- Email: [email protected]
Determination of heavy metals in leather and fur by microwave plasma-atomic emission spectrometry
Spectrochimica Acta Part B 112 (2015) 6–9
Contents lists available at ScienceDirect
Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Analytical Note
Determination of heavy metals in leather and fur by microwave plasma-atomic emission spectrometry Yang Zhao a,b, Zenghe Li a,⁎, Ashdown Ross c, Zhiding Huang b, Wenkai Chang b, Kun Ou-yang d, Yuhong Chen d, Chunhua Wu d a
Beijing Key Laboratory of Environmentally Harmful Chemical Analysis, College of Science, Beijing University of Chemical Technology, Beijing, 100029, PR China National Footwear Quality Supervision and Inspection Center (Beijing), China Leather and Footwear Industry Research Institute, Beijing, 100015, PR China Department of Product Management, Agilent Technologies, Mulgrave Victoria 3170, Australia d Department of Product Technical Marketing, Agilent Technologies, Beijing, 100102, PR China b c
a r t i c l e
i n f o
Article history: Received 4 February 2015 Accepted 26 June 2015 Available online 2 July 2015 Keywords: Leather Microwave-assisted digestion Heavy metals Microwave plasma-atomic emission spectrometry
a b s t r a c t A sensitive method for the determination of heavy metals (Cd, Co, Cr, Cu, Hg, Ni and Pb) in leather and fur by microwave plasma-atomic emission spectrometry (MP-AES) based on microwave-assisted acid digestion and a multimode sample introduction system was developed. The leather was subjected to microwave-assisted digestion in a mixed solution of nitric acid and hydrogen peroxide before analysis by MP-AES. Results were compared with those from the standardized method instrument, inductively coupled plasma-atomic emission spectrometry (ICP-AES). Examination of a leather certified reference material (CRM, GSB 16-3087-2013) was used to validate the proposed method by statistical test, with good accuracy and precision shown for all heavy metals analyzed. Under optimum conditions, recoveries of 98.1%–102.6% and relative standard deviations (RSD) of 0.7%–3.0% were achieved, with limits of detection (LOD) ranging from 0.6 mg/kg (Cd) to 5.0 mg/kg (Pb). Furthermore, the MP-AES heavy metals analysis carried out on 23 test leather and fur samples showed an adequate concordance with results obtained from ICP-AES. The benefits of the MP-AES are related to low analysis cost and improved operational safety with nitrogen used for plasma generation. The MP-AES system investigated could offer comparable performance compare to ICP-AES and may be applied as an optional method for routine analysis in leather and fur testing institution. © 2015 Elsevier B.V. All rights reserved.
1. Introduction It is well known that the presence of heavy metals (for example, Cd, Co, Cr, Cu, Hg, Ni and Pb) is of considerable concern to human health, and agricultural, livestock and aquatic industries [1]. Heavy metals are usually found in leather and other textile products due to the use of tanning agents, dyes and additives containing metal salts in conditional tanning processes [2]. Therefore, the concentration of heavy metals in leather and leather products is regulated in several countries and it is clear that monitoring of heavy metals in leather is extremely important. A variety of analytical methods for the determination of heavy metals have been developed and reported, including atomic absorption spectrometry (AAS) [3,4], inductively coupled plasma-atomic emission spectrometry (ICP-AES) [5,6], inductively coupled plasma-mass spectrometry (ICP-MS) [7–10], and others [11–13]. Among these techniques, ICP-AES is the most widely employed [14], while AAS provides the highest analytical sensitivity particularly when using an electrothermal atomizer for atoms generation, however, this requires elements to ⁎ Corresponding author. Tel./fax: +86 10 64421310. E-mail address: [email protected] (Z. Li).
http://dx.doi.org/10.1016/j.sab.2015.06.017 0584-8547/© 2015 Elsevier B.V. All rights reserved.
be analyzed one by one. Microwave plasma-atomic emission spectrometry (MP-AES) is an alternative spectroscopic analysis technology in which a stable nitrogen plasma is produced using microwave energy [15]. Although microwave plasmas have been in existence for a couple of decades, the main usage has been restricted to specific research groups with very few studies in commercially available analytical instruments. Recently, the use of nitrogen as a plasma gas has also been studied. In addition, comparisons of microwave and inductively coupled plasma sources suggest that MP performance approaches that of ICP [16]. Therefore the MP-AES, which runs on nitrogen, is of interest for many fields of analytical chemistry, as operating costs are significantly lower than for argon or helium dependent instruments [17,18]. Studies have reported the successful use of an MP-AES instrument (Agilent Technologies, 4100 MP-AES) for element analysis on agricultural materials [19]. However, there is no study that outlines the performance of the nitrogen MP-AES technique for analysis of leather and fur materials, with even less scientific information available on heavy metals. The aim of this work is to develop an inexpensive method for the determination of heavy metals in leather and fur. In this work, samples underwent microwave-assisted digestion and detection by MP-AES
Y. Zhao et al. / Spectrochimica Acta Part B 112 (2015) 6–9
and ICP-AES. Verification of the proposed method, including accuracy, precision and limit of detection (LOD) data, are discussed in this paper. The method was applied for the analysis of Cd, Co, Cr, Cu, Hg, Ni and Pb in leather and fur samples by MP-AES, and results were compared with the those from ICP-AES.
