Properties of subcritical water as an eluent for reversed-phase liquid chromatography—Disruption of the hydrogen-bond network at elevated temperature and its consequences

The use of subcritical water as an eluent for reversed-phase liquid chromatography is further explored. Shape selectivity as well as thermodynamic values for solute transfer were measured and compared to those seen with traditional ambient methanol/water and acetonitrile/water mobile phases. Linear solvation energy analysis was also used to analyze extrapolated values of the retention factor in pure water at ambient temperatures (k^′_w${k^{\prime }}_w$) for subcritical water and ambient hydroorganic mobile phases. Results indicate that it is likely that a large disruption in the hydrogen-bonding network of water at high temperatures causes unique chromatographic selectivity, as well as prohibits accurate extrapolation from high temperature to ambient conditions using pure water. Additionally, subcritical water was not found to be a suitable mobile phase for determining k^′_w${k^{\prime }}_w$ for use in estimating octanol/water partition coefficients.     

Characterization of Ascentis RP-Amide column: Lipophilicity measurement and linear solvation energy relationships

This study investigates lipophilicity determination by chromatographic measurements using the polar embedded Ascentis RP-Amide stationary phase. As a new generation of amide-functionalized silica stationary phase, the Ascentis RP-Amide column is evaluated as a possible substitution to the n  -octanol/water partitioning system for lipophilicity measurements. For this evaluation, extrapolated retention factors, log k^′_w$\text{log}{k^{\prime }}_w$, of a set of diverse compounds were determined using different methanol contents in the mobile phase. The use of n-octanol enriched mobile phase enhances the relationship between the slope (S  ) of the extrapolation lines and the extrapolated log k^′_w$\text{log}{k^{\prime }}_w$ (the intercept of the extrapolation), as well as the correlation between log P   values and the extrapolated log k^′_w$\text{log}{k^{\prime }}_w$ (1:1 correlation, r² = 0.966). In addition, the use of isocratic retention factors, at 40% methanol in the mobile phase, provides a rapid tool for lipophilicity determination. The intermolecular interactions that contribute to the retention process in the Ascentis RP-Amide phase are characterized using the solvation parameter model of Abraham. The LSER system constants for the column are very similar to the LSER constants of the n-octanol/water extraction system. Tanaka radar plots are used for quick visual comparison of the system constants of the Ascentis RP-Amide column and the n-octanol/water extraction system. The results all indicate that the Ascentis RP-Amide stationary phase can provide reliable lipophilic data.     

Iron organic speciation determination in rainwater using cathodic stripping voltammetry

A sensitive method using Competitive Ligand Exchange-Adsorptive Cathodic Stripping Voltammetry (CLE-ACSV) has been developed to determine for the first time iron (Fe) organic speciation in rainwater over the typical natural range of pH. We have adapted techniques previously developed in other natural waters to rainwater samples, using the competing ligand 1-nitroso-2-naphthol (NN). The blank was equal to 0.17 ± 0.05 nM (n = 14) and the detection limit (DL) for labile Fe was 0.15 nM which is 10–70 times lower than that of previously published methods. The conditional stability constant for NN under rainwater conditions was calibrated over the pH range 5.52–6.20 through competition with ethylenediaminetetraacetic acid (EDTA). The calculated value of the logarithm of β^′_Fe3+(NN)3${{\beta }^{\prime }}_{{\text{Fe}}^{3+}{(\text{NN})}_3}$ increased linearly with increasing pH according to log β^′_Fe3+(NN)3=2.4±0.6×pH+11.9±3.5$\text{log}{{\beta }^{\prime }}_{{\text{Fe}}^{3+}{(\text{NN})}_3}=2.4\pm 0.6\times \text{pH}+11.9\pm 3.5$ (salinity = 2.9, T = 20 °C). The validation of the method was carried out using desferrioxamine mesylate B (DFOB) as a natural model ligand for Fe. Adequate detection windows were defined to detect this class of ligands in rainwater with 40 μM of NN from pH 5.52 to 6.20. The concentration of Fe-complexing natural ligands was determined for the first time in three unfiltered and one filtered rainwater samples. Organic Fe-complexing ligand concentrations varied from 104.2 ± 4.1 nM equivalent of Fe(III) to 336.2 ± 19.0 nM equivalent of Fe(III) and the logarithm of the conditional stability constants, with respect to Fe³⁺, varied from 21.1 ± 0.2 to 22.8 ± 0.3. This method will provide important data for improving our understanding of the role of wet deposition in the biogeochemical cycling of iron.     

