- Email: [email protected]
Heat-assisted slurry sampling GFAAS method for determination of lead in food standard reference materials
Journal of Food Composition and Analysis 42 (2015) 78–83
Contents lists available at ScienceDirect
Journal of Food Composition and Analysis journal homepage: www.elsevier.com/locate/jfca
Short Communication
Heat-assisted slurry sampling GFAAS method for determination of lead in food standard reference materials Yue’e Peng a,b, Wei Guo a,*, Ping Zhang c, Lanlan Jin a a
State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, PR China Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China c Beijing CKC, PerkinElmer, Inc., 100015, PR China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 September 2014 Received in revised form 17 March 2015 Accepted 23 March 2015 Available online 13 April 2015
A valid method based on heat-assisted slurry sampling graphic furnace atomic absorption spectrometry (HASS-GFAAS) was developed for the accurate determination of trace Pb in food standard reference materials (SRMs). The HASS technique significantly improved Pb recovery and precision compared to conventional slurry sampling techniques. The optimized HASS procedure was performed as follows: first, the sample (particle size 150 mm) was diluted with 0.05% (v/v) Triton X-100 containing 2% HNO3 and 1% H2O2 followed by heating for 20 min at 120 8C on a heating block. Next, the obtained slurry was sonicated in an autosampler cup, and finally, the slurry was introduced into a graphite tube and analyzed by the GFAAS with a Pb electrodeless discharge lamp (EDL). Calibration with aqueous standard solutions was used for Pb determination in food samples. The characteristic mass and limit of detection for Pb based on the integrated absorbance for a 2% (m/v) sample were 12 0.6 pg and 0.003 mg kg 1, respectively. The accuracy (95.1–102% recovery) and good precision (0.1–3.6%) of this procedure are illustrated by the results obtained for the 12 food reference materials. The proposed method is suitable for determination of trace Pb in solid food samples. ß 2015 Elsevier Inc. All rights reserved.
Keywords: Food analysis Food composition Heat-assisted slurry sampling Trace element in food Heavy metal contamination Lead Food certified material Method validation Standard reference material
1. Introduction Lead (Pb) is toxic to humans even at low levels; the most important sources of Pb exposure are industrial emission, soil, and automobile exhaust (Vinas et al., 2000). Vegetables with a relatively large leaf area, such as spinach and cabbage, can contain high levels of Pb when grown near such Pb sources. Therefore, the accurate and rapid determination of trace Pb in various food samples is considered important step in contamination studies. Over the last two decades, graphite furnace atomic absorption spectrometry (GFAAS) (Alkis et al., 2014; Antoine et al., 2012; Chahid et al., 2014; Chen et al., 2014; de Andrade et al., 2014; Gunduz and Akman, 2013; Itkonen et al., 2012; Skrbic et al., 2013; Soares and Nascentes, 2013) and inductively coupled plasma mass spectrometry (ICP-MS) (Gimou et al., 2014; Itkonen et al., 2012; Julshamn et al., 2013; Koyyalamudi et al., 2013; Millour et al., 2012, 2011; Mol, 2011; Park et al., 2011; Potorti et al., 2013; Rodushkin et al., 2011; Scancar et al., 2013; Tormen et al., 2011; Vassileva et al., 2014) techniques have emerged as the dominant analytical
* Corresponding author. Tel.: +86 27 67883495; fax: +86 27 67883495. E-mail address: [email protected] (W. Guo). http://dx.doi.org/10.1016/j.jfca.2015.03.007 0889-1575/ß 2015 Elsevier Inc. All rights reserved.
methods for trace element determination in various food samples. However, GFAAS is proving to be a more useful method than ICPMS owing to its relative simplicity, and its capability for direct determination of Pb in complex food matrices. It is well-known that in the determination of Pb in dried food samples, a microwave acid digestion step is frequently carried out prior to measurement of Pb by GFAAS; but the main drawbacks of this step are low sample throughput, risk of occasional explosions, and production of a significant amount of hazardous waste from large amounts of concentrated acid; it is also time-consuming (Santos et al., 2002; Sardans et al., 2010). In recent years, two improved techniques, direct solid sampling GFAAS (SS-GFAAS) and slurry sampling GFAAS have been used to simplify sample preparation procedures, and to avoid sample contamination (Lemaire et al., 2013; Rego et al., 2012). The SS-GFAAS is characterized by several interesting advantages, such as high sensitivity, greatly reduced risk of contamination and analyte loss, high sample throughput, and relatively moderate cost (Araujo et al., 2008; Torok and Zemberyova, 2012). However, this method was not widely accepted until recently, primarily owing to the following disadvantages: (1) the difficulty in handling and introducing small sample masses; (2) the high imprecision of the results due to the heterogeneity of some natural
Y. Peng et al. / Journal of Food Composition and Analysis 42 (2015) 78–83
79
samples; (3) the difficulty in calibration due to the requirement of solid standards with similar matrix composition and structure; and (4) the difficulty in diluting solid samples (Sardans et al., 2010). Another attractive alternative is the use of slurry samples introduction in GFAAS, which combines the advantages of liquid and solid sampling and avoids many problems associated with direct solid sampling: (1) simple aqueous standards are used instead of solid-sample matrix calibration; (2) the sample concentration can easily be changed by dilution, it can avoid weighted errors and inhomogeneity problems are eliminated, and the analyte concentration falls within the linear range; (3) slurry can be directly sampled using the conventional autosampler of GFAAS, thus avoiding the need for specialized expensive equipment such as a microbalance, and solid sampling accessories (Meeravali and Kumar, 1998; Sardans et al., 2010). Some researchers have reported that the Pb recovery for some biological samples (plankton, algae, etc.) was between 95% and 105%; however, it was only 80% for cabbage and oyster tissue (Maduro et al., 2006; Vinas et al., 2000), which could be due to the lower ratios of analyte in the liquid phase than in the solid phase (Millerihli, 1994). It has been demonstrated that the use of 5–15% HNO3 as the stabilizing medium could improve the ratio of analyte in the liquid phase (from 15% to 75%) and the Pb recovery in pine needles up to 95% (Millerihli, 1994; Sanchez-Moreno et al., 2010); however, the use of concentrated acid involves a risk of decreasing the useful lifetime of the expensive graphite tube (Vinas et al., 2000). Additionally, the required particle size of <32 mm for the traditional slurry sampling procedure increases the difficulty of sample grinding and the risk of contamination (Meeravali and Kumar, 1998; Sanchez-Moreno et al., 2010). In this study, a simple heating step was added prior to the slurry sampling procedure to improve the accuracy and precision of Pb determination in food samples. The proposed heat-assisted slurry sampling graphite furnace atomic absorption spectrometry (HASSGFAAS) method was evaluated by analysis various solid food reference materials.
et al., 1999). When a W–Rh permanent modifier was used, the tube lifetime was 815 firings, which is 31% longer than that when using a conventional NH4H2PO4 + Mg (NO3)2 modifiers (see also Supplementary data, Table 1S). After 300 firings, the coated graphite tube platform should be treated again. The operating parameters of the GFAAS used in this study are summarized in Table 1. When a W–Rh permanent modifier was used, the tube lifetime was 815 firings, which is 31% longer than that when using a conventional NH4H2PO4 + Mg (NO3)2 modifier.
2. Materials and methods
Each sample was accurately weighted (25–250 mg) into a 15 mL conical polypropylene tube and subsequently diluted with a mixed reagent (2%, v/v HNO3 + 1%, v/v H2O2 + 0.05%, v/v Triton X100) to a volume of 10 mL. The tubes were heated for 20 min on a heating block (120 8C). After cooling, the slurries were manually shaken before they were introduced into the autosampler cups. Just prior to each injection, the slurries were homogenized for 2 min in an ultrasonic bath (Branson, North Olmsted, OH, USA). The calibration was performed against aqueous standard solutions. The washing water used to clean the sampling capillary was replaced with a solution containing 0.1% (v/v) HNO3 + 0.05% (v/v) Triton X100 to avoid clogging of the autosampler pipette, thus improving the introduction of the slurry onto the platform.
