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Determination of As, Cd, Hg and Pb in herbs using slurry sampling electrothermal vaporisation inductively coupled plasma mass spectrometry
Food Chemistry 141 (2013) 2158–2162
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Food Chemistry journal homepage: www.elsevier.com/locate/foodchem
Analytical Methods
Determination of As, Cd, Hg and Pb in herbs using slurry sampling electrothermal vaporisation inductively coupled plasma mass spectrometry Mei-Ling Lin a, Shiuh-Jen Jiang a,b,⇑ a b
Department of Chemistry, National Sun Yat-sen University, Kaohsiung 80424, Taiwan Department of Medical Laboratory Science and Biotechnology, Kaohsiung Medical University, Kaohsiung 80708, Taiwan
a r t i c l e
i n f o
Article history: Received 9 March 2012 Received in revised form 2 July 2012 Accepted 23 April 2013 Available online 10 May 2013 Keywords: Inductively coupled plasma mass spectrometry coupled Electrothermal vaporisation Ultrasonic slurry sampling Herbs As, Cd, Hg and Pb
a b s t r a c t Inductively coupled plasma mass spectrometry coupled with ultrasonic slurry sampling electrothermal vaporisation (USS-ETV-ICP-MS) has been applied to determine As, Cd, Hg and Pb in 0.5% m/v slurries of several herb samples. 1% m/v 8-Hydroxyquinoline was used as the modifier to enhance the ion signals. The influences of instrument operating conditions, slurry preparation and interferences on the ion signals were reported. This method has been applied to the determination of As, Cd, Hg and Pb in NIST SRM 1547 peach leaves and SRM 1573a tomato leaves reference materials and three herb samples purchased from the local market and ground to 150 lm. The analysis results of the standard reference materials agreed with the certified values which are at sub lg g 1 levels. Precision between sample replicates was better than 4% for all the determinations. The method detection limits estimated from standard addition curves were about 0.3, 0.1, 0.1 and 0.2 ng g 1 for As, Cd, Hg and Pb, respectively, in original herb samples. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Herbs are an important source of diets and/or medicines for human beings. However, environmental pollution and the contamination during processing can cause the herbs to contain toxic elements and then to produce detrimental effects on human health. Thus, the determination of trace elements in herbs in order to measure the levels of toxic elements is important. Moreover, some of the metals are the subject of food legislation. According to the regulations of China and Hong Kong governments, the maximum allowable concentration of As, Cd, Hg and Pb in herbs is 2, 0.3, 0.2 and 5 lg g 1, respectively. Several methods based on atomic absorption spectrometry (AAS) (Huang et al., 2006; Chen, Wang, & Lee, 2001), inductively coupled plasma optical emission spectrometry (ICP-OES) (Chen et al., 2002; Liu, Ke, Ye, & Ding, 2007) and inductively coupled plasma mass spectrometry (ICP-MS) (Chen, Zhuang et al., 2001; Chen et al., 2002; Kou, Xu, & Gu, 2007; Martena et al., 2010; Wang, Feng, & Wang, 2010) have been successfully applied to the determination of trace elements in herb samples. Most of the analyses need tedious sample dissolution and pretreatment step. Furthermore
⇑ Corresponding author at: Department of Chemistry, National Sun Yat-sen University, Kaohsiung 80424, Taiwan. Fax: +886 7 5253908. E-mail address: [email protected] (S.-J. Jiang). 0308-8146/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodchem.2013.04.105
the volatile species might be lost during sample pretreatment. In this study, ultrasonic slurry sampling (USS) electrothermal vaporisation (ETV) ICP-MS is proposed as an alternative method for the simultaneous determination of As, Cd, Hg and Pb in herb samples. Electrothermal vaporisation is one of the sample introduction techniques, which are currently employed in ICP-MS (Aramendia, Resano, & Vanhaecke, 2009; Maia, Pozebon, & Curtius, 2003; Maia, Vale, Welz, & Curtius, 2001; Miller-Ihli & Baker, 2001; Peschel, Herdering, & Broekaert, 2007; Resano, Aramendia, Devos, & Vanhaecke, 2006; Zhang, Jiang, He, & Hu, 2007). This alternative technique to solution nebulization presents several advantages such as improved sensitivity, small sample size, reduced sample preparation time, contamination and analyte loss before analysis. But perhaps the most notable benefit of ETV-ICP-MS is the possibility to perform solids analysis (Aramendia et al., 2009; Maia et al., 2001, 2003; Miller-Ihli & Baker, 2001; Peschel et al., 2007; Zhang et al., 2007). Ultrasonic slurry sampling is one of the methods for solid sample introduction, which has been successfully used in electrothermal atomic absorption spectrometry (ETAAS) (Dias et al., 2005; Lopez-Garcia, Rivas, & Hernandez-Cordoba, 2008; Munoz & Aller, 2006). This approach has been extended to ETV-ICP-MS (Aramendia et al., 2009; Peschel et al., 2007; Zhang et al., 2007). Furthermore, ETV sample introduction technique has been employed to eliminate various spectral interferences in ICP-MS analysis (Li, Jiang, & Chen, 1998; Liao & Jiang, 1999; Liaw & Jiang, 1996).
M.-L. Lin, S.-J. Jiang / Food Chemistry 141 (2013) 2158–2162
In the present work, an USS-ETV-ICP-MS instrument and technique was employed for the determination of As, Cd, Hg and Pb in herb samples. The optimisation of the USS-ETV-ICP-MS technique and its analytical figures of merit, as well as its application to the determination of As, Cd, Hg and Pb in selected herb samples, are described in this paper. 2. Materials and methods 2.1. Apparatus and conditions A Perkin-Elmer Sciex (Concord, ON, Canada) ELAN 6100 DRCII ICP-MS spectrometer equipped with a HGA-600MS electrothermal vaporizer was used. Pyrolytic coated graphite tubes with same material platforms were used throughout. The transfer line consisted of an 80-cm-long, 6-mm-i.d. PTFE tubing. The sample introduction system included a Model AS-60 autosampler equipped with an USS-100 ultrasonic slurry sampler. Polystyrene autosampler cups were used. The USS-100 was set at 25 W (40% power), and a 5-s mix time was used to mix slurries before injection of 20-lL sample aliquots for analysis. The ICP conditions were selected to maximise As, Cd, Hg and Pb ion signals while a solution containing 1 ng mL 1 of these elements in 1% HNO3 was continuously introduced with an U-5000 AT+ ultrasonic nebulizer (CETAC, Omaha, NB, USA). Since an ETV sampling device was used and the signal obtained by ICP-MS was a transient signal, 10 ms dwell time was used in the following experiments. Peak area of the transient signal was used for data handling. The ICP-MS and ETV operating conditions used throughout this work are summarised in Table 1 and Table 2, respectively. 2.2. Reagents Trace metal grade HNO3 (70% m/m) was obtained from Fisher (Fair Lawn, NJ, USA). 8-Hydroxyquinoline was obtained from JANSSEN CHIMICA (Geel, Belgium). Disodium ethylenediamine
Table 1 Equipment and operating conditions. ICP mass spectrometer Outer gas flow rate/L min 1 Auxiliary gas flow rate/L min Carrier gas flow rate/L min 1 RF power/kW Sampler/skimmer Data acquisition Dwell time/ms Scan mode Sweeps per reading Readings per replicate Signal measurement mode Isotopes monitored
Perkin-Elmer Sciex ELAN 6100 DRCII 14.2 1.33 0.98 1.20 Platinum
1
10 Peak-hopping 2 260 Integrated 75 As, m/z 77, 111Cd,
202
Hg,
206
Pb
Table 2 HGA-600MS temperature programming.a
a
Program step
Temp (°C)
Ramp (s)
Hold (s)
Gas flow rate (mL min 1)
Read
Dry 1 Dry 2 Pyrolysis Vaporisation Cooling Condition Cooling
90 120 180 1900 20 2700 20
5 5 5 0 5 1 5
5 5 10 8 2 5 5
300 300 300 – 300 300 300
– – – ON – – –
Slurry volume: 10 lL.
