrations before and after the additions of Cd, Pb, Se and As in white, parboiled and brown rice cooked samples (n = 3). Samples
Found value, ng g−1 ± SD HNO3 (cold finger)
WR PR BR
WR PR BR
WR PR BR
WR PR BR AV = added value.
AV (ng g−1) 0.0 20.0 0.0 20.0 0.0 20.0 0.0 400.0 0.0 400.0 0.0 400.0 0.0 2000.0 0.0 2000.0 0.0 2000.0 0.0 1000.0 0.0 1000.0 0.0 1000.0
Cd 3.9 ± 0.20 25.1 ± 0.33 6.3 ± 0.50 28.0 ± 0.45 7.0 ± 0.36 28.1 ± 0.42 Pb 171.9 ± 5.67 536.4 ± 27.35 155.9 ± 17.00 530.8 ± 57.85 201.0 ± 0.20 614.0 ± 0.12 Se bLOD 2114.8 ± 95.23 bLOD 2214.0 ± 63.89 bLOD 2159.2 ± 45.34 As bLOD 903.2 ± 1.81 bLOD 828.8 ± 40.92 bLOD 835.2 ± 71.82
HNO3 (ultrasound)
TMAH
3.8 ± 0.14 23.3 ± 0.63 6.2 ± 0.23 25.9 ± 0.73 6.6 ± 0.28 25.3 ± 1.29
4.4 ± 0.04 22.4 ± 0.63 5.9 ± 0.24 25.0 ± 0.28 6.5 ± 0.67 26.6 ± 1.40
165.0 ± 7.09 602.2 ± 14.45 153.4 ± 9.35 613.2 ± 7.97 196.0 ± 17.00 623.2 ± 15.58
176.6 ± 4.77 593.0 ± 7.12 156.7 ± 14.10 586.5 ± 18.77 206.6 ± 6.61 536.5 ± 6.44
bLOD 1910.0 ± 112.69 bLOD 2220.0 ± 17.76 bLOD 1900.0 ± 5.70
bLOD 2302.0 ± 6.91 bLOD 2216.4 ± 73.14 bLOD 2060.0 ± 109.18
bLOD 1098.2 ± 46.56 bLOD 1101.6 ± 22.03 bLOD 856.8 ± 5.14
bLOD 888.0 ± 108.33 bLOD 860.0 ± 94.60 bLOD 836.0 ± 38.46
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Table 7 Cd, Pb, Se and As concentration in CRM NIST 1568a (n = 3). Analyte
Cert. value ng g−1 (x ± SD)
Cd Pb Se As
Found concentration, x ± SD (RSD) ng g−1 (%)
22.0 ± 2.0 b10.0a 380.0 ± 40.0 290.0 ± 30.0
HNO3 (cold finger)
TMAH
HNO3 (ultrasound)
21.0 ± 0.5 (2.5) Nd 385.0 ± 21.0 (5.5) 270.0 ± 10.3 (3.8)
24.0 ± 0.3 (1.3) Nd 400.0 ± 7.0 (1.8) 304.0 ± 19.2 (6.3)
21.0 ± 0.5 (2.3) Nd 405.0 ± 21.0 (5.2) 300.0 ± 4.2 (1.4)
Nd = Not detected. a Value not certified.
3.2. Pyrolysis and atomization temperature optimization The pyrolysis and atomization curves for Cd are shown in Fig. 3. Without chemical modifiers, there is a low thermal stability for the analyte in both HNO3 (Fig. 3A) and for TMAH (Fig. 3B) medium. Moreover, high thermal stability was applied as chemical modifiers in both media. In the pyrolysis curve, the analyte was stable up to a temperature of 600 to 900 °C for TMAH and HNO3, respectively, while for atomization curves, the absorbance signal remained at low variations, between 1100 and 1300 °C. In HNO3 and TMAH medium, the pyrolysis temperatures used for the study were 400 and 680 °C, respectively, while the optimum temperature for atomization was 1200 °C for the two media. Pyrolysis and atomization temperatures were also optimized for Pb, Se and As (Table 2). The use of a chemical modifier resulted in improved thermal stability for all analytes in both media, eliminating matrix interferents.
3.3. Analytical results Figures of merit for the calibration curves for Cd, Pb, Se, and As using different sample treatments are shown in Table 4. Good linear correlation coefficients in the curves were obtained (R N 0.99) independent of the method used for sample preparation. Regarding the sensitivity given by the slope of the calibration curves, a better sensitivity for the 1.0 0.8
Integrated absorbance, s
0.6 0.4 0.2
A
0.0 300
450
600
750
900
1050
1200
1350
1500
1.0 0.8 0.6 0.4 0.2
B
0.0 300
450
600
750
900
1050
1200
1350
1500
Temperature (°C) Fig. 3. Pyrolysis and atomization curves for 10.0 pg Cd. (A) aqueous solution in HNO3 medium: without (-■-) and with (
) the addition of 3 μg Mg + 5 μg Pd; In matrix medi-
um: without ( ) and with ( ) the addition of 3 μg Mg + 5 μg of Pd. (B) aqueous solution in TMAH medium: without (-□-) and with ( ) the addition of 3 μg Mg + 5 μg Pd; In matrix medium: without ( ) and with ( ) the addition of 3 μg Mg + 5 μg of Pd.
