Reverse-High Performance Liquid Chromatography Mechanism Explained by Polarization of Stationary Phase

ABSTRACT Original results and conclusions on Reverse Phase High Performance Liquid Chromatography (RPLC) mechanism are here presented. So far, none of the theoretical approaches applied to the RPLC mechanism can explain the retention and elution mechanisms of most of the analytes by RPLC, especially neutral organic compounds. Our experiences allowed us to state that RPLC retention mechanism most likely occurs through polarization of stationary phase (usually dielectric surfaces) submerged into solvents with huge dielectric constant and high dipole moment (i.e. water and/or acetonitrile) at high pressures as those applied in RPLC systems. The surface polarized interacts with polarizable target analytes (i.e. naphthalene, pyrene or benzo(ghi)perylene) in such a way that the retention depends on the medium polarizability of the target compound. The higher is the medium polarizability of the compound retained the higher is its retention time. By using Polycyclic Aromatic Hydro-carbons (PAHs) and 6-nitrochrysene as probes, we found that although the dependence of retention times on pressure is not evident in most cases, the interaction between pressure and the mobile phase dipole moment is evidenced at the light of the constructed mathematical models. Two very different cases can be distinguished: 1. On one hand, when low dipole moment mobile phases are used, high pressure means big work developed by the system in such a way that high pressures inside the system reduce retention times compared to low pressures. 2. On the another one, when high dipole moment solvents are used as mobile phase, high pressure entails an outstanding increase of the retention times compared to low dipole moment mobile phases. This is most likely due to high pressures that entail closer alignment of mobile phase dipoles and lesser tilt of them, originating an outstanding electric field inside the analytical column. In addition to this, reduced retention times for dipole solutes (6-nitrochrysene, dipole molecule) compared to non-polar solutes (PAHs) were observed when pressure drops. This conclusions would allow to extend the model to basic/acid molecules at different pH. In this case, electrostatic repulsions among analyte molecules should be considered

A new HPLC method for simultaneously measuring chloride, sugars, organic acids and alcohols in food samples

This paper introduces an original, rapid, efficient and reliable HPLC method for the accurate and simultaneous quantification (g/L) of chloride in samples containing sugars, organic acids and alcohols. Separation was achieved using a HI-Plex H column at 35oC, with H2SO4 (0.005 N) as the mobile phase at a flow rate of 0.4 mL/min. The column effluent was monitored by a Refractive Index (RI) detector. A linear response was achieved over NaCl concentrations of 0.25 – 2.5 g/L and 5 – 40 g/L. The analytical method inter- and intra-run accuracy and precision were better than ±10.0%. Investigating the mechanism of detection using different chloride and sodium s reviled that this method can be used for determining the total concentration of chloride salts when in suspension. This method was successfully applied to 15 samples of commercial food products and the salt content obtained from this method was compared with 3 other methods for salt determination. The (HI-Plex H) column was designed for determining the concentrations of sugars, organic acids and alcohols when in solution. Hence, application of our new methodology would allow the determination of sugars, alcohols and organic acids in samples derived from seawater-based fermentation media as well as samples from salty food and dairy products

Screening of antioxidant properties of the apple juice using the front-face synchronous fluorescence and chemometrics

Fluorescence spectroscopy is gaining increasing attention in food analysis due to its higher sensitivity and selectivity as compared to other spectroscopic techniques. Synchronous scanning fluorescence technique is particularly useful in studies of multi-fluorophoric food samples, providing a further improvement of selectivity by reduction in the spectral overlapping and suppressing light-scattering interferences. Presently, we study the feasibility of the prediction of the total phenolics, flavonoids, and antioxidant capacity using front-face synchronous fluorescence spectra of apple juices. Commercial apple juices from different product ranges were studied. Principal component analysis (PCA) applied to the unfolded synchronous fluorescence spectra was used to compare the fluorescence of the entire sample set. The regression analysis was performed using partial least squares (PLS1 and PLS2) methods on the unfolded total synchronous and on the single-offset synchronous fluorescence spectra. The best calibration models for all of the studied parameters were obtained using the PLS1 method for the single-offset synchronous spectra. The models for the prediction of the total flavonoid content had the best performance; the optimal model was obtained for the analysis of the synchronous fluorescence spectra at Delta lambda = 110 nm (R (2) = 0.870, residual predictive deviation (RPD) = 2.7). The optimal calibration models for the prediction of the total phenolic content (Delta lambda = 80 nm, R (2) = 0.766, RPD = 2.0) and the total antioxidant capacity (Delta lambda = 70 nm, R (2) = 0.787, RPD = 2.1) had only an approximate predictive ability. These results demonstrate that synchronous fluorescence could be a useful tool in fast semi-quantitative screening for the antioxidant properties of the apple juices.info:eu-repo/semantics/publishedVersio

Determination of Anthracene on Ag-Au Alloy Nanoparticles/Overoxidized-Polypyrrole Composite Modified Glassy Carbon Electrodes

