Pavel Májek - Academia.edu (original) (raw)

Papers by Pavel Májek

Journal of chromatography. A, 2021

The rate constants for (L)-N-acetyl homocysteine thiolactone enantiomerization have been obtained... more The rate constants for (L)-N-acetyl homocysteine thiolactone enantiomerization have been obtained from batch-wise studies and by dynamic gas chromatography of racemic mixtures. Results from the batch-wise experiments show that the kinetics of racemization at 150 °C is the same for vials made of glass, silanized glass or Teflon-coated glass so that the vial surface exhibited no effect on the kinetics of racemization. From the temperature dependence of the rate constants the preexponential factor, activation energy, the activation Gibbs energy and activation entropy have been obtained from transition state theory. The catalytic effect of G-DP, G-BP and B-DP GC chiral stationary phases on racemization has been observed and quantified by the values of rate constants; B-DP exhibited the greatest activity. The Eyring activation parameters obtained from batch-wise experiment were compared with theoretical values acquired from quantum chemical modelling. Agreement between the experimental a...

Nowadays, a large number of chiral analyses are needed, simple and rapid methods for the determin... more Nowadays, a large number of chiral analyses are needed, simple and rapid methods for the determination of the enantiomeric composition in pharmaceutical products should be developed. The fluoxetine belongs to the most prescribed antidepressant chiral drugs and its enantiomers have a different duration of serotonin inhibition. This study presents cheap and fast alternative to traditional chiral techniques for the determination of the fluoxetine enantiomeric composition by the combination of UV/VIS spectrometry and multivariate calibration. The chiral recognition of the fluoxetine was based on the creating of the diastereomeric complexes with α- and β- cyclodextrin. Multivariate calibration methods, including principal component regression (PCR) and partial least square method (PLS), were used for spectral data evaluation. Small differences in results between each cyclodextrins and both calibration models were obtained by multivariate calibration. PLS model for determination of the en...

Food Analytical Methods, 2014

This study compares the use of UV absorption, excitation-emission matrix (EEM) fluorescence and s... more This study compares the use of UV absorption, excitation-emission matrix (EEM) fluorescence and synchronous fluorescence spectroscopy and HPLC with fluorescence detection combined with principal component analysis (PCA), parallel factor analysis (PARAFAC) and linear discriminant analysis (LDA) for distinguishing between commercial samples of Slovak, Belgian, German, Czech and British juniper-flavoured spirit drinks. Overall, 97 %, 88 % and 79 % of samples were properly classified by applying the LDA to the first five principal components of the PCA performed on the synchronous fluorescence spectra (constant wavelength difference 10 nm, 250-450 nm), UV absorption spectra (250-325 nm) and the areas of eight common HPLC peaks, respectively. EEM fluorescence spectroscopy could not discriminate British drinks, while HPLC failed to discriminate Belgian samples. When the areas of eight common HPLC peaks were used as model parameters instead of the five PCs initially used in LDA, the method accuracy was enhanced significantly and 97 % of predicting samples were properly classified. Keywords Beverage. Juniper drinks. Fluorescence. UV absorption spectroscopy. HPLC. Chemometrics communis L. and/or Juniperus oxycedrus L.) berries. Geographical indications shall be reserved for spirit drinks, which acquire their quality, reputation or other characteristics during the production stage in a certain geographic area (Regulation (EC) No. 110/2008; European Union (EC) 2008). The use of geographical indications allows producers to obtain a premium price and market recognition. In the case of premium spirits, there is a large economic interest to mix or completely substitute one brand with another less expensive brand. For this reason, there is a need for a rapid method for determining the geographical origin of products. Although JFSDs are well known and widely consumed (gin is the most popular), there are only few studies on their volatile/ semivolatile composition (

European Food Research and Technology, 2005

The phenolic content of South African wines was investigated for the purpose of their classificat... more The phenolic content of South African wines was investigated for the purpose of their classification according to variety or cultivar. Twenty-two phenolic compounds were quantified by liquid chromatography in 55 red wines of five varieties and in 38 white wines of three varieties. Data were compared to literature values reported for wines from across the globe. ANOVA was used to determine significant concentration differences between the different varieties. Further grouping of the data was investigated by principal component analysis and cluster analysis. Finally, linear discriminant analysis was used to derive classification functions for the effective recognition of red and white wine varieties which allowed 100.0% and 97.4% correct recognition of red and white wines, respectively.

