Microwave-assisted sample preparation of medicines for determination of elemental impurities in compliance with United States Pharmacopeia: How simple can it be?

This work proposed a procedure for microwave-assisted sample preparation of medicines using diluted nitric acid followed by determination of elemental impurities using inductively coupled plasma optical emission spectrometry (ICP OES) and inductively coupled plasma mass spectrometry (ICP-MS) according to the United States Pharmacopeia Chapters 232 and 233. Three solutions, i.e. inverse aqua regia, 7.0 and 2.0 mol L-1 HNO3, were evaluated for microwave-assisted digestion of nine drugs samples. The applicability of each digestion procedure was assessed by comparison of analyte concentrations determined using total (reference procedure) and partial digestions (proposed procedure) as well as by determining dissolved carbon content and evaluating matrix effects. There were none significant differences at a 95% confidence level among the concentrations determined applying reference and proposed procedures. Internal standardization (ICP OES) and aerosol dilution (ICP-MS) were applied for minimization and correction of matrix effects. Addition and recovery experiments were performed according to oral permissible daily exposures values specific for each element and each sample was spiked with element concentrations of 0.5J and 1.5J in order to check accuracies for 24 analytes. Recoveries ranged from 70 to 138% for ICP OES and from 72 to 128% for ICP-MS, for all elements but Os. All analytes were below the respective limits of quantification when applying all sample preparation procedures, except As, Ba, Co, Cu, Cr, Mo, Ni, Pb, Sb, Sn, Tl and V, however the determined concentrations for these elements were lower than the limits proposed by Chapter 232.