Evaluation of Sample Preparation Procedures for Trace Element Determination in Brazilian Propolis by Inductively Coupled Plasma Optical Emission Spectrometry and Their Discrimination According to Geographic Region

Abstract

Propolis is a complex mixture of substances collected by honeybees from buds or exudates of plants, beeswax, and other constituents, as pollen and sugars. The main purpose of this study was to evaluate two digestion procedures for determination of major, minor, and trace elements (Ba, Ca, Cd, Cr, Co, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, and Zn) in natura propolis samples by inductively coupled plasma optical emission spectrometry (ICP OES). The first procedure studied was an open-vessel digestion using HNO3 + H2SO4 + H2O2 in a heating block and the second one was a microwave-assisted concentrated acid digestion using HNO3 + H2O2. Both digestion procedures led to similar results and quantitative recoveries. The residual carbon contents (RCCs) for propolis sample digests were 0.269 ± 0.012 % when using the first procedure with conventional heating and 0.458 ± 0.023 % by microwave-assisted closed vessel digestion, demonstrating high efficiency of both procedures. Accuracy of the results was demonstrated using a certified reference material and by comparison with a recommended official method. The t test (unpaired) at 95 % confidence level showed that there was no significant difference between determined and certified values of all analytes under investigation, except Ca concentration employing conventional procedure. The optimized microwave-assisted digestion procedure led to recoveries around 89–103 % and precision better than 5 % for most samples. The second procedure was faster, safer, and more accurate than the one based on conductive heating. Additionally, principal component analysis (PCA) was applied for checking if there was correlation between inorganic composition and source of propolis samples collected around Bahia State in the Northeast of Brazil.