7
The instrument operating conditions and settings to determine Cd, Co, Cr, Cu, Hg, Ni and Pb by MP-AES and ICP-AES are listed in Table 1. Viewing positions were not included as this parameter was optimized before running each experimental batch. 2.3. Microwave-assisted digestion
2. Experimental 2.1. CRM, reagents and samples The stock standard solution (1000 mg/L) of Cd, Co, Cr, Cu, Hg, Ni and Pb were certified reference materials (CRM), which were purchased from National Institute of Metrology (China). 65% (w/w) nitric acid (HNO3), 30% (w/w) hydrogen peroxide solution (H2O2), sodium borohydride (NaBH4), and sodium hydroxide (NaOH) solutions were obtained from Beijing Chemical Works, all of guaranteed reagent (GR) grade. A reductant solution of 2% (w/v) NaBH4 stabilized with 1% (w/v) NaOH solution was introduced during determination by MP-AES. The CRM for Cd, Co, Cr, Cu, Hg, Ni and Pb in leather (GSB 16-3087-2013) was obtained from China Leather and Footwear Industry Research Institute. All solutions were freshly prepared in pure water (18.2 MΩ · cm) before use. The 23 test leather and fur samples of different origin were collected from several large leather factories in China (see Table S1 in the Appendix A. Supplementary data). 2.2. Apparatus The MDS-10 microwave digestion system from Sineo Co., Ltd was used for the microwave-assisted acid digestion of all leather samples. The Agilent 4100 MP-AES with Multimode Sample Introduction System (MSIS) (allowing simultaneous reductant solution introduction) was used for the analysis of heavy metals in leather and fur samples. An Agilent 710-ES ICP-AES was used as a reference for all determinations. As schematically shown in Fig. 1, the MSIS permitted the use of both vapor generation (for Hg) and routine pneumatic nebulization (for Cd, Co, Cr, Cu, Ni and Pb) sample introduction routes without having to change the sample introduction system.
The leather and fur samples were cut into pieces of up to 0.5 cm edge length and 0.5 cm thickness. Approximately 0.2 g (weighed to 0.1 mg) of sample was transferred into the microwave digestion flask, which was made of polytetrafluoroethylene with maximum allowed pressure and temperature of 5 MPa and 260 °C respectively. 4 mL of 14.4 M nitric acid and 1 mL of 9.8 M hydrogen peroxide solution were added to each flask. The samples were processed by microwave-assisted digestion as follows: ramp temperature to 130 °C and hold for 5 min, ramp and hold at 180 °C for 10 min, finally ramp to 220 °C and hold for 20 min. The total ramp time from ambient to final temperature should take more than 20 min. After digesting, the microwave digestion flasks were allowed to cool to ambient temperature before handling. The digested samples were transferred and made up to 25 mL in a volumetric flask with an acidity of 0.7 mol/L, and then filtered for instrumental analysis. A blank containing identical reagent quantities, without the addition of sample, was also prepared. The leather certified reference material instead of samples with same weight (0.2 g) was digested in accordance with the above procedure to determine the heavy metal recovery. 2.4. Determination After sample preparation, wavelength optimization was performed in order to select the optimal spectral wavelengths for measurement. Multi element standard solutions were analyzed by MP-AES and ICPAES over the wavelength range of interest to monitor spectral interferences. The selected wavelengths are presented in Table 1. For calibration, mixed standard solutions were prepared from the stock standard solution of 1000 μg/mL by dilution with 5% (w/v) nitric acid.
Fig. 1. Schematic diagram of the multimode sample introduction system for MP-AES.
