Ewa Sikorska

Characterization of Edible Oils Using Total Luminescence Spectroscopy

Total luminescence spectroscopy was used to characterise and differentiate edible oils and additionally, to control one of the major problems in the oil quality—the effect of thermal and photo-oxidation. We studied several vegetable oils available on the Polish market, including soybean, rapeseed, corn, sunflower, linseed and olive oils. Total luminescence spectroscopy measurements were performed using two different sample geometries: front-face for pure oil samples and right-angle for transparent samples, diluted in n-hexane. All the samples studied as n-hexane solutions exhibit an intense peak, which appears at 320 nm in emission and 290 nm in excitation, attributed to tocopherols. Some of the oils exhibit a second long-wavelength peak, appearing at 670 nm in emission and 405 nm in excitation, belonging to pigments of the chlorophyll group. Additional bands were present in the intermediate range of excitation and emission wavelengths; however, the compounds responsible for this emission were not identified. The front-face spectra for pure oils included chlorophyll peaks for most samples, and some additional peaks in the intermediate range, while the tocopherol peaks were comparatively less intense. The results presented demonstrate the capability of the total luminescence techniques to characterise and differentiate vegetable oil products, and additionally, to characterize the effect of thermal and photo-oxidation on such products. In the photo-oxidation experiments, special attention was paid to possible involvement of singlet oxygen. Experiments were done to monitor the highly specific O2 (1Δg) → O2(3Σ−) singlet oxygen emission at 1270 nm. Thus, total luminescence spectroscopy presents an interesting alternative to time-consuming and expensive techniques such as gas or liquid chromatography, mass spectrometry and other methods requiring wet chemistry steps.

Simultaneous analysis of riboflavin and aromatic amino acids in beer using fluorescence and multivariate calibration methods

The study demonstrates an application of the front-face fluorescence spectroscopy combined with multivariate regression methods to the analysis of fluorescent beer components. Partial least-squares regressions (PLS1, PLS2, and N-way PLS) were utilized to develop calibration models between synchronous fluorescence spectra and excitation-emission matrices of beers, on one hand, and analytical concentrations of riboflavin and aromatic amino acids, on the other hand. The best results were obtained in the analysis of excitation-emission matrices using the N-way PLS2 method. The respective correlation coefficients, and the values of the root mean-square error of cross-validation (RMSECV), expressed as percentages of the respective mean analytic concentrations, were: 0.963 and 14% for riboflavin, 0.974 and 4% for tryptophan, 0.980 and 4% for tyrosine, and 0.982 and 19% for phenylalanine.

Photophysical properties of lumichromes in water.

The photophysics of lumichrome, 1-methyllumichrome, and lumiflavin in water solutions have been investigated. Fluorescence lifetimes of 2.7 and 2.2 ns were observed for lumichrome and 1-methyllumichrome, respectively, the corresponding triplet state lifetimes of 17 and 18 micros have been obtained from the transient absorption spectra. Evidence for long lived species with absorption maxima near 450 nm and lifetimes of ca. 400 micros has been found in the transient absorption spectra of both lumichromes. Quantum yields for the sensitised production of singlet oxygen, phi(Delta), are 0.36 and 0.41 for lumichrome and 1-methyllumichrome, respectively, in D(2)O.

Spectroscopy and Photophysics of Iso- and Alloxazines: Experimental and Theoretical Study

We present a systematic study of the effect of methyl substitution on iso- and alloxazines in acetonitrile solutions. Substitution patterns have profound effects on both spectral and photophysical properties, with fluorescence quantum yields varying by more than an order of magnitude. TD-DFT calculation were used for the first time to correlate electronic structure changes with the substitution patterns, with good agreement between calculated and theoretical band positions and oscillator strengths. Both n−π* and π −π* states in these compounds are predicted, with the oscillator strengths indicating that only the π −π* states should be observable in the absorption spectra. Substitution patterns are shown to be responsible for energy order inversion between these states.

Efficiency of singlet oxygen generation by alloxazines and isoalloxazines

Some spectroscopic properties of alloxazines and the lifetimes of their singlet and triplet states in acetonitrile are reported. In addition, the efficiencies of singlet oxygen production by energy transfer from the excited states of a range of substituted alloxazines and lumiflavine have been determined in aerated acetonitrile solution. Triplet state quantum yields have also been measured and it has been shown that the efficiency of singlet oxygen production from the triplet state is equal to unity within experimental error in all cases.