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tetraacetic acid (EDTA) was from Fisher. Ammonium pyrrolidinecarbodithioate (APDC) was from Sigma–Aldrich (Buchs, Switzerland). 8-Hydroxyquinoline-5-sulfonic acid was from Koch-Light Limited (Haverhill, Suffolk, England). Thioacetamide (TAC) was from TCI (Tokyo, Japan). L-cysteine was from Fluka (Buchs, Switzerland). Ascorbic acid and Pd element standard solution were from Merck (Darmstadt, Germany). Triton X-100 was obtained from Sigma Chemicals (St. Louis, MO, USA). Element standard solutions were obtained from Spex (Edison, NJ, USA). 8-Hydroxyquinoline stock solution was passed through the Chelex-100 column to remove the trace metals in the solution (Pai & Fang, 1990). 2.3. Slurries preparation The applicability of the method to real samples was demonstrated by the analysis of SRM 1547 peach leaves and SRM 1573a tomato leaves reference materials (National Institute of Standards and Technology, USA). The herb powders namely Angelica sinensis, Astragalus membranaceus, Glycyrrhiza uralensis were purchased from the local market. The purchased herb powder samples were sieved by a Retsch VE1000 sieving machine (Retsch, Haan, Germany). The powders with particle size less than 150 lm were collected for following experiments. Various herb samples were analysed by standard addition method. The slurry was prepared as the following procedure. A 0.5-g portion of the powder material was transferred into a 10 mL standard flask and diluted to mark with pure water. 1-mL aliquots of the stock slurry were transferred to 10 mL standard flasks. Suitable amount of 8-Hydroxyquinoline was added to make the final solution containing 0.5% m/v herb powder and 1% m/v 8Hydroxyquinoline. After various amounts of As, Cd, Hg and Pb element standard solutions were added; these slurries were diluted to the mark with pure water. The slurry was then sonicated for 10 min in an ultrasonic bath, and 1-mL aliquots were removed as needed for analysis. A blank containing 1% m/v 8-Hydroxyquinoline was carried through the procedure to correct any analyte in the reagent used for sample preparation. The analyte concentration in the sample was then calculated from the standard addition calibration curves. For the studies such as the effect of ETV conditions and slurry preparations on the ion signals, an A. membranaceus slurry sample was prepared as the procedure described above. Since there is no reference value for the real-world samples, in order to check the accuracy of the ETV-ICP-MS method, we need to compare the total analyte concentrations in the samples using the present procedure and complete dissolution method. Hence the samples were digested completely using the procedure mentioned below for the determination of the elements studied. The real-world samples (0.2 g) were weighed into closed Teflon PFA vessels and 3 mL HNO3 and 1 mL HF were added. The mixtures were heated inside a CEM MARS 5 (CEM, Matthews, NC, USA) microwave digester to decompose the sample. The microwave digester was operated at a power 600 W (80%) and a pressure of 100 psi; the ramp time and the hold time were 20 min and 15 min, respectively. And then the solution was heated again at 175 psi while the ramp time and the hold time were set at 10 min and 15 min, respectively. The stock solutions were diluted to the appropriate volume followed by introducing into the ICP-MS for As, Cd, Hg and Pb determinations. 3. Results and discussion 3.1. Selection of modifier In this study, several modifiers, including Pd, 8-Hydroxyquinoline, ascorbic acid, EDTA, thioacetamide, APDC, 8-hydroxyquinoline-5-sulfonic acid and L-cysteine, were tested to obtain best
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signals of the elements studied. The use of modifier would change the chemical or physical characteristics of the sample and/or the atomizer surface in order to improve sensitivity. From the experiment it was found that the signals of Cd, Hg and Pb increased significantly when 1% m/v 8-Hydroxyquinoline was tested as the modifier. For the simultaneous determination of these elements, after evaluation, 8-Hydroxyquinoline was selected as the modifier in this study. 8-Hydroxyquinoline has been used as the chemical modifier to improve the signals of certain elements in previous ETV-ICP-OES applications (Wu, Hu, Jiang, & Chen, 2002). Fig. 1 shows the effect of the amount of 8-Hydroxyquinoline on the ion signals. The pyrolysis temperature was set at 200 °C and the vaporisation temperature was set at 1900 °C in this experiment (Huang & Jiang, 2010). As shown, the signal of Hg increased with the increase of 8-Hydroxyquinoline concentration. The signals of Cd and Pb increased slightly with the increase of 8Hydroxyquinoline concentration when 8-Hydroxyquinoline concentration was less than 1% m/v. After evaluation, 1% m/v of 8-Hydroxyquinoline was selected in the following ETV-ICP-MS analysis. Effects of the addition of HNO3, H2O2 and Triton X-100 in the slurry sample on the ion signals were also studied. From the experiments, we found that the signals of the elements studied decreased significantly when various amounts of HNO3, H2O2 and surfactant were added. In order to simplify slurry preparation and to reduce the reagent blank, no other reagents were added in the slurry preparation. Another important factor in the slurry technique is the slurry concentration. However, dilution of the slurry can only be carried out within a limited range. The effect of slurry concentration on the ion signals is shown in Table 3. As shown the sensitivity (counts/mass of sample) of Cd, Hg and Pb decreased with the decrease of dilution factor when slurry concentration was larger than 0.5% m/v. In order to balance sample homogeneity, analyte signals and the complete vaporisation of introduced sample, a dilution factor of 200 (0.5% m/v) was used in the following experiments.
3.2. Selection of pyrolysis and vaporisation temperatures The effect of pyrolysis temperature on ion signals was studied. From the experiment it was found that the ion signals of As and Hg decreased with the increase of pyrolysis temperature when the temperature was larger than 200 °C. This means that a considerable amount of As and Hg volatilised before the vaporisation stage. For the simultaneous determination of As, Cd, Hg and Pb, in the following experiments, the pyrolysis temperature was set at 180 °C.
Relative signal
6 5 4
75
As Cd 202 Hg 206 Pb 111
3
Table 3 Effect of dilution factor on ion signalsa (n = 5). Dilution factor
400 (0.25% m/v) 200 (0.5% m/v) 100 (1% m/v) 67 (1.5% m/v) 50 (2% m/v)
Conc of 8-Hydroxyquinoline / % m/v Fig. 1. Effect of 8-Hydroxyquinoline concentration on the ion signals. Slurry solution contained 0.5% m/v Astragalus membranaceus powders and spiked with various amounts of 8-Hydroxyquinoline. Each data point represents the mean of five measurements. All data were relative to the first point.
