Calculation of retention indices and peak widths in temperature programmed gas-liquid chromatography

Summary Isothermal chromatographic measurements lead directly to H _v ^o and A (entropy term) of solutes, and three constants of an empirical relationship between peak width and column temperature. From the thermodynamic parameters H _v ^o and A retention temperatures have been computed by means of a theoretical model including temperature dependence of carrier gas viscosity, and subsequently retention times; programmed retention indices have been determined by linear and polynomial interpolations. By substituting the value of the calculated retention temperature in the above-mentioned relationship, peak width at half-height for a linear temperature may be estimated. Predicted retentions correlate with observed data, with a P-value 0.01; simulation accuracy is generally 6–10% for peak widths.Retention indices of some organochlorine species, separated on an OV-101 capillary column, may differ by as much as 26 units depending on the method of calculation. Polynomial-calculated indices are more consistent with the retention index scheme, and have smaller standard deviations and better constancy at different heating rates.     

Feature Based Retention Time Alignment for Improved HDX MS Analysis

An algorithm for retention time alignment of mass shifted hydrogen-deuterium exchange (HDX) data based on an iterative distance minimization procedure is described. The algorithm performs pairwise comparisons in an iterative fashion between a list of features from a reference file and a file to be time aligned to calculate a retention time mapping function. Features are characterized by their charge, retention time and mass of the monoisotopic peak. The algorithm is able to align datasets with mass shifted features, which is a prerequisite for aligning hydrogen-deuterium exchange mass spectrometry datasets. Confidence assignments from the fully automated processing of a commercial HDX software package are shown to benefit significantly from retention time alignment prior to extraction of deuterium incorporation values.      

Prediction of internal standards in reversed-phase liquid chromatography: IV. Correlation and prediction of retention in reversed-phase ion-pair chromatography based on linear solvation energy relationships

This paper describes the results of the evaluation of a new solvation parameter model for reversed-phase ion-pair chromatography by linear gradient elution. This model is described as . The first six terms are the usual solvation parameter equation for neutral solutes, and the seventh term represents the contribution to retention from solute’s ionization. The last term describes the retention increase due to ion-pair effect. Retention times obtained for 60 solutes (neutral, acidic and basic) in acetonitrile/aqueous mobile phases with different ion-pair reagents (phosphoric acid, trifluoroacetic acid, heptafluorobutyric acid, perchloric acid, and hexafluorophosphoric acid) are used to evaluate the capability of the function. It is concluded that the model describes the retention of ionizable/ionized compounds under ion-pair conditions very well. Accordingly, the function extends the application of linear solvation energy relationships (LSERs) to ionizable compounds in ion-pair chromatography, and allows us to easily predict their retention for chromatographic optimization, including selectivity optimization and internal standard selection. Finally, the conclusion can be extended to ioscratic elution.     

Gas chromatography of halogenated adamantanes

The main thermodynamic characteristics of the adsorption of haloadamantanes on graphitized thermal carbon black were determined by experimental measurements and by calculations in terms of molecular-statistic theory of adsorption. Using experimental data, the Kovac retention indices were calculated and the optimum conditions for gas-chromatographic separation of haloadamantanes on stationary phases with different polarity were elucidated. The influence of the cage effect in the adamantane unit on the chromatographic properties of haloadamantanes was established.     

Extended Thermodynamic Approach to Ion Interaction Chromatography: Influence of Organic Modifier Concentration

Summary The influence of organic modifier and ion interaction reagent (IIR) concentrations on analyte retention in ion interaction chromatography (IIC) was investigated via a bivariate approach. The system examined is as follows: ODS-Hypersils was used as stationary phase; the eluent was a variable mixture of a phosphate buffer pH 2.1 and methanol ranging 0%–40% (v/v). The organic modifier decreases both the analyte and the IIR free energy of adsorption hence their interaction is lower when the organic modifier concentration is higher. Retention equations describe how these two factors interact with each another. The estimated equilibrium constant for the IIR adsorption corresponds to G° = –17.3 KJ mol^–1. This value is reliable because it makes sense physically. The chi-squared (k _calc – k _exp)² is 1.11E + 00 and 2.85E-02, for octopamine and 2-naphthalenesulfonate, respectively. Retention equations were compared with those obtained from the outstanding retention models in IIC. For the modelled system, the extended thermodynamic approach is able to explain experimental evidence that cannot be rationalised by existing retention models: it proves superior for fitting experimental data and is also able to predict when the purely electrostatic approach may work properly. In the absence of IIR the present retention equation reduces, as expected, to the relationship that describes the influence of organic modifier on retention behaviour in reversed-phase liquid chromatography.     