acid dissolution procedure was observed in ultrasonic bath for Cd and As compared to the other procedures. For Pb, the sensitivity was greater using the cold finger system. For Se, both curves in acid medium showed the same sensitivity, higher than in TMAH medium. Procedures in acid medium showed lower limits of detection (LOD) and quantification (LOQ) for all analytes. The high values found for LOD and LOQ for Se and As are due to the low radiation intensity emitted by the respective hollow cathode lamps, resulting in a low repeatability among the blank replicates. 3.4. Total amount of Cd, Pb, Se and As and method validation The amounts for As, Cd, Pb and Se in WR, PR and BR raw samples are presented in Tables 5 and 6. The highest concentrations of Cd and Pb were found in BR samples. This indicates that the distribution of these two analytes is mainly in the external parts of grains, such as bran, which remains intact in BR. The other evaluated forms showed average concentrations of 11.3 and 7.6 ng g−1 for Cd and 120.9 and 150.2 ng g−1 for Pb in raw samples of WR and PR, respectively. White rice showed higher concentration of Pb against parboiled rice. The reason for this is that a diffusion of metals occurs for the outer layer by the parboiling process. By the final of this process, the outer layer is removed, reducing the concentration of the metals [41]. In cooked samples, a decrease of 46.9, 46.0 and 61.0% was observed in the concentration of Cd to WR, PR and BR raw samples, respectively. Conversely, a slight increase of 12.2 and 22.2% in Pb concentration was noted for WR and PR, respectively, whereas 22.6% reduction was observed in BR samples. The increase of Pb concentration can be associated with the presence of anti-nutrients such as phytic acid. In raw rice, phytic acid can form an insoluble complex with metals. However, the metal–phytate complex bonds are broken by the cooking process, releasing some metals [42]. Domingo et al. [43] reported an increase in the concentration of As, Cd and Pb after cooking rice that was 158.0 to 237.0 ng g− 1, 0 to 2.0 ng g−1, and 0 to 16.0 ng g− 1, respectively. Rahmanikhah and coworkers [44] reported reductions in the concentrations of Cd and Pb after cooking rice: 60 to 78% for Pb and 14 to 74% for Cd. However, it is important to mention that in the mentioned study, rice grains were washed prior to cooking, which may drastically affect the removal of these metals. Furthermore, different cooking methods were applied to the sample, which can result in the loss or increase of analytes. [43,44] The highest concentration of Cd found in this work (18.7 ng g−1 for raw BR) showed approximately 21.4 and 10.7 times lower than the maximum levels established by Agencia Nacional de Vigilancia Sanitária (Anvisa), European Food Safety Authority (EFSA) and Joint FAO/WHO food standard program codex alimentarius commission (JECFA), which are 400.0 and 200.0 ng g−1, respectively. On the contrary, only Pb concentrations in raw and cooked BR samples (average of 260.0 and 201.0 ng g−1, respectively) are approximately 1.3 and 1.0 times greater than the maximum allowed, which is 200.0 ng g−1 for all standards [45–47]. Concentrations of Se and As of all the forms and procedures used in this study were below the LOD, and thus their
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quantification was not possible. The average relative standard deviation (RSD) of all procedures was 4.0%, proving the accuracy. Regardless the sample preparation used, there were no significant differences in the analyte concentration, when paired Student t test at the 95% confidence level was applied. In order to validate procedures, the recovery test of the analytes was applied in WR, PR and BR (raw and cooked) for all three procedures. Samples were added to concentrations of 20, 400, 2000 and 1000 ng g−1 of Cd, Pb, Se and As, respectively. Recoveries varied from 80.0 to 115.1%, indicating a good recovery range. 4. Conclusion Advances and alternative searches for the sample preparation of rice for inorganic contaminant determination by atomic spectrometric techniques such as GF AAS have been focus of a great number of studies lately. In this study, the three methods showed to be easily handled and low cost. However, when the efficiency between the methods was compared, the decomposition procedure with the cold finger was safer, more effective in organic decomposition, without losses of analytes by volatilization and with optimum values of sensitivity and LODs. Procedures using ultrasonic bath and solubilization with TMAH medium obtained suitable analytical response and excellent recoveries in the recovery test, as well as CRM. However, some disadvantages were also found such as low homogeneity of the ultrasonic waves, and the risk of explosion caused mainly by the release of gases in the acid decomposition. In relation to the levels of the elements, it became clear that BR was the form that presented the highest retention capacity of these metals from anthropogenic activity, especially by having bran in its constitution. For the cooked samples, it could be observed an attenuation of the Cd concentration, whereas an increase in the concentration of Pb for WR and PR samples was observed when compared to the raw samples. The Se and As concentrations could not be quantified as they were below the LOD obtained in all preparation methods investigated by GF AAS. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.microc.2015.09.018. Conflict of interest There is no conflict of interest. Acknowledgments The authors gratefully acknowledge the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) (Process no 447373/2014-5) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)—Edital Casadinho Process no 552197/ 2011-4 for their financial support and scholarships. References [1] FAO—Food Agricultural Organization Statistical Yearbook, World Food and Agriculture, 2013. [2] B.O. Juliano, Rice in Human Nutrition, FAO, Rome, 1993. [3] FAO—Food and Agriculture Organization of the United Nations, Statistical databases, http://www.fao.org (Accessed in May 2015). [4] M.A.F. Gomes, R.R.M. Barizon, Panorama da Contaminação Ambiental por Agrotóxicos e Nitrato de origem Agrícola no Brasil: cenário 1992/2011, Embrapa Meio Ambiente, Jaguariúna, 2014 (36 pp. Embrapa Meio Ambiente, Documentos, 98). [5] Q. Yongzhong, C. Chen, Z. Qi, L. Yun, C. Zhijun, L. Min, Concentrations of cadmium, lead, mercury and arsenic in Chinese market milled rice and associated population health risk, Food Control 21 (2010) 1757–1763. [6] L.B. Batista, J.M.O. Souza, S.S. de Souza, F.F. Barbosa, Speciation of arsenic in rice and estimation of daily intake of different arsenic species by Brazilians through rice consumption, J. Hazard. Mater. 191 (2011) 342–348. [7] H. Xiaoshuai, W. Huoyan, Z. Jianmin, M. Chengling, D. Changwen, C. Xiaoqin, Risk assessment of potentially toxic element pollution in soils and rice (Oryza sativa) in a typical area of the Yangtze River Delta, Environ. Pollut. 157 (2009) 2542–2549.
