A sample preparation method for fluoride detection by potentiometry with ion-selective electrode in medicinal plants

Abstract

In this study, the fluoride concentration in medicinal plants was determined in a range from 2.5–250 μg g−1. Fluoride was determined using potentiometry with ion-selective electrode (ISE) after sample preparation. Sample masses ranging from 0.1–1.0 g were pressed into pellets and burned using the microwave-induced combustion (MIC) method. After combustion, fluoride was absorbed in diluted ammonia for further determination by using ISE. Recoveries close to 100% were obtained with a low relative standard deviation (5%), using 20 bar of oxygen and 5 min of reflux. Solutions obtained using the MIC method presented low carbon content (< 25 mg L−1) and avoided additional dilution prior to analysis. Up to 1.0 g of medicinal plant was efficiently digested, facilitating fluoride determination at low levels (limit of quantification was 2.5 μg g−1). No statistical difference (t-test, 95 % of confidence level) was observed between the values obtained using ISE after MIC digestion and the values of certified reference materials (CRMs) of aquatic plant (BCR 60) and olive leaves (BCR 62). In addition, results presented no statistical difference compared to those acquired using the AOAC’s recommended Method 975.04 (Fluoride in plants). A microwave-assisted extraction (MAE) method was also evaluated using the optimized conditions for MIC (1 g, 6 mL of 25 mmol L−1 ammonia solution and 5 min of reflux). However, MAE digests presented high carbon content in final solutions (around 5 g L−1) and interference was observed during fluoride determination using ISE. Therefore, the MIC method allowed to efficiently digest up to 1.0 g of sample, and the analyte was absorbed in a suitable alkaline solution. Using this method, fluoride detection by ISE was possible even for samples at low concentration being considered a rather attractive quality control method for fluoride levels in medicinal plants.