Methodology for the estimation of chlorothalonil and its metabolite in mustard crop by gas liquid chromatography

A simplified method for the determination of chlorothalonil (I) and its metabolite 4-hydroxy-2,5,6-trichloroisophthalonitrile (II) in mustard crop is described. It involves extraction, derivatisation, clean-up on a silica-gel column and gas-liquid chromatography. The retention times and detection limits are 4.49, 5.39 min and 0.01, 0.005 g/g for I and II, respectively.     

Particle densities and non-equilibrium in a low-pressure argon plasma jet

Plasma diagnostic techniques have been employed to determine particle densities and temperatures in a low-pressure argon plasma jet generated by a cascade arc. These measurements allow characterization of the extent to which the plasma jet deviates from thermodynamic equilibrium and provide a basis for predicting how reactive gases will interact with the excited and ionized species in the plasma jet. It was found that the distribution of atomic states in the plasma jet is not adequately described by either local thermodynamic equilibrium (LTE) or partial local thermodynamic equilibrium (pLTE), and the jet was optically thick for 3p4s transitions across the jet radius. Excited argon neutrals outnumber ions by a large ratio, and dominate subsequent dissociation/excitation phenomena. The rate of methane destruction in the plasma jet shows that estimates for particle densities, temperature, and jet velocity are self-consistent.     

Chemically bonded multifunctional stationary phases for high-performance liquid chromatography

Summary A new class of stationary phases for high-performance liquid chromatography (HPLC) are described which simulate in their retention chracteristics ion-pair separations. The phases consist of mixtures of chemically dissimilar ligands chemically bonded to silica supports. These phases are largely reversed-phase in nature, but also contain significant ion-exchange properties, at levels similar to those demonstrated to occur in ionpairing. By bonding both ionic and hydrophobic groups in the correct proportions, mixed retention mechanisms are created, resulting in unique selectivities, while retaining the excellent stabilities and efficiencies characteristic of bonded phases. The ratio of hydrophobic to ionic character can be controlled during the synthesis, and is used as a tool to vary the stationary phase, rather than only the mobile phase, to effect the separation desired. The synthesis and behavior of both anionic and cationic/reversed-phase materials are described, and are applied to the simultaneous separation of nucleosides and nucleotides, and to the separation of the catecholamines.Presented at the 14th International Symposium on Chromatography London, September, 1982.     

Determination of thermodynamic characteristics from retention data in GC

Summary Retention data have been measured for model substances of different polarities under isothermal conditions at several temperatures using capillary columns coated with Carbowax-20M and SP-2100 stationary phases. By using the k capacity ratios obtained for various temperatures the thermodynamic characteristics of the partition process were determined. For homologous series linear relationships can be established between thermodynamic characteristics and carbon number. Thermodynamic characteristics can be used both to compare and evaluate column quality and to predict retention indices for different temperatures and linear programmed temperature conditions.Dedicated to Professor J. F. K. Huber on the occasion of his 60th birthday.     

Determination of thermodynamic functions of sorption for cyclohexyl-, piperidyl-, morpholyl-, and thiomorpholyl-n-alkanes by capillary gas-liquid chromatography

The values of partial molar free energy (G), enthalpy (H), and entropy (S) of sorption in the homologous series ofN-alkylpiperidines,N-alkylmorpholines,N-alkyl thiomorpholines, and alkylcyclohexanes were determined. It was found that the free energy of sorption is determined to a greater extent by the enthalpy term than by the entropy one. The free energy of sorption of the first homolog decreases when then-alkyl chain is attached directly to the carbon atom of the cycle and increases in the case ofN-alkylsubstituted heterocycles. The influence of the heteroatom nature on intermolecular interactions of homologs with the nonpolar stationary phase was quantitatively estimated on the basis of thermodynamic data.Dead time necessary for calculation of the retention factor was determined by the retention of methane injected into the column simultaneously with the sample.Translated from Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 2030–2032, August, 1996.     

