María Paz San Andrés

Separation and Determination of Phenolic Antioxidants by HPLC with Surfactant/n‐Propanol Mobile Phases

The influence of cationic and anionic surfactants and short-chain alcohols in the mobile phase on the retention of five antioxidants has been studied. The solutes chosen were butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), and propyl, octyl, and dodecyl gallates (PG, OG, DG).The surfactants were hexadecyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS), and n-propanol (PrOH) was the selected alcohol. A simple isocratic reversed-phase method for the antioxidant determination is proposed. Separation of five primary antioxidants takes 18 min with the mobile phase SDS 0.10 M/H 3 PO 4 0.01 M/PrOH 30%. Variation of the percentage of alcohol in the mobile phase permits optimization of the retention times of the antioxidants. Detection limits in the pg range were obtained for the all solutes. The method was used to determine the antioxidants in olive oil at three different levels, giving mean recoveries close to 100% for all the solutes (BHA 102%, PG 99%, OG 99%, DG 99%) except BHT (84%).

Antioxidant, antibacterial and ACE-inhibitory activity of four monofloral honeys in relation to their chemical composition

Different monofloral honeys from Castilla-La Mancha (Spain) have been studied in order to determine their main functional and biological properties. Thyme honey and chestnut honey possess the highest antioxidant capacity, which is due to their high vitamin C (in thyme honey) and total polyphenolic content (in chestnut honey). On the other hand, chestnut honey showed high antimicrobial activity against Staphylococcus aureus and Escherichia coli, whilst others had no activity against S. aureus and showed very small activity against E. coli. Moreover it was found that the antimicrobial activity measured in chestnut honey was partly due to its lysozyme content. In addition the angiotensin I-converting enzyme (ACE) inhibitory activity was measured, and the ACE inhibition is one mechanism by which antihypertensive activity is exerted in vivo. All the types of honey showed some activity but chestnut honey had the highest ACE inhibitory activity.

Vitamin C and Sugar Levels as Simple Markers for Discriminating Spanish Honey Sources

UNLABELLED In this work, 7 Spanish honeys with different botanical origins were studied. The honey origins were rosemary, chestnut, lavender, echium, thyme, multifloral, and honeydew. The chemical compounds determined were ascorbic acid (vitamin C), hydroxymethylfurfural, and major sugar contents (glucose and fructose). The physicochemical parameters, pH, conductivity, moisture, free acidity, and color, were also measured. Vitamin C is an important antioxidant in food, and the possibility to use it as discriminate parameter among different honeys was studied. The determination of vitamin C in honey samples was carried out by 2 different methods, volumetric and chromatographic comparing the results by both statistically. Vitamin C content was higher in thyme honeys than in the other types; however a wide dispersion in the values was found. Through a linear discriminant analysis (LDA), conductivity, glucose, fructose, and vitamin C content were the most important discriminant parameters. PRACTICAL APPLICATION Vitamin C content in different honey sources has been determined by a simple and rapid chromatographic method (less than 3 min) in honeys from 6 botanical origins. The results together with glucose and fructose content and some physicochemical parameters have been studied in order to discriminate the botanical origin of honeys and in the future certified their quality. A statistical LDA was applied to the data, and differentiation of honey sources was possible with very good agreement. The vitamin C content found in thymus honeys was significantly higher than in other types. This fact makes vitamin C a special marker for thymus honeys that have a higher antioxidant effect than the others giving it special properties. The identification of honey sources is essential for beekeepers in order to certify honeys for consumers.

Determination of Ni(II), Co(II) and Cu(II) as diethyldithiocarbamate complexes by high-performance liquid chromatography using hexadecyltrimethylammonium bromide in the mobile phase

Abstract The enhancement of the selectivity and sensitivity in high-performance liquid chromatography due to the presence of the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) in the mobile phase in the determination of three metal ions as complexes with sodium diethyldithiocarbamate was studied. The presence of an organic modifier such as 1-propanol in the mobile phase contributes to the decrease in the retention times of the complexes. The results show that under these conditions, the separation of the three metal ions is possible whereas in the absence of surfactant it is not possible. Also, the detection limits and selectivity are better in the presence of CTAB than in its absence. The method was applied to the determination of nickel and copper in real samples.