2.1. Instrumentation All measurements were carried out using a PinAAcleTM 900T atomic absorption spectrometry (PerkinElmer, Inc., Shelton, CT, USA), that was equipped with a transversely heated graphite atomizer (THGA), a longitudinal AC Zeeman background correction system, a Pb electrodeless discharge lamps (EDL), a TubeViewTM color furnace camera, and an AS 900 autosampler. A tungsten carbide-rhodium coating on the integrated platform of a standard THGA graphite tube with the end cap was used as a permanent chemical modifier deposition using the previously reported (Lima
2.2. Reagents and standards De-ionized water (18.2 MV cm 1) used for the preparation of all blanks, sample solutions, and standards was obtained from a water purification system (Merck Millipore, Fontenay-sous-Bois, France). The Pb standard solution (1000 mg L 1) was purchased from the National Center for Analysis and Testing of Steel Materials, China. Nitric acid (HNO3, 65–70%, w/w, 99.999%) and hydrogen peroxide (H2O2, 65–70%, w/w, 99.999%) were purchased from Alfa Aesar. (Alfa Aesar Ltd., Tianjin, China). Twelve standard reference materials (SRMs) were used for validation of the proposed method. Two SRMs (mussel tissue NIST 1570a and bovine liver NIST 1577c) were obtained from NIST Office of Reference Materials (Gaithersburg, MD, USA), and ten SRMs (spiral algae GBW 10025, pollen GBW 10026, ginseng GBW 10027, radix astragali GBW 10028, rice GBW 10045, carrot GBW 10047, celery GBW 10048, prawn GBW 10050, pig liver GBW 10051, green tea GBW 10052) were obtained from the National Institute of Metrology, China. To evaluate the effect of sample particle size on the proposed method, a randomly selected unknown ginseng sample was ground to different particle sizes: w > 250 mm, 150 < w < 250 mm, 75 < w < 150 mm, 38 < w < 75 mm, 30 < w < 38 mm, and w < 30 mm. 2.3. Heat-assisted slurry sampling procedure
Table 1 Operating parameters of the graphite furnace and spectrometer. Step
Temp. (8C)
Ramp. time (s)
Hold time (s)
Drying
130 150 450 600 1600 2500
5 15 15 10 0 1
30 250 30 250 15 50 20 250 3 0 5 250 Measurement mode: peak area Zeeman-effect background correction system Pyrolytic graphite tubes with platform Injection volume (mL): 20
Pyrolysis
Atomization Cleaning Wavelength (nm): 283.3 Spectral bandwidth (nm): 0.7 EDLb lamp intensity (mA): 440 Integration time (s): 5 a b
Air comes from the air compressor. Electrodeless discharge lamps.
Inter. flow (mL min
1
)
Gas type Ar Ar Aira Ar – Ar
80
Y. Peng et al. / Journal of Food Composition and Analysis 42 (2015) 78–83
2.4. Microwave acid digestion procedure For comparison, the microwave acid digestion procedure was used in this study. The food sample (0.2500 g) was weighed into a Teflon vessel and 3.0 mL HNO3 and 2.0 mL H2O2 were added. Then, the vessel was sealed. Microwave (CEM, Inc., Matthews, NC, USA) digestion was carried out as the follow: first, the temperature was ramped to 120 8C in 10 min at 800 W, held for 5 min, ramped to 160 8C in 10 min at 800 W, and held for 15 min; next, after cooling, the opened vessel was placed on a heating block (120 8C) where the sample was boiled to near dryness; finally, it was transferred to a 25 mL volumetric flask and diluted up to the mark with a 1% HNO3 solution. 2.5. Pb partitioning in slurry Pb partitioning in the slurry refers to the ratio of the amount of Pb found in the liquid fraction of the slurry to the total amount of Pb in the slurry (Millerihli, 1994). The degree of Pb extraction into the liquid phase was evaluated by transferring 1.2 mL of 1% m/v slurries into the autosampler cups, sonicating for 2 min, and then introducing the sample into the graphite tube for analysis. After three measurements, the remaining slurry was passed through a 0.45 mm membrane filter to separate the liquid phase from any particles, and then 20 mL of the liquid phase was delivered (n = 5) into the graphite tube. 3. Results and discussion 3.1. Optimization of HASS-GFAAS conditions Preliminary experiments were carried out to investigate the Pb recovery and precision of the traditional slurry sampling GFAAS and microwave acid digestion GFAAS (MV-GFAAS) techniques. Five food reference materials (RMs), spinach leaves (NIST 1570a), pollen (GBW 10026), ginseng (GBW 10027), radix astragali (GBW 10028), and bovine liver (NIST 1577c) were selected for analysis. As shown in Fig. 1, the recoveries of Pb for five RMs by slurry sampling GF-AAS ranged from 34.1 to 101%, but three RMs (pollen, ginseng, and radix astragali) were not quantitatively measured, and the different recoveries for the selected SRMs may be due to the sample matrix. The recoveries of Pb for these RMs by MV-GFAAS
Fig. 1. The recovery and precision of Pb in five food reference materials by the conventional slurry sampling graphic furnace atomic absorption spectrometry (Slurry sampling GFAAS) and the microwave acid digestion GFAAS (MV-GFAAS) techniques.
ranged from 97 to 102%. Additionally, the precision (RSD) of the slurry sampling GFAAS technique (RSD, 5–8%) was lower than that of the MV-GFAAS technique (RSD, 1.7–3.1%). To improve the accuracy and precision of determination of Pb in food samples, a simple method involving the addition of a heating step before the conventional slurry procedure was evaluated. As it is typical a volatile element (Acar, 2005), Pb is lost from the graphite atomizer at temperature higher than 500 8C (see also Supplementary data, Fig. 1S). In our study, when the W–Rh (250 mg W + 200 mg Rh) coating on the integrated platform of a transversely heated graphite tube was used as a permanent chemical modifier, the Pb recoveries in the food RMs ranged from 96 to 101%. Other conditions, such as pyrolysis and atomization temperatures, were also optimized for the slurries. Fig. 2 shows the pyrolysis and atomization curves of Pb, which was obtained for optimization of the heat-assisted slurry, conventional slurry, and the digested solutions (using ginseng RM GBW 10027). The optimal pyrolysis and atomization temperatures for the heatassisted slurry procedure were 600 8C and 1600 8C, respectively, which were similar to those of the conventional slurry and the digestion procedure. In addition to the graphite furnace temperature programs described in Table 1, a pre-pyrolysis step (450 8C) was added, using air to avoid carbon residues. The parameters that influence the heat-assisted slurry procedures were studied for determination of Pb in food samples. The recovery of Pb in ginseng reference material GBW10027 (Pb content was 1.44 0.1 mg kg 1) was used to optimize the parameters of the heat-assisted slurry sampling procedure such as HNO3 concentration, heating temperature, and heating duration. The absorbance curves of Pb for the GBW10027 ginseng slurries with HNO3 concentration varied between 0.5% and 5% (v/v) and with a constant particle size (<150 mm), heating temperature (120 8C), heating duration (20 min), and slurry concentration (2% m/v) are shown in Fig. 3a. As is evident from the figure, the average Pb recovery was 97% in the 1–5% HNO3 range. A 2% value was selected because a higher acid concentration provided no advantages but had the disadvantage of shortening the useful lifetime of the graphite tube. Similar optimization procedures for the heating temperature and heating duration are shown in Fig. 3b and c, respectively. The temperature of the heat-assisted slurry sampling procedure was optimized as 120 8C. Under this temperature (120 8C), the heating time optimized as 20 min to guarantee quantitative recovery for different food samples, some of which may have a
Fig. 2. Effects of the pyrolysis and atomization temperatures on Pb absorbance (ginseng reference material GBW 10027) with different preparation procedures: heating assisted slurry sampling (HASS), conventional slurry sampling (SS), and microwave acid digestion (MV).
