| The oil pastels are widely used for art supplies by children. It is important to determine the content of heavy metals and it is relevant directly to health of human being and industrial development. As the limit values of heavy metals for oil pastels are getting much stricter by European Union Safety Standard of Toys, it is imperative to establish a rapid and accurate detection method.In the paper, three detection methods for determination of the content of heavy metals in oil pastels were established and the "total"and "migration"content of trace cadmium, chromium and lead in oil pastels bought in the market were determined by the methods. The main contents and results are as follows:1. Three pretreatments of oil pastels, i.e., microwave digestion, wet digestion and dry ashing digestion were investigated and the three digestion methods were compared. The optimal conditions were as follows:acid of wet digestion:nitric acid-perchloric acid system; ashing temperature:500℃, ashing time:5 hours; and the conditions of microwave digestion:temperature:100℃, power:800 W, run time:90 min. The results showed that microwave digestion was more suitable for pretreatments of oil pastels. A graphite furnace atomic absorption spectrometric method was developed for the determination of the "total" content of trace cadmium, chromium and lead in oil pastels based on microwave digestion. The results showed that the best method of microwave digestion had good recovery rate that varied from 97.1 to 102.1%，the relative standard deviation(n=11) was between 1.50 and 3.01%, the detection limits varied from 0.065×10-3 to 2.51 ×10-3 μg·g-1, with the limits of quantification between 0.217×10-3 and 8.37×10-3 μg·g-1. The method can be used for the determination of the "total" content of trace cadmium, chromium and lead in oil pastels.2. The main factors which increased migration quantity of heavy metals, such as the mixing time, mixing speed and the experimental environment on simulated gastric juice were studied. The optimal conditions were as follows:mixing time:60 min, mixing speed: 200 rmin-1 and shading conditions. A graphite furnace atomic absorption spectrometric method was developed for the determination of "migration" content of cadmium, chromium and lead in oil pastels under the condition of simulated gastric acid. The results showed that the method had good recovery rate that varied from 93.0 to 103.0%, the relative standard deviation(n=11) was between 1.06 and 1.59%, the detection limits varied from 0.098×10-3 to 4.71×10-3 μg·g-1, with the limits of quantification between 0.327× 10’3 and 15.7×10-3 μg·g-1. The method is applied in determination of the migration of trace cadmium, chromium and lead in oil pastels.3. A flame atomic absorption spectrometric method was developed for the determination of the "migration" of trace cadmium in oil pastels using ionic liquids based ultrasonic assisted extraction. The main factors affecting the extraction efficiency, such as pH value, amount of metal chelating, ionic liquid and nitric acid, ultrasonic extraction time, different interfering ions and chain length of ionic liquid were studied. The optimal conditions were as follows:pH value:9.0, amount of dithizone:2.00 mL 0.50 g·L-amount of ionic liquid:2.00 mL, amount of nitric acid:4.00 mL 1.00 mol·L-1, ultrasonic extraction time:5 min. Under the optimized conditions, the extraction efficiency of cadmium could reach 99%, enrichment times were 25. The recoveries varied from 96.0% to 105.0%, the relative standard deviation (n=11) was 1.7%, and the detection limit was 0.016 μg·g-1 and the limit of quantification was 0.053 μg·g-1. The method has low detection limit, low limit of quantification, good precision and recovery rate with more convenient operation and lower cost than the method for the determination of the "migration" content of trace cadmium, chromium and lead in oil pastels under the condition of simulated gastric acid. Therefore, it can be used to determine large quantities of samples. Our experiments also provide some clues for detection of other trace heavy metal ions.4. The "total" and "migration" content of trace cadmium, chromium and lead in oil pastels bought in the market were determined by the methods that established. The results showed that "total" content of trace cadmium, chromium and lead were 1.275-3.691 μg·g-1, 2.493-4.538 μg·g-1 and 1.339～3.533 μg·g-1, respectively. The "migration" of trace cadmium, chromium and lead were 0.198～0.519 μg·g-1, 0.214-0.468 μg·g-1, and 0.137～0.328 μg·g-1, respectively, and the migration rate was just between 7.40 and 16.9%. It showed that the "migration" of trace cadmium, chromium and lead in the oil pastels were very low and which was far below the limit values of European Union Safety Standard of Toys. But, the "total" of trace cadmium, chromium and lead in the oil pastels were higher than the "migration". It showed that the oil pastels bought in the market were qualified to be used safely by children. On the other hand, in order to produce quality toys, the limit values of the "total" should be concerned when we pay attention to the "migration" of heavy metals in toys. |
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