Irena Vovk

Analytical Chemistry



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    ABSTRACT: Since Chinese lantern (Physalis alkekengi L.) represents a rich source of various bioactive secondary metabolites, there is an urge for its detailed characterization. Non-polar flavonoid aglycones represent one of the few bioactive species found in plant’s cuticular waxes. The separation of flavonoids is already extensively covered in the literature, but methods dedicated to separation and identification of methylated flavonoids are rather scarce. In the present study a non-targeted approach for the separation, isolation and identification of methylated flavonoids present in P. alkekengi L. var. franchetii cuticular waxes was established. A rapid and simple separation on HPTLC silica gel was developed for preliminary screening of flavonoids. Fast HPLC–UV–MSn and HPLC–UV methods using a C6-Phenyl and a C18 stationary phase were also developed, respectively. In both cases, the right combination of temperature and tetrahydrofuran, as a mobile phase modifier, were shown to be crucial for a baseline separation of all studied compounds. By employing a semi-preparative analog of the C18 column, a simultaneous isolation of pure unknown analytes was achieved. Using these developed methods in combination with NMR, four 3-O-methylated flavonols were detected and identified in P. alkekengi L. var. franchetii cuticular waxes: myricetin 3,7,3'-trimethyl ether, quercetin 3,7-dimethyl ether, myricetin 3,7,3',5'-tetramethyl ether and quercetin 3,7,3'-trimethyl ether. Moreover, the simple and fast isocratic HPLC–UV–MSn method (under 8 min) should prove useful in quality control of P. alkekengi L. var. franchetii by enabling chromatographic fingerprinting of external methylated flavonols. Finally, a rationale for the mechanism of separation of these metabolites by HPLC is also given, which establishes a foundation for future development of chromatographic methods for methylated flavonols and related compounds.
    No preview · Article · Feb 2016
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    ABSTRACT: The anthocyanin composition of blue (Triticum aestivum L., cv. Skorpion) and purple wheat (Triticum aethiopicum JAKUBZ cv. Abyssinskaja arrasajta cv. Abyssinskaja arrasajta), cultivated in the Czech Republic, and of the prepared whole blue and purple wheat bread was determined. In blue and purple wheat, 19 and 26 anthocyanins, respectively, were tentatively identified by liquid chromatography and mass spectrometry. The total content of anthocyanins determined in blue and purple wheat was 9.26 and 13.23 mgkg(-1), respectively. The breads were baked at 240 and 180 °C. Some significant differences in anthocyanins content were observed between breads prepared at different baking temperatures. The content of cyanidin-3-glucoside, delphinidin-3-glucoside and pelargonidin-3-glucoside was determinated in starting material, whole meal flours and baked breads. These kinds of wheat are suitable for baking bread, since intake of anthocyanins may play an important role in the prevention of human diseases.
    No preview · Article · Jun 2015 · International Journal of Food Sciences and Nutrition
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    ABSTRACT: Cholesterol is an essential component of mammalian cells, but its role in cancer is unclear. We have determined the total cholesterol content in the healthy and the cancerous lung tissues of the same patient. Tissues of 13 patients (7 women and 6 men, different histological type, different stages of the disease) were obtained during surgical intervention. The samples (0.056-2.004 g) were hydrolyzed in alkaline medium, and the total cholesterol was extracted into n-hexane and simultaneously determined by thin-layer chromatography (TLC) on silica gel high-performance thin-layer chromatography (HPTLC) plates and nonaqueous reversed-phase high-performance liquid chromatography (HPLC), which both proved to be suitable for the determination of cholesterol in lung tissues and gave comparable results. This is the first report about the comparison of the total cholesterol content in the healthy and the cancerous tissue of the same patient. The difference in the results for the total cholesterol from both types of tissues was remarkable. For 13 patients, the mean contents and standard deviations determined by TLC were 4.01 ± 0.75 mg g-1 in the healthy lung tissue and 7.75 ± 1.93 mg g-1 in the cancerous lung tissue. Comparable results were obtained by HPLC analyses of the same samples: 3.91 ± 0.73 mg g-1 in the healthy and 6.95 ± 1.83 mg g-1 in the cancerous lung tissue. The range of total cholesterol content determined by TLC was between 3.20 mg g-1 and 5.83 mg g-1 in healthy lung tissues, and between 4.64 mg g-1 and 12.01 mg g-1 in lung cancer tissues, while the ranges obtained by HPLC were between 2.80 mg g-1 and 5.49 mg g-1 for healthy lung tissue, and between 4.38 mg g-1 and 11.24 mg g-1 for lung cancer tissue. The cancerous lung tissue of each of the thirteen patients contained higher amounts of total cholesterol compared to their healthy lung tissue. In 8 of the patients, the total cholesterol in the cancer tissue was more than 60% higher than in the healthy tissue; furthermore, in 5 patients, it was more than 100% higher.