Y. Peng et al. / Journal of Food Composition and Analysis 42 (2015) 78–83
81
Fig. 3. Effects of heat-assisted slurry procedure conditions on Pb recoveries in ginseng reference material GBW10027. (a) Effect of HNO3 concentration, (b) effect of heating temperature, and (c) effect of heating duration.
high Pb content. Particle size was also evaluated for the HASS-GFAAS method, and different particle sizes (w > 250 mm, 150 < w < 250 mm, 75 < w < 150 mm, 38 < w < 75 mm, 30 < w < 38 mm, and w < 30 mm etc.) of an unknown ginseng sample (Pb = 0.642 0.016 mg g 1 measured with the MV-GFAAS method) were investigated. As shown in Fig. 4, recoveries of 98% were attained for particle sizes w < 150 mm, and this size was selected, too small particle sizes required a longer grinding time. However, particle sizes < 30 mm were required for the conventional slurry sampling procedure (Lima et al., 2000), which reduces the sample throughput. The slurry concentration is another important variable that must be optimized in order to obtain suitable results. The optimal slurry concentrations for Pb determination in ginseng SRM (GBW10027) ranged from 0.2% to 3.0% (m/v). For low slurry concentrations (<0.2%, m/v), the precision of measurements was impaired (RSD > 9.3%), because a smaller number of particles were introduced into the atomizer (Millerihli, 1993). However, highly
concentrated slurries (>3.0%, m/v) may need a longer heating duration and an increased acidic concentration, which would decrease the sample throughput and the graphite tube lifetime (Guo et al., 2014), this making the procedure unsuitable for routine analysis. 3.2. Analytical figures of merit Calibrations were carried out using aqueous standard solutions with a linear range from 1 to 40 ng mL 1 Pb, and the fitted linear equation of the analytical curve was Y = 0.0041X + 0.0007 with the correlation coefficient 0.999, and the relative standard deviation ranged from 0.1% to 3.6%. The characteristic mass obtained for Pb was 12 0.6 pg, and the uncertainties were based on the average of six results obtained on different days. The limits of detection (LOD) and limits of quantification (LOQ) were 0.003 mg kg 1 and 0.010 mg kg 1, respectively. The LOD was calculated from 20 consecutive measurements of the blank solutions, and taken into 50-fold dilution factor (for 2%, m/v food samples). 3.3. Food sample analysis
Fig. 4. Effect of different particle sizes on the recoveries of Pb with the heat-assisted slurry procedure. The Pb recovery in an unknown ginseng sample was calculated by normalizing the absorbance value from the heating assisted slurry sampling graphic furnace atomic absorption spectrometry (HASS-GFAAS) method to that of the microwave acid digestion graphic furnace atomic absorption spectrometry (MV-GFAAS) method.
The proposed HASS-GFAAS method was used to analyze 12 food SRM samples. For comparison, the slurry sampling GFAAS and MVGFAAS method were also used. The Pb content obtained by the heat-assisted slurry sampling method was in agreement with the certified values (Table 2), and the recoveries ranged from 96 to 103%. As all results by HASS-GFAAS method were processed in authentic triplicate, all confidence intervals were calculated for t with 2 degrees of freedom at 95% confidence level. There is no statistical difference at a 95% confidence level between the proposed analytical method (HASS-GFAAS) and the certified values for each sample. For the slurry sampling GFAAS method, the recoveries of seven samples ranged from 94.1 to 106.7%, but for other six SRMs (spinach leaves NIST 1570a, pollen GBW 10026, ginseng GBW 10027, radix astragali GBW 10028, celery GBW 10048, and prawn GBW 10050), the Pb recoveries only ranged from 34.1 to 86.5%. Additionally, the precision (RSD) of the proposed HASS method is better than that of the conventional slurry sampling procedure, most likely because a large percentage of the
Y. Peng et al. / Journal of Food Composition and Analysis 42 (2015) 78–83
82
Table 2 Results of Pb values in 12 biological standard reference materials (n = 3). SRMs
NIST1577c (Bovine liver) NIST1570a (Spinach leaves) GBW10025 (Spiral algae) GBW10026 (Pollen) GBW10027 (Ginseng) GBW10028 (Radix astragali) GBW10045 (Rice) GBW10047 (Carrot) GBW10048 (Celery) GBW10050 (Prawn) GBW10051 (Pig liver) GBW10052 (Green tea) a b c d e
Pb values (mean SD), mg kg
1
Certified values
HASS-GFAAS (This work)b
SIS-GFAASc
MV-GFAASe
0.068 0.010 (0.200)a 2.80 0.20 0.250 0.040 1.44 0.10 5.75 0.50 0.070 0.023 0.430 0.070 2.70 0.70 0.200 0.050 0.120 0.030 1.60 0.20
0.067 0.006 0.203 0.002 2.76 0.04 0.240 0.003 1.45 0.02 5.72 0.14 0.068 0.006 0.425 0.051 2.73 0.05 0.204 0.060 0.124 0.018 1.58 0.02
0.065 0.008 0.170 W 0.004d 2.68 0.04 0.185 W 0.006 0.626 0.022 1.96 W 0.28 0.071 0.012 0.413 0.082 2.18 W 0.16 0.173 W 0.090 0.128 0.092 1.50 0.01
0.068 0.007 0.220 0.003 2.78 0.04 0.250 0.004 1.46 0.02 5.77 0.21 0.072 0.020 0.427 0.078 2.72 0.05 0.208 0.080 0.131 0.024 1.62 0.04
Information values, insufficient information is available to assess the uncertainty associated with the value, no uncertainty is provided. Heating assisted slurry sampling GFAAS method. Traditional slurry sampling GFAAS method. Pb recovery less than 85%. Microwave acid digestion GFAAS method.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jfca.2015.03.007. References
Fig. 5. Extraction of Pb from the 2% (m/v) ginseng reference material (GBW 10027) slurry into aqueous phase.
Pb is extracted into the liquid phase (Fig. 5), and therefore, the analysis is more representative of the analyte concentration in the original solid sample. The results of three real food samples also validated the accuracy of the proposed method (see also Supplementary data, Table 2S).
4. Conclusions An improved slurry sampling GFAAS technique for detection of trace Pb in food standard reference materials was evaluated and better recovery and precision were obtained by the proposed heatassisted slurry sampling procedure. As a standard laboratory method, it can be used to rapidly analyze trace Pb in various solid food samples. Acknowledgements This work was supported by the National Nature Science Foundation of China (nos. 21175120 and 21207120) and the National Key Scientific Instrument and Equipment Development Projects of China (no. 2011YQ06010008), and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (no. CUGL140411).
