Dabsyl chloride is a derivatization reagent used for UV/Vis detection of amino compounds. Since the dabsyl group does not fluoresce, it has never been monitored with a fluorescent (FL) detector. The present report shows that dabsylated amino acids exemplified by serine enantiomers produce appreciable negative peaks with an FL detector due to a strong quenching effect of the dabsyl group; thus, their fluorescent determination is possible. This finding extends a repertoire of derivatizing reagents for chromatographs without a UV/Vis detector (but with an FL one). Calibration dependencies for L- and D-Ser were similar with UV/Vis and FL detectors, although the limit of detection for the former (8 mu g (L), 20 mu g (D)) was lower than for the latter (48 mu g (L), 42 mu g (D)) due to larger noise of the FL detector.

The van't Hoff equation is a widely used tool to study adsorption thermodynamics in chromatography. Recent experimental data and some theoretical arguments questioned the accuracy of thermodynamic characteristics determined by using this equation. The present report addresses these concerns and shows that if certain requirements are satisfied, the van't Hoff analysis provide reliable results. These requirements include isothermality of the column not perturbed by the percolation of the mobile phase, mass transfer kinetics allowing sufficient proximity to equilibrium, pressure below 200 bar for low molecular weight compounds and below 50 bar for macromolecules, and knowledge of the phase ratio at all experimental temperatures to allow accurate conversion of the retention factor to the equilibrium constant. Physical meaning of adsorption enthalpy and entropy obtained by means of the van't Hoff analysis particularly in the case of heterogeneous adsorbents is discussed and recommendations on how to perform experiments to obtain reliable results are given.

Soils and subsoils have a complex structure consisting of several elements with different morphology and chemical composition, combining micro-, meso-, and macroporous substructures. Classical approaches, such as Brunauer-Emmett-Teller (BET) or Barrett-Joyner-Halenda (BJH) methods, cannot provide an accurate assessment of morphological characteristics of such solids, nor can do modern sophisticated numerical methods which depend on idealized models of pores that hardly reflect a real morphology of heterogeneous solids in question. The present study describes a way to overcome this difficulty by applying a combination of the αS-plot technique and a modified Remy-Poncelet method. The proposed approach allowed estimating the volumes of the untramicropores, supermicropores and the sum of meso- and macropores as well as the specific surface areas of the solid without ultramicropores and of the solid without all micropores. The former specific surface area is a more reasonable estimate of the area available for layer coverage than the BET specific surface area, and all together these characteristics provide a detail description of solid’s morphology. Subsoil samples excavated from different depths were used as exemplary solids. They mostly consisted of quartz, feldspar, and clay minerals and had different sorts of pores associated with the clay fraction and soil aggregates as well as a relatively large fraction of non-porous sand particles. Relations of the morphological properties with subsoil profile and mineralogical composition are discussed. For citation: Asnin L.D., Samoylov M.S., Pershina M.V., Tselishchev Yu.G., Sliusar N.N. Assessment of specific surface area and porosity of subsoils. ChemChemTech [Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol.]. 2024. V. 67. N 6. P. 55-64. DOI: 10.6060/ivkkt.20246706.6996.

Chromatographic behavior of novel chiral stationary phases with bonded selectors based on Cinchona alkaloids modified with dipeptides was studied using dipeptides as probe molecules. Buffer-free and salt containing hydro-organic solutions were used as the mobile phases. The selectors exhibit pseudoenantiomeric behavior with respect to the L/D or LL/DD enantiomers and do not behave so with respect to the LD/DL enantiomers. The alkaloid part of the selectors is the driver of enantioselectivity, while the dipeptide substituent plays a modulating role. The quinidine-based selectors demonstrate stronger adsorption affinity and higher enantioselectivity as compared to the quinine-based selectors. The dipeptide analytes containing a glycyl fragment are weaker retained and their enantiomers are worse separated comparing to dipeptides with both units being larger amino acids. Moreover, a phenyl group in the structure of a dipeptide analyte facilitates enantioseparation. The effect of the mobile phase composition on retention depends on the hydrophobicity of an analyte. Hydrophobic dipeptides are better eluted by methanol-rich solvents, hydrophilic dipeptides are better eluted with water-rich solvents, and dipeptides with an intermediate hydrophobicity demonstrate a U-shaped or more complicated dependence of the retention factor on the percentage of methanol. Even a small buffer addition to the mobile phase decreases retention, but the ion-exchange mechanism was not confirmed. The effect of an electrolyte is rather due to the shielding of the charged groups of the selector reducing thereby electrostatic interaction between the selector and analyte. Efficiency of the novel columns is comparable to that of other brush-type chiral columns, the highest achieved number of the theoretical plates per 1 m varying between 30000 and 40000.