Comparison of the adsorption mechanisms of pyridine in hydrophilic interaction chromatography and in reversed-phase aqueous liquid chromatography

The adsorption isotherms of pyridine were measured by frontal analysis (FA) on a column packed with shell particles of neat porous silica (Halo), using water–acetonitrile mixtures as the mobile phase at 295 K. The isotherm data were measured for pyridine concentrations covering a dynamic range of four millions. The degree of heterogeneity of the surface was characterized by the adsorption energy distribution (AED) function calculated from the raw adsorption data, using the expectation-maximization (EM) procedure. The results showed that two different retention mechanisms dominate in Per aqueous liquid chromatography (PALC) at low acetonitrile concentrations and in hydrophilic interaction chromatography (HILIC) at high acetonitrile concentrations. In the PALC mode, the adsorption mechanism of pyridine on the silica surface is controlled by hydrophobic interactions that take place on very few and ultra-active adsorption sites, which might be pores on the irregular and rugose surface of the porous silica particles. The surface is seriously heterogeneous, with up to five distinct adsorption sites and five different energy peaks on the AED of the packing material. In contrast, in the HILIC mode, the adsorption behavior is quasi-homogeneous and pyridine retention is governed by its adsorption onto free silanol groups. For intermediate mobile phase compositions, the siloxane and the silanol groups are both significantly saturated with acetonitrile and water, respectively, causing a minimum of the retention factor of pyridine on the Halo column.     

Photophysical Deactivation Mechanisms of the Pyrimidine Analogue 1-Cyclohexyluracil

The photophysical relaxation mechanisms of 1-cyclohexyluracil, in vacuum and water, were investigated by employing the Multi-State CASPT2 (MS-CASPT2, Multi-State Complete Active-Space Second-Order Perturbation Theory) quantum chemical method and Dunning’s cc-pVDZ basis sets. In both environments, our results suggest that the primary photophysical event is the population of the S11(ππ*) bright state. Afterwards, two likely deactivation pathways can take place, which is sustained by linear interpolation in internal coordinates defined via Z-Matrix scans connecting the most important characteristic points. The first one (Route 1) is the same relaxation mechanism observed for uracil, its canonical analogue, i.e., internal conversion to the ground state through an ethylenic-like conical intersection. The other route (Route 2) is the direct population transfer from the S11(ππ*) bright state to the T23(nπ*) triplet state via an intersystem crossing process involving the (S11(ππ*)/T23(nπ*))STCP singlet-triplet crossing point. As the spin-orbit coupling is not too large in either environment, we propose that most of the electronic population initially on the S11(ππ*) state returns to the ground following the same ultrafast deactivation mechanism observed in uracil (Route 1), while a smaller percentage goes to the triplet manifold. The presence of a minimum on the S11(ππ*) potential energy hypersurface in water can help to understand why experimentally it is noticed suppression of the triplet states population in polar protic solvent.     