Acar, O., 2005. Evaluation of V, IrRu, V–Ir, V–Ru, and W–V as permanent chemical modifiers for the determination of cadmium, lead, and zinc in botanic and biological slurries by electrothermal atomic absorption spectrometry. Analytica Chimica Acta 545 (2), 244–251. Alkis, I.M., Oz, S., Atakol, A., Yilmaz, N., Anli, R.E., Atakol, O., 2014. Investigation of heavy metal concentrations in some Turkish wines. Journal of Food Composition and Analysis 33 (1), 105–110. Antoine, J.M.R., Fung, L.A.H., Grant, C.N., Dennis, H.T., Lalor, G.C., 2012. Dietary intake of minerals and trace elements in rice on the Jamaican market. Journal of Food Composition and Analysis 26 (1–2), 111–121. Araujo, R.G.O., Oleszczuk, N., Rampazzo, R.T., Costa, P.A., Silva, M.M., Vale, M.G.R., Welz, B., Ferreira, S.L.C., 2008. Comparison of direct solid sampling and slurry sampling for the determination of cadmium in wheat flour by electrothermal atomic absorption spectrometry. Talanta 77 (1), 400–406. Chahid, A., Hilali, M., Benlhachimi, A., Bouzid, T., 2014. Contents of cadmium, mercury and lead in fish from the Atlantic sea (Morocco) determined by atomic absorption spectrometry. Food Chemistry 147, 357–360. Chen, H., Guo, L., Zhang, M., Gu, J., Zhong, K., Bo, L., Li, J., 2014. Determination of lead in soybean sauces by the diffusive gradients in thin films technique. Food Chemistry 165, 9–13. de Andrade, C.K., dos Anjos, V.E., Felsner, M.L., Torres, Y.R., Quinaia, S.P., 2014. Direct determination of Cd, Pb and Cr in honey by slurry sampling electrothermal atomic absorption spectrometry. Food Chemistry 146, 166–173. Gimou, M.-M., Charrondiere, U.R., Leblanc, J.-C., Pouillot, R., Noel, L., Guerin, T., 2014. Concentration data for 25 elements in foodstuffs in Yaounde: The Cameroonian Total Diet Study. Journal of Food Composition and Analysis 34 (1), 39–55. Gunduz, S., Akman, S., 2013. Determination of lead in rice grains by solid sampling HR-CS GFAAS. Food Chemistry 141 (3), 2634–2638. Guo, W., Zhang, P., Jin, L.L., Hu, S.H., 2014. Rice cadmium monitoring using heatextraction electrothermal atomic absorption spectrometry. Journal of Analytical Atomic Spectrometry 29 (10), 1949–1954. Itkonen, S.T., Ekholm, P.J., Kemi, V.E., Lamberg-Allardt, C.J.E., 2012. Analysis of in vitro digestible phosphorus content in selected processed rye, wheat and barley products. Journal of Food Composition and Analysis 25 (2), 185–189. Julshamn, K., Maage, A., Norli, H.S., Grobecker, K.H., Jorhem, L., Fecher, P., Dowell, D., 2013. Determination of arsenic, cadmium, mercury, and lead in foods by pressure digestion and inductively coupled plasma/mass spectrometry: first action 2013.06. Journal of AOAC International 96 (5), 1101–1102. Koyyalamudi, S.R., Jeong, S.-C., Manavalan, S., Vysetti, B., Pang, G., 2013. Micronutrient mineral content of the fruiting bodies of Australian cultivated Agaricus bisporus white button mushrooms. Journal of Food Composition and Analysis 31 (1), 109–114. Lemaire, R., Del Bianco, D., Garnier, L., Beltramo, J.L., 2013. Determination of lead in lipstick by direct solid sampling high-resolution continuum source graphite furnace atomic absorption spectrometry: comparison of two digestion methods. Analytical Letters 46 (14), 2265–2278. Lima, E.C., Barbosa, F., Krug, F.J., 1999. Tungsten–rhodium permanent chemical modifier for lead determination in sediment slurries by electrothermal atomic
Y. Peng et al. / Journal of Food Composition and Analysis 42 (2015) 78–83 absorption spectrometry. Journal of Analytical Atomic Spectrometry 14 (12), 1913–1918. Lima, E.C., Barbosa, F., Krug, F.J., Silva, M.M., Vale, M.G.R., 2000. Comparison of ultrasound-assisted extraction, slurry sampling and microwave-assisted digestion for cadmium, copper and lead determination in biological and sediment samples by electrothermal atomic absorption spectrometry. Journal of Analytical Atomic Spectrometry 15 (8), 995–1000. Maduro, C., Vale, G., Alves, S., Galesio, M., da Silva, M.D.R.G., Fernandez, C., Catarino, S., Rivas, M.G., Mota, A.M., Capelo, J.L., 2006. Determination of Cd and Pb in biological reference materials by electrothermal atomic absorption spectrometry: a comparison of three ultrasonic-based sample treatment procedures. Talanta 68 (4), 1156–1161. Meeravali, N.N., Kumar, S.J., 1998. Rapid slurry atomization using transverse heated electrothermal atomic absorption spectrometry for the determination of cadmium, copper, manganese and lead in biological reference materials. Journal of Analytical Atomic Spectrometry 13 (7), 647–652. Millerihli, N.J., 1993. Advances in ultrasonic slurry graphite-furnace atomic-absorption spectrometry. Fresenius Journal of Analytical Chemistry 345 (7), 482–489. Millerihli, N.J., 1994. Influence of slurry preparation on the accuracy of ultrasonic slurry electrothermal atomic-absorption spectrometry. Journal of Analytical Atomic Spectrometry 9 (10), 1129–1134. Millour, S., Noel, L., Chekri, R., Vastel, C., Kadar, A., Sirot, V., Leblanc, J.-C., Guerin, T., 2012. Strontium, silver, tin, iron, tellurium, gallium, germanium, barium and vanadium levels in foodstuffs from the Second French Total Diet Study. Journal of Food Composition and Analysis 25 (2), 108–129. Millour, S., Noel, L., Kadar, A., Chekri, R., Vastel, C., Guerin, T., 2011. Simultaneous analysis of 21 elements in foodstuffs by ICP-MS after closed-vessel microwave digestion: method validation. Journal of Food Composition and Analysis 24 (1), 111–120. Mol, S., 2011. Levels of selected trace metals in canned tuna fish produced in Turkey. Journal of Food Composition and Analysis 24 (1), 66–69. Park, K., Min, H., Yim, Y.-H., Yim, Y.-L., Hwang, E.-J., Cho, K., 2011. Instrumental neutron activation analysis (INAA) and isotope dilution-inductively coupled plasma mass spectrometry (ID-ICP/MS) for certification of multielements in a tuna fish candidate certified reference material. Journal of Food Composition and Analysis 24 (7), 1064–1068. Potorti, A.G., Di Bella, G., Lo Turco, V., Rando, R., Dugo, G., 2013. Non-toxic and potentially toxic elements in Italian donkey milk by ICP-MS and multivariate analysis. Journal of Food Composition and Analysis 31 (1), 161–172.
83
Rego, J.F., Virgilio, A., Nobrega, J.A., Neto, J.A.G., 2012. Determination of lead in medicinal plants by high-resolution continuum source graphite furnace atomic absorption spectrometry using direct solid sampling. Talanta 100, 21–26. Rodushkin, I., Baxter, D.C., Engstrom, E., Hoogewerff, J., Horn, P., Papesch, W., Watling, J., Latkoczy, C., van der Peijl, G., Berends-Montero, S., Ehleringer, J., Zdanowicz, V., 2011. Elemental and isotopic characterization of cane and beet sugars. Journal of Food Composition and Analysis 24 (1), 70–78. Sanchez-Moreno, R.A., Gismera, M.J., Sevilla, M.T., Procopio, J.R., 2010. Direct and rapid determination of ultratrace heavy metals in solid plant materials by ET-AAS ultrasonic-assisted slurry sampling. Phytochemical Analysis 21 (4), 340–347. Santos, D., Barbosa, F., Tomazelli, A.C., Krug, F.J., Nobrega, J.A., Arruda, M.A.Z., 2002. Determination of Cd and Pb in food slurries by GRAS using cryogenic grinding for sample preparation. Analytical and Bioanalytical Chemistry 373 (3), 183–189. Sardans, J., Montes, F., Penuelas, J., 2010. Determination of As, Cd, Cu, Hg and Pb in biological samples by modern electrothermal atomic absorption spectrometry. Spectrochimica Acta Part B-Atomic Spectroscopy 65 (2), 97–112. Scancar, J., Zuliani, T., Milacic, R., 2013. Study of nickel content in Ni-rich food products in Slovenia. Journal of Food Composition and Analysis 32 (1), 83–89. Skrbic, B., Zivancev, J., Mrmos, N., 2013. Concentrations of arsenic, cadmium and lead in selected foodstuffs from Serbian market basket: estimated intake by the population from the Serbia. Food and Chemical Toxicology 58, 440–448. Soares, A.R., Nascentes, C.C., 2013. Development of a simple method for the determination of lead in lipstick using alkaline solubilization and graphite furnace atomic absorption spectrometry. Talanta 105, 272–277. Tormen, L., Torres, D.P., Dittert, I.M., Araujo, R.G.O., Frescura, V.L.A., Curtius, A.J., 2011. Rapid assessment of metal contamination in commercial fruit juices by inductively coupled mass spectrometry after a simple dilution. Journal of Food Composition and Analysis 24 (1), 95–102. Torok, P., Zemberyova, M., 2012. Comparison of chemical modifiers for direct determination of Cd, Cu and Zn in food stuffs by solid-sampling-ETAAS. Food Chemistry 132 (1), 554–560. Vassileva, E., Wysocka, I., Betti, M., 2014. Reference measurements for cadmium, copper, mercury, lead, zinc and methyl mercury mass fractions in scallop sample by isotope dilution inductively coupled plasma mass spectrometry. Microchemical Journal 116, 197–205. Vinas, P., Pardo-Martinez, M., Hernandez-Cordoba, M., 2000. Rapid determination of selenium, lead and cadmium in baby food samples using electrothermal atomic absorption spectrometry and slurry atomization. Analytica Chimica Acta 412 (1-2), 121–130.
