Interpretation of the excess adsorption isotherms of organic eluent components on the surface of reversed-phase phenyl modified adsorbents

The adsorption of three organic eluent components (acetonitrile, methanol, and tetrahydrofuran) from water were measured on four phenyl-type bonded phases using the minor disturbance method. The thicknesses of organic layer enriched above the phenyl-type bonded ligands were assessed and interpreted. Acetonitrile and tetrahydrofuran showed multilayer formation while methanol showed monomolecular adsorption. These results were compared to those obtained on alkyl bonded phases.     

Probing the interaction mode in hydrophilic interaction chromatography

This work aims at characterizing interactions between a select set of probes and 22 hydrophilic and polar commercial stationary phases, to develop an understanding of the relationship between the chemical properties of those phases and their interplay with the eluent and solutes in hydrophilic interaction chromatography. "Hydrophilic interaction" is a somewhat inexact term, and an attempt was therefore made to characterize the interactions involved in HILIC as hydrophilic, hydrophobic, electrostatic, hydrogen bonding, dipole-dipole, π-π interaction, and shape-selectivity. Each specific interaction was quantified from the separation factors of a pair of similar substances of which one had properties promoting the interaction mode being probed while the other did not. The effects of particle size and pore size of the phases on retention and selectivity were also studied. The phases investigated covered a wide range of surface functional groups including zwitterionic (sulfobetaine and phosphocholine), neutral (amide and hydroxyl), cationic (amine), and anionic (sulfonic acid and silanol). Principal component analysis of the data showed that partitioning was a dominating mechanism for uncharged solutes in HILIC. However, correlations between functional groups and interactions were also observed, which confirms that the HILIC retention mechanism is partly contributed by adsorption mechanisms involving electrostatic interaction and multipoint hydrogen bonding. Phases with smaller pore diameters yielded longer retention of solutes, but did not significantly change the column selectivities. The particle diameter had no significant effect, neither on retention, nor on the selectivities. An increased water content in the eluent reduced the multipoint hydrogen bonding interactions, while an increased electrolyte concentration lowered the selectivities of the tested columns and made their interaction patterns more similar.     

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.     

Positively charged covalent organic framework and its application in the dispersive solid-phase extraction of ultraviolet-filters from food packaging material migrants

Abstract

Covalent organic frameworks (COFs) show excellent property, such as high porosity and excellent structure stability and were well applied in fields such as catalysis and adsorption, but most of the COFs showed similar structure and thus similar adsorption performance. Modification of simple COFs to enhance its adsorption performance in separation technique is an important issue. In this study, quaternary ammonium groups with long hydrophobic chain were introduced into a simple COF (TpPa-1) for the first time. The positively charged COF (PC-TpPa-1) can form electrostatic interaction and hydrophobic effect with negatively charged analytes, and showed good adsorption performance for ultraviolet-filters (UV filters). Under the optimum conditions, i.e. adsorbent amount 20?mg, pH?=?7, 1.0?mL acetonitrile as eluent, the obtained recoveries for all UV filters were in the range of 86.4–96.7%.The developed method was successfully applied to the determination of UV filters from food packaging material migrants with the aid of HPLC as a detector.     

Separation of Aromatic Carboxylic Acids in IC and the Study on the Relationship Between Retention Behavior and Electrostatic Potential

A method has been developed for separation of 20 kinds of aromatic carboxylic acid using anion-exchange chromatography. A Dionex IonPac AS9-HC guard column (250mm×2mm) was used with a 9mmol/L sodium carbonate solution containing 50% (v/v) acetonitrile as eluent. A set of retention time data has been obtained using a conductivity detector DS6. Furthermore, geometrical optimization and electrostatic potential calculation of 20 kinds of aromatic carboxylic acid have been performed at the HF/6-31G* level of theory. A number of statistically-based parameters derived from molecular surface electrostatic potential have been obtained. Linear relationship between retention time and structural parameters has been established by multiple regression method. The result shows that parameters derived from electrostatic potential Vs +, Vs -, П together with the dipole moment μ can be well used to express the quantitative structure-retention time of this kind of aromatic carboxylic acid. Good predictive capability has also been demonstrated. The result has provided a framework which further proves the general applicability of this electrostatic potential parameter set to a great extent,and with which the ion chromatographic adsorption mechanism can be investigated.     

