Sylvie Héron

General view of molecular interaction mechanisms in reversed-phase liquid chromatography

Abstract The compilation of the results obtained during the last 10 years on the retention behaviour of homologous series in reversed-phase liquid chromatography makes it possible to build a coherent model of molecular interactions for this type of sample. This model takes into account the effects of the chain length and the polarity of the sample molecule. It rationalizes the influence of the bonding density and the function nature (monomeric/polymeric) of the stationary phase and the role of temperature and the nature of the organic modifier in partially aqueous and non-aqueous reversed-phase liquid chromatography. The use of homologous series to investigate retention mechanisms gives the opportunity to distinguish between the solvophobic and the partition theories. Depending on the experimental conditions, both theories can explain the results observed and the changes in retention pattern when altering one of the chromatographic parameters previously mentioned. The description of the structure of the stationary phase as a brush, a picket fence or a stack is also integrated in the model. The experimental results accumulated with homologous series also suggest that the molecular interaction mechanisms governed by molecular recognition sites differ for molecules that are members of different homologous series, polycyclic aromatic hydrocarbons or carotenes.

Comparison of the responses of triacylglycerols with an evaporative visible light scattering detector used in conventional, micro and capillary liquid chromatography☆

Abstract During recent years, there has been a growing interest in the development of packed nano- and microchromatographies. It needs coupling the systems used to detectors adapted for microchromatographies. In the reported results, the properties of two different nebulizers of an evaporative visible light scattering detector have been studied under different flow-rate conditions (conventional HPLC, micro-LC and capillary LC). In micro-LC and capillary LC, it is shown that the calibration curves gained for triacylglycerols analyzed in non-aqueous reversed-phase liquid chromatography could be statistically assimilated to a linear relationship even if they rigorously follow a well-known power relationship such as area=a·massb. This makes the establishment of the calibration curves easier in miniaturized chromatographies than in conventional ones. Depending on the chromatography used, the linear range falls between either 2–50 ng or 14–80 ng. The limit of quantification was 5 ng (S/N=10). On the other hand, a study of the response of modified nebulizer versus flow-rate has shown that the modified evaporative visible light scattering detector is more complex than detectors whose response is really proportional to the concentration or to the mass.

Assessment of Two-Dimensional Separative Systems Using Nearest-Neighbor Distances Approach. Part 1: Orthogonality Aspects

We propose here a new approach to the evaluation of two-dimensional and, more generally, multidimensional separations based on topological methods. We consider the apex plot as a graph, which could further be treated using a topological tool: the measure of distances between the nearest neighbors (NND). Orthogonality can be thus defined as the quality of peak dispersion in normalized separation space, which is characterized by two factors describing the population of distances between nearest neighbors: the lengths (di(o)) of distances and the degree of similarity of all lengths. Orthogonality grows with the increase of both factors. The NND values were used to calculate a number of new descriptors. They inform about the extent of peak distribution, like the arithmetic mean (A̅(o)) of NNDs, as well as about the homogeneity of peak distribution, like the geometric mean (G̅(o)) and the harmonic mean (H̅(o)). Our new, NND-based approach was compared with another recently published method of orthogonality evaluation: the fractal dimensionality (DF). The comparison shows that the geometric mean (G̅(o)) is the descriptor behaving in the most similar way to dimensionality (DF) and the harmonic mean (H̅(o)) displays superior sensitivity to the shortest, critical distances between peaks. The latter descriptor (H̅(o)) can be considered as sufficient to describe the degree of orthogonality based on NND. The method developed is precise, simple, easy to implement, and possible to use for the description of separations in a true or virtual system of any number of dimensions.

Separation of 9,10-anthraquinone derivatives: evaluation of C18 stationary phases.

Chromatographic study of natural products helps to determine their molecular composition and to identify their sources (biological, geographical, etc.). However, identifying anthraquinoids is still a challenge because this chemical family is composed of more than half a thousand molecules. In the present work, a series of C18 stationary phases were systematically studied to evaluate their separation potential for these compounds in high performance reversed phase liquid chromatography (RPLC). The stationary phases with various physicochemical properties were evaluated with respect to the separation of 30 standards. Selectivity of the studied stationary phases and symmetry of the obtained peaks were chosen as the evaluation criteria. Calculations of symmetry and separation capacity parameters have been proposed. These parameters are not dependent on the dimensions of the columns and were used for the classifications of phases. In conclusion, this study is intended to be a guide as to which stationary phases analysts can use for resolving separation of a complex mixture of anthraquinoids.

