Andreas E. Karatapanis

A revisit to the retention mechanism of hydrophilic interaction liquid chromatography using model organic compounds.

In this work, a revisit to the retention mechanism of HILIC was attempted to point out critical factors that contribute to the chromatographic regime as well as to bring out subtle details of the relative contribution of partitioning and surface adsorption. In this vein, the retention behaviour of a set of water-soluble vitamins (WSVs) and toluene on three silica based columns was evaluated under varying chromatographic conditions. The data obtained were associated with the hydration degree of the stationary phases and the ability of the organic solvents to disrupt the formation of the water-enriched layer. Moreover, the elution behaviour of toluene at different buffer salt concentrations in the mobile phase, confirmed the preferential partition of salt ions into the stagnant layer, as ACN content was increased. The results from the fitting of partitioning and surface adsorption models indicated differences in the contribution of the two retention mechanisms to both neutral and charged compounds. The occurrence of surface adsorption and the retentivity differences for neutral WSVs depend on the hydration degree and the hydrogen bonding properties of the solutes and the column surface, respectively. For charged solutes experiencing electrostatic repulsion, the contribution of the adsorption mechanism at highly organic mobile phases, emanates from both the weak effect of buffer salt ions on the electrostatic interaction and the strong effect of hydrophilic interactions. On the other hand, the chromatographic retention of electrostatically attracted solutes indicates that the surface adsorption dominates, even at mobile phases rich in water.

Silica-modified magnetic nanoparticles functionalized with cetylpyridinium bromide for the preconcentration of metals after complexation with 8-hydroxyquinoline.

Magnetically driven separation technology has received considerable attention in recent decade for its great potential application. In this study, we investigate the application of silica-modified magnetite nanoparticles (NPs) coated with a cationic surfactant as adsorbent for microextraction and determination of trace amounts of Cu(II), Ni(II), Co(II), Cd(II), Pb(II) and Mn(II) from environmental water samples. The synthesized silica-coated NPs in combination with cetylpyridinium bromide have the ability to adsolubilize the metal ions after complexation with 8-hydroxyquinoline. The NPs bearing the target metals are easily separated from the aqueous solution by applying an external magnetic field and the complexed metals were desorbed using acidic methanol. The desorbed analytes are introduced into the graphite furnace of an atomic absorption spectrometer. The effect of pH, complexing agent, amount of cetylpyridinium bromide, microextraction time, desorption conditions, ionic strength on extraction efficiency of the metal ions are investigated and optimized. Under the optimized conditions, the detection limits for Cu(II), Ni(II), Co(II), Cd(II), Pb(II) and Mn(II) are 4.7, 9.1, 9.5, 2.3, 7.4 and 15.3 ng L(-1), respectively and the relative standard deviations (n=6) are less than 3.6%. The accuracy of the method was evaluated by recovery measurements on the spiked samples and good recoveries (93-113%) with low RSDs were achieved.

HILIC separation and quantitation of water-soluble vitamins using diol column

Hydrophilic interaction liquid-chromatography (HILIC) in conjunction with diode array detection has been applied for the separation of selected-water-soluble vitamins using an end-capped HILIC-diol column. Vitamins with significant biological importance, such as thiamine (B(1)), riboflavin (B(2)), nicotinic acid (B(3)), nicotinamide (B(3)), pyridoxine (B(6)), folic acid (B(9)), cyanocobalamin (B(12)) and ascorbic acid (vitamin C) were simultaneously separated. Chromatographic conditions including type and percentage of organic modifier in the mobile phase, pH, type and concentration of buffer salt and flow rate were investigated. ACN was shown to offer superior separation for the compounds tested as compared to methanol, isopropanol and THF. Isocratic separation and analysis were achieved for six vitamins (B(1), B(2), nicotinic acid/nicotinamide, B(6) and C) at ACN-H(2)O 90:10, containing ammonium acetate 10 mM, triethylamine 20 mM, pH 5.0, using a flow rate of 0.8 mL/min, while a gradient was necessary to resolve a mixture of all eight water-soluble vitamins. The HILIC method was validated and successfully applied to the analysis of a pharmaceutical formulation and an energy drink negating the need for time consuming clean-up steps.