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Evaluation of sample preparation methods for the determination of As, Cd, Pb, and Se in rice samples by GF AAS Richard Macedo de Oliveira, Ana Clara Nascimento Antunes, Mariana Antunes Vieira, Aline Lisbôa Medina, Anderson Schwingel Ribeiro ⁎ Universidade Federal de Pelotas (UFPel), Laboratório de Metrologia Química, Programa de Pós-Graduação em Química, 96160-000 Capão do Leão, RS, Brazil
a r t i c l e
i n f o
Article history: Received 16 September 2015 Accepted 22 September 2015 Available online xxxx Editor: J. Sneddon Keywords: Rice Inorganic contaminants Sample preparation GF AAS
a b s t r a c t This study presents a comparative evaluation of three different methods for sample preparation in rice for As, Cd, Pb, and Se determination by graphite furnace atomic absorption spectrometry (GF AAS). Raw and cooked white rice, parboiled rice, and brown rice samples were studied with the following procedures: i) HNO3 decomposition in open system with cold finger, ii) HNO3 decomposition in ultrasonic bath, and iii) alkali solubilization with TMAH. The sample preparation using reflux system was optimized and the best conditions were: 750 mg of sample, 10 mL of HNO3, 120 °C (heating), and 90 min in the digester block. In rice samples, medium concentrations of 12.0 ± 4.0 and 177.0 ± 64.0 ng g−1 for Cd and Pb were found, respectively, while for As and Se, the concentrations were below the limit of detection of the studied methods. After cooking, samples presented a maximum decrease of 61% in Cd concentration. The method validations were assured by analysis of certified reference material (NIST 1568a), and results were in accordance with the certified values. The recovery test was carried out and results ranged between 80 and 115% for all analytes in the rice samples, before and after cooking. All evaluated procedures presented accurate and precise results; however, the decomposition with reflux system proved to be more effective, simple and secure. Such characteristics are necessary for the application in routine analysis with medium detection limits of 77.2; 1.0; 35.5 and 192.1 ng g−1 for As, Cd, Pb and Se, respectively. © 2015 Published by Elsevier B.V.
1. Introduction Rice is a commodity of utmost importance to world food security because it feeds approximately 50% of the world population [1]. It is considered a complex matrix because it presents carbohydrates, mainly starch, water, proteins (18 essential amino acids), fiber, vitamins (B1, B2, B3), and minerals [2]. Asia countries are the main producers and consumers of this cereal, with about 90% of all world production. Brazil is the only non-Asian country among the main ten producers of rice, with approximately 1.6% of total world production. As a consequence of this large production, rice is daily consumed by Brazilians [3]. Due to extensive contamination of soil and irrigation water, mainly from the indiscriminate use of pesticides and fertilizers [4], rice intake may also be dangerous to humans, since inorganic contaminants such as Cd, Pb, and As may be absorbed into the cereal composition, being exposed to human body by ingestion [5–7]. Attention is needed to Se because although it is an essential element, it may cause risks to human health if ingested in large amounts [8]. All these elements are associated with different diseases in the human body, such as tumor development in various regions, continuous effects in central nervous system, besides effects on skin, tachycardia, as well as hair, nail and teeth losses [9–12]. ⁎ Corresponding author. Tel.: +55 53 3275 7179. E-mail address: [email protected] (A.S. Ribeiro).
http://dx.doi.org/10.1016/j.microc.2015.09.018 0026-265X/© 2015 Published by Elsevier B.V.