8
Y. Zhao et al. / Spectrochimica Acta Part B 112 (2015) 6–9
Table 1 Operating conditions and selected quantitation wavelengths of MP-AES and ICP-AES for heavy metals determination. (Wavelengths were selected to obtain highest signal/noise ratios for target elements). Instrument parameter
MP-AES
ICP-AES
Nebulizer Spray chamber Radio frequency power (W) Plasma gas flow rate (L/min) Auxiliary argon flow rate (L/min) Nebulizer pressure (kPa) Read-time (s)
Concentric MSIS — — — 100 ~ 240 3 for Cd, Co, Cr, Cu, Ni and Pb; 10 for Hg 3 15 20 15 1.0 Cd 228.802 Co 350.228 Cr 427.480 Cu 327.395 Hg 253.652 Ni 341.476 Pb 405.781
Concentric — 1200 15.0 1.50 240 10
Number of replicates Sample uptake delay (s) Stabilization time (s) Pump speed (rpm) Sample flow rate (mL/min)
Selected wavelengths (nm)
3 10 20 15 0.6 Cd 228.802 Co 238.892 Cr 205.560 Cu 327.395 Hg 194.164 Ni 231.604 Pb 220.353
Calibration ranges (7 points) were selected to match the expected element concentrations, which were 0–1.2 μg/mL for Cd, Co, Cu, Hg, Ni, Pb and 0–60 μg/mL for Cr. In this work, a standard reference solution was used to optimize parameters such as nebulizer pressure, number of replicates, read-time, and stabilization time. 3. Results and discussions 3.1. Limit of detection The limits of detection (LOD) were calculated as three times the standard deviation of ten blank measurements. The obtained LOD for the MP-AES method and ICP-AES method for each heavy metal element in leather samples were listed in Table 2. As presented by the analysis, the LOD for MP-AES were comparable to that of ICP-AES. And the small difference in the LOD suggested that the performance of the MPAES would be comparable to that of ICP-AES. 3.2. Accuracy and precision The accuracy and precision of all procedures were evaluated by determining of Cd, Co, Cr, Cu, Hg, Ni and Pb in a CRM leather sample (GSB 16-3087-2013). Calibration curves for Cd, Co, Cr, Cu, Hg, Ni and Pb show excellent linearity across the calibrated range with all correlation coefficients greater than 0.999. A typical spectrum and calibration graph for Cu are shown in Fig. 2. It was clear that the sample gave a high and well-shaped peaks with freedom of background interference. As observed in the corresponding inset, the MP-AES gave a calibration curve of y = −45 + 72500x at the selected wavelength of 327.395 nm. Table 2 Limit of detection (LOD) of the MP-AES and ICP-AES for each selected element in leather and fur samples (mg/kg). Element
MP-AES
ICP-AES
Cd Co Cr Cu Hg Ni Pb
1.3 1.9 0.9 1.5 2.0 0.9 1.2
0.6 1.0 0.9 1.0 2.0 1.4 5.0
Fig. 2. Signal (black) and background (red) spectra for Cu (327.395 nm) by MP-AES (inset: the corresponding calibration curve).
The data obtained for GSB 16-3087-2013 are reported in Table 3. The data shows an excellent recovery range of 98.1%–102.6% and good precision with relative standard deviations (RSD) for seven digestion replicates in the range from 0.7% to 3.0%. These results demonstrate the ability of MP-AES to accurately determine Cd, Co, Cr, Cu, Hg, Ni and Pb in leather samples. 3.3. Analysis of leather and fur samples The 23 test leather and fur samples are listed in Table S1 (Appendix A. Supplementary data). After initial experiments with the CRM, 23 leather and fur samples were analyzed by both MP-AES and ICP-AES; the results are presented in Table S2 (Appendix A. Supplementary data). The chromium content in these samples varied from 36.1 mg/kg to 34246.0 mg/kg with MP-AES, while the corresponding values for ICP-AES were 36.2-34032.4 mg/kg. Copper was found in most of the samples with the maximum concentration being 65 mg/kg. Other elements (Cd, Co, Hg, Ni and Pb) were found in at least one of the samples. The selected heavy metal elements with their RSD values obtained by MP-AES and ICP-AES for each collected leather and fur sample were showed in Table S2 (Appendix A. Supplementary data). All the quantitative data were statistically analyzed to express as mean ± standard deviation (SD). Significance between the mean values was calculated by using ANOVA one-way statistical analysis. Probability values p greater than 0.05 were considered that there were no statistically significant between MP-AES and ICP-AES. When the data for leather and fur were available from measurements of MP-AES and ICP-AES, the p values were presented in Table S2 (Appendix A. Supplementary data). Statistically significant differences were not observed for the vast majority of samples. However, there were individual p values less than 0.05, which suggested that the data from MP-AES is different to that of ICPAES. The statistically difference may be caused by the inhomogeneity
Table 3 Accuracy and precision data of the MP-AES method obtained by determination of heavy metals in certified reference material for leather (GSB 16-3087-2013). Element
Certified(mg/kg)
Found(mg/kg)*
Recovery (%)
RSD (%)
Cd Co Cr Cu Hg Ni Pb
86.6 ± 3.6 91.4 ± 4.4 5630.0 ± 280.0 97.1 ± 5.2 93.5 ± 5.4 95.5 ± 3.6 94.0 ± 4.0
85.0 ± 0.6 91.2 ± 2.5 5732.3 ± 156.3 99.6 ± 2.1 94.3 ± 1.5 93.7 ± 1.8 93.2 ± 2.7
98.1 99.8 101.8 102.6 100.9 98.2 99.1
0.7 2.8 2.8 2.2 1.6 2.0 3.0
* Mean ± SD of three parallel determinations.
