Retention Increments Research Articles

Hydrophobic hyper-cross-linked polymer shells encapsulating dispersed nanoscale CuS boosting mercury immobilization

Metal sulfides (MSs) hold significant promise for environmental remediation but are hindered by their low accommodation capacity and poor water resistance. Herein, this work reports a hybrid strategy to address these limitations by assembling highly dispersed CuS nanoparticles within the hyper-cross-linked polymer (HCP). Benefiting from the super-hydrophobicity and well-developed porosity of HCP, alongside the high dispersion and multiple active sites of CuS nanoparticles, the resultant CuS@HCP exhibits exceptional water resistance, enhanced surface area, high mass transfer ability, and substantial Hg0 adsorption capacity. The Hg0 adsorption capacity (Q90) of CuS@HCP3 reaches 1.25 and 1.391 mg/g under N2 and simulated flue gas, respectively. Notably, the normalized Q90 of coated CuS surpasses that of pristine CuS by 25-fold under N2 + 15 % H2O and its durability outstrips most previously reported mineral sulfides even in complex simulated flue gas. Universal research confirms MSs@HCP (M = Zn, Sn, and Fe) equally demonstrate over 15-fold increment in Hg0 retention under high humidity conditions compared to the pristine MSs. Such HCP hybrid strategy universally and effectively optimizes the accommodation capacity and water resistance of MSs, endowing MSs@HCP broad application prospects.

The biogenic elements retention in reservoirs of the Yangtze River basin and effect on the nutrient flux into the sea

The Yangtze River catchment is among the most human-impacted basins globally, with a population of approximately 590 million and over 60,000 dams constructed by 2020. While the effect of reservoir dams on runoff and sediment has been extensively studied, assessing the impact of biogenic element retention in reservoirs on nutrient flux into the sea remains a challenge. This study analyzed the total organic carbon (TOC), total organic nitrogen (TN), biogenic silica (BSi), grain size, and 210Pb chronology of sediment cores collected from the Pengshui, Wuqiangxi, Three Gorges, and Danjiangkou reservoirs. The analysis was combined with the database of annual reservoir area and nutrient flux into the sea of the Yangtze River. The results showed that the average annual sedimentary retention of TOC, TN, and BSi in the four reservoirs increased by 16%, 18%, and 26% after dam construction and rapid eutrophication. The total water area of large reservoirs increased from 30 to 132,000 km2 between 1960 and 2016, resulting in total accumulated retention increments of TOC, dissolved inorganic nitrogen (DIN), dissolved inorganic phosphate (DIP), and dissolved silicon (DSi) in reservoirs reaching 27, 3.5, 0.1, and 27.2 million tons, respectively. The retention increments accounted for 20.6%, 7.0%, 8.8%, and 12.1% of the total flux of TOC, DIN, DIP, and DSi into the sea during the same period, respectively. The retention of TOC and DSi in reservoirs led to a decrease in their annual flux into the sea by 67.2% and 61.4% in 2016 compared to 1960. However, the retention of DIN and DIP increased their annual flux by 6.4-fold and 4.2-fold in 2016 compared to 1960, which was attributed to the substantial emission of fertilizer and sewage in the downstream catchment area of dams. These findings provide valuable insights into biogenic element retention in reservoir catchments and their impact on nutrient flux into the sea.

Moringa oleifera protein isolates: In silico analysis of bioactivity, allergenicity, and toxicity, and application in plant-based burgers

Moringa oleifera seeds are excellent sources of protein. In this study, the potential bioactivity, allergenicity, and toxicity of M. oleifera protein isolates (MPI), and the effect of different concentrations of MPI (0%, 5%, 10%, and 15%) on the physicochemical properties and sensory acceptance of plant-based burgers (PBB) were investigated. A total of 100 peptides were identified through LC-MS/MS analysis. Subsequent in silico analyses unveiled their notable bioactivity potential and their potential to cause allergies while showing low levels of toxicity. The addition of MPI improved the nutritional value and cooking properties of PBB, evidenced by a 13% increment in protein content, a 3% increment in moisture retention, and an 11% less cooking loss. Furthermore, burgers containing 15% MPI had the lowest hardness (11.65 ± 0.33 N), cohesiveness (0.25 ± 0.01), springiness (0.42 ± 0.02 mm), and chewiness (1.22 ± 0.11 N × mm). MPI did not impair (p > 0.05) consumers’ sensory acceptance of the PBB. Overall, MPI is a promising protein source for preparing high-quality PBB.