Temperature and Solute Molecular Size Effects on the Retention and Enantioselectivity of a Series of D, L Dansyl Amino Acids on a Vancomycin-Based Chiral Stationary Phase

Summary The influence of column temperature (0–28 °C) and solute molecular size on the retention and enantioselectivity of a series of D, L dansyl amino acids with a non-polar side chain (valine, leucine, phenylalanine and tryptophan) were investigated using a vancomycin-based chiral stationary phase (CSP). The enthalpic and entropic terms for the solute-CSP association were determined from the linear vant Hoff plots. Two solute groups were distinguished in relation to these thermodynamic quantities: the solute group I (dansyl valine, dansyl leucine, dansyl phenylalanine) for which large negative values of enthalpic terms were obtained and the solute group II (dansyl tryptophan) for which H value was much less negative. The enthalpy-entropy compensation study revealed that the interaction mechanism was identical for the group I solute enantiomers but changed for D, L dansyl tryptophan. This was further exemplified as the group I compound enantiomers were resolved over the temperature range while the enantiomers of dansyl tryptophan were not separated in the operating conditions. Relationships between both the solute retention factors and apparent enantioselectivity, and the accessible surface area of the amino acid side chain indicated that when the solute molecular size increased (i) the retention was enhanced by the hydrophobic effect and (ii) the chiral discrimination decreased dependent, at least in part, on a steric hindrance phenomenon at the vancomycin aglycone pocket.     

Enantiomer separation of side-chain fluorinated alkylbenzenes by capillary gas chromatography on cyclodextrin phases

Summary The separation of racemic side-chain fluorinated alkylbenzenes and bromofluorinated analogues by capillary gas chromatography using permethylated , and -cyclodextrins dissolved in polysiloxanes of different polarity as stationary phases is described. The influence of the achiral polysiloxane matrices on the separation of enantiomers is discussed in the light of the results obtained with the different phases. For a part of the tested compounds thermodynamic data are determined which describe the interaction of enantiomers with the stationary phase. The mechanism of separation is discussed on this basis and by comparison with data for structurally similar compounds.     

First-principles study of crystalline mono-amino-2,4,6-trinitrobenzene, 1,3-diamino-2,4,6-trinitrobenzene, and 1,3,5-triamino-2,4,6-trinitrobenzene

The electronic structure and thermodynamic properties of crystalline mono-amino-2,4,6-trinitrobenzene (MATB), 1,3-diamino-2,4,6-trinitrobenzene (DATB), and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) have been comparatively studied using density functional theory in the local density approximation. An analysis of electronic structure shows that the CNO₂ bonds in the three solids are easier to be broken in the thermal decomposition than the CNH₂ bonds. The calculated thermodynamic properties show that the order of their thermodynamic stability is TATB > DATB > MATB and their decomposition reactions are favorable under high temperature. Finally, an attempt is made to correlate the impact sensitivity of the three solids with their band gap. The result shows that there is the relationship between the band gap and impact sensitivity.     

Chiral separations by host-guest complexation with cyclodextrin and crown ether in capillary zone electrophoresis

Summary Capillary zone electrophoresis using cyclodextrins and a chiral crown ether as buffer constituents was studied for the enantiomeric separation of drugs and amino acids. Based on results obtained from separation of racemic -amino acids both chiral selectors are compared with respect to resolution, efficiency and retention time. For (±)-Quinagolide effects of buffer composition and temperature are examined using -cyclodextrin as chiral agent. Optimum conditions were pH 2.5 at 30 mmol L^–1 -cyclodextrin. A linear dependence of retention on -cyclodextrin concentration allowed calculation of formation constants of the host-guest complexes. Buffer concentration and temperature also influence resolution. The application of a chiral crown ether to the separation of optical isomers in capillary zone electrophoresis is described for the first time. Chiral recognition of solutes depends on the formation of protonated alkyl amines and separation is attributed to the formation of diastereomeric host-guest complexes with different interactions for each enantiomer. The effects of crown ether concentration on resolution are presented.     