Chiral resolution of polar organic compounds such as amino acids and peptides represents an important chromatographic task due to increasing significance of natural species, which play important signaling and regulatory roles in the living organisms. Despite the number of available chiral stationary phases, this task remains challenging, since not many of the commercially available systems are capable to resolve non-derivatized zwitterionic species. In this study, we present a target-oriented design of a new class of chiral selectors. Pursuing the goal to separate amino acids, and especially short peptides, we have combined Cinchona alkaloids - quinine and quinidine - with three different biogenic dipeptides. We have synthesized six different chiral stationary phases, with selector loading of similar to 200 mu mol g(-1), and tested their chiral recognition capabilities for acidic, basic and zwitterionic analytes using various mobile phases. We have observed that all chiral stationary phases retain the chiral anion exchange capability known for commercially available Cinchona-based columns leading to baseline or partial resolution of six out of ten analytes. The performance in chiral resolution of basic analytes is not optimum due to the weak cation exchange character of the peptidic residue. However, we report on encouraging results in the chiral resolution of short peptides, for which, depending on their structure, we see the chiral resolution of up to three stereoisomers (from four possible) in a preliminary screening.

The mechanisms of retention and separation of the enantiomers of chiral oxazolopyrroloquinolones on a stationary phase with graftеd macrocyclic antibiotic ristocetin A under conditions of high-performance liquid chromatography with aqueous methanol mobile phases have been studied. The retention factor was found to decrease monotonically when the CH 3 OH concentration in the mobile phase increased to 90 vol %; then it slightly increased as the composition of the mobile phase approached pure methanol. The thermodynamics of adsorption of oxazolopyrroloquinolones in different regions of this dependence was studied. A mathematical model has been proposed; it describes well the experimental data and allows for analyte solvation in the mobile phase and competitive adsorption (with mobile phase components) on a chiral selector. The results were compared with previously obtained data for water–acetonitrile mobile phases. The reasons for the observed differences were discussed.

Retention and separation of enantiomers of amine derivatives of indane and tetralin (rasagiline and its analogues) on chiral stationary phases (CSPs) Chiral-T and Chiral-V with teicoplanin and vancomycin antibiotics grafted onto superficially porous silica particles under conditions of reversed-phase and polar organic chromatography were studied. The mobile phases (MP) were water-methanol and acetonitrile-methanol solvents modified with triethylamine-acetic acid buffer. The effects of molecular structure and physical properties of the analytes on enantioselective retention are discussed. The retention mechanism is hypothesized to involve the ion-ion attraction between the positively charged amino group of an analyte and the carboxylate anion of either antibiotic. The binding occurs outside of the antibiotic’s aglycon basket that accounts for relatively low enantioselectivity observed. The presence of a large substitute at the analyte’s amino group complicates enantiorecognition. The effect of the MP solvent composition on retention and enantioseparation was investigated. It is a complex phenomenon combined of different oppositely directed influences that resulted in different shapes, increasing, decreasing, or U-shaped, of the retention factor vs. composition dependences. A model taking into account the interaction of both solvents of a binary MP with both an analyte and an adsorption site was successfully applied to approximate a majority of the studied systems. Pros and cons of the model are discussed.