Re-exploration of dietary iodine intake in Chinese adult males using a modified iodine balance study.

There is still controversy about optimal dietary iodine intake as the Universal Salt Iodization policy enforcement in China. A modified iodine balance study was thus conducted to explore the suitable iodine intake in Chinese adult males using the iodine overflow hypothesis. In this study, thirty-eight apparently healthy males (19·1 (sd 0·6) years) were recruited and provided with designed diets. After the 14-d iodine depletion, daily iodine intake gradually increased in the 30-d iodine supplementation, consisting of six stages and each of 5 d. All foods and excreta (urine, faeces) were collected to examine daily iodine intake, iodine excretion and the changes of iodine increment in relation to those values at stage 1. The dose-response associations of iodine intake increment with excretion increment were fitted by the mixed effects models, as well as with retention increment. Daily iodine intake and excretion were 16·3 and 54·3 μg/d at stage 1, and iodine intake increment increased from 11·2 μg/d at stage 2 to 118·0 μg/d at stage 6, while excretion increment elevated from 21·5 to 95·0 μg/d. A zero iodine balance was dynamically achieved as 48·0 μg/d of iodine intake. The estimated average requirement and recommended nutrient intake were severally 48·0 and 67·2 μg/d, which could be corresponded to a daily iodine intake of 0·74 and 1·04 μg/kg per d. The results of our study indicate that roughly half of current iodine intakes recommendation could be enough in Chinese adult males, which would be beneficial for the revision of dietary reference intakes.

Transmission reduction, health benefits, and upper-bound costs of interventions to improve retention on antiretroviral therapy: a combined analysis of three mathematical models

SummaryBackgroundIn this so-called treat-all era, antiretroviral therapy (ART) interruptions contribute to an increasing proportion of HIV infections and deaths. Many strategies to improve retention on ART cost more than standard of care. In this study, we aimed to estimate the upper-bound costs at which such interventions should be adopted.MethodsIn this combined analysis, we compared the infections averted, disability-adjusted life-years (DALYs) averted, and upper-bound costs of interventions that improve ART retention in three HIV models with diverse structures, assumptions, and baseline settings: EMOD in South Africa, Optima in Malawi, and Synthesis in sub-Saharan African low-income and middle-income countries (LMICs). We modelled estimates over a 40-year time horizon, from a baseline of Jan 1, 2022, when interventions would be implemented, to Jan 1, 2062. We varied increment of ART retention (25%, 50%, 75%, and 100% retention), the extent to which interventions could be targeted towards individuals at risk of interrupting ART, and cost-effectiveness thresholds in each setting.FindingsDespite simulating different settings and epidemic trends, all three models produced consistent estimates of health benefit (ie, DALYs averted) and transmission reduction per increment in retention. The range of estimates was 1·35–3·55 DALYs and 0·12–0·20 infections averted over the 40-year time horizon per additional person-year retained on ART. Upper-bound costs varied by setting and intervention effectiveness. Improving retention by 25% among all people receiving ART, regardless of risk of ART interruption, gave an upper-bound cost per person-year of US$2–6 in Optima (Malawi), $43–68 in Synthesis (LMICs in sub-Saharan Africa), and $28–180 in EMOD (South Africa). A maximally targeted and effective retention intervention had an upper-bound cost per person-year of US$93–223 in Optima (Malawi), $871–1389 in Synthesis (LMICs in sub-Saharan Africa), and $1013–6518 in EMOD (South Africa).InterpretationUpper-bound costs that could improve ART retention vary across sub-Saharan African settings and are likely to be similar to or higher than was estimated before the start of the treat-all era. Upper-bound costs could be increased by targeting interventions to those most at risk of interrupting ART.FundingBill & Melinda Gates Foundation.