Y. Zhao et al. / Spectrochimica Acta Part B 112 (2015) 6–9
of leather and fur samples. Nevertheless, the results from MP-ASE were very close to obtained data by ICP-AES. This indicated that the analysis performance of MP-AES is comparable to ICP-AES. 4. Conclusions The method of microwave-assisted digestion for the determination of Cd, Co, Cr, Cu, Hg, Ni and Pb in leather and fur with MP-AES was established in this work. 23 leather and fur samples were analyzed by both MP-AES and ICP-AES, and the values obtained in MP-AES were found to be comparable to those gained with ICP-AES, suggesting that the differences between the two methods are acceptable. Due to important advantages of low running costs and high laboratory safety (as no expensive or flammable gasses are required), MP-AES presents broad application prospects in the determination of heavy metals in leather and fur samples. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.sab.2015.06.017. References [1] C. Tonetti, R. Innocenti, Determination of heavy metals in textile materials by atomic absorption spectrometry: verification of the test method, Autex Res. J. 9 (2009) 66–70. [2] E. Matoso, S. Cadore, Determination of inorganic contaminants in polyamide textiles used for manufacturing sport T-shirts, Talanta 88 (2012) 496–501. [3] M. Tüzen, Determination of heavy metals in soil, mushroom and plant samples by atomic absorption spectrometry, Microchem. J. 74 (2003) 289–297. [4] S. Balasubramanian, V. Pugalenthi, Determination of total chromium in tannery waste water by inductively coupled plasma-atomic emission spectrometry, flame atomic absorption spectrometry and UV-visible spectrophotometric methods, Talanta 50 (1999) 457–467. [5] I. Rezić, I. Steffan, ICP-OES determination of metals present in textile materials, Microchem. J. 85 (2007) 46–51.
9
[6] O.T. Yayıntas, S. Yılmaz, M. Turkoglu, Y. Dilgin, Determination of heavy metal pollution with environmental physicochemical parameters in waste water of Kocabas Stream (Biga, Canakkale, Turkey) by ICP-AES, Environ. Monit. Assess. 127 (2007) 389–397. [7] C. Moor, T. Lymberopoulou, V.J. Dietrich, Determination of heavy metals in soils, sediments and geological materials by ICP-AES and ICP-MS, Mikrochim. Acta 136 (2001) 123–128. [8] S. Melaku, T. Wondimu, R. Dams, L. Moens, Simultaneous determination of trace elements in Tinishu Akaki River water sample, Ethiopia, by ICP-MS, Can. J. Anal. Sci. Spectrosc. 49 (2004) 374–384. [9] V.F. Taylor, H.P. Longerich, J.D. Greenough, Multielement analysis of Canadian wines by inductively coupled plasma mass spectrometry (ICP-MS) and multivariate statistics, J. Agric. Food Chem. 51 (2003) 856–860. [10] G.L. Donati, R.S. Amais, J.A. Nóbrega, Strategies to improve accuracy and sensitivity in phosphorus determinations by inductively coupled plasma quadrupole mass spectrometry, J. Braz. Chem. Soc. 23 (2012) 786–791. [11] M. Bettinelli, G.M. Beone, S. Spezia, C. Baffi, Determination of heavy metals in soils and sediments by microwave-assisted digestion and inductively coupled plasma optical emission spectrometry analysis, Anal. Chim. Acta 424 (2000) 289–296. [12] T. Frentiu, A.I. Mihaltan, M. Senila, E. Darvasi, M. Ponta, M. Frentiu, B.P. Pintican, New method for mercury determination in microwave digested soil samples based on cold vapor capacitively coupled plasma microtorch optical emission spectrometry: comparison with atomic fluorescence spectrometry, Microchem. J. 110 (2013) 545–552. [13] H. Matusiewicz, M. Ślachcinski, Simultaneous determination of hydride forming elements (As, Sb, Se, Sn) and Hg in sonicate slurries of biological and environmental reference materials by hydride generation microwave induced plasma optical emission spectrometry (SS-HG-MIP-OES), Microchem. J. 82 (2006) 78–85. [14] M. Seelig, J.A.C. Broekaert, Investigations on the on-line determination of metals in air flows by capacitively coupled microwave plasma atomic emission spectrometry, Spectrochim. Acta B 56 (2001) 1747–1760. [15] J.A.C. Broekaert, V. Siemens, Recent trends in atomic spectrometry with microwaveinduced plasmas, Spectrochim. Acta B 59 (2004) 1823–1839. [16] M.R. Hammer, A magnetically excited microwave plasma source for atomic emission spectroscopy with performance approaching that of the inductively coupled plasma, Spectrochim. Acta B 63 (2008) 456–464. [17] Y. Su, Z. Jin, Y. Duan, M. Koby, V. Majidi, J.A. Olivares, S.P. Abeln, Highly sensitive beryllium detection with microwave plasma source atomic emission spectrometry, Anal. Chim. Acta 422 (2000) 209–216. [18] S. Karlsson, V. Sjöberg, A. Ogar, Comparison of MP AES and ICP-MS for analysis of principal and selected trace elements in nitric acid digests of sunflower (Helianthus annuus), Talanta 135 (2015) 124–132. [19] W. Li, P. Simmons, D. Shrader, T.J. Herrman, S.Y. Dai, Microwave plasma-atomic emission spectroscopy as a tool for the determination of copper, iron, manganese and zinc in animal feed and fertilizer, Talanta 112 (2013) 43–48.