The determination of these elements is usually carried out in the trace level and the technique of graphite furnace atomic absorption spectrometry (GF AAS) is used, since it has the ability to reconcile high sensitivity, low limits of detection, and a small amount of sample for analysis (~20–50 μL) [13–15]. However, extensive sample preparation is required, especially when it is a complex matrix, such as rice [16]. Regarding rice crops, literature have reported wet decomposition methods in open systems, such as open system with heating plates [17, 18], heating in water bath [19] in digesters blocks [20–22], or even dry decomposition through sample calcination and ash production [21]. However, these methods may provide incomplete decomposition, low recovery of some elements [23–26], as well as analyte or acid volatilization [25,27,28]. Moreover, analyzing rice samples by wet decomposition in closed systems, especially in microwave oven with only concentrated HNO3 [29,30] or mixture of H2O2 as complement for organic elimination, appears to be the most appropriate [5,31], since these methods are fast, safe and reduce the risk of contamination and volatilization [32]. Despite this fact, these methods are not simple because they frequently have a low frequency rate and require sophisticated and expensive equipment thus a great number of laboratories do not use this technology, hindering its use in routine analysis. An alternative for sample preparation is the use of cold finger as reflux system for biological decomposition of bovine meat samples for the determination of Cd, Pb, and Sn [33], fish for determining Hg [34] and
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rice samples for quantification of Cu, Fe, Mn, and Zn [35]. However, the efficiency of this system has not been evaluated in the decomposition of rice samples for quantification of elements easily volatilized and in lower concentrations, such as Cd, Pb, Se and As. In addition, the evaluation of dissolution assisted by ultrasound with HNO3 and in alkaline medium with TMAH needs to be better explored for sample rice, since only extraction methods have been applied to this type of sample being not efficient for all elements [36–39]. Therefore, this work aims to carry out a comparative study of three alternative methods of sample preparation of polished white rice (WR), parboiled rice (PR) and brown rice (BR) for the quantification of Cd, Pb, Se and As by GF AAS using: i) decomposition in open system with cold finger, ii) dissolution assisted by ultrasound in HNO3, and iii) solubilization with TMAH. Furthermore, the behavior of these elements after cooking the rice was also evaluated.
403
Table 2 Temperature program of graphite furnace for the determination of Cda, Pbb, Sec and Asd in rice samples treated with HNO3 and TMAH. ⁎Treated with TMAH. Step
Temperature (°C)
Ramp (°C s−1)
Hold (s)
Gas flow rate (mL min−1)
Drying Drying Pyrolysis Atomization Cleaning
110 130 400a; 680a⁎; 900b; 1000c,d 1200a,a⁎; 1700b; 1800c; 2200d 2450
5 15 10 0 1
30 30 20 5 3
250 250 250 0 250
and 3 μg into graphite furnace for each measurement, respectively. Before using, all glass apparatus were conventionally washed, soaked in 10% (v/v) HNO3 for at least 48 h, and then rinsed with ultrapure water prior to use. 2.3. Samples
2. Experimental 2.1. Instrumentation Integrated absorbance signals were measured in an atomic absorption spectrometer PinAAcle 900z (Perkin Elmer, Walthan, EUA) equipped with longitudinal Zeeman-effect based background correction system, autosampler (AS-900 model), color furnace camera (TubeView™), and transverse-heated graphite tube atomizer (THGA). The operational conditions recommended by the manufacturer were employed. Hollow cathode lamps (Perkin Elmer, Walthan, EUA) were used. Argon, 99.996% (Linde, Brazil), was used as protected and purge gas. The instrumental parameters used for each analyte are presented in Table 1 and the temperature program optimized is shown in Table 2. Samples were weighed using an analytical balance model 2140 from Ohaus Adventurer, USA, with a resolution of 0.1 mg and tare maximum of 210 g. For the sample-acid digestion, a heated digester block was used (MA-4025 model, Marconi, Brazil). In each digester tube, a cold finger with continuous water recirculation (~15 °C) was introduced to avoid losses by volatilization of analytes and reagents. Acid dissolution was also performed by the use of ultrasound energy, with an ultrasound bath (Elmasonic S 40 (H) model from Elma, Singen, Germany). This equipment was also used to enhance sample solubilization effect in an alkaline medium. 2.2. Reagents Analytical reagent grade materials were used for all the experiments. All the solutions were prepared using ultrapure water, which was obtained employing a Direct-Q 3 Water Purification System (Millipore Corporation, USA) with a resistivity of 18.3 MΩcm. The following reagents were used for different sample preparation methods: nitric acid (Synth, Brazil) and tetramethylammonium hydroxide 25% m/v in water medium (Sigma Aldrich, Germany). The acid was purified by doubly sub-boiling distillation in a quartz system MA-075 (Marconi, Brazil). Working reference solutions of As and Se (Fluka, Buchs, Switzerland), Cd and Pb (Merck, Darmstadt, Germany) were prepared daily by appropriate dilutions of a stock solution containing 1000 mg L−1 in ultrapure water from a standard concentrate solutions. Palladium and Mg (Sigma Aldrich, Germany) were used as a chemical modifier with addition of 5
Samples of WR, PR and BR were used for method development and analyte quantification. All samples were acquired in the local market. First, for the determination of analytes, the samples were submitted to the miniaturization step, using a commercial processor in order to provide better handling and homogeneity. Then, samples were stored at room temperature (~ 25 °C) in plastic recipients previously cleaned and decontaminated. Second, analytes were prepared and measured in the cooked rice samples. Sample of ~ 50 g of WR, BR and PB were cooked using 115 mL of deionized water (as recommended on the label of the product). After cooking, samples were submitted to miniaturization step, using a commercial processor in order to provide better homogeneity and ease of handling. Finally, samples were stored at 16 °C in plastic recipients previously cleaned and decontaminated. Certified reference materials (CRMs) 1568a (Rice flour) from National Institute of Standards and Technology (NIST, Gaithersburg, EUA) were used to evaluate accuracy. 2.4. Sample preparation procedures Initially, studies to determine sample humidity were carried out. Approximately 1 g of sample was weighed in triplicate and dried at 103 °C in oven until constant weight. Humidity levels of 11.7 and 69.4%; 8.8 and 68.3%; and 8.7 and 67.3% were found in the samples of WR, PR and BR, raw and cooked, respectively. The amount of dry matter of each sample was taken into account in calculating the addition of TMAH during the preparation procedures. 2.5. HNO3 decomposition in open system with cold finger (Procedure 1) Different parameters may affect the efficiency of decomposition of samples with a high content of organic compounds: acid volume, amount of sample used, time, and temperature of decomposition using the digester block. This efficiency in the decomposition is an important factor to let the analytes free in the solution, without interfering or losing analyte and reagent by volatilization. Thus, HNO3 decomposition in open system with cold finger has been explored for the determination of volatile elements (Cd, Pb, As and Se) in WR samples using surface response methodology (SRM). A central composite design Table 3 Factors and their respective levels in the central composite design.