Ligand–receptor binding increments in enantioselective liquid chromatography

A set of N-derivatized amino acids were separated into enantiomers on a tert-butylcarbamoylated quinine-based chiral stationary phase (CSP). Quantitative structure–property relationship (QSPR) studies were then employed to investigate the retention behavior and factors responsible for enantioselectivity. Computations were performed using a general linear model and a Free-Wilson matrix with indicator variables as structural descriptors. The approach allowed calculations of retention increments for first and second eluted enantiomers as well as group contributions to enantioselectivity. The results demonstrated that the additivity principle of group contributions was obeyed for the majority of solutes in the data set. Only a few basic amino acids (Arg, His) needed to be removed as they did not fit to such a linear model leading to outliers. The model was carefully validated and then utilized to investigate retention and enantioselectivity contributions of different protection groups and individual amino acid residues. It turned out that primarily protection groups were driving retention and enantioselectivity. In contrast, the contribution of amino acid residues to enantioselectivity was only significant for secondary amino acids, α-methylated amino acids, aspartic acid and a few sterically bulky aliphatic amino acid residues (Tle, Ile, allo-Ile). Amongst them only the latter group contributed positively to enantioselectivity while the other residues mentioned reduced enantioselectivity significantly. This type of QSPR model may be valuable to analyze retention/selectivity data of closely related congeneric compound series, is illustrative and straightforward to implement. It is thus valuable for interpretation of retention mechanisms, while its utility for prediction of retention and enantioselectivity data is limited to compounds made up of groups included in the solute set used for deriving the increments.

Fouling reduction and retention increment of polyethersulfone nanofiltration membranes embedded by amine-functionalized multi-walled carbon nanotubes

Flat sheet asymmetric amine-functionalized multiwalled carbon nanotubes (NH2-MWCNTs)/polyethersulfone (PES) nanocomposite nanofiltration membranes with different nanofiller contents were prepared by the phase inversion method. The characteristics of the mixed matrix membranes were analyzed by SEM, AFM, contact angle and zeta potential measurements. The performance of the membranes was evaluated in terms of pure water flux (PWF), static protein adsorption and bovine serum albumin (BSA) filtration. Results showed that the hydrophilicity and PWF of the nanocomposite membranes were enhanced with increase of NH2-MWCNTs dosages. Increasing the nanotube dosages from 0wt% to 0.045wt% led to increase of PWF from 13.6 to 23.7L/m2h. The SEM images illustrated that the fabricated membranes possessed a typical asymmetric finger-like structure. The salt rejection experiments and zeta potential measurements evidenced that the surface charge of the NH2-MWCNT embedded membranes was negative and the retention mechanism of the solutes was mainly the Donnan exclusion mechanism. The salt retention sequence for 0.045wt% NH2-MWCNT was Na2SO4 (65%)>MgSO4 (45%)>NaCl (20%) after 180min filtration. The increase of the salt solution pH caused more negative surface charge and higher retention. The addition of the modified MWCNTs, in BSA filtration analysis, led to significant improvement of the fouling recovery ratio (FRR) due to lower roughness, more negatively charged surface and higher hydrophilicity of the membrane surface.

Mixed-mode chromatography with zwitterionic phosphopeptidomimetic selectors from Ugi multicomponent reaction

In the present contribution a novel Ugi multicomponent reaction (MCR) was used to generate zwitterionic chromatographic selectors with capability for application in mixed-mode chromatography featuring complementary selectivities in reversed-phase (RP) and hydrophilic interaction liquid chromatography (HILIC). Aminophosphonate zwitterionic chromatographic ligands were synthesized adopting a one pot microwave assisted three-component UGI-MCR synthesis and, after purification, were immobilized by thiol-ene click chemistry on silica beads. Chromatographic characteristics of these stationary phases were evaluated by variation of experimental conditions for a set of diverse analytes with neutral, acidic, basic and zwitterionic character revealing the presence of multimodal retention capabilities (i.e. tunable retention increments from ion exchange, hydrophobic and hydrophilic interaction were observed). To further investigate and classify the retention properties of the novel stationary phases we performed a comparative chromatographic study with commercially available mixed mode, HILIC and RP columns. The resultant chromatographic data were analyzed by principal component analysis (PCA). PCA revealed that the new reversed-phase/zwitterionic ion-exchangers (RP/ZWIX) are complementary to common RP, HILIC and mixed-mode phases on the market and could be a promising alternative in impurity profiling and 2D-HPLC concepts. Moreover, the adopted synthetic approach offers the capability to generate chemical diversity simply by the variation of the starting aldehyde, aminophosphonic acid and/or isonitrile components. This unique characteristics offer great possibility for the design of novel selectors for mixed mode chromatography like RP/ZWIX, HILIC, affinity and chiral chromatography.