    No preview · Article · Apr 2015 · JPC - Journal of Planar Chromatography - Modern TLC
  • Katerina Naumoska · Irena Vovk
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    ABSTRACT: Three TLC methods were used for an initial screening of some common plant triterpenoids and phytosterols in cuticular wax extracts of different vegetables (zucchini, eggplant, tomato, red pepper, mangold, spinach, lettuce, white-colored radicchio di Castelfranco, raddichio Leonardo, white cabbage, red cabbage and savoy cabbage). The preliminary experiments showed that the studied vegetables are potential sources of triterpenoids and phytosterols. To identify the compounds present in the extracts with high certainty, the first TLC-MS(2) method was developed for the analysis of eight triterpenoids (lupeol, α-amyrin, β-amyrin, cycloartenol, cycloartenol acetate, lupeol acetate, lupenone and friedelin) and two phytosterols (β-sitosterol and stigmasterol). This method takes the advantages of: (1) a satisfactory separation of the target compounds; (2) their differentiation according to the band colors; and (3) the potential of their discrimination by the acquired first-order mass (MS) and product ion (MS(2)) spectra. Since the closely eluting compounds have complex and similar MS(2) spectra, distinguishing between them was possible by the proposed characteristic ions. Using a custom-built mass spectral library, the head to tail MS(2) spectra comparison of sample test solution zones and standard aided the compound identification. In addition to the molecular mass information, the developed atmospheric pressure chemical ionization method (APCI) in positive ion mode provided structural information, regarding the presence of functional group in the molecule. This approach resulted in many positively assigned compounds in the investigated vegetable waxes, from which more than a half are reported for the first time. Copyright © 2015 Elsevier B.V. All rights reserved.
    No preview · Article · Jan 2015 · Journal of Chromatography A
  • Irena Vovk · Vesna Glavnik
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    ABSTRACT: This chapter describes the state of the art and the potential of the instrumental thin-layer chromatography (TLC) in the analysis of dietary supplements. To be discussed are the analytical challenges related to the endless possible combinations of bioactive ingredients and excipients; chemically undefined plant ingredients; adulterants; and consequently, the lack of the analytical methods, reference standards, and standard reference materials (SRMs). Additionally, the chapter presents the instrumental possibilities, especially those used for the detection (densitometry, image analysis, mass spectrometry, Raman spectroscopy) as well as selected published and some new applications in dietary supplement analysis. Apart from being used for chemical fingerprinting, TLC has a tremendous potential (1) in the analysis of adulterants, (2) in supporting the identification and characterization of biomarkers, and (3) in the generation of the SRMs.
    No preview · Chapter · Jan 2015
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    Etil Guzelmeric · Irena Vovk · Erdem Yesilada
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    ABSTRACT: Brewed tea of chamomile flowers (Matricaria recutita L.) (Asteraceae) has been extensively consumed for centuries due to either its pleasant taste or medicinal purposes. On the other hand, the major problem is difficulty in distinguishing the genuine specimen when supplying chamomile through nature-picking. Consequently flowers of other Asteraceae members resembling to chamomile in appearance may frequently be practiced by lay people or marketed in spice shops or bazaars. Evidently detection of such adulterations plays a vital role in terms of public health to avoid risk of toxicity (i.e. pyrazolidin alkaloids) and ineffective treatments (lack or insufficient concentration of the active constituents). This work presents either development and validation of a high performance thin-layer chromatographic (HPTLC) method for apigenin 7-O-glucoside which is one of the active markers in chamomile flowers or its application for the fingerprint discrimination of chamomile-like materials i.e. Anthemis spp., Bellis spp., Chrysanthemum sp. and Tanacetum sp. gathered by local people assuming as chamomile. Separation was performed on the silica gel 60 NH2 F254s HPTLC plates using the developing solvent system of ethyl acetate-formic acid-acetic acid-water (30:1.5:1.5:3, v/v/v/v). The proposed HPTLC method may also be a leading guide for the quality assessment of chamomile tea products on the market. Copyright © 2014 Elsevier B.V. All rights reserved.