Chiral zwitterion ion exchangers represent efficient chiral stationary phases for stereoselective resolution of various analytes including chiral acids, bases, and zwitterions. In this contribution, we have focused on utilization of chiral zwitterionic sorbents, denoted as ZWIX (+A) and ZWIX (-A). These are analogical chiral systems to commercially available columns, Chiralpak ZWIX (+) and Chiralpak ZWIX (-), which are usually operated with buffered mobile phases. In this contribution, we have studied the enantiorecognition power of the ZWIX (+A) and ZWIX (-A) columns on a series of dipeptides operated under buffer-free reversed-phase conditions. Retention characteristics of zwitterionic dipeptides are discussed using an electrostatically driven adsorption model, which provides a good fit with both monotonous and U-shaped curves.

Two chiral stationary phases virtually reproducing the Nautilus-R column were modeled in silico to study the enantiorecognition mechanism of some selected dipeptides, taking into consideration the two different anchoring alternatives to the silica layer involving the two ristocetin A amino groups. A mobile phase composed of water-methanol (40:60, v/v) was included in the system. The analyses of the trajectories supported the experimental L(LL)<D(DD) enantiomeric elution order of Ala-Ala, Gly-Leu, Leu-Gly-and Leu-Leu. In strict accordance with the enthalpy contributions observed in the thermodynamic evaluations of the retention profiles of Ala-Ala-and Leu-Leu, the molecular dynamics indicated that the selector-selectand association process is mainly controlled by electrostatic interactions. A distance analysis indicated that the carboxy-terminal of the D-Leu-D-Leu-enantiomer tends to approach closer the positive charges present on the selector with respect to its antipode. Similarly, the experimental enantiomeric elution order (L)<(D) of Leu-Gly-was explained by a higher hydrogen bond frequency for the D-Leu-Glyin system B with respect to the L-Leu-Gly. A different interaction profile was observed for the Gly-Leu-dipeptide enantiorecognition mechanism that resulted to be mainly driven by van der Waals interactions. The Gly-Leu-and Leu-Gly-results highlighted the importance of the Leu-position in the dipeptide sequence that indeed governs the binding modes of the dipeptide with the chiral selector. The accordance observed among the thermodynamic and the molecular dynamics analyses indicate the adherence of this in silico methodology to the experimental results and its utility in the investigation of the enantiorecognition mechanisms in chiral chromatography. (C) 2022 Elsevier B.V. All rights reserved.

Retention and separation factors of enantiomers of several dipeptides were measured on chiral stationary phases with grafted macrocyclic antibiotic ristocetin A from different manufacturers, Chirobiotic R and Nautilus-R, using buffered and unbuffered methanol–water mixtures with 0 to 90% (v/v) of methanol as mobile phases. A remarkable difference in enantioselectivity toward both hydrophilic and hydrophobic dipeptides was observed between these two stationary phases that were explained by different configurations of the binding sites of the selectors resulted from different anchoring reactions applied to graft the antibiotic moiety to the silica support and also by different number of ionizable amino groups of the grafted selectors.

The adsorption dynamics of the LL- and DD-enantiomers of Leu-Leu and Gly-Gly on a Chirobiotic V column packed with a CSP bearing glycopeptide antibiotic vancomycin was studied and compared with the adsorption dynamics of the same dipeptides on a Chirobiotic R column packed with a CSP with bonded antibiotic ristocetin A. The column efficiencies were essentially different. The heights equivalent to a theoretical plate (HETP) on Chirobiotic R were 2-4 times larger than on Chirobiotic V, and the shape of van Deemter plots, strongly convex-upwards on the former column, demonstrated minor (Leu-Leu) or lacking (Gly-Gly) convexity on the latter column. This difference is attributed to differences in the binding kinetics of dipeptides to the antibiotics, fast for vancomycin and slow for ristocetin A. Since the kinetic C term of the van Deemter equation cannot explain such a large discrepancy in HETP for the same analyte on two columns, and since the A and B terms for the Chirobiotic R column were essentially larger than for the Chirobiotic V column, it is suggested that binding kinetics influences eddy and longitudinal diffusion processes associated with those terms. It is also hypothesized that unusually high values of van Deemter B coefficients occasionally found in chiral chromatography are not experimental artifacts but the manifestation of coupling the effects of slow adsorption kinetics and longitudinal diffusion.