Difference equation model for isothermal gas chromatography expresses retention behavior of homologues of n-alkanes excluding the influence of holdup time

A difference equation (DE) model is developed using the methylene retention increment (Δtz) of n-alkanes to avoid the influence of gas holdup time (tM). The effects of the equation orders (1st–5th) on the accuracy of a curve fitting show that a linear equation (LE) is less satisfactory and it is not necessary to use a complicated cubic or higher order equation. The relationship between the logarithm of Δtz and the carbon number (z) of the n-alkanes under isothermal conditions closely follows the quadratic equation for C3–C30n-alkanes at column temperatures of 24–260°C. The first and second order forward differences of the expression (Δlog Δtz and Δ2log Δtz, respectively) are linear and constant, respectively, which validates the DE model. This DE model lays a necessary foundation for further developing a retention model to accurately describe the relationship between the adjusted retention time and z of n-alkanes.

Effect of epoxidized natural rubber on thermal properties, fatigue life, and natural weathering test of styrene butadiene rubber/recycled acrylonitrile‐butadiene rubber (SBR/NBRr) blends

AbstractThe utilization of waste rubber powder in polymer matrices provides an attractive strategy for polymer waste disposal. Addition of recycled acrylonitrile‐butadiene rubber (NBRr) in rubber compounds gives economic (lowering the cost of rubber compounds) as well as processing advantages. In this study, the properties of styrene butadiene rubber (SBR)/NBRr blends with and without epoxidized natural rubber (ENR‐50) as a compatibilizer were determined. The results such as thermal gravimetric analysis (TGA), fatigue life, and natural weathering test of SBR/NBRr blends with and without ENR‐50 were carried out. Results showed that TG thermograms of SBR/NBRr blends with ENR‐50 show lower thermal stability compared blends without ENR‐50. The incorporation of ENR‐50 into SBR/NBRr blends has reduced char residue compared SBR/NBRr blends without ENR‐50. The incorporation of ENR‐50 in SBR/NBRr blends has increased the rigidity of the blends thus lowering the fatigue life. The increment in tensile properties retention of SBR/NBRr blends with ENR‐50 indicated the enhancement on weathering resistant. The surfaces of SBR/NBRr blends with ENR‐50 after 6 months exposure showed a minimal severity of crack compared with SBR/NBRr blends without ENR‐50. It revealed that the scale of cracks has reduced indicating well‐retaining interfacial adhesion between SBR and NBRr with the presence of ENR‐50 as a compatibilizer. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Chocolate HILIC phases: development and characterization of novel saccharide-based stationary phases by applying non-enzymatic browning (Maillard reaction) on amino-modified silica surfaces

Novel saccharide-based stationary phases were developed by applying non-enzymatic browning (Maillard Reaction) on aminopropyl silica material. During this process, the reducing sugars glucose, lactose, maltose, and cellobiose served as "ligand primers". The reaction cascade using cellobiose resulted in an efficient chromatographic material which further served as our model Chocolate HILIC column. (Chocolate refers to the fact that these phases are brownish.) In this way, an amine backbone was introduced to facilitate convenient manipulation of selectivity by additional attractive or repulsive ionic solute-ligand interactions in addition to the typical HILIC retention mechanism. In total, six different test sets and five different mobile phase compositions were investigated, allowing a comprehensive evaluation of the new polar column. It became evident that, besides the so-called HILIC retention mechanism based on partition phenomena, additional adsorption mechanisms, including ionic interactions, take place. Thus, the new column is another example of a HILIC-type column characterized by mixed-modal retention increments. The glucose-modified materials exhibited the relative highest overall hydrophobicity of all grafted Chocolate HILIC columns which enabled retention of lipophilic analytes with high water content mobile phases.