    Full-text · Article · Dec 2014 · Journal of Pharmaceutical and Biomedical Analysis
  • Alen Albreht · Borut Zupančič · Irena Vovk
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    ABSTRACT: L-Carnitine is used extensively in functional foods and food supplements; consequently, the control of its enantiomeric purity is of paramount importance. A new derivatization procedure and chiral gas chromatographic method with flame ionization detection, using a cyclodextrin based stationary phase, enables prompt, simple, and inexpensive screening of the enantiomeric ratio of L- and D-carnitine in samples with different matrices. Conversion of carnitine to beta-acetoxy-gama-butyrolactone was optimized for maximum conversion (98% of the desired product lactone was formed and 2% of the side product gama-crotonolactone) and minimum racemization (no changes at the chiral center were detected) and time consumption. As it is shown in this study, a fast gas chromatographic method, with total run time of 7 min, together with the new derivatization procedure enables an effective enantiomeric purity screening of L-carnitine in real samples such as food supplements and L-carnitine raw ingredient.
    No preview · Article · Dec 2014 · Acta Chimica Slovenica
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    ABSTRACT: High-performance thin-layer chromatography (HPTLC) combined with image analysis and pattern recognition methods were used for fingerprinting and classification of 52 propolis samples collected from Serbia and one sample from Croatia. Modern thin-layer chromatography equipment in combination with software for image processing and warping was applied for fingerprinting and data acquisition. The three mostly used chemometric techniques for classification, principal component analysis, cluster analysis and partial least square-discriminant analysis, in combination with simple and fast HPTLC method for fingerprint analysis of propolis, were performed in order to favor and encourage their use in planar chromatography. HPTLC fingerprint analysis of propolis was for the first time performed on amino silica plates. All studied propolis samples have been classified in two major types, orange and blue, supporting the idea of existence of two types of European propolis. Signals at specific RF values responsible for classification of studied extracts have also been isolated and underlying compounds targeted for further investigation.
    Full-text · Article · Apr 2014 · Journal of Chemometrics
  • Jasna Uranjek · Irena Vovk · Lidija Kompan
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    ABSTRACT: AimGlutamine administration influences intestinal permeability (IP). Enteral and parenteral glutamine supplementations have different metabolic pathways. In the present study, we investigated the effect of the route of glutamine supplementation on IP. The infection rate, inflammatory parameters and treatment outcome were the secondary end-points in this study. Patients and MethodsA prospective, single-blind study was performed in mechanically-ventilated, early enterally-fed, mixed-surgery, intensive care unit (ICU) patients, randomly assigned to the parenteral group (group P) or enteral group (group E). The supplemented groups were treated with glutamine for 5 days. IP was measured using the lactulose/mannitol (L/M) test at the end of the study. ResultsA total of 81 patients completed the study; 39 from group P and 42 from group E. We found no difference in the L/M index (0.492 ± 0.68 group P, 0.521 ± 0.86 group E; P = 0.88) and the ICU-acquired infection rate (38 per cent group P, 28 per cent group E; P = 0.34). A positive correlation between the start of enteral feeding and the L/M index was confirmed (correlation coefficient: 0.36, P = 0.003), but not with the dose (correlation coefficient: −0.019, P = 0.9) of glutamine supplementation. The outcome data also showed no statistical significant difference. Conclusions The results of this pilot study showed no difference in the IP, infection rate and 6 months' survival between the parenteral or enteral glutamine-supplemented patients.
