Solvent modulation: An accurately modeled method for controlling solute retention in liquid chromatography

An innovative solvent delivery method termed solvent modulation has been developed to control solute retention in liquid chromatography. Solvent modulation is the technique whereby individual solvent zones are introduced onto the chromatographic column in a known random orrepeating sequence. Because the solvent zones are of constant composition and are spatially separated from one another, solute retention is controlled independently in each zone. Hence, the overall retention of a solute is a time-weighted average of the capacity factors in the solvent zones it has encountered. Solvent modulation offers a simple, versatile, and accurately modeled means to control and predict solute retention in liquid chromatography.     

Explicit ion, implicit water solvation for molecular dynamics of nucleic acids and highly charged molecules

An explicit ion, implicit water solvent model for molecular dynamics was developed and tested with DNA and RNA simulations. The implicit water model uses the finite difference Poisson (FDP) model with the smooth permittivity method implemented in the OpenEye ZAP libraries. Explicit counter-ions, co-ions, and nucleic acid were treated with a Langevin dynamics molecular dynamics algorithm. Ion electrostatics is treated within the FDP model when close to the solute, and by the Coulombic model when far from the solute. The two zone model reduces computation time, but retains an accurate treatment of the ion atmosphere electrostatics near the solute. Ion compositions can be set to reproduce specific ionic strengths. The entire ion/water treatment is interfaced with the molecular dynamics package CHARMM. Using the CHARMM-ZAPI software combination, the implicit solvent model was tested on A and B form duplex DNA, and tetraloop RNA, producing stable simulations with structures remaining close to experiment. The model also reproduced the A to B duplex DNA transition. The effect of ionic strength, and the structure of the counterion atmosphere around B form duplex DNA were also examined.     

Voltammetric study of interfacial electron transfer between bis(cyclopentadienyl)iron in organic solvents and hexacyanoferrate in water.

The electron-transfer reaction between bis(cyclopentadienyl)iron(II) ([Fe(II)(C5H5)2]) in nitrobenzene and a hexacyanoferrate redox couple ([Fe(II/III)(CN)6](4-/3-)) in water at the nitrobenzene / water interface was studied using normal pulse voltammetry. The voltammetric results indicate that the electron-transfer reaction takes place by way of a so-called ion-transfer (IT) mechanism, of which the forward and backward rate constants of the homogeneous electron-transfer reaction between [Fe(II/III)(C5H5)2](0/+) and [Fe(II/III)(CN)6](4-/3-) in the water phase have been determined. The electron-transfer reaction between [Fe(II)(C5H5)2] in 1,2-dichloroethane and [Fe(II/III)(CN)6](4-/3-) in water at the 1,2-dichloroethane / water interface was shown to also take place by the IT-mechanism.     

Extraction and separation of antimony(III) and (V) by a liquid cation-exchanger HDEHP

A systematic investigation was carried out on the extraction of Sb(III) and (V) with HDEHP from various acidic, neutral and alkaline solutions. Antimony(III) is best extracted from neutral or slightly acidic solutions, and the E values are nearly the same in the forward and backward extractions. Antimony(V) extraction is high only from concentrated HCl and HClO₄, and the E values are much larger in the backward direction. Extraction and separation of Sb(III) and (V) was studied as a function of acidity, alkalinity, anion and water-miscible organic additives in the aqueous phase, as well as the diluent used and HDEHP molarity. Separation factors obtained for Sb(III) and (V) were higher than when using isopropyl ether as solvent, which was hitherto used for this purpose.     

Broadening of peaks eluted before the solvent in capillary GC Part II: The role of phase soaking

Summary Phase soaking is a solvent effect which tends to reconcentrate peaks eluted after and to broaden peaks eluted before the solvent. The principles of the phase soaking effect on peaks eluted before the solvent are discussed. The broadening effect is distinguished from the broadening effect occurring in the flooded column inlet by partial solvent trapping. It was found that in most cases broadening by partial solvent trapping strongly predominated over broadening by phase soaking. Phase soaking was noticeable only in the neighbourhood of the solvent peak.Phase soaking does not broaden peaks eluted before the solvent as much as it re-concentrates those eluted after it. The two phase soaking effects on the front and the rear of the solvent band (that is, of the soaked zone) differ strongly from each other.Peak broadening by phase soaking is negligible for non-trapped components, because such solutes start their chromatography before a significant quantity of solvent enters the column. Phase soaking only broadens solute bands which were retained by the solvent in the column inlet, that is, bands already broadened by partial solvent trapping.     