Triazolo-linked cinchona alkaloid carbamate anion exchange-type chiral stationary phases: Synthesis by click chemistry and evaluation

Immobilization strategy based on Huisgen 1,3-dipolar cycloaddition ( click chemistry) of 10,11-didehydrocinchona tert-butylcarbamates to azido-grafted silica gels has been evaluated for preparation of novel chiral stationary phases ( CSP 1– 3). The resultant 1,2,3-triazole-linked CSPs were tested under various mobile phase conditions (polar organic and reversed phase mode) with a representative set of structurally diverse racemic acids including N-protected aminoacids, aromatic and aryloxycarboxylic acids as well as binaphthol phosphate. The chiral recognition performance of the C3-triazole-linked CSPs was found to mirror largely that of the known C3-thioether-linked CSP in terms of elution order, enantioselectivity and retention behavior. In an effort to assess the non-specific binding expressed as retention increment of these triazole-linked CSPs, the parent azidopropyl- and triazole-modified silica materials (thus not containing the chiral head ligand) were studied independently. Compared with the corresponding CSPs, the analyte retention on the azidopropyl control column was very low, and practically negligible on the corresponding triazole-modified reference column. Only minor losses in analyte retention behavior (<5%) were observed with triazole-linked CSPs after two month of continuous use with polar–organic and reversed-phase-type mobile phases, highlighting the excellent stability of the 1,2,3-triazole linker.

Temperature-Induced Inversion of the Elution Order of Enantiomers in Gas Chromatography: N-Ethoxycarbonyl Propylamides and N-Trifluoroacetyl Ethyl Esters of α-Amino Acids on Chirasil-Val-C11 and Chirasil-Dex Stationary Phases

Inversion of the elution order of enantiomers caused by enthalpy-entropy compensation at the isoenantioselective temperature (Tiso) was experimentally observed by gas chromatography on the diamide-type chiral stationary phase (CSP), Chirasil-L-Val-C11, with N-ethoxycarbonyl propylamide (ECPA) derivatives of a number of alpha-amino acids. For the first time, a clear visual representation of the increase of the apparent enantioseparation factor alpha app from 1.00 to 1.08 as the temperature is raised from 120 to 170 degrees C is presented. The increase of alpha app is accompanied by a concomitant reduction of the retention factors of the enantiomers. The Tiso values were in the range from 110 to 130 degrees C depending on the nature of the alpha-amino acid. On the contrary, the Tiso values of the N(O)-trifluoroacetyl ethyl ester derivatives (TFA-Et) of the same alpha-amino acids were approximately 80 degrees higher than that of ECPA derivatives. The comprehensive thermodynamic investigation of the enantioseparation of ECPA and TFA-Et derivatives of valine and alanine using the retention increment method showed that the Delta L,D(DeltaH) difference between the diastereomeric selector-selectand associates was almost the same for ECPA and TFA-Et derivatives despite a much stronger bonded selector-selectand association taking place for the ECPA derivatives. On the other hand, the Delta L,D(DeltaS) values were found to be more negative in the case of ECPA derivatives, resulting in the unusually low values of Tiso. A temperature-dependent inversion of the elution order of enantiomers was also observed on the cyclodextrin-type CSP, Chirasil-Dex, with TFA-Et derivatives of several alpha-amino acids. The Tiso values were in the range from 20 to 170 degrees C depending on the nature of the alpha-amino acid. The results obtained demonstrate the necessity to conduct temperature-dependent studies in order to optimize the enantiomeric separation of single racemates isothermally or of mixtures of racemates in temperature-programmed runs using enantioselective GC. It is also shown that consideration of the elution order of enantiomers and the value of the apparent enantioseparation factor alpha app alone, without performing temperature-dependent measurements, can easily lead to wrong conclusions regarding the enantiorecognition mechanism.