Journal of Agricultural and Food Chemistry, 2008

A simple method for the analysis of major wine volatiles and semivolatiles by stir bar sorptive e... more A simple method for the analysis of major wine volatiles and semivolatiles by stir bar sorptive extraction in combination with thermal desorption and gas chromatography-mass spectrometry (SBSE-TD-GC-MS) was developed. Significant experimental parameters such as extraction time, temperature, salt addition, pH, and thermal desorption parameters were optimized to provide a sensitive and robust analytical method. The method provided good repeatability (%RSD < 10%) for 38 major wine volatile compounds, including alcohols, acids, esters, phenols, aldehydes, ketones, and lactones. Quantitative data for 62 South African red and white wines were used to study the suitability of major volatile data for the differentiation of wine samples according to grape variety or cultivar. Principal component analysis (PCA) and cluster analysis (CA) showed that most of the variation in volatile composition between wine samples could be ascribed to differences in wine age, wood contact, and fermentation practices. Despite the contribution of these factors, discriminant analysis (DA) was successfully applied to the classification of red and white wine samples according to cultivar.

Acta Chimica Slovaca, 2020

Antibiotic, soothing and healing properties of panthenol are exploited in various pharmaceutical ... more Antibiotic, soothing and healing properties of panthenol are exploited in various pharmaceutical and cosmetic products. Only D-panthenol is biologically active while its L-form might counteract the biological effectiveness of the D-enantiomer. Hydrating and moisturizing effects are exhibited by both enantiomers. Therefore, it is necessary to develop a rapid and cheap method for the determination of panthenol enantiomeric excess in pharmaceutical preparations. In this study, b-cyclodextrin was used as a chiral selector for the recognition of panthenol enantiomers. Inclusion complexes formed by b-cyclodextrin and the analyte showed small differences in NIR and UV/VIS spectra compared by chemometric assessment. Based on the figure of merit and model characteristics, PLS calibration model in the selected range of NIR spectra is preferred. UV/VIS spectrometry has the disadvantage of complicated sample preparation compared to NIR spectrometry. Results reached by both proposed methods were...

Collection of Czechoslovak Chemical Communications, 2005

Using MP2 treatment, the optimal geometries and corresponding electronic structures of [HN=C(OCH3... more Using MP2 treatment, the optimal geometries and corresponding electronic structures of [HN=C(OCH3)-N-C(OCH3)=NH]- (mici-), HN=C(OCH3)-NH-C(OCH3)=NH (Hmici), [Cu(mici)]2 and [Cu(Hmici)2]2+ were investigated. In agreement with experimental data, the preferred conformation of our systems depends on the ligand: mici- prefers the methoxy groups oriented to the central N atom whereas Hmici prefers the opposite orientations. The shielding by the methoxy groups is the main reason for the preferred site of deprotonation. The deprotonation of the [Cu(Hmici)2]2+ complex is more probable than that of the free Hmici ligand. Due to preferred conformation of this complex, the deprotonation proceeds by cleavage of the central N-H bond.

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Chirality, 2016

Fluoxetine is the most prescribed antidepressant chiral drug worldwide. Its enantiomers have a di... more Fluoxetine is the most prescribed antidepressant chiral drug worldwide. Its enantiomers have a different duration of serotonin inhibition. A novel simple and rapid method for determination of the enantiomeric composition of fluoxetine in pharmaceutical pills is presented. Specifically, emission, excitation, and synchronous fluorescence techniques were employed to obtain the spectral data, which with multivariate calibration methods, namely, principal component regression (PCR) and partial least square (PLS), were investigated. The chiral recognition of fluoxetine enantiomers in the presence of β-cyclodextrin was based on diastereomeric complexes. The results of the multivariate calibration modeling indicated good prediction abilities. The obtained results for tablets were compared with those from chiral HPLC and no significant differences are shown by Fisher&amp;amp;#39;s (F) test and Student&amp;amp;#39;s t-test. The smallest residuals between reference or nominal values and predicted values were achieved by multivariate calibration of synchronous fluorescence spectral data. This conclusion is supported by calculated values of the figure of merit.