    No preview · Article · Nov 2013 · Surgical Practice
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    ABSTRACT: An isocratic HPLC method was developed for the determination of eight xanthophylls (lutein, capsanthin, zeaxanthin, canthaxanthin, β-apo-8'-carotenal, ethyl-8'-apo-β-carotene-8'-oate, citranaxanthin and β-cryptoxanthin; registered as additives in poultry feeding) in egg yolks. Optimum separation of all-E-isomers of these xanthophylls was achieved in less than 18min on a ProntoSIL C30 column at 27°C using acetone-methanol-0.5M triethylammonium acetate buffer pH 7 14:5:1 (v/v) as the mobile phase with a flow rate of 1mL/min using spectrophotometric detection at 450nm. Other mobile phases were also found suitable, including acetone-water 93:7 (v/v) and acetone-methanol 1:4 (v/v) and the influences of column temperature on the separation and addition of triethylammonium acetate buffer pH 7 to the mobile phase on enhancement of the peak areas were evaluated. Preparation of test solution from yolks included a short vortexing of 0.5g of yolk in 10mL of acetone, followed by 15min magnetic stirring under nitrogen and centrifugation. The method was validated for 5 analytes. The calibration range was between 0.04 and 2μg/mL and the mean recovery of the analytes (95%) and the intra-day precision of the method (RSD less than 5%) on three levels in triplicates were obtained. Analyses of eggs from four husbandry classes showed the presence of up to four xanthophylls (lutein, zeaxanthin, canthaxanthin and ethyl-8'-apo-β-carotene-8'-oate) and traces of β-cryptoxanthin.
    No preview · Article · Sep 2013 · Journal of Chromatography A

  • No preview · Article · Sep 2013 · Lc Gc North America

  • No preview · Article · Sep 2013 · LC GC Europe
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    ABSTRACT: During coffee roasting major changes occur in coffee bean composition. Among others dark coloured melanoidins are formed, which are high molecular weight Maillard reaction products. A new approach is presented here to monitor the influence of roasting conditions on the antioxidant capacity of melanoidins and chlorogenic acids (CGAs) in a coffee brew. Validated Folin-Ciocalteu (FC) and ABTS assays were used as on-line antioxidant assays coupled (post-column) with high performance size-exclusion chromatography (HPSEC). HPSEC enabled the separation of melanoidins from CGAs and the determination of the antioxidant capacity of each fraction, within a total elution time of 25 min. Besides the on-line assay measurements, both assays were also applied off-line with flow injection analysis (FIA). The maximum antioxidant capacity was determined to be at a light-to-medium roast degree, measured with both ABTS-FIA and FC-FIA assays as well as on-line ABTS assay. With FC on-line assay the maximum was found to be at a very light roast degree. Based on the peak areas obtained with the new coupled technique the roasting effects on the variability of melanoidin and CGA contents in coffee brews were studied. The majority of melanoidins are already formed in the early stage of the roasting process and the relative contribution of melanoidins to the total antioxidant capacity increases towards darker roasts, mainly because CGAs degrade during roasting. A new parameter, the ratio of melanoidin to CGA peak area, was introduced as a possible predictor of the roast degree.
    Full-text · Article · Apr 2013
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    ABSTRACT: Separation of three triterpenic acids (ursolic, oleanolic and betulinic acid) was achieved on different thin-layer chromatography (TLC) (silica gel 60) and high-performance thin-layer chromatography (HPTLC) sorbents (silica gel 60, C2 RP and C18 RP) using several developing solvents, based on the non-polar diluent n-hexane, and ester (methyl acetate, ethyl acetate, ethyl propionate) as selector. Anisaldehyde and molybdophosphoric acid detection reagents were used. Finally, a simple method on a C18 RP HPTLC plate was developed using n-hexane-ethyl acetate (5:1 v/v) as a developing solvent in a horizontal developing chamber. The method was used for the screening of ursolic, oleanolic and betulinic acids in different vegetable extracts. Other plant triterpenoids (lupeol, α-amyrin, β-amyrin, cycloartenol, lupenone, friedelin, lupeol acetate, cycloartenol acetate) and phytosterols (β-sitosterol, stigmasterol) did not interfere. TLC-MS was used as a tool for the additional confirmation of the presence of ursolic, oleanolic, and betulinic acids in some of the studied vegetable extracts. Ursolic and oleanolic acids were found in radicchio Leonardo and white-colored radicchio di Castelfranco extracts for the first time, while betulinic acid was not detected in the eggplant extract by MS, although it was suggested at first by TLC analysis. Pre-chromatographic bromination on the HPTLC silica gel 60 plates and subsequent development in toluene-chloroform-diethyl ether-formic acid (20:16:4:0.1, v/v) provided a superior resolution of these compounds.