Thermodynamic study of N-trifluoroacetyl-O-alkyl nipecotic acid ester enantiomers on diluted permethylated β-cyclodextrin stationary phase

Thermodynamic studies were performed on 12 pairs of N-trifluoroacetyl-O-alkyl nipecotic acid ester enantiomers on diluted permethylated beta-cyclodextrin stationary phase (CP Chirasil-Dex CB). The influence of ester alkyl group structure on interaction with permethylated beta-cyclodextrin (Me-CD) and enantioselectivity was studied. The types of alkyl groups studied included n-alkyl (C1-C5) and groups containing branching at differing locations relative to the chiral center of the molecule. The results show that for a given molecular weight, the n-alkyl esters have stronger interactions with Me-CD than esters containing branched alkyl groups. However, although having weaker interactions with Me-CD, esters containing alpha-branched alkyl groups exhibit higher enantioselectivity than the corresponding n-alkyl or beta-branched isobutyl esters. From the retention data, thermodynamic parameters were estimated using the retention increment method and enthalpy-entropy compensation plots (ln R' versus deltaH) were constructed. The results suggest that ester enantiomers with branching at the alpha-carbon of the ester alkyl group have additional and/or different types of enantioselective interactions with Me-CD than the C1-C5 n-alkyl esters or beta-branched isobutyl ester. In order to obtain a qualitative sense of the interaction with Me-CD, structures of the diastereomeric complexes formed between Me-CD and some of the ester enantiomers were modeled using simulated annealing molecular dynamics.

GC—MS in Space Research: Decoding Complex Isothermal Chromatograms Recovered from Space Missions

An analytical procedure is described to study GC-MS isothermal chromatograms simulating those recovered from space missions: in fact GC plays a predominant role in space missions devoted to characterizing the chemical composition of extra-terrestrial atmospheres. SIM (selected ion monitoring) detection was used for monitoring selected chemical classes: a simplified chromatogram can be obtained giving information on the chemical composition of the complex mixture. Since only isothermal GC chromatograms are allowed by flight constraints, a time axis transformation is required to make them homogeneous: i.e., constant retention increments for CH2 additions in terms of a homologous series. The order in the linearized chromatogram can be simply singled out with a chemometric approach based on the study of the Autocovariance Function (ACVF) computed on the digitized chromatogram: the plot of the experimental autocorrelation function (EACF) shows well-shaped peaks if constant interdistances are repeated in different regions of the chromatogram. The method was applied to standard mixtures representative of planetary atmospheres--hydrocarbons, nitriles and oxygenated compounds with between 3 and 12 carbon atoms--analyzed in flight simulating conditions. The coupling of the selectivity of SIM detection with the interpretation power of the EACF procedure proves to be a powerful tool for interpreting data recovered from space missions: the chemical composition of the mixture can be identified by handling the raw SIM chromatograms.

Decoding of complex isothermal chromatograms recovered from space missions: Identification of molecular structure

A chemometric approach, based on the study of the autocovariance function, is described to study isothermal GC chromatograms of multicomponent mixtures: isothermal GC analysis is the method of choice in space missions since it is, to date, the only method compatible with flight constraints. Isothermal GC chromatograms look inhomogeneous and disordered with peak density decreasing at higher retention times: a time axis transformation is proposed to make retention an homogeneous process so that CH 2 addition in terms of an homologous series yields a constant retention increment. The time axis is transformed into a new scale based on the retention times of n-alkanes, as they are the basis of the universal Kovats indices procedure. The order introduced into the chromatogram by retention time linearization can be simply singled out by the experimental autocorrelation function (EACF) plot: if constant inter-distances are repeated in different regions of the chromatogram, well-shaped peaks are evident in the EACF plot. By comparison, with a standard mixture it is possible to identify peaks diagnostic of specific molecular structures: study of the EACF plot provides information on sample chemical composition. The procedure was applied to standard mixtures containing compounds representative of the planetary atmospheres that will be investigated in the near future: in particular, those related to Titan’s atmosphere (Cassini–Huygens mission) and cometary’s nucleus (Rosetta mission). The employed experimental conditions simulated those applied to GC instruments installed on space probes and landers in space missions. The method was applied to two specific investigations related to space research, i.e., a comparison of retention selectivity of different GC columns and identification of the chemical composition of an unknown mixture.