Contents lists available at ScienceDirect
Journal of Food Composition and Analysis journal homepage: www.elsevier.com/locate/jfca
Short Communication
Heat-assisted slurry sampling GFAAS method for determination of lead in food standard reference materials Yue’e Peng a,b, Wei Guo a,*, Ping Zhang c, Lanlan Jin a a
State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, PR China Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China c Beijing CKC, PerkinElmer, Inc., 100015, PR China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 September 2014 Received in revised form 17 March 2015 Accepted 23 March 2015 Available online 13 April 2015
A valid method based on heat-assisted slurry sampling graphic furnace atomic absorption spectrometry (HASS-GFAAS) was developed for the accurate determination of trace Pb in food standard reference materials (SRMs). The HASS technique significantly improved Pb recovery and precision compared to conventional slurry sampling techniques. The optimized HASS procedure was performed as follows: first, the sample (particle size 150 mm) was diluted with 0.05% (v/v) Triton X-100 containing 2% HNO3 and 1% H2O2 followed by heating for 20 min at 120 8C on a heating block. Next, the obtained slurry was sonicated in an autosampler cup, and finally, the slurry was introduced into a graphite tube and analyzed by the GFAAS with a Pb electrodeless discharge lamp (EDL). Calibration with aqueous standard solutions was used for Pb determination in food samples. The characteristic mass and limit of detection for Pb based on the integrated absorbance for a 2% (m/v) sample were 12 0.6 pg and 0.003 mg kg 1, respectively. The accuracy (95.1–102% recovery) and good precision (0.1–3.6%) of this procedure are illustrated by the results obtained for the 12 food reference materials. The proposed method is suitable for determination of trace Pb in solid food samples. ß 2015 Elsevier Inc. All rights reserved.
Keywords: Food analysis Food composition Heat-assisted slurry sampling Trace element in food Heavy metal contamination Lead Food certified material Method validation Standard reference material
1. Introduction Lead (Pb) is toxic to humans even at low levels; the most important sources of Pb exposure are industrial emission, soil, and automobile exhaust (Vinas et al., 2000). Vegetables with a relatively large leaf area, such as spinach and cabbage, can contain high levels of Pb when grown near such Pb sources. Therefore, the accurate and rapid determination of trace Pb in various food samples is considered important step in contamination studies. Over the last two decades, graphite furnace atomic absorption spectrometry (GFAAS) (Alkis et al., 2014; Antoine et al., 2012; Chahid et al., 2014; Chen et al., 2014; de Andrade et al., 2014; Gunduz and Akman, 2013; Itkonen et al., 2012; Skrbic et al., 2013; Soares and Nascentes, 2013) and inductively coupled plasma mass spectrometry (ICP-MS) (Gimou et al., 2014; Itkonen et al., 2012; Julshamn et al., 2013; Koyyalamudi et al., 2013; Millour et al., 2012, 2011; Mol, 2011; Park et al., 2011; Potorti et al., 2013; Rodushkin et al., 2011; Scancar et al., 2013; Tormen et al., 2011; Vassileva et al., 2014) techniques have emerged as the dominant analytical
* Corresponding author. Tel.: +86 27 67883495; fax: +86 27 67883495. E-mail address: [email protected] (W. Guo). http://dx.doi.org/10.1016/j.jfca.2015.03.007 0889-1575/ß 2015 Elsevier Inc. All rights reserved.
methods for trace element determination in various food samples. However, GFAAS is proving to be a more useful method than ICPMS owing to its relative simplicity, and its capability for direct determination of Pb in complex food matrices. It is well-known that in the determination of Pb in dried food samples, a microwave acid digestion step is frequently carried out prior to measurement of Pb by GFAAS; but the main drawbacks of this step are low sample throughput, risk of occasional explosions, and production of a significant amount of hazardous waste from large amounts of concentrated acid; it is also time-consuming (Santos et al., 2002; Sardans et al., 2010). In recent years, two improved techniques, direct solid sampling GFAAS (SS-GFAAS) and slurry sampling GFAAS have been used to simplify sample preparation procedures, and to avoid sample contamination (Lemaire et al., 2013; Rego et al., 2012). The SS-GFAAS is characterized by several interesting advantages, such as high sensitivity, greatly reduced risk of contamination and analyte loss, high sample throughput, and relatively moderate cost (Araujo et al., 2008; Torok and Zemberyova, 2012). However, this method was not widely accepted until recently, primarily owing to the following disadvantages: (1) the difficulty in handling and introducing small sample masses; (2) the high imprecision of the results due to the heterogeneity of some natural
Y. Peng et al. / Journal of Food Composition and Analysis 42 (2015) 78–83
79
samples; (3) the difficulty in calibration due to the requirement of solid standards with similar matrix composition and structure; and (4) the difficulty in diluting solid samples (Sardans et al., 2010). Another attractive alternative is the use of slurry samples introduction in GFAAS, which combines the advantages of liquid and solid sampling and avoids many problems associated with direct solid sampling: (1) simple aqueous standards are used instead of solid-sample matrix calibration; (2) the sample concentration can easily be changed by dilution, it can avoid weighted errors and inhomogeneity problems are eliminated, and the analyte concentration falls within the linear range; (3) slurry can be directly sampled using the conventional autosampler of GFAAS, thus avoiding the need for specialized expensive equipment such as a microbalance, and solid sampling accessories (Meeravali and Kumar, 1998; Sardans et al., 2010). Some researchers have reported that the Pb recovery for some biological samples (plankton, algae, etc.) was between 95% and 105%; however, it was only 80% for cabbage and oyster tissue (Maduro et al., 2006; Vinas et al., 2000), which could be due to the lower ratios of analyte in the liquid phase than in the solid phase (Millerihli, 1994). It has been demonstrated that the use of 5–15% HNO3 as the stabilizing medium could improve the ratio of analyte in the liquid phase (from 15% to 75%) and the Pb recovery in pine needles up to 95% (Millerihli, 1994; Sanchez-Moreno et al., 2010); however, the use of concentrated acid involves a risk of decreasing the useful lifetime of the expensive graphite tube (Vinas et al., 2000). Additionally, the required particle size of <32 mm for the traditional slurry sampling procedure increases the difficulty of sample grinding and the risk of contamination (Meeravali and Kumar, 1998; Sanchez-Moreno et al., 2010). In this study, a simple heating step was added prior to the slurry sampling procedure to improve the accuracy and precision of Pb determination in food samples. The proposed heat-assisted slurry sampling graphite furnace atomic absorption spectrometry (HASSGFAAS) method was evaluated by analysis various solid food reference materials.
et al., 1999). When a W–Rh permanent modifier was used, the tube lifetime was 815 firings, which is 31% longer than that when using a conventional NH4H2PO4 + Mg (NO3)2 modifiers (see also Supplementary data, Table 1S). After 300 firings, the coated graphite tube platform should be treated again. The operating parameters of the GFAAS used in this study are summarized in Table 1. When a W–Rh permanent modifier was used, the tube lifetime was 815 firings, which is 31% longer than that when using a conventional NH4H2PO4 + Mg (NO3)2 modifier.
2. Materials and methods
Each sample was accurately weighted (25–250 mg) into a 15 mL conical polypropylene tube and subsequently diluted with a mixed reagent (2%, v/v HNO3 + 1%, v/v H2O2 + 0.05%, v/v Triton X100) to a volume of 10 mL. The tubes were heated for 20 min on a heating block (120 8C). After cooling, the slurries were manually shaken before they were introduced into the autosampler cups. Just prior to each injection, the slurries were homogenized for 2 min in an ultrasonic bath (Branson, North Olmsted, OH, USA). The calibration was performed against aqueous standard solutions. The washing water used to clean the sampling capillary was replaced with a solution containing 0.1% (v/v) HNO3 + 0.05% (v/v) Triton X100 to avoid clogging of the autosampler pipette, thus improving the introduction of the slurry onto the platform.
