Assessment of supporting electrolyte contributions in electrochemically modulated liquid chromatography

This paper reports the results of an investigation on the role of the supporting electrolyte in separations using electrochemically modulated liquid chromatography (EMLC) with a porous graphitic carbon stationary phase. With respect to the identity of the supporting electrolyte, the elution strength of the electrolyte anion increased as F- < OH- < BF4- < ClO4- < PF6- for injections of negatively charged aromatic molecules, whereas a 10-fold increase in electrolyte concentration induced a 60% change in retention for the same solutes. Furthermore, both the concentration and composition of the supporting electrolyte affected retention in a manner that varied with the charge of the analyte and applied potential. This behavior is explained using Gouy-Chapman diffuse double layer theory, coupled with comparisons of this theory with closely related models for ion-pair chromatography. Insights into the retention mechanism reveal that an ion-exchange mechanism controls the retention of negatively charged solutes at applied potentials removed from the potential of zero charge (PZC). At potentials close to the PZC, the electrostatic model is less effective with the predominant retention mechanism likely involving hydrophobic interactions with the carbonaceous stationary phase. The combined effects of these findings are demonstrated by using a temporal gradient in supporting electrolyte concentration to optimize an EMLC separation.     

Carbon nanotubes‐A resin for electrochemically modulated liquid chromatography

Electrochemically modulated liquid chromatography is a special form of ion exchange chromatography in which the separation process is controlled by applying an electric potential to the stationary phase. This form of chromatography has so far only been applied in research studies. The present study shows that multiwalled carbon nanotubes are an effective resin material for an electrochemically modulated chromatography process. The experiments are carried out in a newly designed column that enables the packing of nanomaterials. We investigate the influence of the applied potential on the retention and elution of maleic acid, determine the dynamic binding capacity, and calculate the utilization degree of the electrical charge in the adsorption process. Moreover, the stability of the resin and the membrane over more than 200 working hours are presented. In addition to the stability, their sturdiness and inexpensive price are important qualities that make multiwalled carbon nanotubes interesting for application as the stationary phase in an electrochemically driven process. The investigated chromatography technique represents a promising separation process for future applications as a preparative step in biotechnology as well as other life science fields.     

The retention of some azoles in reversed-phase liquid chromatography

The influence of the structure-related physicochemical properties of derivatives of nitrogen-containing heterocyclic compounds on their retention under the conditions of reversed-phase liquid chromatography was studied. The dependence of the chromatographic retention of azole derivatives on the composition of the mobile phase was determined. The presence of azole protonated forms was substantiated by conductometric data.     

On the potential of electrochemically modulated liquid chromatography of proteins in a micro open parallel plate separator

Electrochemically Modulated Liquid Chromatography (EMLC) is a technique in which an electrical potential is applied to the stationary phase in order to alter its adsorptive properties. The micro Open Parallel Plate Separator (microOPPS) is an open-channel micro-chromatographic system with the unique characteristic of an adjustable rectangular flow cross-section. In this publication, the performance of the microOPPS in the EMLC mode was simulated using a comprehensive mathematical model. The separation of two proteins was analyzed by performing a parametric study for the effects of the potential applied to the stationary phase, the dimensions of the device, and the operating conditions. It is shown that the combination of the microOPPS and EMLC is promising for protein separations. These separations can be optimized by carefully selecting the applied potential and the geometry of the microOPPS. The marriage of the microOPPS and EMLC adds two additional dimensions for control, to improve the separation efficiency in analytical applications and in preparative separations of trace compounds.     

Studies on the retention behaviour of some thiazolylazo derivatives in reversed phase liquid chromatography

The retention behaviour of thiazolylazo derivatives, 4-(2-thiazolylazo) resorcinol (TAR), 4-(2-thiazolylazo)-orcinol (TAO), 2-(2-thiazolylazo)-4-methylphenol (TAC) and 1-(2-thiazolylazo)-2-naphtol (TAN) was studied by reversed phase liquid chromatography. The optimum conditions for the separation of four thiazolylazo derivatives were examined with respect to column, flow rate, mobile phase composition and pH of mobile phase. These derivatives were separated simultaneously on Symmetry C₈ column using composition of acetonitrile/water (60/40, v/v) as mobile phase. The capacity factor (k′) has been decreased at higher pH than pK_a of solute which may due to the increasing concentration of the ionized species as increase the pH of mobile phase. The dependence of log k′ on the volume faction of water in the binary mobile phase and k′ on the liquid–liquid extraction distribution ratio (D_c) in acetonitrile–water (60/40, v/v)/n-octane extraction system for thiazolylazo derivatives were obtained good linear relationship. The results showed that the retention behaviour of these derivatives was mainly affected by the hydrophobic interaction between thiazolylazo as solute and mobile phase.     