1.00 ± 0.06 2.30 ± 0.07 4.10 ± 0.07 5.86 ± 0.15 6.08 ± 0.09
1.00 ± 0.04 2.01 ± 0.06 2.75 ± 0.05 3.37 ± 0.19 3.43 ± 0.09
Cd
202
206
1.00 ± 0.03 1.76 ± 0.03 1.92 ± 0.05 2.12 ± 0.07 3.72 ± 0.19
1.00 ± 0.05 1.97 ± 0.09 2.62 ± 0.05 3.18 ± 0.13 3.18 ± 0.03
Hg
Pb
a Values are means of five measurements ± standard deviation. Astragalus membranaceus slurry solution contained various amounts of Astragalus membranaceus in 1.0% m/v 8-Hydroxyquinoline. All data were relative to 0.25% m/v.
The effect of the vaporisation temperature on the ion signals was also studied. From the experiments we found that the ion signals increased slightly with the increase of the vaporisation temperature when the temperature was less than 1900 °C. In the following experiments, 1900 °C was selected as the vaporisation temperature. The summary of the ETV operating conditions is listed in Table 2.
3.3. Spectral interferences study For ICP-MS analysis, the components of the sample could produce several polyatomic ions which interfere in the determination of various elements. The determination of As and Cd was interfered by various background ions. For instance, 75As+ (100%) is interfered by 40Ar35Cl+ and the isotopes of cadmium are interfered by various molybdenum oxide ions. In order to evaluate the significance of these interferences, the following experiments were performed to check the interferences caused by Cl and Mo in the herb samples. An A. membranaceus slurry was prepared as the method described previously. And then the prepared slurry solution was spiked with 0, 10, 100 lg mL 1 of Cl and 0, 0.05, 0.1, 0.25 lg mL 1 of Mo. These slurries were injected for the determination of the ion signals at m/z 75 and 77 and the 111Cd/113Cd isotope ratio using ETV-ICP-MS. The isotope ratio was calculated from the peak areas of each injection peak. The spiked concentrations were much higher than the concentration of matrix elements in the prepared slurry solution. From the experiments we found that the ion signal of As was not affected by the additional Cl and the isotope ratio of 111 Cd/113Cd was not affected by Mo at these concentrations in accordance with the Student’s t-test for a confidence level of 95%. The alleviation of the interferences could be due to the introduction of much more dry aerosol and the elimination of chloride during the pyrolysis step with ETV sample introduction device (Liaw, & Jiang, 1996; Liao & Jiang, 1999; Li et al., 1998). These experiments demonstrated that the concentrations of As and Cd in the herb samples can be determined directly by ETV-ICP-MS without significant interferences.
1
lg L 1) of external calibration and standard addition calibration.
Analyte
External calibration slope
Standard addition (Astragalus membranaceus) slope
Standard addition (Glycyrrhiza uralensis) slope
75
22,100 30,200 109,000 37,200
336,000 70,300 343,000 95,200
289,000 ( 14%)a 74,900 (7%) 330,000 ( 4%) 103,000 (8%)
1 0.00 0.25 0.50 0.75 1.00 1.25 1.50
111
As
Table 4 Slopes (counts s
2
Relative signal 75
As Cd 202 Hg 206 Pb
111
a The value shown in parenthesis is the deviation of the slope between Astragalus membranaceus slurry and Glycyrrhiza uralensis slurry.
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M.-L. Lin, S.-J. Jiang / Food Chemistry 141 (2013) 2158–2162 Table 5 Determination of As, Cd, Hg and Pb by USS-ETV-ICP-MSa (n = 3).
a b c
Sample
Methodb
As
Cd
Hg
Pb
SRM 1547 Peach leaves
Method 1 Method 2 Certified
0.0631 ± 0.0016 0.126 ± 0.006 0.060 ± 0.018
0.0268 ± 0.0006 0.0257 ± 0.0004 0.026 ± 0.003
0.0315 ± 0.0014 0.0347 ± 0.0015 0.031 ± 0.007
0.868 ± 0.015 0.849 ± 0.011 0.87 ± 0.03
SRM 1573a Tomato leaves
Method 1 Method 2 Certified
0.111 ± 0.002 0.115 ± 0.004 0.112 ± 0.004
1.52 ± 0.02 1.53 ± 0.02 1.52 ± 0.04
0.0349 ± 0.0016 0.0370 ± 0.0018 0.034 ± 0.004
0.599 ± 0.010 0.600 ± 0.015 nac
Astragalus membranaceus
Method 1 Method 2
0.177 ± 0.004 0.180 ± 0.002
0.104 ± 0.003 0.100 ± 0.001
0.0101 ± 0.0004 0.0107 ± 0.0003
0.448 ± 0.008 0.440 ± 0.012
Glycyrrhiza uralensis
Method 1 Method 2
0.184 ± 0.004 0.185 ± 0.003
0.0146 ± 0.0003 0.0142 ± 0.0002
0.0199 ± 0.0007 0.0205 ± 0.0007
0.332 ± 0.008 0.335 ± 0.004
Angelica sinensis
Method 1 Method 2
0.399 ± 0.007 0.397 ± 0.007
0.0145 ± 0.0002 0.0144 ± 0.0002
0.241 ± 0.005 0.234 ± 0.007
4.04 ± 0.09 4.13 ± 0.18
Concentration/lg g
1
Results are means of three measurements ± standard deviation. Method 1: standard addition method. Method 2: determined by pneumatic nebulization ICP-MS after sample dissolution. Certified: NIST certified value. na = not available.
3.4. Determination of As, Cd, Hg and Pb in herbs In order to validate the USS-ETV-ICP-MS method, the concentrations of As, Cd, Hg and Pb were determined in NIST SRM 1547 peach leaves and SRM 1573a tomato leaves reference samples and three herb samples purchased locally. In order to evaluate the possibility of using external calibration method for the determination of trace elements in herbs by USS-ETV-ICP-MS, calibration curves obtained by standard addition method of 0.5% m/v herb slurry solution and external calibration of aqueous standard were compared. The results are listed in Table 4. As shown, the sensitivities of the elements studied were different in aqueous solution and in herb slurry though the correlation coefficients (r2) were >0.9992. Therefore, external calibration method could not be used for the quantification of these elements in the herb samples. It is interesting to see that except for arsenic, the slopes of the elements studied were quite similar between the A. membranaceus slurry and the G. uralensis slurry. To obtain accurate quantification results, the standard addition method was used for the determination of As, Cd, Hg and Pb in these samples. Analysis results are shown in Table 5. As shown the determined concentrations in the leaves reference samples were in good agreement with the certified values. The pneumatic nebulization ICP-MS result of arsenic in peach leaves reference sample was higher than the certified value. It could be due to the spectral interference caused by ArCl+. However the ETV-ICP-MS result was in good agreement with the certified value. As shown in Table 5, the results for the samples for which no reference values were available were also found to be in good agreement with those of digested samples analyzed by pneumatic nebulization ICP-MS in accordance with the Student’s t-test for a confidence level of 95%. The content of Hg and Pb in A. sinensis was much higher than the other samples analysed. However, the Hg content in A. sinensis exceeded the maximum allowable concentration. The high concentration of Hg and Pb in this sample could be due to the environmental pollution and/or the contamination problem during the processing. This experiment indicated that As, Cd, Hg and Pb in herbs could be readily quantified by the proposed USS-ETV-ICP-MS method. Detection limits of As, Cd, Hg and Pb were determined from the standard addition calibration curves. It was based on the usual definition as the concentration of the analyte yielding a signal equivalent to three times the standard deviation of the reagent blank
signal (n = 7). Detection limits estimated from standard addition curves were about 0.3, 0.1, 0.1 and 0.2 ng g 1 for As, Cd, Hg and Pb, respectively, in original herbs. In conclusion, the utility of 8-Hydroxyquinoline as modifier for USS-ETV-ICP-MS has been reported. The advantage of passing dry aerosol generated by ETV in alleviating the isobaric interferences on the determination of As and Cd by ICP-MS has been demonstrated. The analysis results agreed with the reference values. Precision between sample replicates was better than 4% for all the determinations. The procedure can be used as an alternative to the existing procedures for the analysis of herbs.