2.1. Instrumentation All measurements were carried out using a PinAAcleTM 900T atomic absorption spectrometry (PerkinElmer, Inc., Shelton, CT, USA), that was equipped with a transversely heated graphite atomizer (THGA), a longitudinal AC Zeeman background correction system, a Pb electrodeless discharge lamps (EDL), a TubeViewTM color furnace camera, and an AS 900 autosampler. A tungsten carbide-rhodium coating on the integrated platform of a standard THGA graphite tube with the end cap was used as a permanent chemical modifier deposition using the previously reported (Lima
2.2. Reagents and standards De-ionized water (18.2 MV cm 1) used for the preparation of all blanks, sample solutions, and standards was obtained from a water purification system (Merck Millipore, Fontenay-sous-Bois, France). The Pb standard solution (1000 mg L 1) was purchased from the National Center for Analysis and Testing of Steel Materials, China. Nitric acid (HNO3, 65–70%, w/w, 99.999%) and hydrogen peroxide (H2O2, 65–70%, w/w, 99.999%) were purchased from Alfa Aesar. (Alfa Aesar Ltd., Tianjin, China). Twelve standard reference materials (SRMs) were used for validation of the proposed method. Two SRMs (mussel tissue NIST 1570a and bovine liver NIST 1577c) were obtained from NIST Office of Reference Materials (Gaithersburg, MD, USA), and ten SRMs (spiral algae GBW 10025, pollen GBW 10026, ginseng GBW 10027, radix astragali GBW 10028, rice GBW 10045, carrot GBW 10047, celery GBW 10048, prawn GBW 10050, pig liver GBW 10051, green tea GBW 10052) were obtained from the National Institute of Metrology, China. To evaluate the effect of sample particle size on the proposed method, a randomly selected unknown ginseng sample was ground to different particle sizes: w > 250 mm, 150 < w < 250 mm, 75 < w < 150 mm, 38 < w < 75 mm, 30 < w < 38 mm, and w < 30 mm. 2.3. Heat-assisted slurry sampling procedure
Table 1 Operating parameters of the graphite furnace and spectrometer. Step
Temp. (8C)
Ramp. time (s)
Hold time (s)
Drying
130 150 450 600 1600 2500
5 15 15 10 0 1
30 250 30 250 15 50 20 250 3 0 5 250 Measurement mode: peak area Zeeman-effect background correction system Pyrolytic graphite tubes with platform Injection volume (mL): 20
Pyrolysis
Atomization Cleaning Wavelength (nm): 283.3 Spectral bandwidth (nm): 0.7 EDLb lamp intensity (mA): 440 Integration time (s): 5 a b
Air comes from the air compressor. Electrodeless discharge lamps.
Inter. flow (mL min
1
)
Gas type Ar Ar Aira Ar – Ar
80
Y. Peng et al. / Journal of Food Composition and Analysis 42 (2015) 78–83
2.4. Microwave acid digestion procedure For comparison, the microwave acid digestion procedure was used in this study. The food sample (0.2500 g) was weighed into a Teflon vessel and 3.0 mL HNO3 and 2.0 mL H2O2 were added. Then, the vessel was sealed. Microwave (CEM, Inc., Matthews, NC, USA) digestion was carried out as the follow: first, the temperature was ramped to 120 8C in 10 min at 800 W, held for 5 min, ramped to 160 8C in 10 min at 800 W, and held for 15 min; next, after cooling, the opened vessel was placed on a heating block (120 8C) where the sample was boiled to near dryness; finally, it was transferred to a 25 mL volumetric flask and diluted up to the mark with a 1% HNO3 solution. 2.5. Pb partitioning in slurry Pb partitioning in the slurry refers to the ratio of the amount of Pb found in the liquid fraction of the slurry to the total amount of Pb in the slurry (Millerihli, 1994). The degree of Pb extraction into the liquid phase was evaluated by transferring 1.2 mL of 1% m/v slurries into the autosampler cups, sonicating for 2 min, and then introducing the sample into the graphite tube for analysis. After three measurements, the remaining slurry was passed through a 0.45 mm membrane filter to separate the liquid phase from any particles, and then 20 mL of the liquid phase was delivered (n = 5) into the graphite tube. 3. Results and discussion 3.1. Optimization of HASS-GFAAS conditions Preliminary experiments were carried out to investigate the Pb recovery and precision of the traditional slurry sampling GFAAS and microwave acid digestion GFAAS (MV-GFAAS) techniques. Five food reference materials (RMs), spinach leaves (NIST 1570a), pollen (GBW 10026), ginseng (GBW 10027), radix astragali (GBW 10028), and bovine liver (NIST 1577c) were selected for analysis. As shown in Fig. 1, the recoveries of Pb for five RMs by slurry sampling GF-AAS ranged from 34.1 to 101%, but three RMs (pollen, ginseng, and radix astragali) were not quantitatively measured, and the different recoveries for the selected SRMs may be due to the sample matrix. The recoveries of Pb for these RMs by MV-GFAAS
Fig. 1. The recovery and precision of Pb in five food reference materials by the conventional slurry sampling graphic furnace atomic absorption spectrometry (Slurry sampling GFAAS) and the microwave acid digestion GFAAS (MV-GFAAS) techniques.
ranged from 97 to 102%. Additionally, the precision (RSD) of the slurry sampling GFAAS technique (RSD, 5–8%) was lower than that of the MV-GFAAS technique (RSD, 1.7–3.1%). To improve the accuracy and precision of determination of Pb in food samples, a simple method involving the addition of a heating step before the conventional slurry procedure was evaluated. As it is typical a volatile element (Acar, 2005), Pb is lost from the graphite atomizer at temperature higher than 500 8C (see also Supplementary data, Fig. 1S). In our study, when the W–Rh (250 mg W + 200 mg Rh) coating on the integrated platform of a transversely heated graphite tube was used as a permanent chemical modifier, the Pb recoveries in the food RMs ranged from 96 to 101%. Other conditions, such as pyrolysis and atomization temperatures, were also optimized for the slurries. Fig. 2 shows the pyrolysis and atomization curves of Pb, which was obtained for optimization of the heat-assisted slurry, conventional slurry, and the digested solutions (using ginseng RM GBW 10027). The optimal pyrolysis and atomization temperatures for the heatassisted slurry procedure were 600 8C and 1600 8C, respectively, which were similar to those of the conventional slurry and the digestion procedure. In addition to the graphite furnace temperature programs described in Table 1, a pre-pyrolysis step (450 8C) was added, using air to avoid carbon residues. The parameters that influence the heat-assisted slurry procedures were studied for determination of Pb in food samples. The recovery of Pb in ginseng reference material GBW10027 (Pb content was 1.44 0.1 mg kg 1) was used to optimize the parameters of the heat-assisted slurry sampling procedure such as HNO3 concentration, heating temperature, and heating duration. The absorbance curves of Pb for the GBW10027 ginseng slurries with HNO3 concentration varied between 0.5% and 5% (v/v) and with a constant particle size (<150 mm), heating temperature (120 8C), heating duration (20 min), and slurry concentration (2% m/v) are shown in Fig. 3a. As is evident from the figure, the average Pb recovery was 97% in the 1–5% HNO3 range. A 2% value was selected because a higher acid concentration provided no advantages but had the disadvantage of shortening the useful lifetime of the graphite tube. Similar optimization procedures for the heating temperature and heating duration are shown in Fig. 3b and c, respectively. The temperature of the heat-assisted slurry sampling procedure was optimized as 120 8C. Under this temperature (120 8C), the heating time optimized as 20 min to guarantee quantitative recovery for different food samples, some of which may have a
Fig. 2. Effects of the pyrolysis and atomization temperatures on Pb absorbance (ginseng reference material GBW 10027) with different preparation procedures: heating assisted slurry sampling (HASS), conventional slurry sampling (SS), and microwave acid digestion (MV).
