SIMULTANEOUS QUANTIFICATION OF 16 TOXIC AND HAZARDOUS ELEMENTS IN TOYS BY LASER-ABLATION ICP-MS

Transcription

1 Journal of Materials Science and Engineering with Advanced Technology Volume 3, Number 2, 2011, Pages SIMULTANEOUS QUANTIFICATION OF 16 TOXIC AND HAZARDOUS ELEMENTS IN TOYS BY LASER-ABLATION ICP-MS BIWEN WEI, LI LIN, WENJIA YU and YI ZHENG Shanghai Entry-Exit Inspection and Quarantine Bureau Shanghai P. R. China Abstract A fast and simple method for determination of B, Al, Cr, Mn, Ni, Co, Cu, Zn, As, Se, Sr, Cd, Sb, Ba, Hg, and Pb in toys by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been developed. Using 13 C as an internal standard and self-made polyethylene-based materials containing multielement as an external standard, high sensitivity, and stable accuracy of both standard reference materials and real samples have been gained by the method. The detection limits (MDLs) of 16 elements ranged from to 6.56mg/kg, and the corresponding precisions of these elements ranged from 3.5% to 9% (n = 8). The linear ranges of the calibration curves are over two orders of magnitudes. This method has been applied to the quick quantification of 16 toxic and hazardous elements required by EU toy directive (2009/48/EC) [2]. Most of these elements were detected at concentration levels ranging from 4.2 to 249mg/kg and from 13.9 to 254mg/kg in paint coating (wood toys) and in polyurethane leather (toy models), respectively. 1. Introduction Paintings and polymers in toys often contain a lot of potentially toxic and hazardous chemical elements. Quality control of toys for avoiding children exposure to these substances is of utmost relevance, and it is a Keywords and phrases: laser ablation, ICP-MS, EU toy directive, element quantification. Received April 14, Scientific Advances Publishers

2 90 BIWEN WEI et al. common requirement in national and/or international norms for health and safety reasons. There is a tendency that permitted amounts of toxic and hazardous chemicals in toys are getting less and less, while the scope of chemical substances concerned is getting wider and wider. In the edition of EU toy directive (88/378/EEC) [1], 8 chemical elements are restricted in migration test, but in the new edition of EU toy directive (2009/48/EC) [2], 19 elements have been restricted and the limits of many elements are lowered by 1~3 orders of magnitudes. Sensitivity of those commonly used methods like ICP-OES [7, 9, 10, 13] and AAS (atomic absorption spectrometry) [3, 4, 11] can not meet the need. On the other hand, sample preparations like cutting, scratching, and acid leaching before detection procedure are usually time-consuming and effort-taking. Rapid methods with high sensitivity and ability for in situ detection are required to solve these problems. Some researches have found some in situ detection methods for toys. For example, Brouwer developed a direct AAS detection of Pb and Cd in plastic toys by using a wire to transfer solid samples to AAS flame [6]. Godoi et al. reported a rapid quantification method for Ba, Cd, Cr, and Pb in toys by laser-induced breakdown spectrometry (LIBS) [8]. In this paper, a method for 16 toxic elements quantification based on laser ablation ICP-MS (LA-ICP-MS) has been developed. LA-ICP-MS has been considered a simple and rapid analytical methods, it offers 3~4 orders of magnitudes higher sensitivity than AAS, ICP-OES, and LIBS. To gain matrix-matched standard, pure polymer substrate was mixed with multi-element solution and molded to form calibration standard. Test results showed that these self-made standards are effective in real sample quantification. Thus, this method offers an attractive alternative to the commonly used methods. 2. Experimental 2.1. Instrumentation LA-ICP-MS measurements were performed by using a laser ablation system (LSX-213, CETAC, USA) in combination with an ICP-MS (X Series II, Thermofisher Scientific, USA). All the analyses were carried out under collision cell technique (CCT) mode, with 7% H2 in He as reactive gas to remove 40 Ar 35 Cl interferent in 75 As detection. Sample