Acknowledgements This research was supported by a grant from the National Science Council of the Republic of China under Contract NSC 972113-M-110-008-MY3 and NSC 100-2627-M-110-003-.
References Aramendia, M., Resano, M., & Vanhaecke, F. (2009). Determination of toxic trace impurities in titanium dioxide by solid sampling-electrothermal vaporizationinductively coupled plasma mass spectrometry. Journal of Analytical Atomic Spectrometry, 24, 41–50. Chen, F., Liang, P., Hu, B., Zhao, L., Sun, D. A., & Wang, Y. R. (2002). The application of inductively coupled plasma atomic emission spectrometry/mass spectrometry in the trace elements and speciation analysis of traditional Chinese medicine. Spectroscopy and Spectral Analysis, 22, 1019–1024. Chen, B., Wang, X. R., & Lee, F. S. C. (2001). Pyrolysis coupled with atomic absorption spectrometry for the determination of mercury in Chinese medicinal materials. Analytica Chimica Acta, 447, 161–169. Chen, B., Zhuang, Z. X., Wang, X. R., & Lee, F. S. C. (2001). A novel sample introduction technique for the simultaneous determination of As, Se, Ge and Hg in Chinese medicinal material. Chemical Research in Chinese Universities, 17, 400–406. Dias, L. F., Miranda, G. R., Saint’Pierre, T. D., Maia, S. M., Frescura, V. L. A., & Curtius, A. J. (2005). Method development for the determination of cadmium, copper, lead, selenium and thallium in sediments by slurry sampling electrothermal vaporization inductively coupled plasma mass spectrometry and isotopic dilution calibration. Spectrochimica Acta, Part B, 60, 117–124. Huang, S.-Y., & Jiang, S.-J. (2010). 8-Hydroxyquinoline-5-sulfonic acid as the modifier for the determination of trace elements in cereals by slurry sampling electrothermal vaporization ICP-MS. Analytical Methods, 2, 1310–1315. Huang, R. J., Zhuang, Z. X., Tai, Y., Huang, R. F., Wang, X. R., & Lee, F. S. C. (2006). Direct analysis of mercury in traditional Chinese medicines using thermolysis coupled with on-line atomic absorption spectrometry. Talanta, 68, 728–734.
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M.-L. Lin, S.-J. Jiang / Food Chemistry 141 (2013) 2158–2162
Kou, X. M., Xu, M., & Gu, Y. Z. (2007). Determination of trace heavy metal elements in cortex phellodendri chinensis by ICP-MS after microwave-assisted digestion. Spectroscopy and Spectral Analysis, 27, 1197–1200. Li, Y.-C., Jiang, S.-J., & Chen, S.-F. (1998). Determination of Ge, As, Se, Cd and Pb in plant materials by slurry sampling–electrothermal vaporization–inductively coupled plasma-mass spectrometry. Analytica Chimica Acta, 372, 365–372. Liao, H.-C., & Jiang, S.-J. (1999). EDTA as the modifier for the determination of Cd, Hg and Pb in fish by slurry sampling electrothermal vaporization inductively coupled plasma mass spectrometry. Journal of Analytical Atomic Spectrometry, 14, 1583–1588. Liaw, M.-J., & Jiang, S.-J. (1996). Determination of copper, cadmium and lead in sediment samples by slurry sampling electrothermal vaporization inductively coupled plasma mass spectrometry. Journal of Analytical Atomic Spectrometry, 11, 555–560. Liu, D. L., Ke, S. Y., Ye, R., & Ding, M. Y. (2007). Determination of trace lead in traditional Chinese herbal medicine astragalus by microwave digestion-CTAB enhancing-continual flow injection hydride generation-ICP-AES. Spectroscopy and Spectral Analysis, 27, 2337–2340. Lopez-Garcia, I., Rivas, R. E., & Hernandez-Cordoba, M. (2008). Use of sodium tungstate as a permanent chemical modifier for slurry sampling electrothermal atomic absorption spectrometric determination of indium in soils. Analytical and Bioanalytical Chemistry, 391, 1469–1474. Maia, S. M., Pozebon, D., & Curtius, A. J. (2003). Determination of Cd, Hg, Pb and Tl in coal and coal fly ash slurries using electrothermal vaporization inductively coupled plasma mass spectrometry and isotopic dilution. Journal of Analytical Atomic Spectrometry, 18, 330–337. Maia, S. M., Vale, M. G. R., Welz, B., & Curtius, A. J. (2001). Feasibility of isotope dilution calibration for the determination of thallium in sediment using slurry sampling electrothermal vaporization inductively coupled plasma mass spectrometry. Spectrochimica Acta, Part B, 56, 1263–1275. Martena, M. J., Van Der Wielen, J. C. A., Rietjens, I., Klerx, W. N. M., De Groot, H. N., et al. (2010). Monitoring of mercury, arsenic, and lead in traditional Asian herbal preparations on the Dutch market and estimation of associated risks.
Food Additives & Contaminants: Part A Chemistry, Analysis, Control, Exposure & Risk Assessment, 27, 190–205. Miller-Ihli, N. J., & Baker, S. A. (2001). Microhomogeneity assessments using ultrasonic slurry sampling coupled with electrothermal vaporization isotope dilution inductively coupled plasma mass spectrometry. Spectrochimica Acta, Part B, 56, 1673–1686. Munoz, M. L., & Aller, A. J. (2006). Determination of Cd, Hg, Pb and Tl in coal and coal fly ash slurries using electrothermal vaporization inductively coupled plasma mass spectrometry and isotopic dilution. Journal of Analytical Atomic Spectrometry, 21, 329–337. Pai, S.-C., & Fang, T.-H. (1990). A low contamination Chelex-100 technique for shipboard preconcentration of heavy-metals in seawater. Marine Chemistry, 29, 295–306. Peschel, B. U., Herdering, W., & Broekaert, J. A. C. (2007). A radiotracer study on the volatilization and transport effects of thermochemical reagents used in the analysis of alumina powders by slurry electrothermal vaporization inductively coupled plasma mass spectrometry. Spectrochimica Acta, Part B, 62, 109–115. Resano, M., Aramendia, M., Devos, W., & Vanhaecke, F. (2006). Direct multi-element analysis of a fluorocarbon polymer via solid sampling-electrothermal vaporization-inductively coupled plasma mass spectrometry. Journal of Analytical Atomic Spectrometry, 21, 891–898. Wang, Y., Feng, F., & Wang, Z. (2010). Determination of selected elements in aqueous extractions of a traditional Chinese medicine formula by ICP-MS and FAAS: Evaluation of formula rationality. Analytical Letters, 43, 983–992. Wu, Y. L., Hu, B., Jiang, Z. C., & Chen, S. Z. (2002). Low temperature vaporization for ICP-AES determination of palladium in geological samples using sample introduction of gaseous palladium oxinate. Journal of Analytical Atomic Spectrometry, 17, 121–124. Zhang, Y. F., Jiang, Z. C., He, M., & Hu, B. (2007). Determination of trace rare earth elements in coal fly ash and atmospheric particulates by electrothermal vaporization inductively coupled plasma mass spectrometry with slurry sampling. Environmental Pollution, 148, 459–467.