Influence of Physicochemical Parameters of Some Ring-Substituted Phenol Derivatives on Their Retention on a Porous Graphitized Carbon Column

Abstract

The retention characteristics of 29 phenol derivatives were determined on a porous graphitized carbon column in unbuffered acetonitrile—water and methanol—water eluent mixtures at various organic phase concentrations. Each phenol derivative showed symmetric peaks in each eluent without buffers. Good linear correlations were found between the log k' value and the organic mobile phase concentration in the eluent. Principal component analysis indicated that methanol and acetonitrile expose different selectivities. Stepwise regression analysis proved that the retention of ring—substituted phenol derivatives is mainly governed by the sterical parameters, electron-withdrawing power and hydrogen donor capacity of substituents. According to the results of Free-Wilson analysis, the substituents with large steric parameters, strong electron-withdrawing power and hydrogen donor capacity have the highest impact on the retention. The lipophilicity of phenol derivatives did not affect significantly the retention, although the eluents were typical reversed-phase eluents.     

Application of retention modelling to the simulation of separation of organic anions in suppressed ion chromatography

The ion-exchange separation of organic anions of varying molecular mass has been demonstrated using ion chromatography with isocratic, gradient and multi-step eluent profiles on commercially available columns with UV detection. A retention model derived previously for inorganic ions and based solely on electrostatic interactions between the analytes and the stationary phase was applied. This model was found to accurately describe the observed elution of all the anions under isocratic, gradient and multi-step eluent conditions. Hydrophobic interactions, although likely to be present to varying degrees, did not limit the applicability of the ion-exchange retention model. Various instrumental configurations were investigated to overcome problems associated with the use of organic modifiers in the eluent which caused compatibility issues with the electrolytically derived, and subsequently suppressed, eluent. The preferred configuration allowed the organic modifier stream to bypass the eluent generator, followed by subsequent mixing before entering the injection valve and column. Accurate elution prediction was achieved even when using 5-step eluent profiles with errors in retention time generally being less than 1% relative standard deviation (RSD) and all being less than 5% RSD. Peak widths for linear gradient separations were also modelled and showed good agreement with experimentally determined values.     

Hydrophilic interaction liquid chromatography with alcohol as a weak eluent

There has been a significant increase of interest in polar compound separation by hydrophilic interaction liquid chromatography (HILIC), in which acetonitrile is mostly used as a weak eluent. Although replacing acetonitrile with alcohols as organic modifiers has been previously reported, the separation mechanism was poorly understood. In this paper we explored the separation mechanism through the method development for the analysis of the trace amounts of polar and basic hydrazines, which were genotoxic in nature. Separation parameters such as the type and concentration of alcohol, acid modifier, and buffer in mobile phase as well as the choice of stationary phase and column temperature were studied. The data indicated that both electrostatic and hydrophilic interactions contributed to the retention and separation of the hydrazines. The results presented here provide insight into the adjustment of the retention and separation of analytes in HILIC mode with alcohol as a weak eluent. The optimized HILIC method coupled with chemiluminescent nitrogen detection (CLND) is simple and sensitive (reporting limit at 0.02%) and was applied to simultaneous analysis of hydrazine and 1,1-dimethylhydrazine in a pharmaceutical intermediate.     

Influence of intermolecular interaction with the eluent on the retention of phenol and aniline on polar and non-polar surfaces

Summary The retention order of aniline and phenol on hydroxylated silica gel surface is reversed with the increase of the concentration of a polar component in the eluent. At minor (about 1%) concentrations of isopropanol in hexane aniline emerges first followed by phenol, the elution order being reversed with the increase of isopropanol concentration above 2%. The same behaviour is observed for silica gel with chemically bonded C₁₆ n-alkyl groups and for porous styrene-divinylbenzene copolymer. In all these cases the retention depends to a great extent on substance-eluent intermolecular interactions. At high isopropanol concentration in hexane the rise of column temperature changes the retention order of aniline and phenol on silica gel with hydroxylated surface since the association of these molecules with isopropanol as well as the isopropanol adsorption weaken as the temperature increases.     

Hydrophobic and electrostatic forces control the retention of membrane peptides and proteins with an immobilised phosphatidic acid column

The retention behaviour of four membrane-associated peptides and proteins with an immobilized phosphatidic acid (PA) stationary phase was evaluated. The solutes included the cytolytic peptides gramicidin A and melittin, the integral membrane protein bacteriorhodpsin and cytochrome c, a peripheral membrane protein. Gramicidin has no nett charge and exhibited normal reversed phase-like behaviour which was largely independent of mobile phase pH. In contrast, melittin, which has a positively charged C-terminal tail, exhibited reversed phase like retention at pH 5.4 and 7.4, and was not retained at pH 3 reflecting the influence of electrostatic interactions with the negatively charged phosphatidic acid ligand. Bacteriorhodpsin was eluted at high acetonitrile concentrations at pH 3 and 5.4 and cytochrome c was only eluted at pH 3. Moreover, cytochrome c eluted in the breakthrough peak between 0 and 100% acetonitrile, demonstrating the role of electrostatic interactions with the PA surface. Overall, the results demonstrate that pH can be used to optimize the fractionation and separation of membrane proteins with immobilized lipid stationary phases.     

