Identification of drying oils used in pictorial works of art by liquid chromatography of the 2-nitrophenylhydrazides derivatives of fatty acids

A new HPLC-UV-Vis method for identification of drying oils from binding media or protective film used in pictorial works of art prior to conservation or restoration is proposed. Chromophore derivatization of fatty acids released by hydrolysis of structural drying oils is studied. The derivatization reagent selected was 2-nitrophenylhydrazine with 1-ethyl-3-(3-dimethyl animopropyl)carbodiimide hydrochloride/pyridine as catalyst. This reaction was carried out using microwave heating. Mobile phase was methanol/water/n-propanol/acetic acid (80:14:5:1) running in isocratic mode. Absorbance was measured at 400 nm. In these conditions, hydrazides of myristic, palmitic, oleic, and stearic acids were satisfactorily resolved. Method shows good sensitivity, with a detection limit of 15 μmol l⁻¹, and good linearity between 0.03 and 3 mmol l⁻¹. Peak area ratios among fatty acids derivatives allows identification of the drying oils. The stearic/palmitic ratio is the most important, because it allows to differentiate among the different drying oils. The proposed method has been successfully applied to real samples from items of the cultural heritage of Valencia (Spain).