Contents lists available at SciVerse ScienceDirect
Food Chemistry journal homepage: www.elsevier.com/locate/foodchem
Analytical Methods
Determination of As, Cd, Hg and Pb in herbs using slurry sampling electrothermal vaporisation inductively coupled plasma mass spectrometry Mei-Ling Lin a, Shiuh-Jen Jiang a,b,⇑ a b
Department of Chemistry, National Sun Yat-sen University, Kaohsiung 80424, Taiwan Department of Medical Laboratory Science and Biotechnology, Kaohsiung Medical University, Kaohsiung 80708, Taiwan
a r t i c l e
i n f o
Article history: Received 9 March 2012 Received in revised form 2 July 2012 Accepted 23 April 2013 Available online 10 May 2013 Keywords: Inductively coupled plasma mass spectrometry coupled Electrothermal vaporisation Ultrasonic slurry sampling Herbs As, Cd, Hg and Pb
a b s t r a c t Inductively coupled plasma mass spectrometry coupled with ultrasonic slurry sampling electrothermal vaporisation (USS-ETV-ICP-MS) has been applied to determine As, Cd, Hg and Pb in 0.5% m/v slurries of several herb samples. 1% m/v 8-Hydroxyquinoline was used as the modifier to enhance the ion signals. The influences of instrument operating conditions, slurry preparation and interferences on the ion signals were reported. This method has been applied to the determination of As, Cd, Hg and Pb in NIST SRM 1547 peach leaves and SRM 1573a tomato leaves reference materials and three herb samples purchased from the local market and ground to 150 lm. The analysis results of the standard reference materials agreed with the certified values which are at sub lg g 1 levels. Precision between sample replicates was better than 4% for all the determinations. The method detection limits estimated from standard addition curves were about 0.3, 0.1, 0.1 and 0.2 ng g 1 for As, Cd, Hg and Pb, respectively, in original herb samples. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Herbs are an important source of diets and/or medicines for human beings. However, environmental pollution and the contamination during processing can cause the herbs to contain toxic elements and then to produce detrimental effects on human health. Thus, the determination of trace elements in herbs in order to measure the levels of toxic elements is important. Moreover, some of the metals are the subject of food legislation. According to the regulations of China and Hong Kong governments, the maximum allowable concentration of As, Cd, Hg and Pb in herbs is 2, 0.3, 0.2 and 5 lg g 1, respectively. Several methods based on atomic absorption spectrometry (AAS) (Huang et al., 2006; Chen, Wang, & Lee, 2001), inductively coupled plasma optical emission spectrometry (ICP-OES) (Chen et al., 2002; Liu, Ke, Ye, & Ding, 2007) and inductively coupled plasma mass spectrometry (ICP-MS) (Chen, Zhuang et al., 2001; Chen et al., 2002; Kou, Xu, & Gu, 2007; Martena et al., 2010; Wang, Feng, & Wang, 2010) have been successfully applied to the determination of trace elements in herb samples. Most of the analyses need tedious sample dissolution and pretreatment step. Furthermore
⇑ Corresponding author at: Department of Chemistry, National Sun Yat-sen University, Kaohsiung 80424, Taiwan. Fax: +886 7 5253908. E-mail address: [email protected] (S.-J. Jiang). 0308-8146/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodchem.2013.04.105
the volatile species might be lost during sample pretreatment. In this study, ultrasonic slurry sampling (USS) electrothermal vaporisation (ETV) ICP-MS is proposed as an alternative method for the simultaneous determination of As, Cd, Hg and Pb in herb samples. Electrothermal vaporisation is one of the sample introduction techniques, which are currently employed in ICP-MS (Aramendia, Resano, & Vanhaecke, 2009; Maia, Pozebon, & Curtius, 2003; Maia, Vale, Welz, & Curtius, 2001; Miller-Ihli & Baker, 2001; Peschel, Herdering, & Broekaert, 2007; Resano, Aramendia, Devos, & Vanhaecke, 2006; Zhang, Jiang, He, & Hu, 2007). This alternative technique to solution nebulization presents several advantages such as improved sensitivity, small sample size, reduced sample preparation time, contamination and analyte loss before analysis. But perhaps the most notable benefit of ETV-ICP-MS is the possibility to perform solids analysis (Aramendia et al., 2009; Maia et al., 2001, 2003; Miller-Ihli & Baker, 2001; Peschel et al., 2007; Zhang et al., 2007). Ultrasonic slurry sampling is one of the methods for solid sample introduction, which has been successfully used in electrothermal atomic absorption spectrometry (ETAAS) (Dias et al., 2005; Lopez-Garcia, Rivas, & Hernandez-Cordoba, 2008; Munoz & Aller, 2006). This approach has been extended to ETV-ICP-MS (Aramendia et al., 2009; Peschel et al., 2007; Zhang et al., 2007). Furthermore, ETV sample introduction technique has been employed to eliminate various spectral interferences in ICP-MS analysis (Li, Jiang, & Chen, 1998; Liao & Jiang, 1999; Liaw & Jiang, 1996).
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In the present work, an USS-ETV-ICP-MS instrument and technique was employed for the determination of As, Cd, Hg and Pb in herb samples. The optimisation of the USS-ETV-ICP-MS technique and its analytical figures of merit, as well as its application to the determination of As, Cd, Hg and Pb in selected herb samples, are described in this paper. 2. Materials and methods 2.1. Apparatus and conditions A Perkin-Elmer Sciex (Concord, ON, Canada) ELAN 6100 DRCII ICP-MS spectrometer equipped with a HGA-600MS electrothermal vaporizer was used. Pyrolytic coated graphite tubes with same material platforms were used throughout. The transfer line consisted of an 80-cm-long, 6-mm-i.d. PTFE tubing. The sample introduction system included a Model AS-60 autosampler equipped with an USS-100 ultrasonic slurry sampler. Polystyrene autosampler cups were used. The USS-100 was set at 25 W (40% power), and a 5-s mix time was used to mix slurries before injection of 20-lL sample aliquots for analysis. The ICP conditions were selected to maximise As, Cd, Hg and Pb ion signals while a solution containing 1 ng mL 1 of these elements in 1% HNO3 was continuously introduced with an U-5000 AT+ ultrasonic nebulizer (CETAC, Omaha, NB, USA). Since an ETV sampling device was used and the signal obtained by ICP-MS was a transient signal, 10 ms dwell time was used in the following experiments. Peak area of the transient signal was used for data handling. The ICP-MS and ETV operating conditions used throughout this work are summarised in Table 1 and Table 2, respectively. 2.2. Reagents Trace metal grade HNO3 (70% m/m) was obtained from Fisher (Fair Lawn, NJ, USA). 8-Hydroxyquinoline was obtained from JANSSEN CHIMICA (Geel, Belgium). Disodium ethylenediamine
Table 1 Equipment and operating conditions. ICP mass spectrometer Outer gas flow rate/L min 1 Auxiliary gas flow rate/L min Carrier gas flow rate/L min 1 RF power/kW Sampler/skimmer Data acquisition Dwell time/ms Scan mode Sweeps per reading Readings per replicate Signal measurement mode Isotopes monitored
Perkin-Elmer Sciex ELAN 6100 DRCII 14.2 1.33 0.98 1.20 Platinum
1
10 Peak-hopping 2 260 Integrated 75 As, m/z 77, 111Cd,
202
Hg,
206
Pb
Table 2 HGA-600MS temperature programming.a
a
Program step
Temp (°C)
Ramp (s)
Hold (s)
Gas flow rate (mL min 1)
Read
Dry 1 Dry 2 Pyrolysis Vaporisation Cooling Condition Cooling
90 120 180 1900 20 2700 20
5 5 5 0 5 1 5
5 5 10 8 2 5 5
300 300 300 – 300 300 300
– – – ON – – –
Slurry volume: 10 lL.