Assessment of Two-Dimensional Separative Systems Using the Nearest Neighbor Distances Approach. Part 2: Separation Quality Aspects

In this paper we propose a method for the evaluation of real separation quality in multidimensional separations based on the nearest neighbor distances (NND). This approach allows us to overcome the principal drawback of the orthogonality measurement which does not evaluate how good the real separation obtained with one system is, especially when compared to another one. Separation quality evaluation takes into account the distances (di(s)) between peaks in whole separation space. The distances between nearest neighbors were calculated in resolution scaled analysis space to overcome statistically different peak widths in each dimension. The obtained separation quality is ranked by harmonic mean (H̅(s)) of the distances di(s). The extent of peak spreading, described by arithmetic mean (A̅(s)), gives an appreciation of the effective analysis space of 2D separation. The classifications of systems obtained with the same retention data using separation quality and orthogonality approaches show important differences in ranking orders depending on two different targets of these evaluations: the separation potential of a 2D system and the divergence of selectivity between both separation directions. This study shows separation quality and orthogonality merit to be evaluated in parallel for the same systems. The other new threshold descriptor, minimal limit distance (dilim) derived from resolution dependent peak capacity scale, was used to predict the separation quality as a function of desired resolution. We also introduce here a composed descriptor for 2D systems: the optimality coefficient (Oc), which may be useful in the 2D separation optimization process. It takes into account the maximization of homogeneity of peak spreading (H̅(s)/A̅(s)) and the minimization of effective analysis space (or compactness, dilim/A̅(s)) terms.

Comparison of iso-eluotropic mobile phases at different temperatures for the separation of triacylglycerols in Non-Aqueous Reversed Phase Liquid Chromatography

Triacylglycerols (TAGs) are a large class of neutral lipids that naturally occur in both plant and animal oils and fats. Their analyses in Non-Aqueous Reversed Phase Liquid Chromatography (NARP) require a mixture of weak solvent (mostly acetonitrile) and strong solvent. In the present work, we have established eluotropic solvent strength scale of several binary mobile phases on C18 bonded silica at different temperatures (acetonitrile/methylene chloride, acetonitrile/acetone, acetonitrile/ethyl acetate, acetonitrile/propan-2-ol, and acetonitrile/butan-1-ol at 25°C, 43°C, 63°C and 85°C); it is based on the methylene selectivity and the use of homologous series. We show that this scale is well suited to the TAGs analysis. The analysis of nine seed oils (Aleurites fordii, Calophyllum inophyllum, Glycina max, Olea europea, Orbignya olifeira, Pinus koraiensis, Pistacia lentiscus, Punica granatum and Ribes nigrum) in iso-eluotropic conditions leads to propose unambiguously the couple MeCN/BuOH at 25°C as the best system to separate TAGs. The use of butanol, as strong solvent, provides very good TAGs congeners separations and avoids the use of chlorinated solvents which gave to this day the best separations.

Determination of Triacylglycerol Regioisomers using Electrospray Ionization-Quadrupole Ion Trap Mass Spectrometry with a Kinetic Method

The kinetic method was applied to differentiate and quantify mixtures of regioisomeric triacylglycerols (TAGs) by generating and mass selecting alkali ion bound metal dimeric clusters with a TAG chosen as reference (ref) and examining their competitive dissociations in a quadrupole ion trap mass spectrometer. This methodology readily distinguished pairs of regioisomers (AAB/ABA) such as LLO/LOL, OOP/OPO and SSP/SPS and consequently distinguished sn-1/sn-3, sn-2 substituents on the glycerol backbone. The dimeric complex ions [ref, Li, TAG((AAB and/or ABA))](+) generated by electrospray ionization mass spectrometry were subjected to collision induced dissociation causing competitive loss of either the neutral TAG reference (ref) leading to [Li(AAB and/or ABA)](+) or the neutral TAG molecule (TAG((AAB and/or ABA))) leading to [ref, Li](+). The ratio of the two competitive dissociation rates, defined by the product ion branching ratio (R(iso)), was related via the kinetic method to the regioisomeric composition of the investigated TAG mixture. In this work, a linear correlation was established between composition of the mixture of each TAG regioisomer and the logarithm of the branching ratio for competitive fragmentation. Depending on the availability of at least one TAG regioisomer as standard, the kinetic method and the standard additions method led to the quantitative analysis of natural TAG mixtures.

A new methodology to determine the isoeluotropic conditions on ultra-performance flash purification stationary phases from analytical reversed liquid chromatography stationary phase

Nowadays, the determination of the experimental chromatographic conditions to be used in Reversed Phase Liquid Ultra-Performance Flash Purification is still challenging. This is due to four different items. In most cases, flash purification stationary phases are not available with geometry of column used in analytical chromatography. The flash purification columns are single-use only. From the point of view of selectivity and retention, few RPLC phases exist with properties of separation identical for analytical and flash purification supports. Characterization methods and databases used for comparing analytical RPLC columns do not include stationary phases for RP flash purification columns. The goal of this work is to develop a new method development strategy which permits the determination of the experimental chromatographic conditions on RP ultra-performance flash purification columns. It relies on the knowledge of any isocratic conditions obtained on any given initial reversed stationary phase. The final conditions to implement on the RP ultra-performance flash purification phase enable either to keep the retention range of a selected solute constant, or to set it around a previously chosen value. The rules of transfer in linear gradient mode are also described. The methodology was valid, whatever the initial RP stationary and mobile phases, for different chemical classes, whatever the bonding, particle diameter, porous or core shell particle, towards different RP alkyl and analogues stationary and mobile phases.