A layered magnetic iron/iron oxide nanoscavenger for the analytical enrichment of ng-L−1 concentration levels of heavy metals from water

Magnetically driven separation techniques have received considerable attention in recent decade because of their great potential application. In this study, we investigate the application of an unmodified layered magnetic Fe/Fe(2)O(3) nanoscavenger for the analytical enrichment and determination of sub-parts per billion concentrations of Cd(II), Pb(II), Ni(II), Cr(VI) and As(V) from water samples. The synthesized nanoscavenger was characterized by BET, TGA, XRD and IR and the parameters influencing the extraction and recovery of the preconcentration process were assessed by atomic absorption spectrometry. The possible mechanism of the enrichment of heavy metals on Fe/Fe(2)O(3) was proposed, which involved the dominant adsorption and reduction. The nanoscale size offers large surface area and high reactivity of sorption and reduction reactions. The obtained limits of detection for the metals studied were in the range of 20-125 ng L(-1) and the applicability of the nanomaterial was verified using a real sample matrix. The method is environmentally friendly as only 15 mg of nanoscavenger are used, no organic solvent is required for the extraction and the experiment is performed without the need for filtration or preparation of packed preconcentration columns.

Riboflavin’s time-dependent degradation rate induced by ultraviolet A irradiation

Purpose To evaluate the time-dependent degradation rate of riboflavin after ultraviolet A (UVA) irradiation. Methods Two solutions of commercially available riboflavin solution (0.1%) were used; one served as control, while the second was irradiated using UVA light at 370 nm wavelength. Four samples of riboflavin solution were retrieved prior to irradiance and at 1, 5, 15, 30, and 60 minutes after irradiation (group A); at the same time points samples of riboflavin were retrieved from the control solution in order to assess environmental time-induced degradation of riboflavin (group B). All samples were immediately analyzed using liquid chromatograph mass spectrometry to detect riboflavin and its 2 subproducts, lumiflavin (LF) and lumichrome (LC). Results Mean percentage of riboflavin degradation was 0.0, 5.3, 9.1, 15.3, 20.6, and 33.3 at 0, 1, 5, 15, 30, and 60 minutes after UVA irradiation, respectively (group A). The time-dependent riboflavin degradation was statistically significant (p<0.05), while for group B there was no change in riboflavin concentration at all time intervals. In group A, mean LC concentration demonstrated a gradual concentration increase, reaching 2.386±1.526 ppm after 60 minutes of UVA exposure. Conclusions The time-dependent degradation of riboflavin solution is significant, reaching 20.6% after 30 minutes of UVA exposure. It seems that only a small fraction of the overall riboflavin molecules break down since more than 65% remain intact even after 1 hour of UVA irradiation. Control riboflavin solution seems to be stable, as no degradation is evident even after 60 minutes.

Determination of saccharides in aerosols by hydrophilic interaction liquid chromatography-evaporative light scattering detection after preconcentration/clean-up on magnetic titania

We report a simple analysis method for some common saccharides in aerosol samples on a diol-silica column, in connection with high performance liquid chromatography-evaporative light scattering detection. Under the established chromatographic conditions, the saccharides exhibit typical hydrophilic interaction behaviour with the stationary phase. Prior to chromatographic separation, the effectiveness of titania was studied as a specific trapping material toward saccharides in atmospheric samples. This approach simplifies the sample preconcentration and clean-up rendering the method fast and effective. For convenience during the isolation, magnetic particles were coated with a layer of titania. The actual LOD and LOQ values of the method range from 0.30 to 1.50 μg mL−1 and 1.00 to 5.00 μg mL−1, respectively, for the selected saccharides. The within-day repeatability (n = 3) was found to be less than 1.4% for concentrations in the range of 1.0–50.0 μg mL−1. An aerosol sample originating from the city centre of Ioannina was analyzed to verify the applicability of the method. Most of the saccharides were detected at concentrations from 83 ng m−3 (mannitol) to 200 ng m−3 (mannosan and levoglucosan) after the sample had been subjected to the preconcentration/clean-up procedure.