Table 1 Instrumental parameters for Cd, Pb, Se and As determinations.
Wavelength (nm) Lamp current (mA) Spectral band pass (nm)
Cd
Pb
Se
As
228.8 4 0.7
283.3 10 0.7
196.0 16 2.0
193.7 18 0.7
Variables
Level −2
−1
0
+1
+2
Temperature (°C) Time (min) Sample mass (g) Acid volume (mL)
120 30 0.25 5.0
145 60 0.50 7.5
170 90 0.75 10.0
195 120 1.0 12.5
220 150 1.5 15.0
404
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(CCD) was carried out, 24 with 3 central points and 8 axial points, totaling 27 experiments (supplementary material, Table S1). Parameters and values used in the design are shown in Table 3. Statistical analysis was performed using Statistica version 8.0 for Windows software (Tulsa, OK, USA).
decomposition in ultrasonic bath with heating at 80 °C for 2 h with half open for pressure relief to avoid explosion risk during the procedure. After cooling to room temperature, the flask was filled up to a volume of 30 mL with deionized water for subsequent analysis. 2.8. Alkaline solubilization with TMAH (Procedure 3)
2.6. Method After the optimization of HNO3 decomposition using statistical design, the decomposition of the samples was performed as described below. Approximately 750 mg of WR, PR and BR samples, raw and cooked, or 250 mg of CRM was weighed directly in digester tubes, followed by the addition of 10.0 mL of HNO3. After coupling the cold finger in each tube, it was placed in the digester block and subjected to a heating step at 120 ° C for 90 min, with cold finger system with water recirculation at 15 °C. The digested samples were cooled to room temperature, and the resulting solutions of the samples and CRM were transferred to volumetric flasks, adjusting the volume with deionized water to 30 and 14 mL, respectively. The same sample amount and adjusted volume used in this study was applied in the other procedures. The adjustment needed for the CRM was necessary due to the available CRM amount, following the recommendation of the minimum amount reported by the certificate. 2.7. HNO3 dissolution in ultrasonic bath (Procedure 2) All samples were directly weighed in 50.0 mL polyethylene vials, followed by the addition of 5.0 mL of HNO3 and submitted to
All samples were directly weighed in 15 or 50 mL polyethylene flasks, followed by the addition of 2.3 and 1.2 mL of tetramethylammonium hydroxide solution (25% w/v in H2O) for raw and cooked samples, respectively, while for CRM 1.25 mL was added. The TMAH volume was added following the ratio stipulated by Torres et al. [40]. Subsequently, all flasks were placed in ultrasonic bath with heating at 60 °C for 2 h in order to improve the solubilization rate of the samples. After cooling to room temperature, the flask was filled up to a volume final with deionized water for subsequent analysis. 3. Results and discussion 3.1. HNO3 decomposition in open system with cold finger optimization According to the results of the absorbance signal for As, Cd, Pb, and Se generated by CCD, it was possible to evaluate the parameters that were statistically significant in the method of decomposition with HNO3, enabling the generation of the Pareto diagram for each analyte (Fig. 1). Only the sample mass for Cd and As was statistically significant at 95% confidence level, whereas the acid volume and the sample weight were statistically significant for Pb. At 90% confidence level, the block
A
B
C
D
Fig. 1. Pareto diagram for Cd (A), As (B), Pb (D) at 95% confidence level, and Se (C) at 90% confidence level.
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Fig. 2. Surface response graphics: (A) As, mass (mg) × time (min), (B) Cd, temperature (°C) × mass (mg), (C) Se, mass (mg) × temperature (°C) and (D) Pb, HNO 3 volume (mL) × mass (mg).
temperature was statistically significant for Se. Significant values were considered for the analysis of variance calculation (ANOVA). Therefore, the models are significant and suitable to describe the results from the surface response graphics (Fig. 2), since the F-values calculated for As, Cd, Pb and Se were 7.310, 125.170, 6.269, and 5.283, respectively. As presented in Fig. 2A and B, Cd absorbance response was proportional to the mass (the greater the mass, the grater the response), whereas the best results for As were found in intermediated rice mass (500.0 and 750.0 mg). For Se (Fig. 2C), the surface response showed that the lower the temperature applied in the digestion block, the greater the absorbance signal, independently of the other variables.
