Determination of pistachio (Pistacia vera L.) hull (exo- and mesocarp) phenolics by HPLC-DAD-ESI/MSn and UHPLC-DAD-ELSD after ultrasound-assisted extraction

A method for the simultaneous extraction and quantitation of gallotannins, flavonoids and anacardic acids from pistachio hulls is presented. After screening of various extraction parameters and solvent systems with variable polarity and acidity, a three-fold repeated extraction with methanol/water/formic acid (80/19/1, v/v/v) at a sample-to-solvent ratio of 1/20 (w/v) under probe-sonication was found to be most suitable for extracting ≥95% of the abovementioned phenolics. A newly developed UHPLC-DAD-ELSD method was compared to an HPLC method for compound separation and quantitation. While analyses by UHPLC were 3.7 times faster, the core-shell particle HPLC column used allowed a superior peak resolution, particularly regarding anacardic acids. While phenolic acids were virtually undetectable by ELSD, it may be applicable for the determination of anacardic acids. However, ELSD sensitivity was inferior to that of the DAD used. Furthermore, validation parameters such as signal linearity, detection and quantitation limits, extraction recoveries, intra- and inter-day repeatability are presented. The new method was applied to pistachio hulls of three different varieties. Phenolic contents ranged from 61.2 ± 0.45 to 100 ± 0.48 g/kg dry matter. Among phenolic constituents, anacardic acids prevailed (64.6-80.4% of total phenolics) followed by gallotannins (13.4-21.2%), such as β-glucogallin, gallic acid, and penta-O-galloyl-β-d-glucose, and flavonol glycosides (5.7-16.3%).