    No preview · Article · Apr 2013 · JPC - Journal of Planar Chromatography - Modern TLC
  • Samo Smrke · Irena Vovk
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    ABSTRACT: The coupling of thin-layer chromatography with mass spectrometry (TLC-MS) for the analysis of monomeric flavanols and proanthocyanidins in samples presented as complex matrices has been studied. The elution conditions for TLC-MS were optimised and full scans were compared with selected reaction monitoring for the MS detection of compounds. The performance of silica gel and cellulose plates with different developing solvents in TLC-MS was assessed. Cellulose plates provided superior sensitivity while ionisation suppression was encountered with silica plates. The use of a HILIC guard column beyond the elution head was found to facilitate detection of monomer compounds on silica plates. A new comprehensive TLC×MS procedure for screening flavanols in the entire chromatogram was developed as an alternative to the use of 4-dimethylaminocinnamaldehyde to determine the locations of compounds on the plate. This new procedure was applied to detect flavanols in the peel of Punica granatum L. fruits and in seeds of Juniperus communis L., in which flavanols and proanthocyanidin dimers and trimers were detected for the first time.
    No preview · Article · Mar 2013 · Journal of Chromatography A
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    ABSTRACT: A major factor in the direct determination of lutein in spinach extracts proved to be obtaining reproducible and stable chromatography of lutein. This was achieved on a C30 column with the mobile phase acetone-0.1M triethylammonium acetate (TEAA) buffer (pH 7) 9:1 (v/v). Extraction of 10mg of lyophilized spinach with 10mL of extraction solvent (ethanol, acetone, ethanol-ethyl acetate 1:1 (v/v), methanol-THF 1:1 (v/v)) for 15min with magnetic stirring under nitrogen resulted in equal yields of lutein. The yields were enhanced by addition of 15% of 1M TEAA buffer pH 7 to all four extraction solvents. As confirmed by recovery experiments, no loss of lutein occurred during the extraction. The relative standard deviation from triplicate extractions was less than 5%. The addition of 15% TEAA pH 7 to acetone enhanced the extraction yield of lutein also from unlyophilized spinach. The content of lutein in different spinach samples ranged from 5 to 15mg/100g of fresh weight. The first separation is reported of all the carotenoids and chlorophylls on a C18 core-shell column and the addition of 15% of 1M TEAA buffer pH 7 to acetone also enhanced the extraction yield of β-carotene compared to the yield produced by pure acetone.
    No preview · Article · Dec 2012 · Journal of Chromatography A
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    ABSTRACT: The performed Quantitative Structure-Mobility Relationship (QSMR) study has investigated relative migration times (RMT) of eleven guanidine/imidazoline derivatives, imidazoline receptor ligands, in capillary electrophoresis (CE) system containing one of cyclodextrins (CDs), α, β, or γ cyclodextrin, using linear and nonlinear modeling methods. The analyzed ligands and their inclusion complexes with the CDs were fully examined and optimized at semi-empirical PM3 level. The density functional theory, such as B3LYP/6-31G+(d,p)/3-21G(d)/STO-3G(d,p)/STO-3G(d), and ab initio theory such as HF/3-21G(d)/STO-3G(d), were applied for molecular descriptors computation of the optimized ligands and their complexes. Predictive performances of the developed QSMR models were tested by use of the cross-validation and external test set prediction. Obtained results for Q(2) values (0.869, 0.911, and 0.966 for CE system containing α, β, and γCD, respectively) and root mean squared error of prediction, RMSEP (0.239, 0.242, and 0.288, for α, β, and γCD, respectively) were proved high predictive power of the proposed models. Finally, multi-target QSMR model (mQSMR), using the ligands descriptors (X) and the relative migration time in presence of αCD (Y1), βCD (Y2), and γCD (Y3), has been created. The mQSMR model can be used as initial screening predictive tool for CE migration behavior of other related guanidine/imidazoline derivatives in presence of αCD, βCD, and γCD.