Chromatographic screening for drugs of abuse using capillary columns II. Peak identification on support coated open tubular columns for a series of central nervous system stimulant drugs

Summary A system based on Kovats' Retention Indices is described for the identification of CNS stimulant drugs recovered from body fluids. Use is made of the difference in retention indices found on polar and non-polar GLC columns (l values) and the effect of operating temperature is discussed.     

Chromatography with a two-dimensional column

Summary A chromatographic method using a flat, square column developed along one direction and then eluted along the perpendicular direction, using a second retention mechanism widely different from the first one is described. Such a method would offer a potential peak capacity of several thousands, at least ten times more than either conventional column chromatography or two-dimensional thin-layer chromatography. Equipment capable of satisfying the requirements derived from a theoretical study of the operation of a two-dimensional column is also described. Results are satisfactory.Presented at the 14th International Symposium on Chromatography London, September, 1982 as a plenary lecture.     

Liquid chromatographic separation of metal ions on several silica-bound chelating-agent stationary phases

Summary The separation of metal-ions on several new silica-bound azo-coupled chelating stationary phases is described. Retention on 2-methyl-8-quinolinol and N-(1-naphthyl)-ethylenediamine phases is in the order of known homogeneous-solution stability constants. Very short retention times were obtained on the 2-methyl-8-quinolinol phase compared to an 8-quinolinol phase. A reversed order of retention was observed on -diketone and dithizone stationary phases in tartrate mobile phase. Two different mechanisms of metal-column interaction appear to occur. Large pH and mobile-phase buffer concentration effects were exhibited, especially by the diketone and dithizone phases. An excellent separation of a six-metal mixture is demonstrated on a dithizone column.     

Mobile phase effects in reversed-phase chromatography VI. Thermodynamic models for retention and its dependence on mobile phase composition and temperature

Summary The physico-chemical framework is examined by comparing the predictions of three models for the combined effects of the composition of the hydroorganic mobile phase and the column temperature on the retention ofn-alkylbenzenes on hydrocarbonaceous bonded stationary phases. The well-mixed model leads to expressions for the dependence of retention on three factors which are equivalent to those derived previously from linear extrathermodynamic relationships. The diachoric model stems from the assumption that the mobile phase is microscopically heterogeneous and the displacement model is identical to the retention model most widely used in chromatography with polar sorbents and less polar solvents. Over limited ranges of mobile phase composition and temperature, each model does describe retention behavior. However, only the wellmixed model describes retention well over the entire range of mobile phase composition and temperature studied here. The success of the well-mixed model, and its limits, give insight into the role of the organic solvent in determining the magnitude of chromatographic retention on non-polar stationary phases with hydro-organic eluents.Dedicated to Professor S. R. Lipsky on the occasion of his 60th birthday.     

Influence of Modifier-Free Hydroxide Eluents on the Separation of Carbohydrates by Anion Exchange Chromatography

Eluents based on low hydroxide concentrations are favourable in terms of selectivity for the separation of selected carbohydrates with anion exchange chromatography (AEC) and pulsed amperometric detection. The order of retention of many carbohydrates such as sucrose can be changed simply by selecting the appropriate hydroxide concentration.Instable retention times and weak detection efficiency were observed for hydroxide concentrations below 30mM without modifier. The time dependence of this retention loss was studied by step-changes of the eluent concentration. Linear behaviour with time is observed for all carbohydrates investigated. The slope of the relative decrease of retention is dependent on column capacity and presumably on its carbonate contents.The decrease in retention can be effectively suppressed by using a scavenger column filled with a high-capacity, strong anion exchanger. The variation of retention times were lowered from up to 100% to typically less than 1% during 72h of operation for a 10mM NaOH eluent without any modifier.     

The pyrolysis of hexachloroethane

The thermal decomposition of hexachloroethane in the presence of chlorine has been studied over the temperature range 340-400°C and a pressure range of 0.5°2.5 atm. The rate of the unimolecular C? C bond spliting reaction can be described by the Arrhenius equation Comparison of these rate data with thermodynamic data suggests a combination rate for CCl₃ radicals which is consistent with earlier measurements.