A study is performed of the adsorption of water–methanol and water–acetonitrile mixtures on chiral stationary phases (CSPs) Chirobiotic R, Chirobiotic T, and Nautilus-E with grafted macrocyclic antibiotics ristocetin A, teicoplanin, and eremomycin, respectively. The patterns of adsorption on the indicated CSPs are qualitatively the same, and differ only by quantitative indicators. Adsorption isotherms of excess water from binary solvents have adsorption azeotrope points and show the preferred absorption of water in the range of pure organic component to an azeotrope point in the range of 60–75 mol % for H2О–МеОН and 80–90 mol % for H2O–MeCN systems. It is shown that the thickness of the adsorption phase in the first case is less than one nominal molecular layer (0.10–0.13 nm). For H2O–MeCN, it is 3–4 molecular layers (0.88–1.05 nm). Activity coefficients are calculated for the components of solutions in surface layers. The coefficients indicate the systems deviate considerably from the properties of an ideal adsorption solution. Reasons for this behavior are discussed.

Background: Chiral separation involves many phenomena in which the elution order of the enantiomers has its unique position. The phenomenon of elution order of the enantiomers has also been used in the determination of optical purity which is favorable to elute the major component after minor enantiomeric impurity but the main problem is that, this phenomenon is rare. Results: This review rumors the reversal order of elution of many chiral molecules in HPLC. Besides, this review pronounces the effects of pH, derivatisation of drugs, the composition of the mobile phase, and temperature on the reversal order of elution of chiral drugs. The efforts are also made to discuss the possible future perspectives of reversal order of elution. Conclusion: Various parameters such as pH, mobile phase composition, temperature, and chemical structure of the analytes play a role in the phenomena of the reversal order of elution of many chiral molecules which are discussed in the article.

Effect of mobile phase (water-methanol) composition on the enantioseparation of dipeptides on the chiral stationary phase Chirobiotic R was investigated using Ala-Ala, Leu-Leu, Gly-Leu, and Leu-Gly as case studies. The lipophilicity of dipeptides was found to be an essential factor in the dependence of their retention on the methanol percentage, the retention factor of lipophobic dipeptides increasing monotonously and that of lipophilic dipeptides changing according to an asymmetric U-shaped trajectory as methanol concentration increases. The behavior of enantioselectivity as a function of the methanol content also depends on the lipophilicity of dipeptide. For lipophilic Leu-Leu, the dependence has a dome-like shape, and for more lipophobic dipeptides, Ala-Ala and Gly-Leu, it is an increasing function of the methanol concentration. The importance of solvation equilibria in the bulk liquid and on the surface of the stationary phase for the total retention is discussed from the thermodynamic point of view. Special consideration is given to the adsorption of the water-methanol mixture on the surface of the Chirobiotic R stationary phase.

The chromatographic behavior and adsorption of enantiomers of glycylaspartic acid on the Nautilus-E chiral stationary phase with grafted eremomycin antibiotic were studied. Acetate buf er mixtures with variable pH prepared in a water—methanol mixed solvent (volume ratio 60 : 40) were used as acetate buf ers. The retention of Gly—Asp was found to be enantioselective and depending on the ratio of various ionic forms of dipeptide in the mobile phase. The ratio is determined by the pH. A model of adsorption equilibrium assuming independent binding of diverse ionic forms with the stationary phase, which is characteristic of energy uniformity, was proposed. This model is well consistent with the experimental data. Features of application of the equilibrium dispersion and transport dispersion dynamic elution models for the determination of the adsorption isotherms by the inverse method are discussed.

