Isolation and determination of phenolic acids in rowanberry fruits (Sorbus L.) by gas chromatography

Species of the genus Sorbus L. (rowan) are woody plants growing in the Northern Hemisphere; they are of interest as decorative, food and medicinal plants. Rowan fruits contain a unique complex of macro- and microelements, as well as biologically active substances, including phenolic compounds. Complexity of the composition of plant samples requires the development of highly selective and sensitive methods for determining the content of organic components of different classes. A method for determining phenolic acids in rowan fruits is proposed. The method is based on two-stage chemical modification of the acids, and involves preparing methyl esters under acidic methanolysis conditions, liquid extraction with toluene, intermediate re-extraction into an aqueous alkaline solution, subsequent silylation with N, O-bis-(trimethylsilyl)trifluoroacetamide (BSTFA) and gas chromatographic determination of the obtained derivatives using a flame ionization or mass spectrometric detector. Conditions for extraction of phenolic acids from rowan fruits were studied according to the following parameters: type of extractant, sample to the extractant mass ratio, and duration of extraction (mechanical stirring). The stage of gas chromatographic determination of obtained phenolic acid derivatives was optimized, and their chromatographic characteristics established. The possibility of achieving practically complete separation of aliphatic acids and other accompanying components of a non-acidic nature from phenolic compounds in the course of sample preparation was demonstrated; which significantly increased both the selectivity and sensitivity of the determination of analytes. The range of determined content of phenolic acids (4-hydroxybenzoic acid, 2-hydroxybenzoic acid, 4-hydroxy-3-methoxy-benzoic acid, 4-hydroxycinnamic acid, 4-hydroxy-3-methoxy-cinnamic acid) in rowan fruits was 0.005–0.5 mg/g, the limit of detection was 0.001–0.002 mg/g, the relative error was 10–15%, the sample weight was 0.5 g, the total duration of the analysis was 4 hours.