Leo de Galan

Influence of organic modifiers on the rentention behaviour in reversed-phase liquid chromatography and its consequences for gradient elution

Abstract In a previous paper it was shown that the key to the description and understanding of retention behaviour under gradient conditions in reversed-phase liquid chromatography lies in the relationship between the isocratic capacity factor ( K ) and the volume fraciton of organic modifier (ϕ). Hence, the presentation of extensive data on this relationship in different organic modifier systems offers possibilities for evaluating and refining existing gradient elution models, such as the one of Snyder and co-workers. An extensive study of 32 aromatic compounds in aqueous system containing 10–100% of methanol, acetonitrile and tetrahydrofuran as organic modifiers under isocratic conditions is described. Two major conclusions are drawn that have consequences for gradient elution. Firstly, the relationship between in k and (ϕ) is generally non-linear, in accordance with previous statements. Secondly, the rate of change of in k and (ϕ) can be solute dependent. Systematic changes in the slope of the in k versus (ϕ) curves with absolute retention are observed for methanol and tetrahydro- furan as organic modifiers. It is shown that this conclusion leads to optimal gradients, which are convex rather than linear.

Preparation and Chromatographic Properties of Some Chemically Bonded Phases for Reversed-Phase Liquid Chromatography

Abstract Four different monomeric chemically bonded phases suitable for reversed-phase liquid chromatography have been prepared. Duplicate preparations, elemental analysis, post-treatment with TMS and chromatographic properties all indicate that maximum coverage has been obtained. The chromatographic activity of the silica surface is discussed on the basis of TMS-phases with various surface coverages. It is observed that logarithmic retention data for homologous series and different RP-phases-partially taken from the literature—form a set of straight lines with a common intersection point.

Role of the chain length of chemically bonded phases and the retention mechanism in reversed-phase liquid chromatography

The influence of the chain length of n-alkyldimethylsilyl bonded phases (RP-1 to RP-22) on retention has been investigated using methanol-water mixtures as the mobile phase. It was found that the capacity factor increases exponentially up to a certain chain length, after which it remains constant. For a given solute the onset of retention, denoted as the “critical chain length”, is independent of the mobile phase composition. It increases with the size of the solute molecule from a minimum chain length of 6 up to about 14 carbon atoms. Up to the critical chain length the selectivity increases slightly with increasing chain length. However, the influence of the stationary phase on retention and selectivity is far outweighed by the influence of the mobile phase. The role of the chemically bonded stationary phase in the separation process is proposed as a “compulsory absorption mechanism”. Only a limited part of the bonded chains contributes to the retention mechanism.

Systematic study of ternary solvent behaviour in reversed-phase liquid chromatography☆

Abstract An extensive experimental survey of the retention behaviour of 32 solutes in the system methanol-tetrahydrofuran-water and 49 solutes in the system methanol-acetonitrile-water is presented. The retention data are fitted to a second order six-parameter equation, describing the capacity factor as a function of mobile phase composition. Iso-eluotropic lines, i.e., lines that connect solvents of equal eluotropic strength, are constructed in the ternary diagrams for the two systems and compared with theoretical lines, predicted from solubility parameter theory. Specific effects are defined as variations in retention for a particular solute, using solvents of equal eluotropic strength. Such effects appear to be larger between different binary mixtures than within the ternary triangle. Ternary solvents thus provide a smooth transition between two limiting binary mixtures.

Use of gradient elution for rapid selection of isocratic conditions in reversed-phase high-performance liquid chormatography

Abstract It is shown that an initial scan of an unknown sample with a gradient elution programme offers a very rapid and easy procedure for deciding whether the sample is suitable for isocratic elution and, if so, for establishing the appropriate binary composition for isocratic separation. The resulting chromatogram is a good first-order approximation for the best possible separation in the give binary mobile phase system. In this way the selection of optimal binary compositions ca be greatly accelerated and simplified. It is shown that the transfer from one organic modifier to another can readily be mad using simple trasfer rules. In this way the selectivity can be varied, while the retention remains approximately constant.

