A new approach to the prediction of RP-HPLC retention times in gradient elution from isocratic data

Summary A general chromatographic model has been set up starting from a set of equations based on the concept of the velocity of a solute along the column. The composition of the mobile phase is taken into account solely as a numerical factor entering into suitable equations and totally independent of the chemical-properties of the constituents. A few isocratic experimental runs are necessary as input data, and subsequently a small amount of computational effort is sufficient to make predictions of retention times under gradient elution conditions for solutes of whatever chemical structure. The prediction errors are dependent on the steepness of the linear gradient chosen but are, in any case, acceptably low.     

Comparing columns for gas chromatography with the two-parameter model for retention prediction

The retention times of selected compounds in temperature programmed gas chromatography were predicted using a two-parameter model, on the basis of thermodynamic data obtained from isothermal runs on seven capillary columns, primarily substituted with 5% diphenylsiloxane. The scope for using thermodynamic data obtained from isothermal runs on one column to optimize separation on a different column or a different instrument setup was investigated. Additionally, the predictive utility of thermodynamic data obtained using a DB-5 column that had been in use for three years was compared to that of a new column of the same model. It was found that satisfactory separation could be achieved on one capillary column or instrument setup on the basis of thermodynamic data obtained using a different column or instrument set-up.     

An approach to the prediction of retention times in liquid chromatography

The utility of Rekker's hydrophobic fragmental constant has been examined for optimization of reversed-phase mode liquid chromatographic separations. The chromatographic behavior of about 60 non-ionic compounds was measured in different acetonitrile/water mixtures and the logarithm of their capacity factors (log k) was correlated with their calculated hydrophobicities (log P). Linear relations were found in each case between log k and log P. The slope of the various lines was related to the percentage concentration of acetonitrile in the mobile phase. It was shown that, by using nine stand ard compounds and measuring their capacity factors in five eluents with different acetonitrile concentrations, the retention time could be predicted for 60 compounds. Calculation of the concentration of the organic modifier was also possible in a system of well coated octadecyl bonded packings with acetonitrile/water mixtures as eluent. Prediction of the capacity factor was accomplished to within 5% error.     

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

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

Monte-carlo simulation of gas chromatographic separation for the prediction of retention times and peak half widths

Summary A new method for prediction of gas chromatographic retention times and peak half widths is based on the renewal theory. The only requirements are the heats of vaporization of the compounds to be separated and one calibration measurement. With this data, retention times and peak half widths can be predicted for isothermal as well as temperature-programmed gas chromatography. For the separation of non-polar substances on non-polar stationary phases the prediction error for retention times is approx. 1–2%. First simulations of polar molecules and polar stationary phases indicate that this method is also applicable in these cases but some extension will be required.     

Extrapolation of isothermal retention data in gas chromatography for chiral recognition studies

Summary The calculation of capacity factors, k, from net retention times, t_R, and the corresponding dead times, t_M, at different temperatures suffers from the limited accuracy of the t_M values. If the temperature coefficient racy of the t_M values. If the temperature coefficient d ln k/d (1/T) only is required, it is sufficient to determine net retention times (t_R)_p at constant inlet pressure p_i for different temperatures, since the temperature dependence of (t_M)_p can be assumed as (t_M)_p=A·e^B/T, with B being approximately independent of the column inlet pressure and of the nature of the carrier gas. The extrapolation and interpolation of (t_R)_p may be either performed by linear regression or graphically with a nomogram for ln (t_R)_p versus 1/T. The resolution factor, , of two components, e.g. enantiomers which are resolved on a chiral stationary phase, can be treated in a similar way. Examples are given for the resolution of enantiomers of two non-proteinogenic amino acids on the new polysiloxane phase L-Chirasil-CPG.     

Observations on the effect of temperature on performance and stability of anion exchange columns in ion chromatography

Summary The influence of column temperature on elution behaviour of several ions on anion-exchange resin beds of the Dionex Ion Pac type was investigated. Iodate, bromate, bromide and iodide ions were separated on Ion Pac AS9SC column in the temperature range of 10°C–55°C, using NaHCO₃/Na₂CO₃ and NaHCO₃ eluents. Free energy, enthalpy and entropy changes for respective ion exchange reactions were calculated and compared with literature data for classical gel type exchangers. In most cases the enthalpy change was a function of temperature. The work at elevated temperatures caused progressing resin degradation i.e. loss of strongly basic groups, accompanied by formation of weakly basic groups and also some weakly acidic groups. Similar resin degradation was observed for Ion Pac AS5 column. Observations on the changes of the plate height with the retention factor and temperature lead to conclusion that in some circumstances longitudinal diffusion in the resin phase also contributes to the total plate height.     