Near and mid infrared spectroscopy and multivariate data analysis in studies of oxidation of edible oils

Infrared spectroscopic techniques and chemometric methods were used to study oxidation of olive, sunflower and rapeseed oils. Accelerated oxidative degradation of oils at 60°C was monitored using peroxide values and FT-MIR ATR and FT-NIR transmittance spectroscopy. Principal component analysis (PCA) facilitated visualization and interpretation of spectral changes occurring during oxidation. Multivariate curve resolution (MCR) method found three spectral components in the NIR and MIR spectral matrix, corresponding to the oxidation products, and saturated and unsaturated structures. Good quantitative relation was found between peroxide value and contribution of oxidation products evaluated using MCR--based on NIR (R(2) = 0.890), MIR (R(2) = 0.707) and combined NIR and MIR (R(2) = 0.747) data. Calibration models for prediction peroxide value established using partial least squares (PLS) regression were characterized for MIR (R(2) = 0.701, RPD = 1.7), NIR (R(2) = 0.970, RPD = 5.3), and combined NIR and MIR data (R(2) = 0.954, RPD = 3.1).

Analysis of Olive Oils by Fluorescence Spectroscopy: Methods and Applications

Fluorescence spectroscopy is a well established and extensively used research and analytical tool in many disciplines. In recent years, a remarkable growth in the use of fluorescence in food analysis has been observed (Christensen et al., 2006; Sadecka & Tothova, 2007; Karoui & Blecker, 2011). Vegetable oils including olive oil constitute an important group of food products for which fluorescence was successfully applied. Fluorescence is a type of photoluminescence, a process in which a molecule, promoted to an electronically excited state by absorption of UV, VIS or NIR radiation, decays back to its ground state by emission of a photon. Fluorescence is emission from an excited state, in which the electronic spin is equal to that in the ground state, and typically equal to zero. Such transitions are spin allowed, and occur at relatively high rates, typically 108 s-1 (Lakowicz, 2006).

Excited state proton transfer of methyl- and cyano- substituted alloxazines in the presence of acetic acid

Abstract The photoinduced excited state double proton transfer reaction of methyl- and cyano-substituted alloxazines was examined by steady state and time-resolved methods. The fluorescence decay times of the alloxazinic forms in 1,2-dichloroethane and 1,2-dichloroethane in the presence of acetic acid are similar and of the order of hundreds of picoseconds. The fluorescence decay times of the isoalloxazinic forms, created by excited state proton transfer, are of the order of nanoseconds. On the basis of the function describing the emission decay of the bands corresponding to the isoalloxazinic and alloxazinic forms and the lifetimes obtained, it was found that the excited alloxazinic form is a precursor of the excited isoalloxazinic form, and the rate constant of the proton transfer process is lower than 10 9 s −1 . These results and those of stationary studies, indicating that the isoalloxazinic and alloxazinic species derive from different forms in the ground state, have enabled a model of the proton transfer reaction of alloxazines to be proposed. In this model, the possible formation of alloxazine-acetic acid complexes in a conformation permitting proton transfer, in both the ground and excited states, was assumed.

Acid–Base Equilibriums of Lumichrome and its 1-Methyl, 3-Methyl, and 1,3-Dimethyl Derivatives

Lumichrome photophysical properties at different pH were characterized by UV-vis spectroscopy and steady-state and time-resolved fluorescence techniques, in four forms of protonation/deprotonation: neutral form, two monoanions, and dianion. The excited-state lifetimes of these forms of lumichrome were measured and discussed. The results were compared to those obtained for similar forms of alloxazine and/or isoalloxazine, and also to those of 1-methyl- and 3-methyllumichrome and 1,3-dimethyllumichrome. The absorption, emission, and synchronous spectra of lumichrome, 1-methyl- and 3-methyllumichrome, and 1,3-dimethyllumichrome at different pH were measured and used in discussion of fluorescence of neutral and deprotonated forms of lumichrome. The analysis of steady-state and time-resolved spectra and the DFT calculations both predict that the N(1) monoanion and the N(1,3) dianion of lumichrome have predominantly isoalloxazinic structures. Additionally, we confirmed that neutral lumichrome exists in its alloxazinic form only, in both the ground and the excited state. We also confirmed the existence and the alloxazinic structure of a second N(3) monoanion. The estimated values of pK(a) = 8.2 are for the equilibrium between neutral lumichrome and alloxazinic and isoalloxazinic monoanions, with proton dissociation from N(1)-H and N(3)-H groups proceeding at the almost the same pH, while the second value pK(a) = 11.4 refers to the formation of the isoalloxazinic dianion in the ground state.

Spectroscopy and photophysics of alloxazines studied in their ground and first excited singlet states

The acid–base properties of alloxazine (All) and its methyl derivatives have been studied in their ground and first excited singlet states. The concept of an effective electronic valence potential was applied to predict the changes in basicity and acidity of heteroatoms upon excitation and substitution. Changes in the acid–base properties of N(1) and N(10) nitrogen atoms are particularly important from the point of view of the excited state proton transfer in alloxazines from N(1) to N(10) to form isoalloxazinic structures. A good linear correlation was obtained between the calculated electronic potentials of N(1) and N(3) nitrogen atoms and the experimental pKa values for ground and excited state deprotonation.