High-Performance Liquid Chromatography of Amphibian Peptides. Selectivity Changes Induced by pH

Abstract

The effect of pH on the retention behavior under reversed- phase liquid chromatography conditions of a series of peptides was examined. Isocratic conditions were used with either methanol or acetonitrile as organic modifiers. The intrinsic hydrophobicity of the peptides was altered by changes in the pH of the eluent mixture. Increased retention at pH 7 relative to pH 4 was correlated with the presence of a histidine residue in a hydrophobic environment. An experimental parameter, α_pH, was defined as the positive quotient of capacity factors at pH 4 and pH 7 for a given eluent. These α_pH values are interpreted as reflecting changes in peptide hydrophobicity introduced by variations in solvent and pH. Identical α_pH values were obtained for homologous peptides, particularly histidine containing peptides. This approach to selectivity effects yielded diagnostic conditions for the analysis of bombesin, a peptide touted as a potential marker for human small-cell lung carcinoma.     

Adsorption behavior of hexafluorophosphate on selected bonded phases

The adsorption behavior of ammonium hexafluorophosphate was studied on four HPLC columns packed with adsorbents of different ability for dispersive interactions using frontal chromatography with LC/MS detection in negative ESI mode. Hexafluorophosphate (PF(6)(-)) adsorption isotherms were measured from acetonitrile/water and methanol/water mixtures. Increased PF(6)(-) adsorption with increased acetonitrile content was found between 0 and 15% of acetonitrile in the eluent. Further increase of the acetonitrile concentration leads to an exponential decrease of PF(6)(-) adsorption. Methanol, on the other hand, causes a steady decrease of PF(6)(-) adsorption with increased organic concentration in the mobile phase.     

Behavior of peptides and computer-assisted optimization of peptides separations in a normal-phase thin-layer chromatography system with and without the addition of ionic liquid in the eluent

The addition of an ionic liquid into the mobile phase appeared to be useful in optimization of chromatographic separation of peptides. Different behavior of peptides in thin-layer chromatography (TLC) was observed after addition of 1-ethyl-3-methylimidazolium tetra fluoroborate to the eluent in comparison to the system without the ionic liquid. Nonlinear dependence of the retention coefficient, R(M), of peptides on the volume percentage of acetonitrile in the eluent was found in normal-phase TLC with and without immidazolium tetra fluoroborate in the mobile phase. In general, R(M) increased with increasing concentration of acetonitrile. In TLC systems without the ionic liquid, R(M) can be described well with a quadratic function. On the other hand, in a TLC system with an ionic liquid as the additive to the mobile phase, the retention behavior is better described with a third-degree polynomial function. The potential usefulness of ionic liquids for optimization of separation of peptides was demonstrated. Optimization of the separation conditions was supported by a commercially available computer program.     

Excess Adsorption of Organic Eluent Components from Mobile Phases Containing Electrolytes

The excess adsorption isotherms of organic eluent components from solutions containing electrolytes on a C18-bonded stationary phase are investigated by frontal analysis in staircase mode. The excess adsorption of acetonitrile increases when NaHSO₄, NaH₂PO₄, NaCl, or NaOAc is added to the eluent, but decreases upon addition of NaBr or NaClO₄. The excess adsorption of acetonitrile increases in the order of NaCl, NaHSO₄, NaH₂PO₄?>?NaOAc?>?NaBr, NaClO₄. On the other hand, the effect of electrolyte addition on the excess adsorption of methanol is not significant. The effect of electrolytes on the retention of alkylbenzenes in reversed-phase liquid chromatography is discussed on the basis of the excess adsorption of organic eluent components. The retention of alkylbenzenes shows negative correlation with the excess adsorption of acetonitrile. This indicates that the acetonitrile layer on the stationary phase does not act as a part of the stationary phase. A developed acetonitrile layer reduces the retention of alkylbenzenes by the competitive adsorption at the interface between the organic layer and the stationary phase.     

Novel dye removing agent based on CTS-g-P(AA-co-NIPAM)/GO composite

A novel composite Chitosan graft poly (acrylic acid-co-N-isopropylacrylamide)/graphite oxide (CTS-g-P(AA-co-NIPAM/GO) is synthesized and used to remove methylene blue (MB) and fuchsin basic (FB) from aqueous solutions by adsorption. Small amount of GO brings about great improvement of the thermostability together with the adsorption amount. Adsorption capacities of MB and FB increase from 842.1 and 633.7 mg/g, respectively, to 1496.3 and 1000.8 mg/g, respectively, with 0.02 g intercalation amount of GO. The interactions between GO and main body of CTS-g-P(AA-co-NIPAM) graft copolymer are hydrogen and amide bonds, whereas that between dye molecules and CTS-g-P(AA-co-NIPAM)/GO composite is hydrogen bond as well as electrostatic interaction. Effect of various conditions on the adsorption capacities is discussed. Adsorption isotherms and thermodynamics are studied. The adsorption of both MB and FB are spontaneous and satisfy the Redlich-Peterson equation. Kinetic study shows that the adsorption of both dyes is in accordance with the Pseudo first-order kinetic model.     