Prediction of peptides retention behavior in reversed-phase liquid chromatography based on their hydrophobicity

Hydrophobicity is an important physicochemical property of peptides and proteins. It is responsible for their conformational changes, stability, as well as various chemical intramolecular and intermolecular interactions. Enormous efforts have been invested to study the extent of hydrophobicity and how it could influence various biological processes, in addition to its crucial role in the separation and purification endeavor as well. Here, we have reviewed various studies that were carried out to determine the hydrophobicity starting from (i) simple amino acids solubility behavior, (ii) experimental approach that was undertaken in the reversed-phase liquid chromatography mode, and ending with (iii) some examples of more advanced computational and machine learning models.     

糖类化合物亲水作用色谱保留行为评价

以糖类化合物为研究对象,系统评价了其在亲水模式下的色谱保留行为。分别考察了流动相、固定相和缓冲盐等对糖类化合物保留的影响,建立了糖类化合物在亲水模式下的保留方程。结果表明,糖类化合物随着流动相中乙腈比例的降低,保留时间减小;随着缓冲盐浓度的增加,保留时间增加;同时,糖类化合物的保留行为还会受到有机溶剂种类和固定相类型的影响;其保留行为可使用顶替吸附-液相相互作用模型定量描述。将该模型进一步用于实际样品中糖类化合物保留行为的预测,获得了较好的实验结果,预测保留时间与实测保留时间的相对误差小于0.3%。对糖类化合物亲水模式下的保留行为进行了系统的评价和定量描述,该研究结果将有助于糖类化合物亲水作用色谱分离方法的发展。     

Effect of the organic modifier on the retention of retinoids in reversed-phase liquid chromatography

Summary The retention of retinoids in reversed-phase liquid chromatography was studied using aqueous mobile phases of different composition (methanol 94–86% and acetonityrile 92–82%) at five temperatures (40–60 °C). With both organic modifiers the effect of the molecular structure increased as the water content and the polarity of the mobile phase increased. The temperature-dependence increased in the same manner with aqueous acetonitrile mobile phases. The - interactions between the retinoids and acetonitrile diminish when the water content of the mobile phase is increased, as happens also to the hydrophobic interactions with both organic modifiers. The net effect of these changes depends on the composition of the mobile phase. There was excellent correlation of retention with all polarity parameters studied(, P, x_e, x_d, x_n, E _T ^N , _T, , _o and _d), when the calculations were made separately with methanol and acetonitrile. The volume fraction of the organic modifier, , was the only parameter describing the retention well in both organic modifiers simultaneously.     

Matrix effect on the retention in reversed-phase liquid chromatography

Summary A polymer-based, reversed-phase column (VA-C18), prepared by grafting octadecyl chain onto vinyl alcohol copolymer gel, was investigated for its chromatographic characteristics. n-Alkanes and n-alkyl alcohols were found to be retained only by hydrophobic interaction between the solutes and the octadecyl chain. In the case of aromatic hydrocarbons, in addition to the hydrophobic interaction, - interaction between the solutes and the based material was elucidated to contribute to the retention. For aromatic tertiary amines which are known to strongly interact with the residual silanol group of the silica-based reversed-phase columns to produce broadened and skewed peakes, the VA-C18 column also retained these substrates strongly by the combination of hydrophobic, -, and ionic interactions. In this case, however, symmetrical peaks were observed. From these results, it was determined that in the case of VA-C18, the base material was found not to produce undesirable effect although the solutes interact with the base. Further conclusion obtained was that in reversed phase liquid chromatography, chromatographic properties of base matrix is highly responsible for the overall retention.     

Effect of stationary phase polarity on the retention of ionic liquid cations in reversed phase liquid chromatography

Chromatographic analysis of ionic liquids on different types of packings offers interesting possibility to determine their retention mechanism. As a consequence, the major interactions between stationary phase ligands and analyzed chemical entities can be defined. The main aim of this work was to analyze cations of ionic liquids on chemically bonded stationary phases with specific structural properties. The attempt to predict the main interactions between positive ions of ionic liquids and stationary phase ligands was undertaken. For that purpose, butyl, octyl, octadecyl, phenyl, aryl, mixed, alkylamide, and cholesterolic packings were chosen and applied to the analysis of six most commonly used ionic liquids' cations. Obtained results indicate mainly dispersive and pi-pi type of interaction part in the retention mechanism of analyzed compounds.     

Effect of the counter-anion type and concentration on the liquid chromatography retention of beta-blockers

Zirconia beads (25-38 microm in diameter) were modified with N,N,N',N'-ethylenediaminetetramethylenephosphonic acid to generate a zirconia based pseudoaffinity support, further referred to as r_PEZ. The influence of pH, salt concentration and temperature on the binding of human immunoglobulin G (hIgG) to r_PEZ was studied. Temperature had no significant impact on the maximum binding capacity (Qmax), and the equilibrium-binding constant (Kd), whereas pH and the salt concentration had a noticeable impact on both Qmax and Kd. The Qmax value of 55 mg hIgG/ml of bead was obtained at a pH of 5.5 and found to decrease with an increase of pH. The modified zirconia support allowed the separation of immunoglobulins (IgG, IgA and IgM) from untreated human serum. Elution was possible under mild conditions with a step salt gradient. Overall protein recoveries in the range of 109-125% were obtained with human serum. Human IgG, human IgA, and human IgM yields of 29.50+/-6.3, 3.22+/-0.7, and 6.84+/-0.7%, respectively, were obtained at a linear velocity of 4.32 cm/min. Purity of products, obtained from a single chromatographic step was estimated to be greater than 89.0+/-2.6%. The utility of r_PEZ in the selective removal of immunoglobulins, as in immunoadsorption was discussed.     