Covalently bonded polysaccharide-modified stationary phase for per aqueous liquid chromatography and hydrophilic interaction chromatography

The mixed sulfated/methacryloyl polysaccharide derivative was prepared and successfully immobilized onto the surface of porous silica particles by polymerization. Polysaccharide derivative was calculated as 10.33% in the stationary phase prepared. The new stationary phase (PMSP) showed both hydrophilic interaction (HILIC) and per aqueous liquid chromatography (PALC) characteristics. The effects of column temperature, the water content, pH and ion strength of mobile phase on the retention time of test compounds in highly aqueous eluents were investigated to evaluate the PALC features of PMSP. The column efficiency is about 31,000 plates/m for benzoic acid in water/ACN (97/3, v/v) mobile phase at a flow rate of 1.0 mL/min. Compared with C18 column, the PMSP had shorter retention time for weak polar and non-polar compounds, but also showed stronger retention for strong polar compounds. It indicated that PALC was a suitable mode of chromatography as replacement of HILIC and complementarity of reversed-phase liquid chromatography (RPLC).     

Galvano-Fenton Engineering Solution with Spontaneous Catalyst’s Generation from Waste: Experimental Efficiency,Parametric Analysis and Modeling Interpretation Applied to a Clean Technology for Dyes Degradation in Water

In this paper, the degradation of the diazo dye naphthol blue black (NBB) using the Galvano-Fenton process is studied experimentally and numerically. The simulations are carried out based on the anodic, cathodic, and 34 elementary reactions evolving in the electrolyte, in addition to the oxidative attack of NBB by HO• at a constant rate of 3.35×107 mol−1·m3·s−1 during the initiation stage of the chain reactions. The selection of the operating conditions including the pH of the electrolyte, the stirring speed, and the electrodes disposition is performed by assessing the kinetics of NBB degradation; these parameters are set to 3, 350 rpm and a parallel disposition with a 3 cm inter-electrode distance, respectively. The kinetics of Fe(III) in the electrolyte were monitored using the principles of Fricke dosimetry and simulated numerically. The model showed more than a 96% correlation with the experimental results in both the blank test and the presence of the dye. The effects of H2O2 and NBB concentrations on the degradation of the dye were examined jointly with the evolution of the simulated H2O2, Fe2+, and HO• concentrations in the electrolyte. The model demonstrated a good correlation with the experimental results in terms of the initial degradation rates, with correlation coefficients exceeding 98%.     

Synthesis and characterization of the azido-bound five-coordinate high-spin iron(II) “picket fence” porphyrin complex

An Fe(II)-azido five-coordinate picket fence porphyrin complex with the formula [Na(2,2,2-crypt)][Fe^II(TpivPP)(N₃)] · 3C₆H₅Cl (TpivPP = α,α,α,α-tetrakis(o-pivalamidophenyl)porphinato, known as a picket fence porphyrin, and 2,2,2-crypt is the cryptand-222) has been synthesized and characterized. The synthesis utilizes cryptand-222 to solubilize sodium azide in the preparation procedure. The UV–Vis and IR spectroscopic data are consistent with an azido ferrous porphyrinate. The X-ray structural analysis and the Mössbauer results indicate that the ion complex [Fe^II(TpivPP)(N₃)]⁻ is high-spin and has the (d_xy)²(d_xz)¹(d_yz)¹(d_z2)¹(d_x2-y2)¹$({\text{d}}_{\mathit{xy}}{)}^2({\text{d}}_{\mathit{xz}}{)}^1({\text{d}}_{\mathit{yz}}{)}^1({\text{d}}_{z^2}{)}^1({\text{d}}_{x^2-y^2}{)}^1$ ground state electronic configuration.     

Application of the generalised SAFT-VR approach for long-ranged square-well potentials to model the phase behaviour of real fluids