Contents lists available at ScienceDirect
Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Analytical Note
Determination of heavy metals in leather and fur by microwave plasma-atomic emission spectrometry Yang Zhao a,b, Zenghe Li a,⁎, Ashdown Ross c, Zhiding Huang b, Wenkai Chang b, Kun Ou-yang d, Yuhong Chen d, Chunhua Wu d a
Beijing Key Laboratory of Environmentally Harmful Chemical Analysis, College of Science, Beijing University of Chemical Technology, Beijing, 100029, PR China National Footwear Quality Supervision and Inspection Center (Beijing), China Leather and Footwear Industry Research Institute, Beijing, 100015, PR China Department of Product Management, Agilent Technologies, Mulgrave Victoria 3170, Australia d Department of Product Technical Marketing, Agilent Technologies, Beijing, 100102, PR China b c
a r t i c l e
i n f o
Article history: Received 4 February 2015 Accepted 26 June 2015 Available online 2 July 2015 Keywords: Leather Microwave-assisted digestion Heavy metals Microwave plasma-atomic emission spectrometry
a b s t r a c t A sensitive method for the determination of heavy metals (Cd, Co, Cr, Cu, Hg, Ni and Pb) in leather and fur by microwave plasma-atomic emission spectrometry (MP-AES) based on microwave-assisted acid digestion and a multimode sample introduction system was developed. The leather was subjected to microwave-assisted digestion in a mixed solution of nitric acid and hydrogen peroxide before analysis by MP-AES. Results were compared with those from the standardized method instrument, inductively coupled plasma-atomic emission spectrometry (ICP-AES). Examination of a leather certified reference material (CRM, GSB 16-3087-2013) was used to validate the proposed method by statistical test, with good accuracy and precision shown for all heavy metals analyzed. Under optimum conditions, recoveries of 98.1%–102.6% and relative standard deviations (RSD) of 0.7%–3.0% were achieved, with limits of detection (LOD) ranging from 0.6 mg/kg (Cd) to 5.0 mg/kg (Pb). Furthermore, the MP-AES heavy metals analysis carried out on 23 test leather and fur samples showed an adequate concordance with results obtained from ICP-AES. The benefits of the MP-AES are related to low analysis cost and improved operational safety with nitrogen used for plasma generation. The MP-AES system investigated could offer comparable performance compare to ICP-AES and may be applied as an optional method for routine analysis in leather and fur testing institution. © 2015 Elsevier B.V. All rights reserved.
1. Introduction It is well known that the presence of heavy metals (for example, Cd, Co, Cr, Cu, Hg, Ni and Pb) is of considerable concern to human health, and agricultural, livestock and aquatic industries [1]. Heavy metals are usually found in leather and other textile products due to the use of tanning agents, dyes and additives containing metal salts in conditional tanning processes [2]. Therefore, the concentration of heavy metals in leather and leather products is regulated in several countries and it is clear that monitoring of heavy metals in leather is extremely important. A variety of analytical methods for the determination of heavy metals have been developed and reported, including atomic absorption spectrometry (AAS) [3,4], inductively coupled plasma-atomic emission spectrometry (ICP-AES) [5,6], inductively coupled plasma-mass spectrometry (ICP-MS) [7–10], and others [11–13]. Among these techniques, ICP-AES is the most widely employed [14], while AAS provides the highest analytical sensitivity particularly when using an electrothermal atomizer for atoms generation, however, this requires elements to ⁎ Corresponding author. Tel./fax: +86 10 64421310. E-mail address: [email protected] (Z. Li).
http://dx.doi.org/10.1016/j.sab.2015.06.017 0584-8547/© 2015 Elsevier B.V. All rights reserved.
be analyzed one by one. Microwave plasma-atomic emission spectrometry (MP-AES) is an alternative spectroscopic analysis technology in which a stable nitrogen plasma is produced using microwave energy [15]. Although microwave plasmas have been in existence for a couple of decades, the main usage has been restricted to specific research groups with very few studies in commercially available analytical instruments. Recently, the use of nitrogen as a plasma gas has also been studied. In addition, comparisons of microwave and inductively coupled plasma sources suggest that MP performance approaches that of ICP [16]. Therefore the MP-AES, which runs on nitrogen, is of interest for many fields of analytical chemistry, as operating costs are significantly lower than for argon or helium dependent instruments [17,18]. Studies have reported the successful use of an MP-AES instrument (Agilent Technologies, 4100 MP-AES) for element analysis on agricultural materials [19]. However, there is no study that outlines the performance of the nitrogen MP-AES technique for analysis of leather and fur materials, with even less scientific information available on heavy metals. The aim of this work is to develop an inexpensive method for the determination of heavy metals in leather and fur. In this work, samples underwent microwave-assisted digestion and detection by MP-AES
Y. Zhao et al. / Spectrochimica Acta Part B 112 (2015) 6–9
and ICP-AES. Verification of the proposed method, including accuracy, precision and limit of detection (LOD) data, are discussed in this paper. The method was applied for the analysis of Cd, Co, Cr, Cu, Hg, Ni and Pb in leather and fur samples by MP-AES, and results were compared with the those from ICP-AES.