Multi-spot paper chromatograms of sodium orthophosphate

The formation of two zones on paper chromatograms was investigated by chromatographing orthophosphoric acid and the primary, secondary and tertiary sodium salts, using pyridine-ethyl acetate/water as mobile phase. Movement of the species from the origin depends on washing the paper with acid and suggests an exchange of protons for sodium as a prerequisite condition. Washing with EDTA does not alter this requirement. Zone movement and multi-spot formation require that the forming solvent contain at least 5% of water (v/v). Between this value and saturation, increasing water content enhances zone disengagement. A solute spot of a sodium salt exchanges with the protons to produce a fast-migrating protonated phosphate species. If the solute sample has a sodium content less than the amount of exchangeable protons, a single fast spot is produced ; if the concentration exceeds this capacity, a second slower spot results.     

Two-step solvent gradients in simulated moving bed chromatography. Numerical study for linear equilibria

The application of gradients in simulated moving bed (SMB) chromatography has recently attracted interest as a method for further improving the performance of this continuous separation process. One possible implementation of gradients consists in setting the solvent strength in the desorbent stream higher than that in the feed stream. As a result, the components to be separated are more retained in the zones upstream of the feed position and more easily eluted in the zones downstream of the feed position. If a liquid mobile phase is used, gradients can be created by dosing different solvents into the feed and desorbent ports. In a closed-loop gradient SMB arrangement the solvent strength within the unit will depend on the two feed compositions and on the characteristic flow-rates of the process. In this work an equilibrium stage model describing a true moving bed process is used to analyze numerically the main features of a two-step gradient SMB process. The adsorption isotherms are assumed to be always linear under isocratic conditions. The relevant Henry constants depend in a nonlinear manner on the composition of the solvent. Based on numerical simulations the impact of the two inlet solvent compositions is demonstrated in terms of the size and shape of regions of applicable flow-rates. Different strategies of designing the process are discussed and compared with respect to maximizing productivities and minimizing desorbent requirements.     

Speciation of dissolved iron(II) and iron(III) in environmental water samples by gallic acid-modified nanometer-sized alumina micro-column separation and ICP-MS determination

A method has been developed for the speciation of trace dissolved Fe(II) and Fe(III) in water by coupling gallic acid (GA) modified nanometer-sized alumina micro-column separation with inductively coupled plasma mass spectrometry (ICP-MS). The separation of Fe(II) and Fe(III) was achieved based on the obvious difference in reaction kinetics between Fe(II) and Fe(III) with GA. Fe(III) was selectively retained on the micro-column at pH 5.5-6.5, while Fe(II) could not be retained by the micro-column at the whole tested pH range of 1.0-6.5, and passed through the micro-column. The Fe(II) can be determined by ICP-MS directly without preconcentration/separation procedure, while Fe(III) retained on the micro-column was then eluted with 1.0 mL of 1 mol L(-1) HCl and determined by ICP-MS. The parameters affecting the separation of Fe(II) and Fe(III) were investigated systematically and the optimum separation conditions were established. Under the optimized conditions, the detection limits of 0.48 microg L(-1) and 0.24 microg L(-1) with relative standard deviation of 5.6% and 4.3%(C= 5 microg L(-1), n= 7) for Fe(II) and Fe(III) were found, respectively. No obvious effect on the speciation of Fe(II) and Fe(III) was found with the change of the ratio of Fe(II) and Fe(III) from 0 ratio 10 to 10 ratio 0. The proposed method was applied for the determination of trace Fe(II) and Fe(III) in environmental water and the recoveries for spiked samples were found to be in the range of 97-105%.     

