Digestion of herbal products, assisted by microwave, for spectrophotometric analysis of lead

The phytotherapeutic products are used extensively throughout the world, for this reason governments must ensure quality, safety and efficacy of them. The herbal products may be a source of toxic metals, either by natural contamination or by the same human activities, including adulteration. Toxic metals exposure, such as lead and mercury, causes serious adverse health effects, therefore it must be controlled. Wet digestion with oxidizing acids is the most common sample preparation for metallic elements determination. Nonetheless, today is known that the use of microwave ovens improves greatly the efficiency of both dissolution and digestion. In this paper, we evaluate a microwave-assisted acid digestion by using an acid/oxidant mixture solution and a domestic microwave-equipment as method preparation for determining lead in a phytotherapeutic product (Taraxacum officinale). Digestion was carried out in three sequential steps: firstly, acid/oxidant treatment (HNO3) for 30 seconds by using the microwave-assisted acid digestion method; after, an intermediate cool treatment procedure for 5 minutes; and finally, oxidant treatment with H2O2 for 30 seconds. The quantitative analysis was performed using atomic absorption spectrophotometry with electrothermal atomization. The analyzed sample did not exceed the Pb concentration of 10 mg kg-1 for herbal products suggested by the WHO. Nevertheless, the method could be applied in the determination of Pb at levels very close to the limit set by WHO, and even above it, with a variation coefficient better than 10% (including the digestion system and analytical system). This preliminary proposal represented a significant time/cost saving in the development of the proposed technique.