Effect of polar solvent acetonitrile on the electrochemical behavior of polyaniline in ionic liquid electrolytes

Polyaniline film was electropolymerized in organic acidic media (CF3COOH) and then investigated by cyclic voltammetry, AC impedance, and galvanostatic charging and discharging tests in ionic liquid-1-methyl-3-butylimidazolium hexafluorophosphate (BMIPF6) and the mixture electrolytes of BMIPF6 and acetonitrile (ACN) with different ratios. The results showed that the polymer in mixture of BMIPF6 and ACN have lower solution resistance, higher cycle life, and higher electrochemical capacitance. The relationship of the peak current to the scan rates provides some insight into the nature of the polyaniline film switching reaction in different electrolytes.     

Effect of eluant composition on the retention of water-soluble vitamins in reversed-phase high-performance liquid chromatography

The effects of the quantitative composition and pH of water-acetonitrile eluants and temperature on the retention of water-soluble vitamins in reversed-phase high-performance liquid chromatography were studied. The retention parameters of the analytes were calculated and the limits of the applicability of the main adsorption models of retention to the test chromatographic system were discussed.     

Study on the retention equation in hydrophilic interaction liquid chromatography

Hydrophilic interaction liquid chromatography (HILIC) is an effective technique for separating polar compounds. But its retention equation has not been studied systematically yet. In this study, an appropriate retention equation was established by using eight nucleosides as model analytes and by comparing four retention models on six different HILIC columns. As a result, retention equation Ink = a + b x InC(B) + cxC(B) could be quantitatively described the retention factors with good accuracy in HILIC mode. Based on this equation, the retention times of eight nucleosides under the conditions by other mobile phase can be predicted on each column. All of the predicted relative errors of retention times were smaller than 5%. The established retention model was also successfully applied to predict retention times of the real Traditional Chinese Medicine-Carthaus tinctorius L. sample.     

Effect of temperature on the adsorption behavior of tryptophan in reversed-phase liquid chromatography

Single-component adsorption isotherm data of l-tryptophan on a C(18)-bonded silica column were acquired by frontal analysis (FA), with aqueous mobile phases containing 5% of acetonitrile at five different temperatures between 23 and 62 degrees C. The non-linear fitting of these data provided the bi-Moreau model for all temperatures as the best isotherm model. The inverse method (IM) was used to derive the parameters at these temperatures from the parameters of the 25 degrees C isotherm. The adsorption constants and the saturation capacities of the low and high-energy sites decreases by increasing the temperature, while the adsorbate-adsorbate parameters of both sites increase. An excellent agreement was found between the experimental and calculated overloaded band profiles at all the temperatures used. The breakthrough curves obtained and the overloaded band profiles obtained were found to have different shapes according to the range of concentration studied and the temperatures. At low concentration 0.05-0.5 g/L the breakthrough curves and the overloaded band profiles have a front shock and diffuse rear, which indicates langmuirian behavior, but at intermediate 1-2 g/L and high concentration 8 g/L they start to have diffuse fronts and shocks at the rear or more than one shock at the rear which indicates non-langmuirian behavior. At 23 degrees C the isotherm has another langmuirian part, which appears at high concentration. The behavior of the breakthrough curves is explained by the shape of the isotherm in which all of the isotherms have a langmuirian part (the isotherm is concave upward) and an antilangmuirian part (the isotherm is concave downward). The temperature affected the breakthrough curves by decreasing the time of the appearance of the fronts for all concentration ranges studied, and by decreasing the time difference between the highest concentration and lowest concentration of the fronts, especially the low concentration range at 0.5 g/L. The fronts of the breakthrough curves at high concentration seems to be the most affected by temperature.     

The influence of the structure of some aromatic heterocyclic derivatives on their retention in reversed-phase high-performance liquid chromatography

The chromatographic behavior of aromatic heterocyclic derivatives in reversed-phase high-performance liquid chromatography was investigated. The retention characteristics of the substances under the conditions of chromatography with water-acetonitrile mobile phases (retention factors, relative retentions, distribution coefficients, Henry adsorption constants, differences in the differential molar energy of sorption, and Gibbs energies of sorption) were determined. It was shown that the chromatographic retention of the sorbates depended on their molecular structure. The influence of the nature of heteroatoms and their number on the sorption of heterocyclic compounds was discussed. Original Russian Text ? S.V. Kurbatova, B.R. Saifutdinov, O.G. Larionov, V.V. Meshkovaya, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 3, pp. 557–564.