7
The instrument operating conditions and settings to determine Cd, Co, Cr, Cu, Hg, Ni and Pb by MP-AES and ICP-AES are listed in Table 1. Viewing positions were not included as this parameter was optimized before running each experimental batch. 2.3. Microwave-assisted digestion
2. Experimental 2.1. CRM, reagents and samples The stock standard solution (1000 mg/L) of Cd, Co, Cr, Cu, Hg, Ni and Pb were certified reference materials (CRM), which were purchased from National Institute of Metrology (China). 65% (w/w) nitric acid (HNO3), 30% (w/w) hydrogen peroxide solution (H2O2), sodium borohydride (NaBH4), and sodium hydroxide (NaOH) solutions were obtained from Beijing Chemical Works, all of guaranteed reagent (GR) grade. A reductant solution of 2% (w/v) NaBH4 stabilized with 1% (w/v) NaOH solution was introduced during determination by MP-AES. The CRM for Cd, Co, Cr, Cu, Hg, Ni and Pb in leather (GSB 16-3087-2013) was obtained from China Leather and Footwear Industry Research Institute. All solutions were freshly prepared in pure water (18.2 MΩ · cm) before use. The 23 test leather and fur samples of different origin were collected from several large leather factories in China (see Table S1 in the Appendix A. Supplementary data). 2.2. Apparatus The MDS-10 microwave digestion system from Sineo Co., Ltd was used for the microwave-assisted acid digestion of all leather samples. The Agilent 4100 MP-AES with Multimode Sample Introduction System (MSIS) (allowing simultaneous reductant solution introduction) was used for the analysis of heavy metals in leather and fur samples. An Agilent 710-ES ICP-AES was used as a reference for all determinations. As schematically shown in Fig. 1, the MSIS permitted the use of both vapor generation (for Hg) and routine pneumatic nebulization (for Cd, Co, Cr, Cu, Ni and Pb) sample introduction routes without having to change the sample introduction system.
The leather and fur samples were cut into pieces of up to 0.5 cm edge length and 0.5 cm thickness. Approximately 0.2 g (weighed to 0.1 mg) of sample was transferred into the microwave digestion flask, which was made of polytetrafluoroethylene with maximum allowed pressure and temperature of 5 MPa and 260 °C respectively. 4 mL of 14.4 M nitric acid and 1 mL of 9.8 M hydrogen peroxide solution were added to each flask. The samples were processed by microwave-assisted digestion as follows: ramp temperature to 130 °C and hold for 5 min, ramp and hold at 180 °C for 10 min, finally ramp to 220 °C and hold for 20 min. The total ramp time from ambient to final temperature should take more than 20 min. After digesting, the microwave digestion flasks were allowed to cool to ambient temperature before handling. The digested samples were transferred and made up to 25 mL in a volumetric flask with an acidity of 0.7 mol/L, and then filtered for instrumental analysis. A blank containing identical reagent quantities, without the addition of sample, was also prepared. The leather certified reference material instead of samples with same weight (0.2 g) was digested in accordance with the above procedure to determine the heavy metal recovery. 2.4. Determination After sample preparation, wavelength optimization was performed in order to select the optimal spectral wavelengths for measurement. Multi element standard solutions were analyzed by MP-AES and ICPAES over the wavelength range of interest to monitor spectral interferences. The selected wavelengths are presented in Table 1. For calibration, mixed standard solutions were prepared from the stock standard solution of 1000 μg/mL by dilution with 5% (w/v) nitric acid.
Fig. 1. Schematic diagram of the multimode sample introduction system for MP-AES.