Conversely, for Pb (Fig. 2D), central values (750.0 mg of sample and 10.0 mL of HNO3) presented the highest absorbance signal. The other combinations of the parameters follow the same trend for all analytes, and for this reason, surface responses are not shown. In order to standardize the method in the digester block with cold finger as a reflux system, the following parameters for the simultaneous quantification of the four analytes were used: 120 °C in the digester block, 10.0 mL of HNO3, 90 min of sample exposure and 750.0 mg sample. These parameters were selected because they were appropriate for the mineralization and determination for all investigated analytes. For method validation, the optimized condition was repeated and the
Table 4 Figures of merit for Cd, Pb, Se and As. Range (μg L−1)
Cd Pb Se As
0.25–1.0 5.0–20.0 25.0–100.0 10.0–40.0
HNO3 (reflux)
HNO3 (ultrasound)
TMAH
a (L μg−1)
LOD (ng g−1)
LOQ (ng g−1)
a (L μg−1)
LOD (ng g−1)
LOQ (ng g−1)
a (L μg−1)
LOD (ng g−1)
LOQ (ng g−1)
0.0505 0.0008 0.0012 0.0018
1.20 26.6 193.2 39.6
3.40 88.6 644.4 132.0
0.0538 0.0006 0.0012 0.0019
2.70 38.5 127.0 89.8
8.20 128.5 423.2 299.2
0.0304 0.0006 0.0007 0.0015
1.94 41.4 256.0 102.3
5.90 138.0 852.8 364.3
a: slope; LOD: limit of detection; LOQ: limit of quantification.
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Table 5 Concentrations before and after the additions of Cd, Pb, Se and As in white, parboiled and brown rice raw samples (n = 3). Samples
Found value, ng g−1 ± SD HNO3 (cold finger)
WR PR BR
WR PR BR
WR PR BR
WR PR BR
AV (ng g−1) 0.0 20.0 0.0 20.0 0.0 20.0 0.0 400.0 0.0 400.0 0.0 400.0 0.0 2000.0 0.0 2000.0 0.0 2000.0 0.0 1000.0 0.0 1000.0 0.0 1000.0
Cd 7.4 ± 0.12 29.2 ± 0.38 11.8 ± 0.45 34.7 ± 0.72 17.0 ± 0.21 39.5 ± 0.51 Pb 148.3 ± 17.50 488.8 ± 13.19 120.1 ± 7.21 524.32 ± 1.05 262.5 ± 12.64 631.2 ± 17.04 Se bLOD 1983.2 ± 1.98 bLOD 2067.8 ± 45.49 bLOD 1681.2 ± 7.6 As bLOD 979.6 ± 59.76 bLD 960.0 ± 34.56 bLOD 1109.6 ± 90.69
HNO3(ultrasound)
TMAH
7.8 ± 0.50 27.5 ± 0.79 10.9 ± 0.11 29.9 ± 0.27 16.0 ± 0.53 35.2 ± 0.81
7.7 ± 0.08 27.2 ± 1.13 11.3 ± 1.14 30.5 ± 1.10 18.7 ± 1.40 37.6 ± 0.04
149.0 ± 11.3 506.5 ± 44.57 120.0 ± 10.00 502.8 ± 3.02 260.0 ± 5.72 678.4 ± 2.03
153.3 ± 18.86 493.3 ± 9.37 122.5 ± 10.65 557.1 ± 17.82 257.7 ± 10.05 625.6 ± 26.90
bLOD 1818.0 ± 45.45 bLOD 1617.2 ± 4.51 bLOD 1703.2 ± 105.60
bLOD 2008.8 ± 4.96 bLOD 1885.2 ± 56.56 bLOD 2254.8 ± 1.10
bLOD 844.0 ± 5.90 bLD 982.4 ± 23.57 bLOD 820.0 ± 12.3
bLOD 836.0 ± 88.61 bLD 806.4 ± 34.67 bLOD 800.0 ± 40.00
AV = added value.
recovery test was carried out in WR sample to verify the reliability of these optimized parameters. Recoveries were 106.0; 91.1; 105.7 and 90.3% after the addition of 20, 400, 2000, and 1000 ng g−1 of Cd, Pb, Se, and As, respectively (Table 5). In addition, in order to evaluate the accuracy of the results obtained, one certified reference material of
rice was analyzed and results are presented in Table 7. The application of the Student t-test for a confidence level of 95% showed good agreement between the measured values with the certified values and no statically significant differences were found, proving the accuracy of the results for all analytes studied.
Table 6 Concentrations before and after the additions of Cd, Pb, Se and As in white, parboiled and brown rice cooked samples (n = 3). Samples
Found value, ng g−1 ± SD HNO3 (cold finger)
WR PR BR
WR PR BR
WR PR BR
WR PR BR AV = added value.
