In this reported method, dithizone extraction solution was washed with 1% HCl rather than 1% of HNO3 as in the USP method (24) because a trace amount of HNO3 left in the dithizone solution could rapidly decompose the dithizone. After replacing 1% of HNO3 with 1% of HCl, the decomposition problem did not happen again.
To maximally reduce the consumption of chloroform, some preliminary experiments were performed for the recycling of the dithizone
chloroform solution. We found the following procedure could be used for recycling the dithizone chloroform solution: After
lead extraction, the 0.003% dithizone solution is washed with 20 mL of DI water, followed by 20 mL of 1% of HC, and then washed
again with 20 mL of DI water. After washing, a small amount of ethanol is added into the dithizone solution, 1 mL to 100 mL
(1:100 v/v), to stabilize the dithizone. This solution should be kept in a refrigerator until next time of use. Based on our
experimental results, the dithizone solution could be reused for as many as 10 times, as long as the greenish-blue color retains
stable.
Conclusion
This method was used in our quality control laboratory for several months with various samples and proved to be reliable for
the determination of trace amounts of lead in all samples. From sample digestion to interfering element masking and to dithizone
chloroform solution recycling, each step has been optimized. For certain mixture samples of organic ingredients and minerals,
repeated treatment with H2SO4–H2O2 and H2SO4–HNO3 is very effective and could reduce the digestion time dramatically. For the determination of ppm levels of lead in salts
of alkaline earth metals, sulfuric acid should be avoided and an extra amount of citrate reagent should be used to prevent
the precipitation. Recycling the dithizone solution can not only reduce the consumption of chloroform but also maximally lower
the overall cost. This method is very rugged and could be used for routine analysis of trace amounts of lead as well as for
the validation of other instrumental methods such as ICP-MS or GFAA.
Lang Lang is a senior student at School of Pharmacy & Allied Health Science, University of Montana, Missoula, MT. Konghwa Chiu is a professor in the department of applied science, National Hualien University of Education, Taiwan, ROC. Qingyong Lang* is a senior analytical chemist in the R&D department at Nutritional Laboratories International, Missoula, MT, drlang@nutritionallabs.com
*To whom all correspondence should be addressed.
Submitted: Aug. 21, 2007. Accepted: Dec.3, 2007
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