Y. Peng et al. / Journal of Food Composition and Analysis 42 (2015) 78–83
81
Fig. 3. Effects of heat-assisted slurry procedure conditions on Pb recoveries in ginseng reference material GBW10027. (a) Effect of HNO3 concentration, (b) effect of heating temperature, and (c) effect of heating duration.
high Pb content. Particle size was also evaluated for the HASS-GFAAS method, and different particle sizes (w > 250 mm, 150 < w < 250 mm, 75 < w < 150 mm, 38 < w < 75 mm, 30 < w < 38 mm, and w < 30 mm etc.) of an unknown ginseng sample (Pb = 0.642 0.016 mg g 1 measured with the MV-GFAAS method) were investigated. As shown in Fig. 4, recoveries of 98% were attained for particle sizes w < 150 mm, and this size was selected, too small particle sizes required a longer grinding time. However, particle sizes < 30 mm were required for the conventional slurry sampling procedure (Lima et al., 2000), which reduces the sample throughput. The slurry concentration is another important variable that must be optimized in order to obtain suitable results. The optimal slurry concentrations for Pb determination in ginseng SRM (GBW10027) ranged from 0.2% to 3.0% (m/v). For low slurry concentrations (<0.2%, m/v), the precision of measurements was impaired (RSD > 9.3%), because a smaller number of particles were introduced into the atomizer (Millerihli, 1993). However, highly
concentrated slurries (>3.0%, m/v) may need a longer heating duration and an increased acidic concentration, which would decrease the sample throughput and the graphite tube lifetime (Guo et al., 2014), this making the procedure unsuitable for routine analysis. 3.2. Analytical figures of merit Calibrations were carried out using aqueous standard solutions with a linear range from 1 to 40 ng mL 1 Pb, and the fitted linear equation of the analytical curve was Y = 0.0041X + 0.0007 with the correlation coefficient 0.999, and the relative standard deviation ranged from 0.1% to 3.6%. The characteristic mass obtained for Pb was 12 0.6 pg, and the uncertainties were based on the average of six results obtained on different days. The limits of detection (LOD) and limits of quantification (LOQ) were 0.003 mg kg 1 and 0.010 mg kg 1, respectively. The LOD was calculated from 20 consecutive measurements of the blank solutions, and taken into 50-fold dilution factor (for 2%, m/v food samples). 3.3. Food sample analysis
Fig. 4. Effect of different particle sizes on the recoveries of Pb with the heat-assisted slurry procedure. The Pb recovery in an unknown ginseng sample was calculated by normalizing the absorbance value from the heating assisted slurry sampling graphic furnace atomic absorption spectrometry (HASS-GFAAS) method to that of the microwave acid digestion graphic furnace atomic absorption spectrometry (MV-GFAAS) method.
The proposed HASS-GFAAS method was used to analyze 12 food SRM samples. For comparison, the slurry sampling GFAAS and MVGFAAS method were also used. The Pb content obtained by the heat-assisted slurry sampling method was in agreement with the certified values (Table 2), and the recoveries ranged from 96 to 103%. As all results by HASS-GFAAS method were processed in authentic triplicate, all confidence intervals were calculated for t with 2 degrees of freedom at 95% confidence level. There is no statistical difference at a 95% confidence level between the proposed analytical method (HASS-GFAAS) and the certified values for each sample. For the slurry sampling GFAAS method, the recoveries of seven samples ranged from 94.1 to 106.7%, but for other six SRMs (spinach leaves NIST 1570a, pollen GBW 10026, ginseng GBW 10027, radix astragali GBW 10028, celery GBW 10048, and prawn GBW 10050), the Pb recoveries only ranged from 34.1 to 86.5%. Additionally, the precision (RSD) of the proposed HASS method is better than that of the conventional slurry sampling procedure, most likely because a large percentage of the
Y. Peng et al. / Journal of Food Composition and Analysis 42 (2015) 78–83
82
Table 2 Results of Pb values in 12 biological standard reference materials (n = 3). SRMs
NIST1577c (Bovine liver) NIST1570a (Spinach leaves) GBW10025 (Spiral algae) GBW10026 (Pollen) GBW10027 (Ginseng) GBW10028 (Radix astragali) GBW10045 (Rice) GBW10047 (Carrot) GBW10048 (Celery) GBW10050 (Prawn) GBW10051 (Pig liver) GBW10052 (Green tea) a b c d e
Pb values (mean SD), mg kg
1
Certified values
HASS-GFAAS (This work)b
SIS-GFAASc
MV-GFAASe
0.068 0.010 (0.200)a 2.80 0.20 0.250 0.040 1.44 0.10 5.75 0.50 0.070 0.023 0.430 0.070 2.70 0.70 0.200 0.050 0.120 0.030 1.60 0.20
0.067 0.006 0.203 0.002 2.76 0.04 0.240 0.003 1.45 0.02 5.72 0.14 0.068 0.006 0.425 0.051 2.73 0.05 0.204 0.060 0.124 0.018 1.58 0.02
0.065 0.008 0.170 W 0.004d 2.68 0.04 0.185 W 0.006 0.626 0.022 1.96 W 0.28 0.071 0.012 0.413 0.082 2.18 W 0.16 0.173 W 0.090 0.128 0.092 1.50 0.01
0.068 0.007 0.220 0.003 2.78 0.04 0.250 0.004 1.46 0.02 5.77 0.21 0.072 0.020 0.427 0.078 2.72 0.05 0.208 0.080 0.131 0.024 1.62 0.04
Information values, insufficient information is available to assess the uncertainty associated with the value, no uncertainty is provided. Heating assisted slurry sampling GFAAS method. Traditional slurry sampling GFAAS method. Pb recovery less than 85%. Microwave acid digestion GFAAS method.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jfca.2015.03.007. References
Fig. 5. Extraction of Pb from the 2% (m/v) ginseng reference material (GBW 10027) slurry into aqueous phase.
Pb is extracted into the liquid phase (Fig. 5), and therefore, the analysis is more representative of the analyte concentration in the original solid sample. The results of three real food samples also validated the accuracy of the proposed method (see also Supplementary data, Table 2S).
4. Conclusions An improved slurry sampling GFAAS technique for detection of trace Pb in food standard reference materials was evaluated and better recovery and precision were obtained by the proposed heatassisted slurry sampling procedure. As a standard laboratory method, it can be used to rapidly analyze trace Pb in various solid food samples. Acknowledgements This work was supported by the National Nature Science Foundation of China (nos. 21175120 and 21207120) and the National Key Scientific Instrument and Equipment Development Projects of China (no. 2011YQ06010008), and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (no. CUGL140411).
