Pavun L., Đurđević P., Jelikić-Stankov M., Đikanović D., Uskoković-Marković S. (2018): Determination of flavonoids and total polyphenol contents in commercial apple juices. Czech J. Food Sci., 36: 233-238.We propose a sensitive and selective spectrofluorimetric method for the determination of flavonoids as expressed in 'quercetin equivalent' in apple juices. The method is based on the strong emission of the aluminium(III)-quercetin complex at 480 nm with excitation at 420 nm, and it is successfully applied for the determination of flavonoids in commercial apple juices and compared with results obtained in reference spectrophotometric determination. The flavonoid content in commercial apple juices was found to range from 5.53 to 15.55 mg/l quercetin equivalent. The very good agreement between the two methods indicates the suitability of the proposed spectrofluorimetric method for the precise and accurate determination of flavonoids. In addition, the total polyphenol content was determined spectrophotometrically using the Folin-Ciocalteu (FC) method and the antioxidative activity of the tested juices was tested in a DPPH assay and these values were correlated with each other. The obtained profiles of compounds with antioxidative ability lead us to conclude that fruit juice labels based only on fruit % might sometimes misinform consumers.

A fluorometric method, based on the fluorescence properties of the aluminium(III)-hesperidin complex, for the determination of hesperidin in human plasma and pharmaceutical forms has been developed and validated. The complex shows a strong emission in the presence of the surfactant betain sulphonate SB 12 at 476 nm with excitation at 390 nm. The linearity range for pharmaceutical forms of hesperidin was 0.06-24.4 μg mL -1 with a limit of detection, LOD, of 0.016 μg mL -1 and a limit of quantification, LOQ, of 0.049 μg mL -1 . Recovery values in the range 99.3-99.7 % indicate good accuracy of the method. A linear dependence of the intensity of fluorescence of the complex on the concentration of hesperidin in plasma was obtained in concentration range from 0.1-12.2 μg mL -1 . The LOD was 0.032 μg mL -1 while LOQ was 0.096 μg mL -1 . Recovery values were in the range 98.4-99.8 %. The reliability of the method was checked by an LC-MS/MS method for plasma samples and an HPLC/UV method for tablets with direct determination of hesperidin after separation. Linearity range in determination of hesperidin in pharmaceutical forms was obtained in the range from 0.05 to 10.00 μg mL -1 . The LOD was 0.01 μg mL -1 and the LOQ was 0.03 μg mL -1 . The linearity range for the determination of hesperidin in plasma was 0.02-10.00 μg mL -1 with an LOD 0.005 μg mL -1 and an LOQ of 0.015 μg mL -1 . The good agreement between the two methods indicates the usability of the proposed fluorometric method for the simple, precise and accurate determination of hesperidin in clinical and quality control laboratories.

A simple, accurate and precise method based on the fluorescence properties of the aluminum(III)-quercetin complex, for the determination of quercetin, has been developed and validated. The complex has strong emission at pH 3.30,  em = 480 nm, with  ex = 420 nm. The linearity range of quercetin determination was 1.5-60.5 ng ml -1 with LOD 0.09 ng ml -1 and LOQ 0.27 ng ml -1. Recovery values in the range of 99.9-100.2% indicate a good accuracy of the method. The established method was applied for the determination of quercetin in capsules, with a recovery value of 98.3%, standard deviation of 0.22% and coefficient of variation of 0.09%.The reliability of the method was checked by the newly developed RP-HPLC/UV method for capsules with the direct determination of quercetin after separation. The good agreement between the two methods indicates the applicability of the proposed spectrofluorimetric method for quercetin determination in pharmaceutical dosage forms, with high reproducibility, and enables the direct and simple determination without its prior extraction from samples.The proposed spectrofluorimetric method has much better sensitivity and LOD and LOQ values that are about 1000 times lower than data reported in the literature.Keywords: quercetin; flavonoids; spectrofluorimetric determination; RP-HPLC; capsules СПЕКТРОФЛУОРОМЕТРИСКО ОПРЕДЕЛУВАЊЕ НА КВЕРЦЕТИН ВО ФАРМАЦЕВТСКИ ДОЗИРАНИ ФОРМИРазвиен и валидиран е едноставен, точен и прецизен метод за определување на кверцетин заснован на флуоресцентните својства на комплексот кверцетин-алуминиум(III). Комплексот покажува силна емисија при pH 3,30,  em = 480 nm и  ex = 420 nm. Линеарниот опсег за определување на кверцетин е 1,5-60,5 ng ml -1 со LOD 0,09 ng ml -1 и LOQ 0,27 ng ml -1. Вредностите на аналитичкиот принос од 99,9-100,2% покажуваат добра точност на методот. Воспоставениот метод е применет за определување на кверцетин во капсули со вредност на аналитичкиот принос од 98.3%, стандардна девијација од 0,22% и коефициент на варијација од 0,09%.Веродостојноста на методот е проверена со новововедениот RP-HPLC/UV метод за капсули со директно определување на кверцетин по негова сепарација. Доброто согласување на двата метода укажува на применливост на предложениот спектрофлуорометриски метод за опреде-лување на кверцетин во фармацевтски дозирани форми со висока репродуцибилност и овозможува директно и едноставно определување без претходна екстракција од примероците.Предложениот спектрофлуорометриски метод има многу подобра осетливост и вредностите LOD и LOQ се за околу 1000 пати пониски од оние дадени во литературата.

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We analyzed low molecular mass phenolics, lignin content and both soluble and cell wall bound peroxidase activity in the needles of three Picea omorika (Pančić) Purkyne lines grown in the generative seed orchard. The highest values of the total phenol content as well as of catechine, caffeic acid, coniferyl alcohol, isoferulic acid and lignin concentration were detected in B5 line ("semidichotomy" line). The soluble guaiacol peroxidase activity was the highest in A3 line (line "borealis"). The highest activity of cell wall bound peroxidases was measured in B5 line, and it was in correlation with lignin content.