A new model to describe the sorption of surfactants on solids in non-aqueous media

A new phenomenological model is developed to describe the sorption of surfactants on solids in non-aqueous media. This is based on the use of interaction parameters (delta) among solid, solute and solvent to assess the degree of the various interactions and computing an effective interaction parameter for the entire system represented by delta eff = abs{A|delta solid - delta solvent| + B|delta solute - delta solvent| - C|delta solid - delta solute|}. The effective interaction parameter determines the extent of adsorption that can occur in a given system. Interaction parameters typically account for dispersive interactions between the different components. This new model is used to describe the sorption behavior of a number of surfactant/solvent/solid systems.     

Thin-layer chromatography of tris(1,10-phenanthroline)iron(II) and tris(2,2′-bipyridine)iron(II) on Sephadex G gels

Summary Gel chromatographic behaviour of tris(1, 10-phenanthroline)iron(II), tris(2,2′-bipyridine)iron(II) and tris(glycinato)cobalt(III) on Sephadex G-10 or G-25 was investigated by TLC with 0.001–1.0M NaCl as the eluent. The zone shapes and R_M values of tris(1,10-phenanthroline)iron(II) and tris(2,2′-bipyridine)iron(II) were appreciably dependent on the sample and eluent concentration, while the neutral complex, tris(glycinato) cobalt(III), exhibited the round zones with constant R_M values. The order of R_M values was found to be tris(glycinato)cobalt(III<tris(2,2∔pyridine)iron(II)<tris-(1,10-phenanthroline)iron(II) in all systems studied, although the reverse trend was expected when assuming the chromatographic behaviour of solute compounds to be controlled by the “sieving effect”. The comparison of the behaviour on Sephadex G gels with that on CM-cellulose revealed that the predominant mechanism involved is not the sieving effect, but ion-exchange and/or hydrophobic interaction.     

Preconcentration of Cr(III), Co(II), Cu(II), Fe(III) and Pb(II) as calmagite chelates on cellulose nitrate membrane filter prior to their flame atomic absorption spectrometric determinations

A method for the preconcentration and determination of Cr(III), Co(II), Cu(II), Fe(III) and Pb(II) ions by atomic absorption spectrometry has been described. The method was based the collection of metal-calmagite complexes on a soluble cellulose nitrate membrane filter. The detection of the solution was obtained by flame atomic absorption spectrometry (FAAS) after completely dissolving the membrane with 0.5 ml of nitric acid at 80 degrees C. The metal ions were recovered quantitatively at pH 8. Various factors which affect the collection and determination of metal ions such as, type and size of the membrane filter, solvent for dissolution of the species retained on the filter were investigated. The detection limits were varying 0.06 mug l(-1) for Cu to 2.5 mug l(-1) for Cr. An application of the proposed method for analyte ions in mineral and tap water samples was also described with satisfactory results (recoveries >95%, relative standard deviations <10%).     

Empirical solvent correction for multiple amide group vibrational modes

Previously proposed solvent correction to the amide I peptide vibration was extended so that it can be applied to a general solvated chromophore. The combined molecular and quantum mechanics (MMQM) method is based on a linear dependence of harmonic force field and intensity tensor components of the solute on solvent electrostatic field. For N-methylacetamide, realistic solvent frequency and intensity changes as well as inhomogeneous band widths were obtained for amide A, I, II , and III modes. A rather anomalous basis set size dependence was observed for the amide A and I vibrations, when bigger basis lead to narrowing of spectral bands and lesser molecular sensibility to the environment. For a model alpha-helical peptide, a W-shape of the vibrational circular dichroism signal observed in deuterated solvent for the amide I band was reproduced correctly, unlike with previous vacuum models.     

Solvent effect on the absorption spectra of coumarin 120 in water: A combined quantum mechanical and molecular mechanical study

