Microwave-assisted extraction for further Cl, Br, and I determination in medicinal plants by ICP-MS: a study of carbon interferences

Abstract

A microwave-assisted extraction (MAE) method for further Cl, Br, and I determination in medicinal plants was developed. Halogen determination was performed by inductively coupled plasma mass spectrometry (ICP-MS) and a study on carbon interferences was carried out using a sample extract. The sample mass used for the optimization of the MAE method was 0.5 g. The following parameters for MAE were evaluated: the extraction solution (6 or 10 mL of water, 100 mmol L⁻¹ NH₄OH, or 100 mmol L⁻¹ TMAH), extraction time (5 or 20 min), and temperature (90 or 240 °C). The accuracy was evaluated by analyte recovery experiments, by using certified reference materials (CRMs), and by comparing the results with those obtained using a reference method (microwave-induced combustion). Using the optimized extraction conditions (6 mL of 100 mmol L⁻¹ TMAH solution and 5 min of microwave heating at 240 °C), no statistical difference was observed in comparison to the certified values. Recoveries were close to 95% and the relative standard deviation was lower than 8%. The carbon concentration in the extracts using H₂O, 100 mmol L⁻¹ NH₄OH or 100 mmol L⁻¹ TMAH was in the range of 980 to 9800 mg L⁻¹. In a systematic study of interferences with the sample extract, no interferences were observed for Cl and Br up to 1000 mg L⁻¹ of carbon in solution. On the other hand, it was possible to observe signal enhancement for I with 500 mg L⁻¹ of carbon. Thus, a dilution step was needed to obtain an accurate analytical response for this element. Blank values were always negligible resulting in relatively low limits of quantification: 125, 0.125, and 0.045 μg g⁻¹ for Cl, Br, and I, respectively. In this way, the proposed MAE method was considered suitable, simple and fast for further Cl, Br, and I determination by ICP-MS. However, care regarding the carbon content to avoid erroneous signal enhancement is necessary.