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tetraacetic acid (EDTA) was from Fisher. Ammonium pyrrolidinecarbodithioate (APDC) was from Sigma–Aldrich (Buchs, Switzerland). 8-Hydroxyquinoline-5-sulfonic acid was from Koch-Light Limited (Haverhill, Suffolk, England). Thioacetamide (TAC) was from TCI (Tokyo, Japan). L-cysteine was from Fluka (Buchs, Switzerland). Ascorbic acid and Pd element standard solution were from Merck (Darmstadt, Germany). Triton X-100 was obtained from Sigma Chemicals (St. Louis, MO, USA). Element standard solutions were obtained from Spex (Edison, NJ, USA). 8-Hydroxyquinoline stock solution was passed through the Chelex-100 column to remove the trace metals in the solution (Pai & Fang, 1990). 2.3. Slurries preparation The applicability of the method to real samples was demonstrated by the analysis of SRM 1547 peach leaves and SRM 1573a tomato leaves reference materials (National Institute of Standards and Technology, USA). The herb powders namely Angelica sinensis, Astragalus membranaceus, Glycyrrhiza uralensis were purchased from the local market. The purchased herb powder samples were sieved by a Retsch VE1000 sieving machine (Retsch, Haan, Germany). The powders with particle size less than 150 lm were collected for following experiments. Various herb samples were analysed by standard addition method. The slurry was prepared as the following procedure. A 0.5-g portion of the powder material was transferred into a 10 mL standard flask and diluted to mark with pure water. 1-mL aliquots of the stock slurry were transferred to 10 mL standard flasks. Suitable amount of 8-Hydroxyquinoline was added to make the final solution containing 0.5% m/v herb powder and 1% m/v 8Hydroxyquinoline. After various amounts of As, Cd, Hg and Pb element standard solutions were added; these slurries were diluted to the mark with pure water. The slurry was then sonicated for 10 min in an ultrasonic bath, and 1-mL aliquots were removed as needed for analysis. A blank containing 1% m/v 8-Hydroxyquinoline was carried through the procedure to correct any analyte in the reagent used for sample preparation. The analyte concentration in the sample was then calculated from the standard addition calibration curves. For the studies such as the effect of ETV conditions and slurry preparations on the ion signals, an A. membranaceus slurry sample was prepared as the procedure described above. Since there is no reference value for the real-world samples, in order to check the accuracy of the ETV-ICP-MS method, we need to compare the total analyte concentrations in the samples using the present procedure and complete dissolution method. Hence the samples were digested completely using the procedure mentioned below for the determination of the elements studied. The real-world samples (0.2 g) were weighed into closed Teflon PFA vessels and 3 mL HNO3 and 1 mL HF were added. The mixtures were heated inside a CEM MARS 5 (CEM, Matthews, NC, USA) microwave digester to decompose the sample. The microwave digester was operated at a power 600 W (80%) and a pressure of 100 psi; the ramp time and the hold time were 20 min and 15 min, respectively. And then the solution was heated again at 175 psi while the ramp time and the hold time were set at 10 min and 15 min, respectively. The stock solutions were diluted to the appropriate volume followed by introducing into the ICP-MS for As, Cd, Hg and Pb determinations. 3. Results and discussion 3.1. Selection of modifier In this study, several modifiers, including Pd, 8-Hydroxyquinoline, ascorbic acid, EDTA, thioacetamide, APDC, 8-hydroxyquinoline-5-sulfonic acid and L-cysteine, were tested to obtain best
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signals of the elements studied. The use of modifier would change the chemical or physical characteristics of the sample and/or the atomizer surface in order to improve sensitivity. From the experiment it was found that the signals of Cd, Hg and Pb increased significantly when 1% m/v 8-Hydroxyquinoline was tested as the modifier. For the simultaneous determination of these elements, after evaluation, 8-Hydroxyquinoline was selected as the modifier in this study. 8-Hydroxyquinoline has been used as the chemical modifier to improve the signals of certain elements in previous ETV-ICP-OES applications (Wu, Hu, Jiang, & Chen, 2002). Fig. 1 shows the effect of the amount of 8-Hydroxyquinoline on the ion signals. The pyrolysis temperature was set at 200 °C and the vaporisation temperature was set at 1900 °C in this experiment (Huang & Jiang, 2010). As shown, the signal of Hg increased with the increase of 8-Hydroxyquinoline concentration. The signals of Cd and Pb increased slightly with the increase of 8Hydroxyquinoline concentration when 8-Hydroxyquinoline concentration was less than 1% m/v. After evaluation, 1% m/v of 8-Hydroxyquinoline was selected in the following ETV-ICP-MS analysis. Effects of the addition of HNO3, H2O2 and Triton X-100 in the slurry sample on the ion signals were also studied. From the experiments, we found that the signals of the elements studied decreased significantly when various amounts of HNO3, H2O2 and surfactant were added. In order to simplify slurry preparation and to reduce the reagent blank, no other reagents were added in the slurry preparation. Another important factor in the slurry technique is the slurry concentration. However, dilution of the slurry can only be carried out within a limited range. The effect of slurry concentration on the ion signals is shown in Table 3. As shown the sensitivity (counts/mass of sample) of Cd, Hg and Pb decreased with the decrease of dilution factor when slurry concentration was larger than 0.5% m/v. In order to balance sample homogeneity, analyte signals and the complete vaporisation of introduced sample, a dilution factor of 200 (0.5% m/v) was used in the following experiments.
3.2. Selection of pyrolysis and vaporisation temperatures The effect of pyrolysis temperature on ion signals was studied. From the experiment it was found that the ion signals of As and Hg decreased with the increase of pyrolysis temperature when the temperature was larger than 200 °C. This means that a considerable amount of As and Hg volatilised before the vaporisation stage. For the simultaneous determination of As, Cd, Hg and Pb, in the following experiments, the pyrolysis temperature was set at 180 °C.
Relative signal
6 5 4
75
As Cd 202 Hg 206 Pb 111
3
Table 3 Effect of dilution factor on ion signalsa (n = 5). Dilution factor
400 (0.25% m/v) 200 (0.5% m/v) 100 (1% m/v) 67 (1.5% m/v) 50 (2% m/v)
Conc of 8-Hydroxyquinoline / % m/v Fig. 1. Effect of 8-Hydroxyquinoline concentration on the ion signals. Slurry solution contained 0.5% m/v Astragalus membranaceus powders and spiked with various amounts of 8-Hydroxyquinoline. Each data point represents the mean of five measurements. All data were relative to the first point.