8
Y. Zhao et al. / Spectrochimica Acta Part B 112 (2015) 6–9
Table 1 Operating conditions and selected quantitation wavelengths of MP-AES and ICP-AES for heavy metals determination. (Wavelengths were selected to obtain highest signal/noise ratios for target elements). Instrument parameter
MP-AES
ICP-AES
Nebulizer Spray chamber Radio frequency power (W) Plasma gas flow rate (L/min) Auxiliary argon flow rate (L/min) Nebulizer pressure (kPa) Read-time (s)
Concentric MSIS — — — 100 ~ 240 3 for Cd, Co, Cr, Cu, Ni and Pb; 10 for Hg 3 15 20 15 1.0 Cd 228.802 Co 350.228 Cr 427.480 Cu 327.395 Hg 253.652 Ni 341.476 Pb 405.781
Concentric — 1200 15.0 1.50 240 10
Number of replicates Sample uptake delay (s) Stabilization time (s) Pump speed (rpm) Sample flow rate (mL/min)
Selected wavelengths (nm)
3 10 20 15 0.6 Cd 228.802 Co 238.892 Cr 205.560 Cu 327.395 Hg 194.164 Ni 231.604 Pb 220.353
Calibration ranges (7 points) were selected to match the expected element concentrations, which were 0–1.2 μg/mL for Cd, Co, Cu, Hg, Ni, Pb and 0–60 μg/mL for Cr. In this work, a standard reference solution was used to optimize parameters such as nebulizer pressure, number of replicates, read-time, and stabilization time. 3. Results and discussions 3.1. Limit of detection The limits of detection (LOD) were calculated as three times the standard deviation of ten blank measurements. The obtained LOD for the MP-AES method and ICP-AES method for each heavy metal element in leather samples were listed in Table 2. As presented by the analysis, the LOD for MP-AES were comparable to that of ICP-AES. And the small difference in the LOD suggested that the performance of the MPAES would be comparable to that of ICP-AES. 3.2. Accuracy and precision The accuracy and precision of all procedures were evaluated by determining of Cd, Co, Cr, Cu, Hg, Ni and Pb in a CRM leather sample (GSB 16-3087-2013). Calibration curves for Cd, Co, Cr, Cu, Hg, Ni and Pb show excellent linearity across the calibrated range with all correlation coefficients greater than 0.999. A typical spectrum and calibration graph for Cu are shown in Fig. 2. It was clear that the sample gave a high and well-shaped peaks with freedom of background interference. As observed in the corresponding inset, the MP-AES gave a calibration curve of y = −45 + 72500x at the selected wavelength of 327.395 nm. Table 2 Limit of detection (LOD) of the MP-AES and ICP-AES for each selected element in leather and fur samples (mg/kg). Element
MP-AES
ICP-AES
Cd Co Cr Cu Hg Ni Pb
1.3 1.9 0.9 1.5 2.0 0.9 1.2
0.6 1.0 0.9 1.0 2.0 1.4 5.0
Fig. 2. Signal (black) and background (red) spectra for Cu (327.395 nm) by MP-AES (inset: the corresponding calibration curve).
The data obtained for GSB 16-3087-2013 are reported in Table 3. The data shows an excellent recovery range of 98.1%–102.6% and good precision with relative standard deviations (RSD) for seven digestion replicates in the range from 0.7% to 3.0%. These results demonstrate the ability of MP-AES to accurately determine Cd, Co, Cr, Cu, Hg, Ni and Pb in leather samples. 3.3. Analysis of leather and fur samples The 23 test leather and fur samples are listed in Table S1 (Appendix A. Supplementary data). After initial experiments with the CRM, 23 leather and fur samples were analyzed by both MP-AES and ICP-AES; the results are presented in Table S2 (Appendix A. Supplementary data). The chromium content in these samples varied from 36.1 mg/kg to 34246.0 mg/kg with MP-AES, while the corresponding values for ICP-AES were 36.2-34032.4 mg/kg. Copper was found in most of the samples with the maximum concentration being 65 mg/kg. Other elements (Cd, Co, Hg, Ni and Pb) were found in at least one of the samples. The selected heavy metal elements with their RSD values obtained by MP-AES and ICP-AES for each collected leather and fur sample were showed in Table S2 (Appendix A. Supplementary data). All the quantitative data were statistically analyzed to express as mean ± standard deviation (SD). Significance between the mean values was calculated by using ANOVA one-way statistical analysis. Probability values p greater than 0.05 were considered that there were no statistically significant between MP-AES and ICP-AES. When the data for leather and fur were available from measurements of MP-AES and ICP-AES, the p values were presented in Table S2 (Appendix A. Supplementary data). Statistically significant differences were not observed for the vast majority of samples. However, there were individual p values less than 0.05, which suggested that the data from MP-AES is different to that of ICPAES. The statistically difference may be caused by the inhomogeneity
Table 3 Accuracy and precision data of the MP-AES method obtained by determination of heavy metals in certified reference material for leather (GSB 16-3087-2013). Element
Certified(mg/kg)
Found(mg/kg)*
Recovery (%)
RSD (%)
Cd Co Cr Cu Hg Ni Pb
86.6 ± 3.6 91.4 ± 4.4 5630.0 ± 280.0 97.1 ± 5.2 93.5 ± 5.4 95.5 ± 3.6 94.0 ± 4.0
85.0 ± 0.6 91.2 ± 2.5 5732.3 ± 156.3 99.6 ± 2.1 94.3 ± 1.5 93.7 ± 1.8 93.2 ± 2.7
98.1 99.8 101.8 102.6 100.9 98.2 99.1
0.7 2.8 2.8 2.2 1.6 2.0 3.0
* Mean ± SD of three parallel determinations.