Preparation of Various Bonded Phases for HPLC Using Monochlorosilanes

Abstract Monochlorosilanes have been prepared with yields of about 80% through a catalytic hydrosilylation of terminal olefins. Subsequently, the silanes are chemically bonded to silica to obtain: (i) n-octyldimethylsilyl bonded phases with reproducible surface coverage ranging from 0.8 to 3.5 μmol/m2, (ii) propyldimethylsilyl bonded phases with different functional groups at the γ-position, all showing a nearly equal surface coverage of some 3.3 μmol/m2, and (iii) n-alkyldimethylsilyl bonded phases with chainlengths ranging from 1 to 22 carbon atoms and with surface coverages ranging from 3.9 μmol/m2 for RP-1 to 3.0 μmol/m2 for the RP-22 bonded phase. A simplified model based on the pore structure of silica allows an explanation and estimation of the maximum surface coverage as a function of the chainlength of the bonded phase.

On the Determination of the Hold-Up Time in Reversed Phase Liquid Chromatography

Abstract The determination of the hold-up time in reversed phase liquid chromatography has been studied extensively for the mobile phase system methanol-water. Hold-up times obtained by static methods, linearization of homologous series and so-called “unretained compounds” are discussed and mutually compared. Several n-alkyldimethylsilyl bonded phases have been used for this investigation. A rough estimate of the hold-up time can be obtained by using components of the mobile phase or highly concentrated salt solutions, but only for mobile phase compositions around 60% (v/v) methanol. Hold-up times accurate to 1% can be obtained over the complete range of mobile phase compositions from the linearization of net retention times of homologous series.

Description of solute retention over the full range of mobile phase compositions in reversed-phase liquid chromatography

Abstract In this paper we investigate the relationship between solute retention and mobile phase composition in reversed-phase liquid chromatography over the entire range of composition, with emphasis on mobile phases with a high water content. It will be shown that a quadratic relationship between the logarithm of the capacity factor and the volume fraction of organic modifier is generally valid for mobile phases containing less than 90% water. When more water is added to the mobile phase, a quadratic equation turns out to be insufficient. An experimental study of ten solutes and three organic modifiers is used to show that an extension of the quadratic equation by a term proportional to the square root of the volume fraction leads to a description of all experimental retention data within approximately 10%.

Retention and selectivity characteristics of a non-polar perfluorinated stationary phase for liquid chromatography

Abstract An experimental study has been made of the chromatographic characteristics of a perfluorinated chemically bonded stationary phase for reversed-phase liquid chromatography. It is demonstrated from the retention data that the perfluorinated material behaves as a less polar stationary phase than an octadecyl (RP-18) material. The gain in selectivity, however, is obscured by large specific effects towards both fluorinated solutes and certain classes of compounds not containing fluorine atoms. The specific fluorine-fluorine interaction can also occur in the mobile phase, when 2,2,2-trifluoroethanol is used as an organic modifier.

Identification of intermediates leading to chloroform and C-4 diacids in the chlorination of humic acid

The chlorination of terrestrial humic acid was studied at pH 7. 2 with varying chlorine to carbon ratios. The principal products are chloroform, di- and trichloroacetic acid, and chlorinated C-4 diacids. At a high chlorine dose many new chlorination products were detected, among them chlorinated aromatic acids. At a low chlorine dose a class of chlorinated compounds was found, which contained a trichloromethyl group. These compounds may be converted into chloroform and in most eases C-4 diacids by oxidation and hydrolysis reactions. Because these compounds are found mainly at low chlorine dosage, they may be regarded as intermediates in the reactions that give chloroform. The intermediates support the hypothesis of Rook that m-dihydroxybenzene moieties in humic acid are responsible for the formation of chloroform. A reaction scheme is proposed that explains the formation of the intermediates.