Determination of riboflavin based on fluorescence quenching by graphene dispersions in polyethylene glycol

The effect of graphene (G) dispersions in a biodegradable polymer, polyethylene glycol (PEG), on the fluorescence emission of vitamin B2 (riboflavin) has been studied. The ultrasonication time and power used for the preparation of the dispersions have been optimized, and their quality has been evaluated by measuring the thickness of the G flakes and their distribution within the polymer by scanning electron microscopy (SEM), and by determining their defect content and degree of exfoliation by Raman spectroscopy. A quenching phenomenon of riboflavin fluorescence has been observed, attributed to π–π stacking interactions between the aromatic rings of the vitamin and G combined with PEG-riboflavin H-bonding interactions. The analysis of the fluorescence spectra confirms that, for mild ultrasonication conditions, the quenching becomes more effective on increasing G concentration, whilst for more intense conditions it is almost independent on the nanomaterial loading. Moreover, for a given G concentration, the fluorescence intensity increases slightly at low PEG contents, while it remains almost constant at high concentrations. The ratio data between the fluorescence intensity in the absence and in the presence of G fit to a second-order polynomial equation, suggesting a combined mechanism of static and dynamic quenching. The analytical characteristics of the fluorimetric method have been calculated for PEG aqueous solutions, for G dispersions in PEG and for the differences of intensities between them, and the best results were obtained in the presence of G. The proposed method was successfully applied to the analysis of riboflavin in multivitamin tablets, and the recoveries found in fortified samples were higher than 90%. The quenching phenomenon observed in this work could be employed for the development of new optical sensors for riboflavin determination.

Determination of Pollutant Phenols by Capillary High‐Performance Liquid Chromatography with UV Detection

This paper describes a liquid chromatographic method using a reversed phase capillary column coupled to an UV detector for the quantitation of thirteen pollutant phenols. Chromatographic separation was carried out with gradient elution at 25.0 ± 0.1°C. The two major anisocratic elution modes (gradient elution and temperature programming) were evaluated. The detection limit range was 10-81 pg (100 nL injected). The chromatographic method combined with liquid-liquid extraction was applied to analysis of these compounds in river water. Recoveries of 75-103% were achieved for most of them.

Comparison of Anionic, Cationic and Nonionic Surfactants as Dispersing Agents for Graphene Based on the Fluorescence of Riboflavin

Fluorescence quenching is a valuable tool to gain insight about dynamic changes of fluorophores in complex systems. Graphene (G), a single-layered 2D nanomaterial with unique properties, was dispersed in surfactant aqueous solutions of different nature: non-ionic polyoxyethylene-23-lauryl ether (Brij L23), anionic sodium dodecylsulphate (SDS), and cationic hexadecyltrimethylammonium bromide (CTAB) and dodecyltrimethylammonium bromide (DTAB). The influence of the surfactant type, chain length and concentration, G total concentration and G/surfactant weight ratio on the fluorescence intensity of vitamin B2 (riboflavin) was investigated. The quality of the different G dispersions was assessed by scanning and transmission electron microscopies (SEM and TEM). A quenching phenomenon of the fluorescence of riboflavin was found for G dispersions in all the surfactants, which generally becomes stronger with increasing G/surfactant weight ratio. For dispersions in the ionic surfactants, the quenching is more pronounced as the surfactant concentration raises, whilst the non-ionic one remains merely unchanged for the different G/Brij L23 weight ratios. More importantly, results indicate that DTAB solutions are the optimum media for dispersing G sheets, leading to an up to 16-fold drop in the fluorescence intensity. Understanding the mechanism in fluorescence quenching of G dispersions in surfactants could be useful for several optical applications.

Effect of Graphene Flakes Modified by Dispersion in Surfactant Solutions on the Fluorescence Behaviour of Pyridoxine

The influence of graphene (G) dispersions in different types of surfactants (anionic, non-ionic, and cationic) on the fluorescence of vitamin B6 (pyridoxine) was studied. Scanning electron microscopy (SEM) was used to evaluate the quality of the G dispersions via measuring their flake thickness. The effect of surfactant type and concentration on the fluorescence intensity was analyzed, and fluorescence quenching effects were found for all of the systems. These turn out to be more intense with increasing both surfactant and G concentrations, albeit they do not depend on the G/surfactant weight ratio. For the same G concentration, the magnitude of the quenching follows the order: cationic > non-ionic ≥ anionic. The cationic surfactants, which strongly adsorb onto G via electrostatic attraction, are the most effective dispersing agents and they enable a stronger interaction with the zwitterionic form of the vitamin; the dispersing power improves with increasing the surfactant chain length. The fit of the experimental data to the Stern-Volmer equation suggests either a static or dynamic quenching mechanism for the dispersions in non-ionic surfactants, while those in ionic surfactants show a combined mechanism. The results that were obtained herein have been compared to those that were reported earlier for the quenching of another vitamin, riboflavin, to elucidate how the change in the vitamin structure influences the interactions with G in the surfactant dispersions.