Influence of silica gel pretreatment and bonding technique on PAH selectivity of octadecyl bonded phases

Summary The effect of silica gel pretreatment and of different bonding techniques, on the chromatographic properties of octadecyl bonded phases is described. The experiments show that a monomeric octadecyl bonded phase, prepared from silica gel that has not been treated before bonding, has a poor selectivity towards polycyclic aromatic hydrocarbons (PAHs) and shows high adsorption of basic components. Silica gel treatment before the bonding of the monofunctional silane does not give a substantial improvement. Polymeric phases on the other hand show a much better PAH selectivity and a lower adsorptivity towards basic components. Further improvement can be achieved with the polymeric modification if silica gel treatment is carried out before bonding.     

A comparative study of hydrophobicity of octadecyl and alkylamide bonded phases based on methylene selectivity

The conformational dynamics of home‐made monomeric and polymeric (octadecyl – C18 and alkylamide – AP) stationary phases have been investigated by analyzing the dependence of retention on the reciprocal temperature for different organic compounds. Theoretical considerations were based on the results of methylene selectivity determination. Two selected PAHs – naphthacene and benzo[a]anthracene, whose structures differ in the position of the ring – were chosen as solutes to study the thermodynamics of retention behavior. Standard enthalpies (ΔH ⁰) were obtained from the slope of the plots of the capacity factor logarithm (ln k) against the reciprocal temperature (1/T), i. e. van't Hoff plots, in a temperature range from 285 to 315 K. The conformation of the stationary phase (monomeric and polymeric) structure and the length of bonded ligands with their specific functional (N‐acylamide) groups had a significant effect on retention.     

Comparison of C18 silica bonded phases selectivity in micellar liquid chromatography

The paper describes a new test designed in micellar LC (MLC) to compare the commercial C18 stationary phase properties. This test provides the total hydrophobicity, hydrophilicity, steric selectivity, hydrogen bonding, and ion‐exchange capacity properties calculation of the ODS stationary phases. Both the test compounds and chromatographic separation conditions choice for column characterization in MLC are detailed. The chromatographic performance of several stationary phases that are used in MLC was evaluated with specific chromatographic test comprising nine test compounds, possessing different physico‐chemical properties, which were injected on different supports with two micellar mobile phases: one at pH 7.0 (0.075 mol/L SDS and 1.5% v/v 1‐pentanol), and other at pH 2.7 (0.075 mol/L SDS and 1.5% v/v 1‐pentanol adjusted to pH by TFA). Fundamental column chromatographic properties were obtained under these conditions and were treated by hierarchical cluster analysis. From the results of cluster analysis, two closely related groups of columns are distinguished, and it was shown that the chosen column characteristic parameters allow characterizing both sorbent and micellar chromatographic system properties. Eleven columns were analyzed by this test, which allows a comparison of columns with the aim of the selection of suitable and analogous column for the analysis with MLC.     

Syntheses of spherical silica and titania from alkoxides on a laboratory scale

Summary A process to synthesize silica and titania as spherical packing materials has been investigated on the laboratory scale by the sol-gel method. The silica and titania obtained were tested under normal-phase separation conditions for comparison of their retention characteristics silica witha a commercial. The silica was found to be similar in its retention behaviour to the commercial silica. The titania showed basic properties and strongly retained acidic compounds.     

Optimization of enantiomeric separations with chiral bonded phases

Summary The preparation and properties of chiral bonded phases with amino acid groups are described. These phases were used in non-polar eluents and in aqueous systems in the presence of Cu²⁺ ions in the ligand-exchange mode. The optimization of chiral resolution is demonstrated for both cases. With non-polar eluents similarity between bonded groups and solutes is required. The elution could be accelerated by non-protonic moderators. Ligand-exchange separation is influenced by the copper content of the eluent and the stationary phase, the organic moderator concentration, the pH and ionic strength of the buffer and by the temperature. Structural requirements of both the bonded groups and of the solutes for chiral separation are discussed.     

Liquid chromatographic behaviour of pyrimidine bases on various bonded phases

Summary The effect of the surface chemistry of bonded-phase column material containing alkyl, phenyl, cyano and amino groups on the adsorption of pyrimidine molecules has been studied by HPLC. The dependence of the retention on the chemical structure of the adsorbed molecules was determined. The influence of the substituents at the pyrimidine ring and their position on the retention character is shown for various bonded phases.     