A novel electrochemical sensor for the detection of anthracene was prepared by modifying a glassy carbon electrode (GCE) with over-oxidized polypyrrole (PPyox) and Ag-Au (1:3) bimetallic nanoparticles (Ag-AuNPs). The composite electrode (PPyox/Ag-AuNPs/GCE) was prepared by potentiodynamic polymerization of pyrrole on GCE followed by its overoxidation in 0.1 M NaOH. Ag-Au bimetallic nanoparticles were chemically prepared by the reduction of AgNO3 and HAuCl4 using C6H5O7Na3 as the reducing agent as well as the capping agent and then immobilized on the surface of the PPyox/GCE. The nanoparticles were characterized by UV-visible spectroscopy technique which confirmed the homogeneous formation of the bimetallic alloy nanoparticles. Transmission electron microscopy showed that the synthesized bimetallic nanoparticles were in the range of 20–50 nm. The electrochemical behaviour of anthracene at the PPyox/Ag-AuNPs/GCE with Ag: Au atomic ratio 25:75 (1:3) exhibited a higher electrocatalytic effect compared to that observed when GCE was modified with each constituent of the composite (i.e., PPyox, Ag-AuNPs) and bare GCE. A linear relationship between anodic current and anthracene concentration was attained over the range of 3.0 × 10−6 to 3.56 × 10−4 M with a detection limit of 1.69 × 10−7 M. The proposed method was simple, less time consuming and showed a high sensitivity

Chitosan-modified cotton thread for the preconcentration and colorimetric trace determination of Co(II)

In this work we propose a thread-based microfluidic device (μTAD) for the preconcentration and colorimetric determination of Co(II) in water using a digital image. The reaction is based on complexation of Co(II) by 4-(2- pyridylazo) resorcinol (PAR), which changes the detection zone from yellow to red. PAR is immobilized in a chitosan membrane to retain the complex in the detection zone. The designed μTAD makes it possible to pre- concentrate and determine cobalt between 25 and 600 µg·L−1 with a relative standard deviation of 4% (n = 5), and a detection limit of 6.5 µg·L−1. The device permits an enhancement factor of 11 by combining the use of a chitosan retention membrane and a sample volume of 50 µL. Recovery experiments were performed in tap water to evaluate the accuracy of the method, and the results obtained compared to a reference method presents an error no higher than 5.7%This work was founded by Spanish “Ministerio de Economía y Competitividad” under Project CTQ2016-78754-C2-1-R and Junta de Andalucía under Projects B-FQM-243-UGR18 and P18-RT-2961. The project was partially supported by European Regional Development Funds (ERDF). Supporting Research in the State of Minas Gerais (Fapemig) (CEX-APQ-02436-15)

Improved optimization of polycyclic aromatic hydrocarbons (PAHs) mixtures resolution in reversed-phase high-performance liquid chromatography by using factorial design and response surface methodology

International audienceOptimization of reversed-high performance liquid chromatography for separation of polycyclic aromatic hydrocarbons (PAHs) mixtures is frequently based on resolution and efficiency criteria. However, when very complex mixtures are analysed (such as diesel or kerosene soot) optimization must be made up based on resolution criterion rather than on efficiency. In fact, low efficiency is recommended in these cases because unexpected compounds not included in synthetic mixtures can appear in real samples. Based on resolution criterion, optimization process for a 16 EPA PAHs mixture was performed on three sets of difficult-to-separate PAHs pairs: acenaphthene–fluorene, benzo[g,h,i]perylene–dibenzo(ah)anthracene and benzo(ghi)perylene-indeno(123cd)pyrene. Resolution of acenaphthene-fluorene pair was used for evaluation of the first part of the chromatogram, and evaluation of the second one was carried out by using the resolution of the pairs dibenzo(ah)anthracene-benzo(ghi)perylene and benzo(ghi)perylene-indeno(123cd)pyrene. Two-level full factorial designs were applied to detect interactions among variables to be optimized: speed flow, temperature of column oven and ramp in both parts of the chromatogram. Experimental data were fitted by multivariate nonlinear regression models. Optimum values of speed flow and temperature were obtained through mathematical analysis of the constructed models. For good resolution of acenaphthene-fluorene 10°C (column oven temperature) and 1.0 mL/min (mobile phase flow rate) are recommended but 40°C and 0.5 mL/min provides us with the best resolution for the pairs dibenzo(ah)anthracene-benzo(ghi)perylene and benzo(ghi)perylene-indeno(123-cd)pyrene. We have found that resolution in the second part of RPLC chromatogram does not depend on conditions used in the first part of the chromatogram. Thus, good resolution was achieved when operating at 1.0 mL/min in the first part of the chromatogram and 0.5 mL/min in the second one. Programmed temperature gradient (10°C until 30 minutes and progressively increasing temperature until reaching 40°C at 45 minutes) was used. Due to the difficulty of achieving 10°C in the column oven and high pressures developed inside the column when so low temperatures are selected, combination of 15°C (in the first part of the chromatogram) with 40°C (the last 45 min of the chromatogram) were finally selected. Speed flow increases from 0.5 mL/min in the first part of the chromatogram to 1 mL/min after 45 minute. Good sensitivities were also achieved