Chromatographic behaviors of dipeptides consisting of leucine and glycine were studied on two antibiotic-based chiral stationary phases (CSPs) with teicoplanin (Chirobiotic T) or ristocetin A (Chirobiotic R) as chiral selectors under reversed-phase conditions. The effect of mobile phase pH on the retention of stereoisomers of dipeptides was investigated and thermodynamic characteristic of adsorption were measured at different pH values. It was shown that the retention of dipeptides depends on the ionization of their molecules in the mobile phase, as different ionic forms have different affinity towards antibiotic selectors. Enantioselectivity of the bound antibiotics with respect to Leu-Leu stereoisomers was achieved via steric modulation of ion-ion interactions between the solute and the selector, while in the case of Gly-Leu enantiomers non-ionic interactions such as hydrogen bonding might play the key role. In both cases, the dipeptides terminating in D-Leu were retained stronger than their optical antipodes, whereas the enantiomers of Leu-Gly were hardly separated. The regression analysis of the retention data applying the Horvath-Melander-Molnar model revealed that different types of enantioselectivity resides in particular ionic forms of the compounds: cations are responsible for the separation of diastereomeric pairs and the anionic and zwitterionic forms have a universal enantioselectivity on the Chirobiotic T CSP, and the anions and zwitterions are the enantioselective forms for the Chirobiotic R CSP.

Unusual dynamic behavior of the enantiomers of 9-bromo-11b-(tert-butyl)-2,3,6,11b-tetrahydrooxazolo[3',2':1 5]pyrrolo[3,4-b]quinoline-5,11-dione (1) was observed on a Nautilus-R column packed with silica grafted with antibiotic ristocetin. It consisted in (i) antibatic behavior of the van Deemter plots of the enantiomers and (ii) high and strongly enantiomer dependent values of the A- and B-terms of the van Deemter equation. Although rare, such a pattern has been found earlier in chiral chromatography, with all reported cases limited to brush-type chiral stationary phases. Adsorption dynamics in this system was studied by means of the moment method and the peak parking technique; hydrodynamic properties of the column were explored by using unretained tracers. It was shown that the peculiar shape of the van Deemter curves for the enantiomers of I is conditioned by imperfect packing of the stationary phase, which result in high transcolumn eddy dispersion, and by slow adsorption/desorption kinetics. It was proven that the whole void volume of the column available to an eluent is not accessible to the studied analyte because it cannot penetrate the space between neighboring grafted ligands. Its mass transfer in pores is also affected by the fact that the stagnant layer of the binary water-acetonitrile mobile phase differs in composition from the bulk liquid due to preferential adsorption of water that influences the apparent molecular diffusivity of solutes. An effect of the structures of analyte and chiral selector on the adsorption kinetics is also briefly discussed. (C) 2019 Elsevier B.V. All rights reserved.

Хроматографическим методом изучена адсорбция энантиомеров ибупрофена на хиральной неподвижной фазе Nautilus-E с привитым антибиотиком эремомицином из водно-этанольных ацетатных буферных растворов. Концентрацию этанола в подвижной фазе варьировали от 40 до 60 об. %. Показано, что изотермы адсорбции обоих энантиомеров имеют сложную форму, характеризующуюся нелэнгмюровского типа кривизной и наличием точки перегиба. Данное обстоятельство объясняется действием двух факторов: энергетической гетерогенностью поверхности неподвижной фазы и диссоциацией ибупрофена в жидкой фазе. Исследовано влияние системного пика на форму хроматограмм целевого компонента. Обсуждается влияние температуры на адсорбционное равновесие.

New chiral high-performance liquid chromatography (HPLC) method for the enantiomeric resolution of quinolones is developed and described. The column used was Chirobiotic T (150 × 4.6 mm, 5.0 μm). Three mobile phases used were MeOH:ACN:Water:TEA (70:10:20:0.1%), (60:30:10:0.1%), and (50:30:20:0.1%). The flow rate of the mobile phases was 1.0 mL/min with UV detection at different wavelengths. The values of retention, resolution, and separation factors ranged from 1.5 to 6.0, 1.80 to 2.25, and 2.86 to 6.0, respectively. The limit of detection and quantification ranged from 4.0 to 12 ng and 40 to 52 ng, respectively. The modeling studies indicated strong interactions of R-enantiomers with teicoplanin chiral selector than S-enantiomers. The supra molecular mechanism of the chiral recognition was established by modeling and chromatographic studies. It was observed that hydrogen bondings and π-π interactions are the major forces for chiral separation. The present chiral HPLC method may be used for enantiomeric resolution of quinolones in any matrices.