Computer-assisted peak deconvolution on chromatograms obtained by GC separation of enantiomers of... more Computer-assisted peak deconvolution on chromatograms obtained by GC separation of enantiomers of 1-chloro-2,2-dimethylaziridine on modified ␣-, ␤-, and ␥-cyclodextrin was used to determine the peak areas of enantiomers, prior Ž. Ž. to A , B and after the separation A , A. Both Gaussian as well as A, 0 B, 0 A B exponentially modified Gaussian functions were used to approximate the peak shapes in the deconvolution procedures. Determined peak areas were used in the calculation of the rate constants and energy barriers to enantiomerization. A comparison of energy barriers determined using the deconvolution of chromatograms with data published in the literature by classical kinetics shows differences within "5%.

Acta Chimica Slovaca, 2015

This study introduces a reliable method to detect adulteration of spirit drinks. Excitation-emiss... more This study introduces a reliable method to detect adulteration of spirit drinks. Excitation-emission matrix (EEM) fluorescence in combination with parallel factor analysis (PARAFAC) and partial least squares (PLS) regression was used to determine the content of water and ethanol in adulterated fruit spirit samples. EEM fluorescence spectra recorded in the emission wavelength range of 315–450 nm and in the excitation wavelength range of 240–305 nm were used for PARAFAC. The model created using PARAFAC-PLS was able to predict the water and ethanol level in adulterated apple spirit with the root mean square error of prediction (RMSEP) values of 1.9 % and 1.8 %, respectively. Regarding adulterated plum spirit, the RMSEP values of 0.7 % and 3.5 % were obtained for water and ethanol, respectively. The aim of this work was to determine whether EEM-PARAFAC can be used to distinguish between plum and apple spirit. Better results were obtained for apple spirit and the method is useful also fo...

Chemical Papers, 2015

This work describes a novel methodology for the recognition of brandies based on direct injection... more This work describes a novel methodology for the recognition of brandies based on direct injection of a raw sample followed by GC-MS analysis. Direct injection was chosen for its simplicity and the fact that the composition of the samples analysed remains unchanged compared to original brandy. The repeatability of the analytical procedure was evaluated by a comparison of the peak areas for randomly selected compounds obtained from 10 parallel measurements. A novel chemometric procedure was investigated in order to separate the samples studied on the basis of their geographical origin, processing technology or maturation time. In this procedure, a principal component analysis was applied to full chromatograms to select the time interval that shows the significant differences between the samples studied. It was shown that the chromatogram recorded at 36-39 min bore the maximal differences, hence it could be used to classify the brandy samples. The chromatographic peaks found within thi...

Journal of chromatography. A, Jan 11, 2015

Methods based on the blank signal as proposed by IUPAC procedure and on the signal to noise ratio... more Methods based on the blank signal as proposed by IUPAC procedure and on the signal to noise ratio (S/N) as listed in the ISO-11843-1 norm for determination of the limit of detection (LOD) and quantitation (LOQ) in one-dimensional capillary gas chromatography (1D-GC) and comprehensive two-dimensional capillary gas chromatography (CG×GC) are described in detail and compared for both techniques. Flame ionization detection was applied and variables were the data acquisition frequency and, for CG×GC, also the modulation time. It has been stated that LOD and LOQ estimated according to IUPAC might be successfully used for 1D-GC-FID method. Moreover, LOD and LOQ decrease with decrease of data acquisition frequency (DAF). For GC×GC-FID, estimation of LOD by IUPAC gave poor reproducibility of results while for LOQ reproducibility was acceptable (within ±10% rel.). The LOD and LOQ determined by the S/N concept both for 1D-GC-FID and GC×GC-FID methods are ca. three times higher than those value...

Neoplasma, 1981

By theoretical methods of quantum mechanics an interaction between carcinogenic hydrocarbons -- b... more By theoretical methods of quantum mechanics an interaction between carcinogenic hydrocarbons -- benz(a)anthracene, benzo(a)pyrene -- and various groups simulating an effect of enzyme oxidase has been studied. If accord is to be achieved with formation of epoxides of these compounds in vivo according to the bay region theory (i. e. in A region) an interaction between the enzyme involving an electron-acceptor group and the bay region of hydrocarbon under simultaneous steric hindrance of the K region is necessary. Another possibility of obtaining the highest reactivity in the A region is to assume the formation of the radical cation as a first intermediate in the interaction with enzyme.