Prediction of retention of O-ethyl,O-aryl and N-isopropyl phosphoroamidothioates in RP-HPLC from molecular structure parameters

Summary Multi-variable regression analysis between lnk_w, c (in retention equation lnk=lnk_w+cC_b) and molecular structure parameters, including hydrophobicity, electric effect, field effect and position-specific effect constant, was carried out with O-ethyl, O-aryl and N-isopropyl phosphoroamidothioates as test solutes. With these quantitative relationships, the retention behaviour of these solutes for different mobile phase compositions was predicted. The results showed that there are only 26.7% of total, experimentally measured, capacity factors with relative deviations larger than 5% and only 2.2% with deviations larger than 10%, which means that it is possible to apply the method reported to predict retention values for qualitative purposes for different mobile phase compositions.     

A new procedure, based on combustion to sulphate and ion chromatography for the analysis of elemental sulphur in sediments

Summary A procedure exploiting the bomb preparation method for the analysis of total sulphur in organic materials has been developed for the determination of zero-valent sulphur (S^o) in sediments. Sediment samples were dried over magnesium oxide and sonicated in the presence of toluene to extract S^o. Toluene solutions were directly oxidized by combustion in a bomb containing sodium carbonate-bicarbonate buffer, which trapped sulphur species generated from S^o as sulphate ions. These were in turn analysed by suppressed anion chromatography with ion-conductivity detection. The method was used to determine the distribution of S^o in saltmarsh sediment cores.     

A method for characterization and optimization of reversed-phase liquid chromatographic separations based on the retention behaviour in homologous series

Summary A method based on the use of the members of homologous series as the calibration standards is suggested for prediction of retention in reversed-phase liquid chromatography. The method respects the dependence of retention on the composition of the mobile phase and makes possible distinguishing between the contributions of non-specific (hydrophobic) and specific (polar) interactions to the retention. The selection of calibration homologous series is described, the method is verified for a number of compounds and compared to the calculations based on the model of interaction indexes. The method suggested can be used for prediction of retention in mobile phases with different compositions and for the optimization of separation conditions. Presented at the 15th International Symposium on Chromatography, Nürnberg, October 1984     

Preparation and chromatographic performance of polymer-based anion exchangers for ion chromatography: A review

In the last decade the developments in the field of ion chromatography (IC) were aimed at increasing the efficiency, sensitivity and rapidity of analysis, as well as on improving separation selectivity. Since selectivity and efficiency to the large extent depend on the surface chemistry of the stationary phase, the development of novel anion exchangers remains one of the priority tasks in modern IC. The exact chemistry of commercially available resins is not known and not many literature data devoted to the procedures of preparing anion exchangers for IC have become available in the last 10–15 years. However, the knowledge about the surface chemistry of anion exchangers can provide understanding of the trends in selectivity and efficiency changes, as well as help with the choice of the stationary phase type suitable for solving a particular analytical task. The current review is devoted to the methods of preparing anion exchangers based on polystyrene-divinylbenzene (PS-DVB) and ethylvinylbenzene-divinylbenzene (EVB-DVB) for IC of inorganic and small organic anions and is aimed at demonstrating the improvement of their performance over the years, which was brought by the development of the new types of stationary phase architecture.     

A new tool for inorganic nitrogen speciation study: simultaneous determination of ammonium ion, nitrite and nitrate by ion chromatography with post-column ammonium derivatization by Nessler reagent and diode-array detection in rain water samples

The paper presents a new method for a simultaneous determination of inorganic nitrogen species in the oxidized (NO₂⁻, NO₃⁻) and reduced (NH₄⁺) form in rain water samples. The method is based on a system of nitrogen species separation employing ion exchange and diode-array detection. The ions are separated in a strong ion-exchanger, nitrites and nitrates are determined directly at 208 and 205 nm, respectively, while the ammonium ions are determined in the column hold-up time after a post-column derivatization by the Nessler reagent, at 425 nm. The use of a diode-array detector permits a simultaneous identification of the inorganic nitrogen species in 8 min. The detection limits obtained are: NO₂⁻, 0.1 mg L⁻¹; NO₃⁻, 0.05 mg L⁻¹; NH₄⁺, 1 mg L⁻¹. The method proposed has been successfully used for speciation analysis of inorganic nitrogen in precipitation.     

Application of ion chromatography to qualitative and quantitative determination of the main inorganic ionic components of samples from a production process of potassium sulphate

This review presents the possibility to use ion chromatography (IC) to analyse solids and brines deriving from treatment procedures of potassium salts. Qualitative and quantitative determination of ions samples has been used to make their chemical reconstruction. By using conventional calibration methods covering the whole concentration interval, analysis were not suitably accurate. During this process, the analysis is considered acceptable when the difference between the sum of the equivalents of cations and anions (ΔE) is ≤2%. For the process, a calibration method has been used employing four reference curves; each defined by three points corresponding to three standards purposely devised in our laboratory. The composition of the standards matched that expected for the samples from the different stages of the process reaction, after their usual sampling, dissolution and dilution. By comparing the conductivity of the analyte in the real sample to the conductivity of the same analyte in a standard, the effects of other analytes present in the medium are practically cancelled and need not be considered. The IC method results are in good agreement with conventional analysis methods (potentiometric titration, atomic absorption, gravimetric analysis).     