In a recent generalisation of the SAFT-VR equation of state the method was extended so as to deal with short as well as long square-well ranges, namely, 1.2≤λ≤3.0$1.2\le \lambda \le 3.0$ [B.H. Patel, H. Docherty, S. Varga, A. Galindo, G.C. Maitland., Mol. Phys. 103 (1) (2005) 129–139]. Here, we confirm the accuracy of the approach by comparison with numerical calculations of the first perturbation term and with vapour pressure and coexistence density computer simulation data. The approach is then used to model a number of real substances, from non-polar to strongly polar. We discuss in particular the values of the square-well potential model found. For this purpose we construct a relative least squares objective function and the percentage absolute average deviation (%AAD) to determine the intermolecular model parameters (m  , λ$\lambda$, σ$\sigma$, ?/_kB$?/k_B$, _?hb/_kB${?}_{hb}/k_B$ and _rc$r_c$) by comparison to experimental vapour-pressure and saturated liquid density data. In order to ensure in each case that the global minimum is identified, the dimensionality of the problem is reduced by discretising the parameter-space [G.N.I. Clark, A.J. Haslam, A. Galindo, G. Jackson., Mol. Phys. 104 (22–24) (2006) 3561–3581]. Applying this method to the study of argon, n  -alkanes, nitrogen, benzene, carbon dioxide, carbon monoxide, the refrigerant R1270, hydrogen chloride hydrogen bromide and water we find that the optimal models always present square-well ranges λ<1.8$\lambda <1.8$, meaning that an upper bound value of λ=1.8$\lambda =1.8$ set in the original approach is sufficient to model real fluids; even polar ones. This finding is explained in terms of the averaged dipole–dipole interaction and of the long-range mean-field limit of the square-well potential.     

Performance of new prototype packed columns for very high pressure liquid chromatography

The reduced heights equivalent to a theoretical plate (HETP) of naphtho[2,3-a]pyrene were measured at room temperature on two sets of new prototype columns designed to be used in very high pressure liquid chromatography (VHPLC). The mobile phase used was pure acetonitrile. The columns are 50, 100, and 150 mm long. Those of the first set are 2.1 mm I.D., those of the second set, 3.0 mm I.D. The performance of these new columns were compared to those of the first generation of VHPLC columns, commercially available in 2.1 mm I.D. The prototype and commercial columns behave similarly at low reduced linear velocities (ν<5$\nu <5$), when the heat effects are negligible. At high flow rates, the shorter prototype columns have a twice better efficiency and less steep C-branches than the commercial columns. In contrast, the C-branch of the 150 mm long prototype columns are slightly steeper than those of the commercial columns. The important contribution to the reduced HETP that is due to the heat effects at high flow rates can in part be accounted for by a band broadening model governed by a flow mechanism with the shortest prototype columns. The sole heat effects cannot, however, explain the mediocre reduced HETPs of the 2.1 and 3.0 I.D. 150 mm long prototype columns. It seems that radial heterogeneity of the flow rate of the long prototype columns is significantly larger than that of the short columns. The contribution of the packing heterogeneity adds up to that of the heat effects to yield a poor column efficiency when sub-2μm$\text{sub-}2\mu \text{m}$ are packed into thin, long column tubes.     

Determination of catecholamines in urine using hydrophilic interaction chromatography with electrochemical detection

The determination of catecholamines in urine was investigated using hydrophilic interaction chromatography (HILIC) as an alternative to the commonly used reversed-phase (RP) method. A number of different approaches were explored, including per-aqueous liquid chromatography (PALC), and HILIC using bare silica, bonded amide and zwitterionic phases. The bonded phases gave superior results in terms of both peak shape and selectivity. The mechanism of the HILIC separation was investigated particularly with respect to the contribution of ion exchange to retention. The electrochemical detection of catecholamines was studied and optimised in typical HILIC mobile phases that contain high concentrations of acetonitrile. HILIC offered a number of advantages over the conventional RP approach, giving good retention of the solutes without use of ion pair reagents, the absence of which also would facilitate detection by mass spectrometry. HILIC used in conjunction with solid phase extraction based on RP also gives orthogonal separation mechanisms in the cleanup and analysis steps. Furthermore, good recoveries from the cleanup stage were obtained, as high concentrations of acetonitrile can be used as eluting solvent that are fully compatible with HILIC, and lipophilic impurities are eluted close to the void volume of the HILIC column.