Chemistry of aqueous silica nanoparticle surfaces and the mechanism of selective peptide adsorption

Control over selective recognition of biomolecules on inorganic nanoparticles is a major challenge for the synthesis of new catalysts, functional carriers for therapeutics, and assembly of renewable biobased materials. We found low sequence similarity among sequences of peptides strongly attracted to amorphous silica nanoparticles of various size (15-450 nm) using combinatorial phage display methods. Characterization of the surface by acid base titrations and zeta potential measurements revealed that the acidity of the silica particles increased with larger particle size, corresponding to between 5% and 20% ionization of silanol groups at pH 7. The wide range of surface ionization results in the attraction of increasingly basic peptides to increasingly acidic nanoparticles, along with major changes in the aqueous interfacial layer as seen in molecular dynamics simulation. We identified the mechanism of peptide adsorption using binding assays, zeta potential measurements, IR spectra, and molecular simulations of the purified peptides (without phage) in contact with uniformly sized silica particles. Positively charged peptides are strongly attracted to anionic silica surfaces by ion pairing of protonated N-termini, Lys side chains, and Arg side chains with negatively charged siloxide groups. Further, attraction of the peptides to the surface involves hydrogen bonds between polar groups in the peptide with silanol and siloxide groups on the silica surface, as well as ion-dipole, dipole-dipole, and van-der-Waals interactions. Electrostatic attraction between peptides and particle surfaces is supported by neutralization of zeta potentials, an inverse correlation between the required peptide concentration for measurable adsorption and the peptide pI, and proximity of cationic groups to the surface in the computation. The importance of hydrogen bonds and polar interactions is supported by adsorption of noncationic peptides containing Ser, His, and Asp residues, including the formation of multilayers. We also demonstrate tuning of interfacial interactions using mutant peptides with an excellent correlation between adsorption measurements, zeta potentials, computed adsorption energies, and the proposed binding mechanism. Follow-on questions about the relation between peptide adsorption on silica nanoparticles and mineralization of silica from peptide-stabilized precursors are raised.     

胺基化PGMA交联微球对胆红素的吸附机理

通过胺基与环氧键之间的开环反应, 用己二胺及多乙烯多胺等小分子胺化试剂对聚甲基丙烯酸缩水甘油酯(PGMA)交联微球进行了化学改性, 制得了胺基化的PGMA交联微球, 研究了该功能微球对胆红素的吸附特性, 考察了胺化试剂的分子结构、介质pH值、离子强度及温度等因素对其吸附性能的影响, 较深入地研究了吸附机理. 实验结果表明, 胺基化微球对胆红素具有强吸附作用, 吸附容量可达17.80 mg·g-1, 等温吸附服从Freundlich方程. 胺基化微球与胆红素分子之间的作用力以静电相互作用为主, 同时也存在氢键作用与疏水相互作用. 在pH 值为6 的介质中二者之间的静电作用最强, 胆红素吸附容量最高. 高离子强度不利于静电相互作用, 盐度增大使吸附容量减小. 温度升高有利于疏水相互作用而不利于氢键作用, 两种作用中占优势者主导温度对吸附容量的影响. 用己二胺改性的微球, 由于疏水相互作用的强化以及较长连接臂导致较小的空间位阻, 使其对胆红素的吸附能力明显高于多乙烯多胺改性的微球.     

Impact factors and thermodynamic characteristics of aquatic humic acid loaded onto kaolin

The adsorption of humic acid (HA) on kaolin particles was studied at various conditions of initial solution pH, ionic strength and solid-to-liquid ratio. The resulting affinity of interactions between humic acid and kaolin was attributed to the surface coordination of HA in ambient suspensions of mineral particles and the strong electrostatic force at low pH. Addition of inorganic salt can also influence the adsorption behavior by affecting the HA molecular structure, the clay particle zeta potential and so on. Equilibrium data were well fitted by the Freundlich model and implied the occurrence of multilayer adsorption in the process. In addition, the enthalpy dependent of system temperature was 79.17 kJ/mol, which proved that the mechanism of HA adsorption onto kaolin was comprehensive, including electrostatic attraction, ligand complexation and hydrogen bonding.