3 SIMULTANEOUS QUANTIFICATION OF 16 TOXIC 91 evaporation was performed with an Nd:YAG laser. The mass spectrometric measurements were carried out under main peak jump mode. Verification measurements were performed by microwave digestion system (Multiwave 3000, Anton Paar, Austria). Operating conditions for the ablation and the analysis were optimized and listed in Table 1. Table 1. Optimal parameters of laser ablation system and ICP-MS Laser ablation system Wavelength Power energy Pulse width Repetition frequency Ablation mode Scan rate 213nm 4.5mJ 5ns 20Hz Raster (3mm 3mm) 50µm/s Spot diameter 100µm Ablation time 90 s* ICP-MS RF power Carrier gas Coolant gas Auxiliary gas CCT gas Acquisition mode Dwell time Acquisition time 1200W 1.1L/min 13.1L/min 0.9L/min 4.1L/min Time resolved 50ms 150s 2.2. Samples and chemicals * Beginning after 20s of ICP-MS acquisition Standard reference materials (SRMs) Trace element reference materials 2586 and 1648a (National institute of reference and technology, NIST) and reference materials EC680K and EC681K (European reference materials) were used in verification test. As the former two were powdered samples, they were pressed at a pressure of 4.0MPa into bricks before the verification test.

4 92 BIWEN WEI et al Synthetic multi-element polyethylene-based standards (SMPS) Powdered low density polyethylene (LDPE) was chosen as a substrate since it has a low melting point and can be easily molded. This technique of SMPS preparation was adapted from a known method [14]. A given mass of fine powdered LDPE was mixed with an excess of 25% HNO3 to remove inorganic contaminants. Then, LDPE was washed thoroughly and repeatedly with Milli-Q water and dried with air flow in an oven under 50 C. Multi-element solution was made to contain 16 elements at a concentration of ~1000ppm (except for As, Cd, Se, Sb, and Hg, which were ~100ppm) and a known amount was then added to 1g dried LDPE. An excess of 3mL water was used to facilitate stirring. The mixture was stirred for 3h and then placed on a hot plate at 50 C to dry. Dried mixture gained was then heated to 180 C, hold 20 min, and pressed at a pressure of 2.0Mpa into bricks. Elements concentrations of SMPS were determined through digestion-icp-ms method with 7mL HNO3 and 1mL H2O2 as the digest solvents. Blank standard was made by the same procedure as that of SMPS without adding of multi-element solution Quantification methods A set of SMPS were used as external standards. 13 C was employed as an internal standard to compensate for signal fluctuations caused by the variation of the ablated sample mass [15]. 3. Results and Discussion 3.1. Verification of SMPS (a) Homogeneity Eight interval areas with uniform distribution were chosen to carry out laser ablation. Plasmalab TM software was utilized for integrating the response value ~ time spectrogram to gain signal intensity. As shown in Figure 1, the relative standard deviation (RSD) for various elements ranges from 3.5% to 9%. RSD values of elements Ba, Co, Cr, Sr, Zn, Ni, Cu, As, and Pb match well with those gained by similar methods [5, 12]. This proved the homogeneity of the SMPS. Homogeneity verification was carried out in every batch of SMPS before calibration and sample test.

5 SIMULTANEOUS QUANTIFICATION OF 16 TOXIC 93 Figure 1. Relative standard deviations of 16 elements in 8 areas measured by SMPS. (b) Calibration and quantification A set of SMPS were measured in LA-ICP-MS system to form calibration curves. Linear range of most elements were 0~500ppm, except for As, Cd, Se, Sb, and Hg, which are of 0~50ppm. Regression coefficients obtained are better than 0.99 for all investigated elements. PE-based European reference materials EC680K and EC681K as well as molded 2568 and 1648a were employed for accuracy evaluation. As listed in Table 2, most values gained from calibration curves agree well with the certified data, except for Al, Mn, and Hg. The Al and Mn concentrations are out of the calibration range. For that reason, both lower concentrations were determined. As for Hg, memory effect and the considerably low concentration lead to the inaccurate result.

6 94 BIWEN WEI et al. Table 2. The quantification of certificated reference materials using calibration curves obtained by SMPS Elements Certified conc. Calibration curve Precision (mg/kg) (mg/kg) (%)(n = 5) B Al Cr Mn Co Ni Cu Zn As Se Sr Cd Sb Ba Hg Pb Limits of detection (LOD) Blank standard was made by the same procedure as that of SMPS without adding of SRMs. Digestion method was employed to determine the blank values of the target elements. Among all target elements, only 7.6mg/kg Al was detected since diethylaluminum chloride is a catalyst for the PE synthesis. LODs were determined by 3σ -criterion (LOD = m b + 3σ b, where m b is the mean value of blank standards and σ is the standard deviation) and are showed in Table 3.

