Submitted to: Analytical Chemist
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/8/2003
Publication Date: 12/1/2003
Citation: Zhang, J., Satterfield, M., Brodbelt, J., Britz, S.J., Clevidence, B.A., Novotny Dura, J. 2003. Structural characterization and improved detection of kale flavonoids by electrospray ionization mass spectrometry. Analytical Chemistry. 75(23):6401-07. Interpretive Summary: Flavonoids are a class of compounds found in fruits and vegetables that appear to protect the body against oxidative damage. Flavonols are a subgroup of flavonoids that contain two compounds of interest, quercetin and kaempferol. Measuring and detecting these compounds in the lab is difficult, and currently available methods are often not sensitive enough to measure the levels of flavonols found in foods, blood, or urine. We report an improved method for measuring quercetin and kaempferol using specialized equipment called high performance liquid chromatography electrospray ionization mass spectrometry (HPLC-ESI-MS), and we confirmed the identity of these compounds by a technique called collision activated dissociation (CAD). Subsequently, we applied this method to a food and biological samples. Commercial kale samples were found to have variable levels of flavonols: 77-244 ppm quercetin and 235-347 ppm kaempferol. Blanching, a technique used prior to freezing of vegetables to maintain quality during freezing, reduced the flavonol concentrations ~60%. We also report that different wavelengths of ultraviolet light can affect flavonol content of kale. This is important for understanding the variability in flavonol content of vegetables and for understanding how to enhance flavonol content of foods. This report will be especially useful to plant scientists studying flavonol production in plants, chemists measuring flavonols in plants or biological samples, and nutritionists and nutrition scientists considering the impact of blanching on flavonol content of vegetables.
Technical Abstract: Sensitive and precise analytical methods are needed for flavonols, a subclass of flavonoids that have strong antioxidant activity. We report an improved method for identifying the predominant flavonols, quercetin and kaempferol, by collision activated dissociation (CAD) and quantifying them by high performance liquid chromatography electrospray ionization mass spectrometry (HPLC-ESI-MS). Applications of the method were demonstrated using a food and biological samples. Commercial kale samples were found to have 77-244 ppm quercetin and 235-347 ppm kaempferol. Blanching reduced the flavonol concentrations ~60%. Isotopically-labeled Vates kale grown in a Plexiglas chamber under an atmosphere of carbon-13 dioxide was found to have much lower flavonol concentrations. UV-A and UV-B supplementation during kale growth in a greenhouse enhanced both quercetin and kaempferol levels in Vates kale. The UV-B supplemented kale not only produced more flavonols, but the quercetin to kaempferol ratio was also higher than that of the UV-A supplemented or the non-supplemented kale. Recovery of flavonols from kale was ~60% based on spike and recovery trials with rutin, a glycoside of quercetin. Recovery of flavonols from biological samples spiked with rutin ranged from 96% for urine to 70% for plasma. Compared to UV detection, ESI-MS in the deprotonation mode provided lower detection limits, and both higher sensitivity and selectivity, in addition to structural characterization of the kale flavonols by CAD.