Acar, O., 2005. Evaluation of V, IrRu, V–Ir, V–Ru, and W–V as permanent chemical modifiers for the determination of cadmium, lead, and zinc in botanic and biological slurries by electrothermal atomic absorption spectrometry. Analytica Chimica Acta 545 (2), 244–251. Alkis, I.M., Oz, S., Atakol, A., Yilmaz, N., Anli, R.E., Atakol, O., 2014. Investigation of heavy metal concentrations in some Turkish wines. Journal of Food Composition and Analysis 33 (1), 105–110. Antoine, J.M.R., Fung, L.A.H., Grant, C.N., Dennis, H.T., Lalor, G.C., 2012. Dietary intake of minerals and trace elements in rice on the Jamaican market. Journal of Food Composition and Analysis 26 (1–2), 111–121. Araujo, R.G.O., Oleszczuk, N., Rampazzo, R.T., Costa, P.A., Silva, M.M., Vale, M.G.R., Welz, B., Ferreira, S.L.C., 2008. Comparison of direct solid sampling and slurry sampling for the determination of cadmium in wheat flour by electrothermal atomic absorption spectrometry. Talanta 77 (1), 400–406. Chahid, A., Hilali, M., Benlhachimi, A., Bouzid, T., 2014. Contents of cadmium, mercury and lead in fish from the Atlantic sea (Morocco) determined by atomic absorption spectrometry. Food Chemistry 147, 357–360. Chen, H., Guo, L., Zhang, M., Gu, J., Zhong, K., Bo, L., Li, J., 2014. Determination of lead in soybean sauces by the diffusive gradients in thin films technique. Food Chemistry 165, 9–13. de Andrade, C.K., dos Anjos, V.E., Felsner, M.L., Torres, Y.R., Quinaia, S.P., 2014. Direct determination of Cd, Pb and Cr in honey by slurry sampling electrothermal atomic absorption spectrometry. Food Chemistry 146, 166–173. Gimou, M.-M., Charrondiere, U.R., Leblanc, J.-C., Pouillot, R., Noel, L., Guerin, T., 2014. Concentration data for 25 elements in foodstuffs in Yaounde: The Cameroonian Total Diet Study. Journal of Food Composition and Analysis 34 (1), 39–55. Gunduz, S., Akman, S., 2013. Determination of lead in rice grains by solid sampling HR-CS GFAAS. Food Chemistry 141 (3), 2634–2638. Guo, W., Zhang, P., Jin, L.L., Hu, S.H., 2014. Rice cadmium monitoring using heatextraction electrothermal atomic absorption spectrometry. Journal of Analytical Atomic Spectrometry 29 (10), 1949–1954. Itkonen, S.T., Ekholm, P.J., Kemi, V.E., Lamberg-Allardt, C.J.E., 2012. Analysis of in vitro digestible phosphorus content in selected processed rye, wheat and barley products. Journal of Food Composition and Analysis 25 (2), 185–189. Julshamn, K., Maage, A., Norli, H.S., Grobecker, K.H., Jorhem, L., Fecher, P., Dowell, D., 2013. Determination of arsenic, cadmium, mercury, and lead in foods by pressure digestion and inductively coupled plasma/mass spectrometry: first action 2013.06. Journal of AOAC International 96 (5), 1101–1102. Koyyalamudi, S.R., Jeong, S.-C., Manavalan, S., Vysetti, B., Pang, G., 2013. Micronutrient mineral content of the fruiting bodies of Australian cultivated Agaricus bisporus white button mushrooms. Journal of Food Composition and Analysis 31 (1), 109–114. Lemaire, R., Del Bianco, D., Garnier, L., Beltramo, J.L., 2013. Determination of lead in lipstick by direct solid sampling high-resolution continuum source graphite furnace atomic absorption spectrometry: comparison of two digestion methods. Analytical Letters 46 (14), 2265–2278. Lima, E.C., Barbosa, F., Krug, F.J., 1999. Tungsten–rhodium permanent chemical modifier for lead determination in sediment slurries by electrothermal atomic
Y. Peng et al. / Journal of Food Composition and Analysis 42 (2015) 78–83 absorption spectrometry. Journal of Analytical Atomic Spectrometry 14 (12), 1913–1918. Lima, E.C., Barbosa, F., Krug, F.J., Silva, M.M., Vale, M.G.R., 2000. Comparison of ultrasound-assisted extraction, slurry sampling and microwave-assisted digestion for cadmium, copper and lead determination in biological and sediment samples by electrothermal atomic absorption spectrometry. Journal of Analytical Atomic Spectrometry 15 (8), 995–1000. Maduro, C., Vale, G., Alves, S., Galesio, M., da Silva, M.D.R.G., Fernandez, C., Catarino, S., Rivas, M.G., Mota, A.M., Capelo, J.L., 2006. Determination of Cd and Pb in biological reference materials by electrothermal atomic absorption spectrometry: a comparison of three ultrasonic-based sample treatment procedures. Talanta 68 (4), 1156–1161. Meeravali, N.N., Kumar, S.J., 1998. Rapid slurry atomization using transverse heated electrothermal atomic absorption spectrometry for the determination of cadmium, copper, manganese and lead in biological reference materials. Journal of Analytical Atomic Spectrometry 13 (7), 647–652. Millerihli, N.J., 1993. Advances in ultrasonic slurry graphite-furnace atomic-absorption spectrometry. Fresenius Journal of Analytical Chemistry 345 (7), 482–489. Millerihli, N.J., 1994. Influence of slurry preparation on the accuracy of ultrasonic slurry electrothermal atomic-absorption spectrometry. Journal of Analytical Atomic Spectrometry 9 (10), 1129–1134. Millour, S., Noel, L., Chekri, R., Vastel, C., Kadar, A., Sirot, V., Leblanc, J.-C., Guerin, T., 2012. Strontium, silver, tin, iron, tellurium, gallium, germanium, barium and vanadium levels in foodstuffs from the Second French Total Diet Study. Journal of Food Composition and Analysis 25 (2), 108–129. Millour, S., Noel, L., Kadar, A., Chekri, R., Vastel, C., Guerin, T., 2011. Simultaneous analysis of 21 elements in foodstuffs by ICP-MS after closed-vessel microwave digestion: method validation. Journal of Food Composition and Analysis 24 (1), 111–120. Mol, S., 2011. Levels of selected trace metals in canned tuna fish produced in Turkey. Journal of Food Composition and Analysis 24 (1), 66–69. Park, K., Min, H., Yim, Y.-H., Yim, Y.-L., Hwang, E.-J., Cho, K., 2011. Instrumental neutron activation analysis (INAA) and isotope dilution-inductively coupled plasma mass spectrometry (ID-ICP/MS) for certification of multielements in a tuna fish candidate certified reference material. Journal of Food Composition and Analysis 24 (7), 1064–1068. Potorti, A.G., Di Bella, G., Lo Turco, V., Rando, R., Dugo, G., 2013. Non-toxic and potentially toxic elements in Italian donkey milk by ICP-MS and multivariate analysis. Journal of Food Composition and Analysis 31 (1), 161–172.
83
Rego, J.F., Virgilio, A., Nobrega, J.A., Neto, J.A.G., 2012. Determination of lead in medicinal plants by high-resolution continuum source graphite furnace atomic absorption spectrometry using direct solid sampling. Talanta 100, 21–26. Rodushkin, I., Baxter, D.C., Engstrom, E., Hoogewerff, J., Horn, P., Papesch, W., Watling, J., Latkoczy, C., van der Peijl, G., Berends-Montero, S., Ehleringer, J., Zdanowicz, V., 2011. Elemental and isotopic characterization of cane and beet sugars. Journal of Food Composition and Analysis 24 (1), 70–78. Sanchez-Moreno, R.A., Gismera, M.J., Sevilla, M.T., Procopio, J.R., 2010. Direct and rapid determination of ultratrace heavy metals in solid plant materials by ET-AAS ultrasonic-assisted slurry sampling. Phytochemical Analysis 21 (4), 340–347. Santos, D., Barbosa, F., Tomazelli, A.C., Krug, F.J., Nobrega, J.A., Arruda, M.A.Z., 2002. Determination of Cd and Pb in food slurries by GRAS using cryogenic grinding for sample preparation. Analytical and Bioanalytical Chemistry 373 (3), 183–189. Sardans, J., Montes, F., Penuelas, J., 2010. Determination of As, Cd, Cu, Hg and Pb in biological samples by modern electrothermal atomic absorption spectrometry. Spectrochimica Acta Part B-Atomic Spectroscopy 65 (2), 97–112. Scancar, J., Zuliani, T., Milacic, R., 2013. Study of nickel content in Ni-rich food products in Slovenia. Journal of Food Composition and Analysis 32 (1), 83–89. Skrbic, B., Zivancev, J., Mrmos, N., 2013. Concentrations of arsenic, cadmium and lead in selected foodstuffs from Serbian market basket: estimated intake by the population from the Serbia. Food and Chemical Toxicology 58, 440–448. Soares, A.R., Nascentes, C.C., 2013. Development of a simple method for the determination of lead in lipstick using alkaline solubilization and graphite furnace atomic absorption spectrometry. Talanta 105, 272–277. Tormen, L., Torres, D.P., Dittert, I.M., Araujo, R.G.O., Frescura, V.L.A., Curtius, A.J., 2011. Rapid assessment of metal contamination in commercial fruit juices by inductively coupled mass spectrometry after a simple dilution. Journal of Food Composition and Analysis 24 (1), 95–102. Torok, P., Zemberyova, M., 2012. Comparison of chemical modifiers for direct determination of Cd, Cu and Zn in food stuffs by solid-sampling-ETAAS. Food Chemistry 132 (1), 554–560. Vassileva, E., Wysocka, I., Betti, M., 2014. Reference measurements for cadmium, copper, mercury, lead, zinc and methyl mercury mass fractions in scallop sample by isotope dilution inductively coupled plasma mass spectrometry. Microchemical Journal 116, 197–205. Vinas, P., Pardo-Martinez, M., Hernandez-Cordoba, M., 2000. Rapid determination of selenium, lead and cadmium in baby food samples using electrothermal atomic absorption spectrometry and slurry atomization. Analytica Chimica Acta 412 (1-2), 121–130.