Direct sample introduction in high-performance liquid chromatography analysis of oily medicinal cannabis samples using surfactant-free microemulsion

Abstract

The therapeutic use of medicinal Cannabis has raised challenges in quality control and cannabinoid quantification in oil extracts, primarily due to incompatibility with liquid chromatography systems. Surfactant-free microemulsion (SFME) systems offer a promising, efficient, and sustainable solution, enabling direct analysis by high-performance liquid chromatography. These microemulsions are dispersions of two immiscible liquids, typically oil and water, made compatible by an amphiphilic substance that interacts with both polar and nonpolar fractions. After SFME testing and critical analysis of the results, clear and homogeneous microemulsions were observed regardless of the oil-to-water ratio studied, provided that sufficient quantities of 1-octanol:1-propanol (3:10 v/v) were added. Based on the established robust working range, the 1:2 (oil:ultrapure water) ratio (% w/w) was chosen as the compromise condition, minimizing medicinal oil use while ensuring compatibility with the chromatographic system. Univariate optimization of chromatographic conditions enabled simultaneous analysis of five cannabinoids, including cannabidiol (CBD), cannabidiolic acid (CBDA), tetrahydrocannabutol (THCB), cannabinol (CBN), and tetrahydrocannabinol (Δ9-THC) using a ZORBAX Eclipse Plus C18 column at 35 °C. The method employed isocratic elution with a mobile phase of 82:18 %v/v methanol:ultrapure water containing 0.15 ± 0.05 % v/v formic acid, a 1.0 mL min⁻¹ flow rate, and a 20 μL injection volume, using the fluorescence and UV absorption detectors. This proposal offers sensitivity (LOD in the ng mL⁻¹), selectivity, and separation efficiency, associated with low cost, minimal waste generation, and low energy demand, aligning with Green Chemistry principles.