Gradient-based High-Performance Thin-Layer Chromatography for an Expanded SARA Analysis of Heavy Petroleum Products
A complete molecular separation of heavy petroleum products is not feasible for their quality control. They are instead characterized in terms of chemical family composition, using classical Hydrocarbon Group Type Analysis such as SARA (Saturates, Aromatics, Resins, Asphaltenes). However, simple SARA does not provide information enough to correlate chemical differences of products with their conversion parameters in the case of heavy products. Automated Multiple Development (AMD) is a technique for HPTLC development that combines incremental multiple development and solvent gradient in short steps. AMD appears as an interesting alternative to obtain different separations with increasing level of complexity for these products. An optimized 20-step, THF-DCM-n-heptane gradient has been used for profiling a variety of samples that covers the whole range of petroleum heavy products. Conditions can be fine tuned to obtain at will either an increased expansion of the asphaltenic-resinic chromatographic zone, or an increased expansion of the apolar zone (saturates-naphthenoaromatics) of products such as vacuum gas oil and residues, bitumes, asphaltenes or base oils. Detection has been carried out by UV and fluorescence densitometry. Saturates can be detected by Fluorescence Detection by Intensity Changes (FDIC) as a positive peak using berberine-impregnated plates. Some examples of application will be presented.