AV (ng g−1) 0.0 20.0 0.0 20.0 0.0 20.0 0.0 400.0 0.0 400.0 0.0 400.0 0.0 2000.0 0.0 2000.0 0.0 2000.0 0.0 1000.0 0.0 1000.0 0.0 1000.0
Cd 3.9 ± 0.20 25.1 ± 0.33 6.3 ± 0.50 28.0 ± 0.45 7.0 ± 0.36 28.1 ± 0.42 Pb 171.9 ± 5.67 536.4 ± 27.35 155.9 ± 17.00 530.8 ± 57.85 201.0 ± 0.20 614.0 ± 0.12 Se bLOD 2114.8 ± 95.23 bLOD 2214.0 ± 63.89 bLOD 2159.2 ± 45.34 As bLOD 903.2 ± 1.81 bLOD 828.8 ± 40.92 bLOD 835.2 ± 71.82
HNO3 (ultrasound)
TMAH
3.8 ± 0.14 23.3 ± 0.63 6.2 ± 0.23 25.9 ± 0.73 6.6 ± 0.28 25.3 ± 1.29
4.4 ± 0.04 22.4 ± 0.63 5.9 ± 0.24 25.0 ± 0.28 6.5 ± 0.67 26.6 ± 1.40
165.0 ± 7.09 602.2 ± 14.45 153.4 ± 9.35 613.2 ± 7.97 196.0 ± 17.00 623.2 ± 15.58
176.6 ± 4.77 593.0 ± 7.12 156.7 ± 14.10 586.5 ± 18.77 206.6 ± 6.61 536.5 ± 6.44
bLOD 1910.0 ± 112.69 bLOD 2220.0 ± 17.76 bLOD 1900.0 ± 5.70
bLOD 2302.0 ± 6.91 bLOD 2216.4 ± 73.14 bLOD 2060.0 ± 109.18
bLOD 1098.2 ± 46.56 bLOD 1101.6 ± 22.03 bLOD 856.8 ± 5.14
bLOD 888.0 ± 108.33 bLOD 860.0 ± 94.60 bLOD 836.0 ± 38.46
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407
Table 7 Cd, Pb, Se and As concentration in CRM NIST 1568a (n = 3). Analyte
Cert. value ng g−1 (x ± SD)
Cd Pb Se As
Found concentration, x ± SD (RSD) ng g−1 (%)
22.0 ± 2.0 b10.0a 380.0 ± 40.0 290.0 ± 30.0
HNO3 (cold finger)
TMAH
HNO3 (ultrasound)
21.0 ± 0.5 (2.5) Nd 385.0 ± 21.0 (5.5) 270.0 ± 10.3 (3.8)
24.0 ± 0.3 (1.3) Nd 400.0 ± 7.0 (1.8) 304.0 ± 19.2 (6.3)
21.0 ± 0.5 (2.3) Nd 405.0 ± 21.0 (5.2) 300.0 ± 4.2 (1.4)
Nd = Not detected. a Value not certified.
3.2. Pyrolysis and atomization temperature optimization The pyrolysis and atomization curves for Cd are shown in Fig. 3. Without chemical modifiers, there is a low thermal stability for the analyte in both HNO3 (Fig. 3A) and for TMAH (Fig. 3B) medium. Moreover, high thermal stability was applied as chemical modifiers in both media. In the pyrolysis curve, the analyte was stable up to a temperature of 600 to 900 °C for TMAH and HNO3, respectively, while for atomization curves, the absorbance signal remained at low variations, between 1100 and 1300 °C. In HNO3 and TMAH medium, the pyrolysis temperatures used for the study were 400 and 680 °C, respectively, while the optimum temperature for atomization was 1200 °C for the two media. Pyrolysis and atomization temperatures were also optimized for Pb, Se and As (Table 2). The use of a chemical modifier resulted in improved thermal stability for all analytes in both media, eliminating matrix interferents.
3.3. Analytical results Figures of merit for the calibration curves for Cd, Pb, Se, and As using different sample treatments are shown in Table 4. Good linear correlation coefficients in the curves were obtained (R N 0.99) independent of the method used for sample preparation. Regarding the sensitivity given by the slope of the calibration curves, a better sensitivity for the 1.0 0.8
Integrated absorbance, s
0.6 0.4 0.2
A
0.0 300
450
600
750
900
1050
1200
1350
1500
1.0 0.8 0.6 0.4 0.2
B
0.0 300
450
600
750
900
1050
1200
1350
1500
Temperature (°C) Fig. 3. Pyrolysis and atomization curves for 10.0 pg Cd. (A) aqueous solution in HNO3 medium: without (-■-) and with (
) the addition of 3 μg Mg + 5 μg Pd; In matrix medi-
um: without ( ) and with ( ) the addition of 3 μg Mg + 5 μg of Pd. (B) aqueous solution in TMAH medium: without (-□-) and with ( ) the addition of 3 μg Mg + 5 μg Pd; In matrix medium: without ( ) and with ( ) the addition of 3 μg Mg + 5 μg of Pd.
