Three-component deep eutectic solvent-based microextraction approach for biodiesel quality control: Determination of water and metals

Biodiesel is an important alternative fuel produced by blending biofuels with petroleum diesel. Biofuel production is usually carried out by catalytic interesterification of animal fats or vegetable oils, followed by the removal of by-products and catalysts through washing with water. Residual amounts of water and catalysts can accelerate fuel biodegradation and contribute to the corrosion of metal surfaces. Considering the need to control the quality of biodiesel fuel, a simple and environmentally friendly analytical approach was first developed for the determination of water and metals (sodium, potassium, magnesium, calcium as catalysts, and iron, copper, tin, zinc, lead as feedstock elements) using a deep eutectic solvent. The technique involved microextraction of analytes from biodiesel into a new three-component deep eutectic solvent (choline chloride, lactic acid, and cobalt chloride) using a laboratory vortex, followed by impregnating the filter paper with the obtained extract to determine the analytes. The colorimetric determination of water is based on a change in the color of the deep eutectic solvent, which goes from blue to pink due to the formation of cobalt aqua complexes. The detection of water on the filter paper was carried out using a conventional scanner. The content of metals on the filter paper was determined by X-ray fluorescence spectrometry. Under optimal conditions, the detection limit was 100 mg kg^-1 for water and 0.03-3 mg kg^-1 for metals. The technique enabled multicomponent analysis to be carried out without the use of dangerous Karl Fischer reagents (for water determination) and microwave mineralization of samples (for elemental analysis). The enrichment factor ranged from 290 to 330, and the RSD was less than 15%.