7 SIMULTANEOUS QUANTIFICATION OF 16 TOXIC 95 Table 3. The detection limits (3σ) of target elements in SMPS Elements LOD (mg/kg) Elements LOD (mg/kg) 11B As Al Se Cr Sr Mn Cd Co Sb Ni Ba Cu Hg Zn Pb 0.72 The comparatively higher LODs of B, Cu, and Zn indicate a contamination of SMPS substrate. Al also got high LOD because of its high content in LDPE. According to EU toy directive (2009/48/EC) [2], the detection limits of these elements are 300mg/kg for B, 1406mg/kg for Al, 156mg/kg for Cu, and 938mg/kg for Zn, which are far beyond the gained LODs Results of toys Four different kinds of plastic toys were chosen. Total digestion method was carried out in comparison with LA-ICP-MS. Due to the difficulty in finding toys that contain all concerned target elements, data below do not cover Se, Hg, As, and Sb. As is shown in Table 4, most detected values of elements are within the analytical precision.

8 96 BIWEN WEI et al. Table 4. Concentrations of 16 target elements in toys (mg/kg) Paint coating (wood toy) Polyurethane leather (rag baby) Elements Precision Precision Precision Precision LA-ICP-MS RM LA-ICP-MS RM (%) (n = 5) (%) (n = 5) (%) (n = 5) (%) (n = 5) B N.D. N.D Al Cr Mn N.D. N.D Ni N.D. N.D Co N.D. N.D. N.D. N.D. Cu N.D. N.D. N.D. N.D. Zn N.D. N.D As N.D. N.D. N.D. N.D. Se N.D. N.D. N.D. N.D. Sr Cd N.D. N.D. N.D. N.D. Sb N.D. N.D. N.D. N.D. Ba Hg N.D. N.D. N.D. N.D. Pb N.D. N.D. Polymer coating (toy model) White plastics (toy model) Elements Precision Precision Precision Precision LA-ICP-MS RM LA-ICP-MS RM (%) (n = 5) (%) (n = 5) (%) (n = 5) (%) (n = 5) B N.D. N.D. N.D. N.D. Al Cr N.D. N.D. N.D. N.D. Mn N.D. N.D. Ni Co N.D. N.D. N.D. N.D. Cu N.D. N.D. Zn As N.D. N.D. N.D. N.D. Se N.D. N.D. N.D. N.D. Sr Cd N.D. N.D Sb N.D. N.D. N.D. N.D. Ba Hg N.D. N.D. N.D. N.D. Pb N.D. N.D. N.D. N.D. RM = Reference method (digestion -ICP-MS). Values below the LODs are expressed as N. D..

9 SIMULTANEOUS QUANTIFICATION OF 16 TOXIC Conclusion Using LA-ICP-MS is possible to analyze large amounts of elements in toys with considerably less work for sample preparation. By the use of simple and easily made synthetic multi-element polyethylene-based standards, rapid quantification of 16 elements in toys could be obtained by LA-ICP-MS. With favourably high sensitivity, this method could qualify the full-fit requirements of EU toy directive 2009/48/EC and thus becomes a good alternative to the commonly used ones. Acknowledgement This work was financially supported by Scientific Research Project of Shanghai Entry-Exit Inspection and Quarantine Bureau (No. HK ). References [1] 88/378/EEC, EU Toy Safety Directive, European Commission [2] 2009/48/EC, EU Toy Safety Directive, European Commission [3] M. A. A. Akl, I. M. M. Kenawy and R. R. Lasheen, Microchem. J. 78 (2004), [4] F. M. A. Alkarkhi, N. Ismail and A. M. Easa, J. Hazard. Mater. 150 (2008), [5] M. A. Amr, H. T. Mohsen, N. F. Zahran and A. J. Helal, Int. J. Mass Spectrom 268 (2007), [6] H. Brouwer, J. Chem. Educ. 82 (2005), [7] B. Feist, B. Mikula, K. Pytlakowska, B. Puzio and F. Buhl, J. Hazard. Mater. 152 (2008), [8] Q. Godoi, F. O. Leme, L. C. Trevizan, E. R. P. Filho, I. A. Rufini, D. Santos Jr. and F. J. Krug, Spectrochimica Acta Part B, in press, doi: /j.sab [9] M. R. Gomez, S. Cerutti, L. L. Sombra, M. F. Silva and L. D. Martinez, Food Chem. Toxicol. 45 (2007), [10] D. Hristozov, C. E. Domini, V. Kmetov, V. Stefanova, D. Georgieva and A. Canals, Anal. Chim. Acta. 516 (2004), [11] M. E. Mahmoud, I. M. M. Kenawy, MM. A. H. Hafez and R. R. Lasheen, Desalination 250 (2010),

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