acid dissolution procedure was observed in ultrasonic bath for Cd and As compared to the other procedures. For Pb, the sensitivity was greater using the cold finger system. For Se, both curves in acid medium showed the same sensitivity, higher than in TMAH medium. Procedures in acid medium showed lower limits of detection (LOD) and quantification (LOQ) for all analytes. The high values found for LOD and LOQ for Se and As are due to the low radiation intensity emitted by the respective hollow cathode lamps, resulting in a low repeatability among the blank replicates. 3.4. Total amount of Cd, Pb, Se and As and method validation The amounts for As, Cd, Pb and Se in WR, PR and BR raw samples are presented in Tables 5 and 6. The highest concentrations of Cd and Pb were found in BR samples. This indicates that the distribution of these two analytes is mainly in the external parts of grains, such as bran, which remains intact in BR. The other evaluated forms showed average concentrations of 11.3 and 7.6 ng g−1 for Cd and 120.9 and 150.2 ng g−1 for Pb in raw samples of WR and PR, respectively. White rice showed higher concentration of Pb against parboiled rice. The reason for this is that a diffusion of metals occurs for the outer layer by the parboiling process. By the final of this process, the outer layer is removed, reducing the concentration of the metals [41]. In cooked samples, a decrease of 46.9, 46.0 and 61.0% was observed in the concentration of Cd to WR, PR and BR raw samples, respectively. Conversely, a slight increase of 12.2 and 22.2% in Pb concentration was noted for WR and PR, respectively, whereas 22.6% reduction was observed in BR samples. The increase of Pb concentration can be associated with the presence of anti-nutrients such as phytic acid. In raw rice, phytic acid can form an insoluble complex with metals. However, the metal–phytate complex bonds are broken by the cooking process, releasing some metals [42]. Domingo et al. [43] reported an increase in the concentration of As, Cd and Pb after cooking rice that was 158.0 to 237.0 ng g− 1, 0 to 2.0 ng g−1, and 0 to 16.0 ng g− 1, respectively. Rahmanikhah and coworkers [44] reported reductions in the concentrations of Cd and Pb after cooking rice: 60 to 78% for Pb and 14 to 74% for Cd. However, it is important to mention that in the mentioned study, rice grains were washed prior to cooking, which may drastically affect the removal of these metals. Furthermore, different cooking methods were applied to the sample, which can result in the loss or increase of analytes. [43,44] The highest concentration of Cd found in this work (18.7 ng g−1 for raw BR) showed approximately 21.4 and 10.7 times lower than the maximum levels established by Agencia Nacional de Vigilancia Sanitária (Anvisa), European Food Safety Authority (EFSA) and Joint FAO/WHO food standard program codex alimentarius commission (JECFA), which are 400.0 and 200.0 ng g−1, respectively. On the contrary, only Pb concentrations in raw and cooked BR samples (average of 260.0 and 201.0 ng g−1, respectively) are approximately 1.3 and 1.0 times greater than the maximum allowed, which is 200.0 ng g−1 for all standards [45–47]. Concentrations of Se and As of all the forms and procedures used in this study were below the LOD, and thus their
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quantification was not possible. The average relative standard deviation (RSD) of all procedures was 4.0%, proving the accuracy. Regardless the sample preparation used, there were no significant differences in the analyte concentration, when paired Student t test at the 95% confidence level was applied. In order to validate procedures, the recovery test of the analytes was applied in WR, PR and BR (raw and cooked) for all three procedures. Samples were added to concentrations of 20, 400, 2000 and 1000 ng g−1 of Cd, Pb, Se and As, respectively. Recoveries varied from 80.0 to 115.1%, indicating a good recovery range. 4. Conclusion Advances and alternative searches for the sample preparation of rice for inorganic contaminant determination by atomic spectrometric techniques such as GF AAS have been focus of a great number of studies lately. In this study, the three methods showed to be easily handled and low cost. However, when the efficiency between the methods was compared, the decomposition procedure with the cold finger was safer, more effective in organic decomposition, without losses of analytes by volatilization and with optimum values of sensitivity and LODs. Procedures using ultrasonic bath and solubilization with TMAH medium obtained suitable analytical response and excellent recoveries in the recovery test, as well as CRM. However, some disadvantages were also found such as low homogeneity of the ultrasonic waves, and the risk of explosion caused mainly by the release of gases in the acid decomposition. In relation to the levels of the elements, it became clear that BR was the form that presented the highest retention capacity of these metals from anthropogenic activity, especially by having bran in its constitution. For the cooked samples, it could be observed an attenuation of the Cd concentration, whereas an increase in the concentration of Pb for WR and PR samples was observed when compared to the raw samples. The Se and As concentrations could not be quantified as they were below the LOD obtained in all preparation methods investigated by GF AAS. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.microc.2015.09.018. Conflict of interest There is no conflict of interest. Acknowledgments The authors gratefully acknowledge the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) (Process no 447373/2014-5) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)—Edital Casadinho Process no 552197/ 2011-4 for their financial support and scholarships. References [1] FAO—Food Agricultural Organization Statistical Yearbook, World Food and Agriculture, 2013. [2] B.O. Juliano, Rice in Human Nutrition, FAO, Rome, 1993. [3] FAO—Food and Agriculture Organization of the United Nations, Statistical databases, http://www.fao.org (Accessed in May 2015). [4] M.A.F. Gomes, R.R.M. Barizon, Panorama da Contaminação Ambiental por Agrotóxicos e Nitrato de origem Agrícola no Brasil: cenário 1992/2011, Embrapa Meio Ambiente, Jaguariúna, 2014 (36 pp. Embrapa Meio Ambiente, Documentos, 98). [5] Q. Yongzhong, C. Chen, Z. Qi, L. Yun, C. Zhijun, L. Min, Concentrations of cadmium, lead, mercury and arsenic in Chinese market milled rice and associated population health risk, Food Control 21 (2010) 1757–1763. [6] L.B. Batista, J.M.O. Souza, S.S. de Souza, F.F. Barbosa, Speciation of arsenic in rice and estimation of daily intake of different arsenic species by Brazilians through rice consumption, J. Hazard. Mater. 191 (2011) 342–348. [7] H. Xiaoshuai, W. Huoyan, Z. Jianmin, M. Chengling, D. Changwen, C. Xiaoqin, Risk assessment of potentially toxic element pollution in soils and rice (Oryza sativa) in a typical area of the Yangtze River Delta, Environ. Pollut. 157 (2009) 2542–2549.
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