Preparation of octadecyl modified column gels using heat-treated silicas and their retention behaviour in high-performance liquid chromatography

Summary The effect of silanol groups on three types of octadecylmodified column gels using heat-treated silicas by calcination has been studied by high-performance liquid chromatography. After heat-treating at 180°C, 500°C and 950°C, the silicas treated with octadecyldimethylchlorosilane were used for the measurement of physical and chemical analysis. From elemental carbon analysis data, the reactive silanol group concentrations, _OH(s), were determined to be 2.0 in the 180°C treated silica, 2.1 in the 500°C treated silica and 1.6 in the 950°C treated silica, respectively (original silica: mean pore diameter 116 Å, specific surface area 298 m²/g, pore volume 1.22 ml/g, particle size 5.0 m). The separation factors, , of pyridine versus phenol were measured to be 0.79 on 180°C treated silica, 0.91 on 500°C treated silica and 1.98 on 950°C treated silica, using acetonitrile-water mixtures as the eluent. And then, on the basis of the physico-chemical and chromatographic data, the three types of octadecyl modified column gels using heat-treated silicas by calcination have been compared.     

Comparison and characterization of reversed phases

Summary A pragmatic test procedure for comparison and evaluation of reversed phases is described. The differences in retention between RP8 and RP18 can be compensated for by adjustment of the eluent composition. A 10% increase in the water content doubles the k values, as does the exchange of an RP8 with an RP18 column. It is possible to differentiate between the two columns with the pair ethylbenzoate and toluene. Under stand-ard conditions (methanol-water, 55–45, v/v; 49–51, w/w) with RP8 ethylbenzoate is always eluted together or after toluene, whereas with RP18 it is always eluted in front of toluene.With the same eluent composition the suitability of stationary phases for the separation of basic solutes can be evaluated. With good phases—symmetrical peaks for basic solutes—aniline is always eluted before phenol and the peak asymmetry relationship of the aniline and phenol peak is less than 1.3. With such good stationary phases the retention of bases is independent of sample size if the linear capacity of 0.1 mg sample/g stationary phase is not exceeded. The test can also be used to study column stability towards hydrolysis.     

Synthesis of acceptor bonded phases for donor-acceptor liquid chromatography

Summary The synthesis of several electron-acceptor stationary phases has been reviewed the materials evaluated for their capacity to separate polyaromatic hydrocarbons (PAHs). Chemically bonded phases were obtained by the same procedure; the organosilane moiety is monofunctional and the HPLC behaviour is compared under identical conditions. Examples of PAHs separations are given.Presentedat the 13th International Symposium on Polynuclear Aromatic Hydrocarbons, Bordeaux, October 1–4, 1991.     

Chromatographic properties of mixed cyano/ODS chemically bonded phases for HPLC

Summary A number of different cyano/ODS polymeric modified silica gel phases were prepared by simultaneous bonding of cyanoalkylsilanes and octadecylsilane. Cyano contents between 10–80% were obtained. High surface coverage (above 4 μmole/m²), as well as low hydroxyl content, characterised all phases. The modified silica was suitable for separations in both straight and reversed phase mode. Significantly reduced retention times were obtained in the reversed phase mode for high mole-cular weight polyaromatic compounds (Mw above 300).     

Effect of temperature on the mechanism of retention of fullerenes in liquid chromatography using various alkyl bonded stationary phases

Summary The temperature-dependency of the separation of fullerenes in liquid chromatography (LC) has been examined using various alkyl bonded stationary phases. It has been found that a maximum retention temperature exists with long alkyl bonded stationary phases, whereas there is no similar effect with the newly synthesized alkyl bonded phases which have two phenyl groups at the base of the bonded phase. The interpretation of the retention behavior of fullerenes in the low temperature region on alkyl bonded stationary phases is discussed using information obtained by CP-MAS solid-state NMR spectroscopy and LC.     

Cholesteric bonded stationary phases for high-performance liquid chromatography: a comparative study of the chromatographic behavior

The properties of four cholesteric bonded stationary phases differing in the nature of the spacer and the end-capping were assessed using simple chromatographic tests based on the retention of nonpolar compounds and of planar or nonplanar probe solutes. All cholesteric columns showed a hydrophobicity close to that of conventional octadecyldimethylsilyl (ODS) materials. Non-end-capped cholesteric bonded phases showed greater selectivity than ODS ones and both end-capped cholesteric bonded phases exhibit behavior intermediate between that of the non-end-capped original material and that of the ODS bonded phase.     

Selectivity studies on branched and linear alkyl bonded stationary phases in reversed-phase liquid chromatography

Summary Reversed-phase liquid chromatographic retention characteristics for the sixteen acyclic C₁−C₅ N-alkylbenzamide congeners were measured on various branched and linear, alkyl bonded hydrocarbon stationary phases. Retention factors, k′, were determined in acetonitrile-water mobile phase compositions on ethyl, n-octyl, n-dodecyl, n-octadecyl, 1-ethyladamantyl, 4-butyloctyl, and 2,4,4-trimethylpentyl stationary phases. Statistical analysis of the two main effects investigated — type of stationary phase and percentage of organic modifier (acetonitrile) in the mobile phase — described greater than ninety percent of the variability in the data for most of the comparisons. Selectivity effects due to variation in the mobile phase dominated the results.     