The solvent effect on the absorption spectra of coumarin 120 (C120) in water was studied utilizing the combined quantum mechanical∕molecular mechanical (QM∕MM) method. In molecular dynamics (MD) simulation, a new sampling scheme was introduced to provide enough samples for both solute and solvent molecules to obtain the average physical properties of the molecules in solution. We sampled the structure of the solute and solvent molecules separately. First, we executed a QM∕MM MD simulation, where we sampled the solute molecule in solution. Next, we chose random solute structures from this simulation and performed classical MD simulation for each chosen solute structure with its geometry fixed. This new scheme allowed us to sample the solute molecule quantum mechanically and sample many solvent structures classically. Excitation energy calculations using the selected samples were carried out by the generalized multiconfigurational perturbation theory. We succeeded in constructing the absorption spectra and realizing the red shift of the absorption spectra found in polar solvents. To understand the motion of C120 in water, we carried out principal component analysis and found that the motion of the methyl group made the largest contribution and the motion of the amino group the second largest. The solvent effect on the absorption spectrum was studied by decomposing it in two components: the effect from the distortion of the solute molecule and the field effect from the solvent molecules. The solvent effect from the solvent molecules shows large contribution to the solvent shift of the peak of the absorption spectrum, while the solvent effect from the solute molecule shows no contribution. The solvent effect from the solute molecule mainly contributes to the broadening of the absorption spectrum. In the solvent effect, the variation in C-C bond length has the largest contribution on the absorption spectrum from the solute molecule. For the solvent effect on the absorption spectrum from the solvent molecules, the solvent structure around the amino group of C120 plays the key role.     

Poly(p-benzamide). II. Thermal behavior and crystal transformations

The thermal behavior and crystal transformation of poly(p-benzamide) are reported. Modification I contains both solvent (N,N-dimethylacetamide) and LiCl in its crystal lattice. Modification II is likely to form a complex crystal with LiCl, whereas crystalline modification III contains neither solvent nor LiCl in its unit cell. The crystal transformation temperature from modification I to II is 214°C, and the crystal–nematic transition temperatures of modification II and III are 475 and 544°C, respectively. Modification III can be obtained from II by heating above 475°C and cooling, or by washing with water. It can also be obtained from modification I by washing with water and annealing.     

How can temperature affect reverse micellar extraction using sucrose fatty acid ester?

The reverse micellar extraction of lysozyme using sucrose fatty acid ester was found to be greatly affected by the temperature in the extraction process. For example, lysozyme was perfectly extracted from the feed aqueous phase to the reverse micellar organic phase at 25°C, while it was not extracted at 5°C at all. After entrapping lysozyme into the reverse micelles, lysozyme was recovered from the reverse micellar organic phase to the recovery aqueous phase only by decreasing the temperature in the backward extraction. Moreover, lysozyme solubilized in the reverse micellar organic solution could be recovered without the recovery aqueous solution at 3°C, and its activity was retained at 95%.     

Surface viscoelastic properties of floating polyelectrolyte multilayers films: a capillary wave study

A capillary wave technique was used to study the viscoelastic properties of floating polyelectrolyte multilayers of (PSS/PAH)(n) at the air-water interface. Oppositely charged polyelectrolyte layers were adsorbed onto two different Langmuir monolayers, either the lipid dimethyldioctadecylammonium bromide (DODAB) or the block copolymer poly(styrene-b-sodium acrylate) (PS-b-PAA). The results allow to propose a schematic representation of the multilayers in three zones: Zone I as a precursor, representing the adhesion between the Langmuir monolayer and the bulk polyelectrolyte multilayer. Zone II forms a bulk or core zone of the multilayer. Zone III as an outer zone in direct contact with the aqueous phase. The results show an increase of the elasticity after the formation of four polyelectrolyte layers accompanied by an apparent negative viscosity. This behaviour was interpreted as a translation of elasticity dominance from zone I to zone II. The Young modulus of seven layers was in the same order of magnitude as observed for planar polyelectrolyte multilayer films.     

Partial Molar Volumes of Hexaamminecobalt(III) Nitrate in Proteated and Deuterated Water

The partial molar volumes (V ₂) for hexaamminecobalt(III) nitrate in proteated and deuterated water were determined at 0.00 (for H₂O only), 5.00, 10.00, 15.00, and 20.00?°C. The increase of the solute??s partial molar volume with increasing concentration and the negative second derivative of the solute??s partial molar volume at infinite dilution with respect to temperature were interpreted in terms of the solvent structure breaking property of the solute. In addition, the difference at each temperature between the solute??s partial molar volume at infinite dilution for proteated and deuterated water was used to estimate the solvent coordination number at each temperature.