    Full-text · Article · Nov 2012 · Electrophoresis
  • Alen Albreht · Irena Vovk · Breda Simonovska
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    ABSTRACT: Shikonin and its ester derivatives belong to a group of secondary metabolites with a wide array of beneficial effects on human health. However, shikonin is principally used in oil-based preparations due to the low solubility of the pigment in aqueous media, and the positive properties of shikonin are not exploited to their full potential. Such low aqueous solubility often results in poor bioavailability, makes shikonin undesirable for oral administration, and restricts its broadened use in the food and pharmaceutical industries. The purpose of this study was to enhance the aqueous solubility of shikonin by the addition of β-lactoglobulin and to characterize the macromolecule–ligand binding interaction by means of spectrophotometry, spectrofluorometry, high-performance liquid chromatography, and mass spectrometry. In the presence of β-lactoglobulin the solubility of shikonin is increased up to 181-fold. One shikonin molecule binds covalently to β-lactoglobulin via Cys121, whereas the remaining pigment molecules most probably bind to the protein via noncovalent interactions.
    No preview · Article · Sep 2012 · Journal of Agricultural and Food Chemistry
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    ABSTRACT: A thin-layer chromatographic (TLC) method for fast screening of trans-resveratrol, pterostilbene, and p-coumaric acid in samples of recombinant bacterial cultures, food supplements, and wine was developed. The separation was performed on high-performance thin-layer chromatography (HPTLC) silica gel 60 plates using n-hexane-ethyl acetate-formic acid (20:19:1, v/v) as developing solvent in tank configuration of horizontal developing chamber, in which better resolution between trans-resveratrol and p-coumaric acid than in sandwich configuration of the same chamber or in automatic developing chamber (ADC) was obtained. Compounds were detected before and after post-chromatographic derivatization (three detection reagents) by image analyzing system (at 366 nm or white light) and by densitometer (absorption-reflectance and fluorescence mode). The lowest densitometric limits of detection (LOD) 2 ng for trans-resveratrol (303 nm), 5 ng for pterostilbene (303 nm), and 15 ng for p-coumaric acid (286 nm) were found before derivatization in absorption-reflectance mode. Post-chromatographic derivatization with anisaldehyde-sulfuric acid detection reagent resulted in higher LOD in the same mode: 13 ng for trans-resveratrol and pterostilbene at 500 nm and 30 ng for p-coumaric acid at 566 nm. Natural fluorescence of both stilbenes was less sensitive than UV absorption and less selective than post-chromatographic derivatization with anisaldehyde reagent at densitometric screening of trans-resveratrol in the samples. A complementary high-performance liquid chromatography (HPLC) method was developed for screening and quantification of the three compounds in recombinant bacterial cultures. Adequate separation of the analytes was performed in 35 min by a gradient elution from a stainless-steel column Hypersil ODS (150 × 4.6 mm I.D., particle size: 5 μm) with the mobile phase consisting of 50 mM sodium acetate buffer pH 5.6 (solvent A) and acetonitrile (solvent B) at the flow rate of 1.5 mLmin−1.
    No preview · Article · May 2012 · JPC - Journal of Planar Chromatography - Modern TLC
  • Z Rodić · B Simonovska · A Albreht · I Vovk
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    ABSTRACT: The main problem in the densitometric determination of carotenoids is their rapid degradation during and immediately after chromatography, respectively. In this study, we show that 15 ng of lutein, lycopene and β-carotene standards applied on C(18) RP high-performance thin-layer chromatography (HPTLC) plates pre-developed with dichloromethane-methanol 1:1 (v/v) remained stable for 1h after the development of chromatogram using methanol-acetone 1:1 (v/v) with 0.1% of 2-tert-butylhydroquinone (TBHQ), which is a substantial improvement of their stability. An HPTLC quantification procedure for free lutein, with densitometry at 450 nm based on the developed method described above, was established and validated. Repeatabilities of the chromatography expressed by the relative standard deviation (RSD) from 6 applications of lutein standard at 5, 15 and 25 ng were 3.41, 1.33 and 1.65%, respectively. The best fit calibration curve from 5 ng to 30 ng of lutein was polynomial. Limit of detection (1.5 ng) and limit of quantification (5 ng) were the best achieved so far. With these chromatographic conditions dietary carotenoids lutein esters, lycopene, free lutein and β-carotene from food supplements were also well separated and were identified by visible absorption spectra scanned in situ and by mass spectra. Some additional developing solvents with the same type of chromatographic layer are proposed for the fast separation of lutein esters from free lutein in food supplements.
    No preview · Article · Mar 2012 · Journal of Chromatography A

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