Analysis of azines and diazines by HPLC on bonded alkyl stationary phases. Pyridine and quinoline in environmental samples

Summary Several heterocyclic compounds have been investigated by HPLC on RP-8 and RP-18 with methanol-water mixtures as mobile phase. The chromatographic systems developed showed good selectivity for determination of azines and diazines. Pyridine and quinoline have been determined in environmental samples and in food. Samples for analysis were preconcentrated by solid phase extraction on-column with octadecyl silane packing.     

Molecular shape recognition capability of liquid-crystal bonded phases in reversed-phase high performance liquid chromatography

Summary The chromatographic retention behaviour of two liquidcrystal bonded phases have been evaluated using polycyclic aromatic hydrocarbons (PAHs) as the probe samples in reversed-phase high performance liquid chromatography (RP-HPLC). The results clearly indicate that these phases have better planarity and shape recognition capabilities than commercially-avaialble polymeric octadecylsilica (ODS) phases whose strong planarity and shape selectivities were found earlier. It can also be concluded from the chromatographic observations that the shape recognition capability of these phases is dependent on both mobile phase composition and column temperature, but that the effect of mobile phase and temperature on the shape selectivity work independently. The retention behaviour can be explained by changes in the phase structure with changes of eluent composition and temperature.     

A comparison of temperature and mobile phase composition effects for bonded liquid crystal and polymeric stationary phases in HPLC

Summary Using two polycyclyic aromatic hydrocarbons as solutes, a comparison is made between a bonded liquid crystal stationary phase and a conventional polymeric C-18 phase. The bonded nematic liquid crystal phase was the silanized form of 4-[4-(allyloxy)benzoyl-oxy]biphenyl and the polymeric phase was Vydac 201TP. Both phases display shape and planarity selectivity as indicated by the results of the variable temperature and mobile phase composition studies. The slot theory of retention can be used to explain these results. However, the liquid crystal phase is more sensitive to molecular geometry, probably due to its more ordered structure on the surface. Variable temperature experiments which compare retention during both heating and cooling provides additional support for this conclusion. With the polymeric bonded C-18 phase, each solute had identical retention at the same temperature during both the heating and cooling cycles. On the bonded liquid crystal phase, measurable differences in retention were observed at identical temperatures depending on whether the column was heated or cooled. This effect is attributed to a degree of partially reversible disordering which occurs as the column temperature was increased. However, conditioning with the appropriate mobile phase can restore the original retention characteristics of the bonded liquid crystal phase.     

Synthesis and characterization of new organometallic complexes bonded stationary phases for high-performance liquid chromatography

Summary The preparation, characterization and potential liquid chromatographic applications of various organometallic iron complexes silica stationary phases are presented. These new supports are synthesized by covalently linking ferrocene, as well as some of its cationic derivatives, to appropriately derivatized silica support matrices. These columns exhibit moderate to high selectivity towards the separation of polycyclic aromatic hydrocarbons (PAHs). A charge transfer retention mechanism has been proposed. A comparison with a reference stationary phase, 3,5-dinitrobenzamide (DNB), to quantify the acceptor power of the new stationary bonded phases, is also reported. Finally, the effect of varying the derivatives of the bonded metallocene on PAHs retention is discussed.     

Comparison of octadecyl-bonded silica and styrene-divinylbenzene copolymer sorbents for trace enrichment purposes fundamental aspects II. Linear chromatographic behaviour of the extracting supports

Summary Two reversed-phase LC systems were investigated by frontal analysis for the determination of linear chromatographic conditions, as defined according to the isotherm concept. The Partisil ODS-3 bonded silica and the PRP-1 polystyrene-divinylbenzene resin were used as stationary phases together withtrans-2-hexen-al as test solute and methanol-water mixtures as mobile phases. Particular attention was paid to the respective influence of the two main parameters which may cause sorbent overloading, that is, the capacity factor (k′) and the solute concentration in the mobile phase (C_m). Provided that k′C_m≤10⁻² M, linear chromatographic behaviour was observed for both sorbents, the maximum capacities of which were found greater than 1mmolg⁻¹.     

The chromatographic quantification of hydrophobicity using micellar mobile phases

Summary In micellar liquid chromatography (MLC), the hydrophobicity of a compound is the predominant factor in its retention and interaction with micelles. A non-linear empirical model can describe the dependence between the retention factor (logk) in MLC and the logarithm of partition coefficients octanol-water (logP). An algorithm based on such a model has been used to makelogP predictions. Retention data for series of neutral compounds eluted with different mobile phases and alkyl-bonded stationary phases have been used to test the predictive ability of the algorithm. The results of this approach are compared with those obtained from automatic computational software packages.