Journal of chromatography. A, Jan 4, 2014

Informational entropy and syentropy percent were used to optimize the flows in the first (1D) and... more Informational entropy and syentropy percent were used to optimize the flows in the first (1D) and in the second (2D) dimension ((1)Fm and (2)Fm, respectively) as well as the temperature program rate (r) for the flow modulated GC×GC-FID separation of C6-C12 aromatic hydrocarbons in a low boiling petrochemical sample. The separations were performed on a column series consisting of a 25m×0.25mm i.d.×0.2μm df of the polar ionic liquid SLB-IL 100 (1,9-di(3-vinylimidazolium)nonane bis(trifluoromethylsulfonyl)imide) in the first dimension and 5m×0.25mm i.d.×0.25μm df apolar HP-5MS (5% phenyl-95% methylpolysiloxane) in the second dimension. A dependence of a distribution of individual aromatic hydrocarbons in the 2D retention plane on the carrier gas flows ((1)Fm, and (2)Fm,) and temperature gradient (r) was examined in this study. It was found that informational entropy and synentropy percent are advantageous criteria to characterize the distribution of peaks in the 2D retention plane. Max...

Journal of Molecular Structure: THEOCHEM, 2004

ABSTRACT

Journal of Microcolumn Separations, 2000

Abstract Computer-assisted peak deconvolution on chromatograms obtained by GC separation of enant... more Abstract Computer-assisted peak deconvolution on chromatograms obtained by GC separation of enantiomers of 1-chloro-2, 2-dimethylaziridine on modified α-, β-, and γ-cyclodextrin was used to determine the peak areas of enantiomers, prior to (AA, 0, BB, 0) ...

Journal of Separation Science, 2006

A classical kinetic method was used to determine the energy barrier for the interconversion of 2,... more A classical kinetic method was used to determine the energy barrier for the interconversion of 2,3-pentadienedioic acid enantiomers. Each individual enantiomer was isolated by collecting the appropriate peaks from the HPLC enantiomeric separation, of racemic 2,3-pentadienedioic acid. The isolated enantiomers were racemized at 228C using various interconversion times. The ratio of enantiomers in each reaction solution was determined by HPLC at 228C. The corresponding peak areas of the enantiomers and the interconversion times obtained from the HPLC chromatograms were used to calculate both the interconversion rate constants describing (+) (-) and (-) (+) interconversions as well as the energy barriers. It was confirmed that the interconversion of 2,3-pentadienedioic acid enantiomers is a firstorder kinetic reaction. Both semiempirical and ab initio methods were used to explore the mechanism of the interconversion of 2,3-pentadienedioic acid enantiomers, and to calculate the interconversion energy barrier. Comparison of the interconversion energy barriers found by the ab initio method (DG # = 110.7 kJ/mol) and by classical kinetics in the mobile phase solution at 228C (DG app = 93.9 l 0.2 kJ/mol) shows a difference which may be attributed to the different conditions assumed in the theoretical calculation (i. e., a gaseous state) and the actual experimental conditions (i. e., liquid solution) and a possible catalytic effect of the solution composition.

Journal of Separation Science, 2005

In this work, the capillary gas chromatographic enantiomer separation of eight congeneric compoun... more In this work, the capillary gas chromatographic enantiomer separation of eight congeneric compounds with general formulae CH3-HCX-COOC2H5 (where X = Cl, Br, I, CN, OH, OC2H5, OC6H5 and NHCOCF3) on four different permethyl- and 2,6-di-O-methyl-3-O-pentyl- beta- and gamma-CD stationary phases has been studied. The separation of enantiomers was evaluated in terms of the interactions of the X substituent of studied derivatives, as well as the nature of the 3-O-alkyl group in the 2,6-di-O-methyl-3-O-alkyl-CDs and the CDs cavity size. The differences in thermodynamic data [deltaH and -deltaS] obtained for studied compounds and the selectivity of modified beta- and gamma-cyclodextrin phases in gas chromatographic separation were evaluated. deltaH values were compared with a deltaH value of an achiral standard (ethyl propionate, where X = H) in order to obtain the contribution of a particular substituent to the overall interaction energy. It was shown that the variation in the enantiomeric separation with temperature and the retention order of these compounds on a given cyclodextrin capillary column depends on the nature of the substituents bonded to stereogenic carbon atom. It was found that the temperature dependencies of selectivity factors, In a on 1/T, were both linear as well as non-linear, inter alia depending on the number of glucopyranose units of the CD derivatives. The enantiospecific thermodynamic data [delta21(deltaH)] and [-delta21(deltaS)] which characterize the chiral recognition in the separation system were used to gain more insight into the mechanistic aspects of the enantioseparations on permethylated and 2,6-di-O-methyl-3-O-pentyl-beta- and gamma-cyclodextrins.