Extraordinary chiral discrimination in inclusion gas chromatography. Thermodynamics of enantioselectivity between a racemic perfluorodiether and a modified γ-cyclodextrin

The enantiomers of the perfluorodiether “compound B” [2-(fluoromethoxy)-3-methoxy-1,1,1,3,3-pentafluoropropane], a decomposition product of the inhalational anesthetic sevoflurane [2-(fluoromethoxy)-1,1,1,3,3,3-hexafluoropropane], were separated by gas chromatography on octakis(3- O-butanoyl-2,6-di- O- n-pentyl)-γ-cyclodextrin (Lipodex E), dissolved in polysiloxane PS 255 (30% w/w), with an unexpectedly high separation factor of α=10.6 at 26 °C. Using the concept of the retention increment R′, non-enantioselective and enantioselective contributions to retention were separated and thus reliable thermodynamic parameters of enantioselectivity, i.e. − Δ S, R (Δ G)=5.7 (0.05) kJ/mol at 303 K, − Δ S, R (Δ H)=20.1 (0.64) kJ/mol, Δ S, R (Δ S)=−47.4 (2.0) J/K mol and T isoenant=424 (30) K or ∼150 °C, were determined by temperature-dependent measurements. The enantiomeric bias represents the largest values ever measured in enantioselective gas chromatography. An equation is presented which allows calculation of the non-enantioselective contributions to retention from measurements at two arbitrary concentrations of Lipodex E in polysiloxane. Surprisingly, the enantioselectivity is greatly reduced when employing the β-cyclodextrin analogue and breaks down completely with the α-cyclodextrin analogue of Lipodex E.

Synthesis and evaluation of a ferrocene-bonded silica phase for reversed-phase high-performance liquid chromatography

The preparation, characterization, and potential liquid chromatographic applications of a ferrocene-bonded silica stationary phase are presented. The phase was prepared by covalent bonding of ferrocene-derivatized aminosilane to silica gel then end-capping with trimethyl-chlorosilane. Use of the new stationary phase enabled good separations of alkylbenzenes, halobenzenes, polycyclic aromatic hydrocarbons, and even basic compounds under reversed-phase conditions. The linear relationship between logk (wherek is the retention factor) and the number of π electrons (n π) in the structures of polyaromatic hydrocarbons indicates that the mechanism of retention on the new stationary phase involves strong π-π interaction with one of the cyclopentadienyl (Cp) groups of the ferrocene. The retention increment for a methylene group was calculated. The value indicates that the mechanism of retention on the new stationary phase also involves a strong hydrophobic interaction, so the phase enables effective separation of compounds with a benzenering.

Stereointegrity of thalidomide: gas-chromatographic determination of the enantiomerization barrier

Enantioselective chromatographic methods, representing the most commonly used techniques for the determination of enantiomeric ratios, can also be used for the evaluation of stereochemical integrity. In the present study, two chromatographic methods, dynamic gas chromatography (DGC) and stopped-flow gas chromatography (SFGC) were used to determine the enantiomerization barrier of thalidomide. In the presence of a chiral stationary phase (CSP), the enantiomers of thalidomide produced characteristic elution profiles exhibiting plateaus and/or peak broadening which were observed between 190 and 220 °C in DGC. To obtain the enantiomerization barrier of thalidomide from experimental data, the fast and efficient simulation program ChromWin was used to simulate the elution profiles and obtain kinetic activation parameters. From temperature-dependent measurements the rate constants k 1 and k −1 and the kinetic activation parameters Δ G # , Δ H # and Δ S # of the enantiomerization of thalidomide were obtained by DGC. The enantiomerization barrier Δ G # was determined to be 154±2 kJ/mol with DGC and 150±3 kJ/mol with the sfGC technique at 200 °C, respectively. The concept of the retention increment R′ has been applied to separate the enantiomerization barrier of thalidomide in the dissolved and complexed state of the CSP.

Relationship between chromatographic properties of aromatic carboxylic acids and their structure

The retention of aromatic carboxylic acids and their esters in reversed-phase HPLC is proportional to the sum of their partition coefficients in octanol–water system and retention increments. This equation can be used for the identification of various side-products formed in monomer production.