Desenvolvimento de método para determinação de glicerol livre em biodiesel a partir da extração em barra de agitação revestida com celulose e quantificação por HPLC-DAD

Abstract

An analytical method based on stir-bar sorptive extraction (SBSE), using monolithic cellulose coated rods, coupled with high-performance liquid chromatography (HPLC), was developed for the determination of free glycerol in biodiesel. The sorbent phases were obtained by reprecipitation of cellulose acetate, followed by in situ deacetylation, resulting in a hierarchically porous structure with high surface area and high affinity for glycerol, mainly attributed to hydrogen bonding interactions between the hydroxyl groups of cellulose and the analyte. Structural characterization by FTIR, N2 adsorption/desorption isotherms, and BET/BJH analyses confirmed the efficient conversion of the precursor and the formation of a mesoporous structure suitable for adsorption and mass transfer processes. The extraction and desorption conditions in the SBSE technique were optimized using a multivariate design via the Doehlert Matrix. The extracts obtained were derivatized with benzoyl chloride, producing glyceryl tribenzoate, a species highly absorbent of UV radiation at 239 nm. Monolithic cellulose rods, 10 mm long, showed better analytical performance, with recoveries close to 100% and low variability in response during glycerol extraction. The maximum adsorption capacity was estimated at 63.0 μg g-1, consistent with the Langmuir model. The method was validated according to document SANTE 11312/2021.v2026, showing linearity between 0.00002 and 0.02000 % (w/w) (r = 0.9991), limits of detection and quantification of 7.8 × 10-6 and 2.4 × 10-5 % (w/w), respectively, as well as satisfactory precision (RSD < 7.5 %) and recovery rates between 94.7 and 113.9 %. The proposed method was successfully applied to the determination of free glycerol in biodiesels obtained from different raw material sources, with results in accordance with ASTM D6751 and EN 14214 standards. Compared to the official AOCS Ca 14 56 method, the proposed method showed greater analytical sensitivity and lower limits of detection. The sorbent bars demonstrated good stability after multiple reuse cycles, while the AGREEprep (0.77) and BAGI (75) metrics evidenced the reduced environmental impact of the methodology. Thus, the developed SBSE-HPLC method constitutes a simple, sensitive, selective, and environmentally sustainable alternative for the determination of free glycerol in biodiesel.