Y. Zhao et al. / Spectrochimica Acta Part B 112 (2015) 6–9
of leather and fur samples. Nevertheless, the results from MP-ASE were very close to obtained data by ICP-AES. This indicated that the analysis performance of MP-AES is comparable to ICP-AES. 4. Conclusions The method of microwave-assisted digestion for the determination of Cd, Co, Cr, Cu, Hg, Ni and Pb in leather and fur with MP-AES was established in this work. 23 leather and fur samples were analyzed by both MP-AES and ICP-AES, and the values obtained in MP-AES were found to be comparable to those gained with ICP-AES, suggesting that the differences between the two methods are acceptable. Due to important advantages of low running costs and high laboratory safety (as no expensive or flammable gasses are required), MP-AES presents broad application prospects in the determination of heavy metals in leather and fur samples. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.sab.2015.06.017. References [1] C. Tonetti, R. Innocenti, Determination of heavy metals in textile materials by atomic absorption spectrometry: verification of the test method, Autex Res. J. 9 (2009) 66–70. [2] E. Matoso, S. Cadore, Determination of inorganic contaminants in polyamide textiles used for manufacturing sport T-shirts, Talanta 88 (2012) 496–501. [3] M. Tüzen, Determination of heavy metals in soil, mushroom and plant samples by atomic absorption spectrometry, Microchem. J. 74 (2003) 289–297. [4] S. Balasubramanian, V. Pugalenthi, Determination of total chromium in tannery waste water by inductively coupled plasma-atomic emission spectrometry, flame atomic absorption spectrometry and UV-visible spectrophotometric methods, Talanta 50 (1999) 457–467. [5] I. Rezić, I. Steffan, ICP-OES determination of metals present in textile materials, Microchem. J. 85 (2007) 46–51.
9
[6] O.T. Yayıntas, S. Yılmaz, M. Turkoglu, Y. Dilgin, Determination of heavy metal pollution with environmental physicochemical parameters in waste water of Kocabas Stream (Biga, Canakkale, Turkey) by ICP-AES, Environ. Monit. Assess. 127 (2007) 389–397. [7] C. Moor, T. Lymberopoulou, V.J. Dietrich, Determination of heavy metals in soils, sediments and geological materials by ICP-AES and ICP-MS, Mikrochim. Acta 136 (2001) 123–128. [8] S. Melaku, T. Wondimu, R. Dams, L. Moens, Simultaneous determination of trace elements in Tinishu Akaki River water sample, Ethiopia, by ICP-MS, Can. J. Anal. Sci. Spectrosc. 49 (2004) 374–384. [9] V.F. Taylor, H.P. Longerich, J.D. Greenough, Multielement analysis of Canadian wines by inductively coupled plasma mass spectrometry (ICP-MS) and multivariate statistics, J. Agric. Food Chem. 51 (2003) 856–860. [10] G.L. Donati, R.S. Amais, J.A. Nóbrega, Strategies to improve accuracy and sensitivity in phosphorus determinations by inductively coupled plasma quadrupole mass spectrometry, J. Braz. Chem. Soc. 23 (2012) 786–791. [11] M. Bettinelli, G.M. Beone, S. Spezia, C. Baffi, Determination of heavy metals in soils and sediments by microwave-assisted digestion and inductively coupled plasma optical emission spectrometry analysis, Anal. Chim. Acta 424 (2000) 289–296. [12] T. Frentiu, A.I. Mihaltan, M. Senila, E. Darvasi, M. Ponta, M. Frentiu, B.P. Pintican, New method for mercury determination in microwave digested soil samples based on cold vapor capacitively coupled plasma microtorch optical emission spectrometry: comparison with atomic fluorescence spectrometry, Microchem. J. 110 (2013) 545–552. [13] H. Matusiewicz, M. Ślachcinski, Simultaneous determination of hydride forming elements (As, Sb, Se, Sn) and Hg in sonicate slurries of biological and environmental reference materials by hydride generation microwave induced plasma optical emission spectrometry (SS-HG-MIP-OES), Microchem. J. 82 (2006) 78–85. [14] M. Seelig, J.A.C. Broekaert, Investigations on the on-line determination of metals in air flows by capacitively coupled microwave plasma atomic emission spectrometry, Spectrochim. Acta B 56 (2001) 1747–1760. [15] J.A.C. Broekaert, V. Siemens, Recent trends in atomic spectrometry with microwaveinduced plasmas, Spectrochim. Acta B 59 (2004) 1823–1839. [16] M.R. Hammer, A magnetically excited microwave plasma source for atomic emission spectroscopy with performance approaching that of the inductively coupled plasma, Spectrochim. Acta B 63 (2008) 456–464. [17] Y. Su, Z. Jin, Y. Duan, M. Koby, V. Majidi, J.A. Olivares, S.P. Abeln, Highly sensitive beryllium detection with microwave plasma source atomic emission spectrometry, Anal. Chim. Acta 422 (2000) 209–216. [18] S. Karlsson, V. Sjöberg, A. Ogar, Comparison of MP AES and ICP-MS for analysis of principal and selected trace elements in nitric acid digests of sunflower (Helianthus annuus), Talanta 135 (2015) 124–132. [19] W. Li, P. Simmons, D. Shrader, T.J. Herrman, S.Y. Dai, Microwave plasma-atomic emission spectroscopy as a tool for the determination of copper, iron, manganese and zinc in animal feed and fertilizer, Talanta 112 (2013) 43–48.