1.00 ± 0.06 2.30 ± 0.07 4.10 ± 0.07 5.86 ± 0.15 6.08 ± 0.09
1.00 ± 0.04 2.01 ± 0.06 2.75 ± 0.05 3.37 ± 0.19 3.43 ± 0.09
Cd
202
206
1.00 ± 0.03 1.76 ± 0.03 1.92 ± 0.05 2.12 ± 0.07 3.72 ± 0.19
1.00 ± 0.05 1.97 ± 0.09 2.62 ± 0.05 3.18 ± 0.13 3.18 ± 0.03
Hg
Pb
a Values are means of five measurements ± standard deviation. Astragalus membranaceus slurry solution contained various amounts of Astragalus membranaceus in 1.0% m/v 8-Hydroxyquinoline. All data were relative to 0.25% m/v.
The effect of the vaporisation temperature on the ion signals was also studied. From the experiments we found that the ion signals increased slightly with the increase of the vaporisation temperature when the temperature was less than 1900 °C. In the following experiments, 1900 °C was selected as the vaporisation temperature. The summary of the ETV operating conditions is listed in Table 2.
3.3. Spectral interferences study For ICP-MS analysis, the components of the sample could produce several polyatomic ions which interfere in the determination of various elements. The determination of As and Cd was interfered by various background ions. For instance, 75As+ (100%) is interfered by 40Ar35Cl+ and the isotopes of cadmium are interfered by various molybdenum oxide ions. In order to evaluate the significance of these interferences, the following experiments were performed to check the interferences caused by Cl and Mo in the herb samples. An A. membranaceus slurry was prepared as the method described previously. And then the prepared slurry solution was spiked with 0, 10, 100 lg mL 1 of Cl and 0, 0.05, 0.1, 0.25 lg mL 1 of Mo. These slurries were injected for the determination of the ion signals at m/z 75 and 77 and the 111Cd/113Cd isotope ratio using ETV-ICP-MS. The isotope ratio was calculated from the peak areas of each injection peak. The spiked concentrations were much higher than the concentration of matrix elements in the prepared slurry solution. From the experiments we found that the ion signal of As was not affected by the additional Cl and the isotope ratio of 111 Cd/113Cd was not affected by Mo at these concentrations in accordance with the Student’s t-test for a confidence level of 95%. The alleviation of the interferences could be due to the introduction of much more dry aerosol and the elimination of chloride during the pyrolysis step with ETV sample introduction device (Liaw, & Jiang, 1996; Liao & Jiang, 1999; Li et al., 1998). These experiments demonstrated that the concentrations of As and Cd in the herb samples can be determined directly by ETV-ICP-MS without significant interferences.
1
lg L 1) of external calibration and standard addition calibration.
Analyte
External calibration slope
Standard addition (Astragalus membranaceus) slope
Standard addition (Glycyrrhiza uralensis) slope
75
22,100 30,200 109,000 37,200
336,000 70,300 343,000 95,200
289,000 ( 14%)a 74,900 (7%) 330,000 ( 4%) 103,000 (8%)
1 0.00 0.25 0.50 0.75 1.00 1.25 1.50
111
As
Table 4 Slopes (counts s
2
Relative signal 75
As Cd 202 Hg 206 Pb
111
a The value shown in parenthesis is the deviation of the slope between Astragalus membranaceus slurry and Glycyrrhiza uralensis slurry.
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a b c
Sample
Methodb
As
Cd
Hg
Pb
SRM 1547 Peach leaves
Method 1 Method 2 Certified
0.0631 ± 0.0016 0.126 ± 0.006 0.060 ± 0.018
0.0268 ± 0.0006 0.0257 ± 0.0004 0.026 ± 0.003
0.0315 ± 0.0014 0.0347 ± 0.0015 0.031 ± 0.007
0.868 ± 0.015 0.849 ± 0.011 0.87 ± 0.03
SRM 1573a Tomato leaves
Method 1 Method 2 Certified
0.111 ± 0.002 0.115 ± 0.004 0.112 ± 0.004
1.52 ± 0.02 1.53 ± 0.02 1.52 ± 0.04
0.0349 ± 0.0016 0.0370 ± 0.0018 0.034 ± 0.004
0.599 ± 0.010 0.600 ± 0.015 nac
Astragalus membranaceus
Method 1 Method 2
0.177 ± 0.004 0.180 ± 0.002
0.104 ± 0.003 0.100 ± 0.001
0.0101 ± 0.0004 0.0107 ± 0.0003
0.448 ± 0.008 0.440 ± 0.012
Glycyrrhiza uralensis
Method 1 Method 2
0.184 ± 0.004 0.185 ± 0.003
0.0146 ± 0.0003 0.0142 ± 0.0002
0.0199 ± 0.0007 0.0205 ± 0.0007
0.332 ± 0.008 0.335 ± 0.004
Angelica sinensis
Method 1 Method 2
0.399 ± 0.007 0.397 ± 0.007
0.0145 ± 0.0002 0.0144 ± 0.0002
0.241 ± 0.005 0.234 ± 0.007
4.04 ± 0.09 4.13 ± 0.18
Concentration/lg g
1
Results are means of three measurements ± standard deviation. Method 1: standard addition method. Method 2: determined by pneumatic nebulization ICP-MS after sample dissolution. Certified: NIST certified value. na = not available.
3.4. Determination of As, Cd, Hg and Pb in herbs In order to validate the USS-ETV-ICP-MS method, the concentrations of As, Cd, Hg and Pb were determined in NIST SRM 1547 peach leaves and SRM 1573a tomato leaves reference samples and three herb samples purchased locally. In order to evaluate the possibility of using external calibration method for the determination of trace elements in herbs by USS-ETV-ICP-MS, calibration curves obtained by standard addition method of 0.5% m/v herb slurry solution and external calibration of aqueous standard were compared. The results are listed in Table 4. As shown, the sensitivities of the elements studied were different in aqueous solution and in herb slurry though the correlation coefficients (r2) were >0.9992. Therefore, external calibration method could not be used for the quantification of these elements in the herb samples. It is interesting to see that except for arsenic, the slopes of the elements studied were quite similar between the A. membranaceus slurry and the G. uralensis slurry. To obtain accurate quantification results, the standard addition method was used for the determination of As, Cd, Hg and Pb in these samples. Analysis results are shown in Table 5. As shown the determined concentrations in the leaves reference samples were in good agreement with the certified values. The pneumatic nebulization ICP-MS result of arsenic in peach leaves reference sample was higher than the certified value. It could be due to the spectral interference caused by ArCl+. However the ETV-ICP-MS result was in good agreement with the certified value. As shown in Table 5, the results for the samples for which no reference values were available were also found to be in good agreement with those of digested samples analyzed by pneumatic nebulization ICP-MS in accordance with the Student’s t-test for a confidence level of 95%. The content of Hg and Pb in A. sinensis was much higher than the other samples analysed. However, the Hg content in A. sinensis exceeded the maximum allowable concentration. The high concentration of Hg and Pb in this sample could be due to the environmental pollution and/or the contamination problem during the processing. This experiment indicated that As, Cd, Hg and Pb in herbs could be readily quantified by the proposed USS-ETV-ICP-MS method. Detection limits of As, Cd, Hg and Pb were determined from the standard addition calibration curves. It was based on the usual definition as the concentration of the analyte yielding a signal equivalent to three times the standard deviation of the reagent blank
signal (n = 7). Detection limits estimated from standard addition curves were about 0.3, 0.1, 0.1 and 0.2 ng g 1 for As, Cd, Hg and Pb, respectively, in original herbs. In conclusion, the utility of 8-Hydroxyquinoline as modifier for USS-ETV-ICP-MS has been reported. The advantage of passing dry aerosol generated by ETV in alleviating the isobaric interferences on the determination of As and Cd by ICP-MS has been demonstrated. The analysis results agreed with the reference values. Precision between sample replicates was better than 4% for all the determinations. The procedure can be used as an alternative to the existing procedures for the analysis of herbs.