Comparison of void volume, mobile phase volume and accessible volume determined from retention data for oligomers in reversed-phase liquid chromatographic systems

The experimental technique of mass spectrometric tracer pulse chromatography was used to determine the void volume, i.e., the total volume of eluent in the column, and the volume of eluent moving freely through the column, i.e., mobile phase volume, for a series of eluents with a C(18)-bonded RPLC column. The interstitial volume of the column was determined by size exclusion chromatography. In order to evaluate the utility of the accessible volumes determined from the retention volumes of homologous solutes, the accessible volume of the column was determined as a function of eluent composition and temperature with polystyrene and polyethylene glycol samples using Martin's Rule. Comparison of these four measured volumes indicated that the experimentally measured accessible volumes did not correspond to either the void volumes, mobile phase volumes or interstitial volumes.     

Nonlinear variation of sorption parameters of n-alkane homologsl in temperature-programmed gas chromatography (tpgc) and new equation for calculation of retention indixces

It has been demonstated that the considaerable difference in temperature increments of sorption parameters of n-alkanes under isothermal conditions is the main reason for nonlinear dependence of sorption parameters on molecular mass of homolog in temperaturre programmed gas chromatography (TPGC). A new nonlinear 4th parameter equation has been given for calculation of the retention indices. Coefficients of the equation are calculated from n-alkanes. The equation allows extrapolation and interpolation calculations of retention indices under TPGC conditions with experimental precision. The results obtained; for fatty acidkl methyl esters demonstrate the advantage of ovr equation in comparison with van den Dool and Kratz's equation.     

Use of a database of plots of pesticide retention (R F) against mobile-phase composition.Part II. TLC as a pilot technique for transferring retention data to HPLC, and use of the data for preliminary fractionation of a mixture of pesticides by micropreparative column chromatography

Summary Relationships betweenR _F values and mobile-phase composition have been determined for moderately polar pesticides in normal-phase systems (NP) of the type silica-non-polar diluent (heptane)-polar modifier (ethyl acetate, tetrahydrofuran, or dioxane) and in reversed-phase systems (RP) of the type octadecyl silica-water-polar modifier (acetonitrile, methanol, or tetrahydrofuran). These relationships constitute a retention database which has enabled choice of the optimum conditions for preparative column chromatographic separation of pesticides into fractions; these were then applied to a silica plate and chromatographed. The plate was videoscanned, furnishing a real picture of the plate showing complete separation of the pesticide fractions.     

Prediction of HPLC retention times in gradient elution from isocratic data application to a set of non-homologous compounds and different stationary phases

Summary Polymethyloctylsiloxane-coated stationary phases have been prepared for liquid chromatography, by thermal reaction. The influence of the reaction conditions on retention and efficiency of test substances with different structures has been discussed. The materials have good stability in both acidic and basic eluents.     

Convex hull: A new method to determine the separation space used and to optimize operating conditions for comprehensive two-dimensional gas chromatography

It is important to develop methods of optimizing the selection of column sets and operating conditions for comprehensive two-dimensional gas chromatography. A new method for the calculation of the percentage of separation space used was developed using Delaunay's triangulation algorithms (convex hull). This approach was compared with an existing method and showed better precision and accuracy. It was successfully applied to the selection of the most convenient column set and the geometrical parameters of second column for the analysis of 49 target compounds in wastewater.     

Synthesis, self-assembling properties, and atom transfer radical polymerization of an alkylated L-phenylalanine-derived monomeric organogel from silica: a new approach to prepare packing materials for high-performance liquid chromatography

The monomer N'-octadecyl-N(alpha)-(4-vinyl)-benzoyl-L-phenylalanineamide (4) based on L-phenylalanine has been simply but effectively synthesized, and its self-assembling properties have been investigated. FTIR and a variable-temperature (1)H NMR spectroscopic investigation demonstrated that the aggregation of compound 4 in various organic solvents is due to the formation of intermolecular hydrogen bonds among the amide moieties. UV/Vis measurements indicated that the multiple pi-pi interactions of the phenyl groups also contribute to the self-assembly. As was observed by (13)C cross-polarization magic-angle spinning (CP/MAS) NMR and variable-temperature (1)H NMR measurements, the ordered alkyl chains also played an important role in the molecular aggregation by van der Waals interactions. Compound 4 was polymerized by surface-initiated atom transfer radical polymerization from porous silica gel to prepare a packing material for HPLC. The results of thermogravimetric analysis showed that a relatively large amount of polymer was grafted onto the silica surface. The organic phase on silica was in a noncrystalline solid form in which the long alkyl chain exists in a less-ordered gauche conformation. Analysis of chromatographic performance for polyaromatic hydrocarbon samples showed higher selectivity than conventional reversed-phase HPLC packing materials.