Acknowledgements This research was supported by a grant from the National Science Council of the Republic of China under Contract NSC 972113-M-110-008-MY3 and NSC 100-2627-M-110-003-.
References Aramendia, M., Resano, M., & Vanhaecke, F. (2009). Determination of toxic trace impurities in titanium dioxide by solid sampling-electrothermal vaporizationinductively coupled plasma mass spectrometry. Journal of Analytical Atomic Spectrometry, 24, 41–50. Chen, F., Liang, P., Hu, B., Zhao, L., Sun, D. A., & Wang, Y. R. (2002). The application of inductively coupled plasma atomic emission spectrometry/mass spectrometry in the trace elements and speciation analysis of traditional Chinese medicine. Spectroscopy and Spectral Analysis, 22, 1019–1024. Chen, B., Wang, X. R., & Lee, F. S. C. (2001). Pyrolysis coupled with atomic absorption spectrometry for the determination of mercury in Chinese medicinal materials. Analytica Chimica Acta, 447, 161–169. Chen, B., Zhuang, Z. X., Wang, X. R., & Lee, F. S. C. (2001). A novel sample introduction technique for the simultaneous determination of As, Se, Ge and Hg in Chinese medicinal material. Chemical Research in Chinese Universities, 17, 400–406. Dias, L. F., Miranda, G. R., Saint’Pierre, T. D., Maia, S. M., Frescura, V. L. A., & Curtius, A. J. (2005). Method development for the determination of cadmium, copper, lead, selenium and thallium in sediments by slurry sampling electrothermal vaporization inductively coupled plasma mass spectrometry and isotopic dilution calibration. Spectrochimica Acta, Part B, 60, 117–124. Huang, S.-Y., & Jiang, S.-J. (2010). 8-Hydroxyquinoline-5-sulfonic acid as the modifier for the determination of trace elements in cereals by slurry sampling electrothermal vaporization ICP-MS. Analytical Methods, 2, 1310–1315. Huang, R. J., Zhuang, Z. X., Tai, Y., Huang, R. F., Wang, X. R., & Lee, F. S. C. (2006). Direct analysis of mercury in traditional Chinese medicines using thermolysis coupled with on-line atomic absorption spectrometry. Talanta, 68, 728–734.
2162
M.-L. Lin, S.-J. Jiang / Food Chemistry 141 (2013) 2158–2162
Kou, X. M., Xu, M., & Gu, Y. Z. (2007). Determination of trace heavy metal elements in cortex phellodendri chinensis by ICP-MS after microwave-assisted digestion. Spectroscopy and Spectral Analysis, 27, 1197–1200. Li, Y.-C., Jiang, S.-J., & Chen, S.-F. (1998). Determination of Ge, As, Se, Cd and Pb in plant materials by slurry sampling–electrothermal vaporization–inductively coupled plasma-mass spectrometry. Analytica Chimica Acta, 372, 365–372. Liao, H.-C., & Jiang, S.-J. (1999). EDTA as the modifier for the determination of Cd, Hg and Pb in fish by slurry sampling electrothermal vaporization inductively coupled plasma mass spectrometry. Journal of Analytical Atomic Spectrometry, 14, 1583–1588. Liaw, M.-J., & Jiang, S.-J. (1996). Determination of copper, cadmium and lead in sediment samples by slurry sampling electrothermal vaporization inductively coupled plasma mass spectrometry. Journal of Analytical Atomic Spectrometry, 11, 555–560. Liu, D. L., Ke, S. Y., Ye, R., & Ding, M. Y. (2007). Determination of trace lead in traditional Chinese herbal medicine astragalus by microwave digestion-CTAB enhancing-continual flow injection hydride generation-ICP-AES. Spectroscopy and Spectral Analysis, 27, 2337–2340. Lopez-Garcia, I., Rivas, R. E., & Hernandez-Cordoba, M. (2008). Use of sodium tungstate as a permanent chemical modifier for slurry sampling electrothermal atomic absorption spectrometric determination of indium in soils. Analytical and Bioanalytical Chemistry, 391, 1469–1474. Maia, S. M., Pozebon, D., & Curtius, A. J. (2003). Determination of Cd, Hg, Pb and Tl in coal and coal fly ash slurries using electrothermal vaporization inductively coupled plasma mass spectrometry and isotopic dilution. Journal of Analytical Atomic Spectrometry, 18, 330–337. Maia, S. M., Vale, M. G. R., Welz, B., & Curtius, A. J. (2001). Feasibility of isotope dilution calibration for the determination of thallium in sediment using slurry sampling electrothermal vaporization inductively coupled plasma mass spectrometry. Spectrochimica Acta, Part B, 56, 1263–1275. Martena, M. J., Van Der Wielen, J. C. A., Rietjens, I., Klerx, W. N. M., De Groot, H. N., et al. (2010). Monitoring of mercury, arsenic, and lead in traditional Asian herbal preparations on the Dutch market and estimation of associated risks.
Food Additives & Contaminants: Part A Chemistry, Analysis, Control, Exposure & Risk Assessment, 27, 190–205. Miller-Ihli, N. J., & Baker, S. A. (2001). Microhomogeneity assessments using ultrasonic slurry sampling coupled with electrothermal vaporization isotope dilution inductively coupled plasma mass spectrometry. Spectrochimica Acta, Part B, 56, 1673–1686. Munoz, M. L., & Aller, A. J. (2006). Determination of Cd, Hg, Pb and Tl in coal and coal fly ash slurries using electrothermal vaporization inductively coupled plasma mass spectrometry and isotopic dilution. Journal of Analytical Atomic Spectrometry, 21, 329–337. Pai, S.-C., & Fang, T.-H. (1990). A low contamination Chelex-100 technique for shipboard preconcentration of heavy-metals in seawater. Marine Chemistry, 29, 295–306. Peschel, B. U., Herdering, W., & Broekaert, J. A. C. (2007). A radiotracer study on the volatilization and transport effects of thermochemical reagents used in the analysis of alumina powders by slurry electrothermal vaporization inductively coupled plasma mass spectrometry. Spectrochimica Acta, Part B, 62, 109–115. Resano, M., Aramendia, M., Devos, W., & Vanhaecke, F. (2006). Direct multi-element analysis of a fluorocarbon polymer via solid sampling-electrothermal vaporization-inductively coupled plasma mass spectrometry. Journal of Analytical Atomic Spectrometry, 21, 891–898. Wang, Y., Feng, F., & Wang, Z. (2010). Determination of selected elements in aqueous extractions of a traditional Chinese medicine formula by ICP-MS and FAAS: Evaluation of formula rationality. Analytical Letters, 43, 983–992. Wu, Y. L., Hu, B., Jiang, Z. C., & Chen, S. Z. (2002). Low temperature vaporization for ICP-AES determination of palladium in geological samples using sample introduction of gaseous palladium oxinate. Journal of Analytical Atomic Spectrometry, 17, 121–124. Zhang, Y. F., Jiang, Z. C., He, M., & Hu, B. (2007). Determination of trace rare earth elements in coal fly ash and atmospheric particulates by electrothermal vaporization inductively coupled plasma mass spectrometry with slurry sampling. Environmental Pollution, 148, 459–467.