Study of the Performances of Thin Layer Chromatography. V. Flow Rate in Reversed Phase Plates

The conventional equation for the development of TLC plates predicts a velocity constant proportional to the cosine of the wetting angle of the surface by the mobile phase. Because silica is wet by all organic solvents used in normal TLC, this dependence was never checked. The development of reversed-phase TLC plates using chemically bonded n-alkyl silica offers a new situation as water-rich solvent mixtures do not wet these surfaces completely. Comparison of velocity constants measured in TLC using plates made with various adsorbents and contact angles measured directly shows the validity of the conventional equation and the cos θdependence. As a consequence, it takes an impractically long time to develop TLC plates using water/alcohol mixtures containing more than ca. 25-40% water (v/v) depending on the adsorbent, which limits the applicability of reversed-phase TLC in practical analysi

Separation of Highly Complex Mixtures by Two-Dimension Liquid Chromatography

This report summarizes the progress made on the title project during the grant period. We developed a new classification of two-dimensional separations based on the observation that separations can be made in time or in space. Thus, two-dimensional separations can be made in time×time, space×space, space×time, or time×space. The two successive separations must use two different modes of chromatography that afford uncorrelated or weakly correlated patterns of retention factors for the components of the samples analyzed. Our attention was mainly focused on the separation of protein digests, particularly, on those of the digests of myoglobin and bovine serum albumin as model systems and extremely efficient temporal separations were developed. We also designed and constructed new instruments to carry out space×space separations (True Bidimensional Chromatography, HPLC2 or spacial separations) and time×space separations (a new hybrid combination of a temporal and a spacial separation that we designed)

SEPARATION OF HIGHLY COMPLEX MIXTURES By TWO-DIMENSION LIQUID CHROMATOGRAPHY Principal Investigator

ABSTRACT This report summarizes the progress made on the title project during the grant period. We developed a new classification of two-dimensional separations based on the observation that separations can be made in time or in space. Thus, two-dimensional separations can be made in time×time, space×space, space×time, or time×space. The two successive separations must use two different modes of chromatography that afford uncorrelated or weakly correlated patterns of retention factors for the components of the samples analyzed. Our attention was mainly focused on the separation of protein digests, particularly, on those of the digests of myoglobin and bovine serum albumin as model systems and extremely efficient temporal separations were developed. We also designed and constructed new instruments to carry out space×space separations (True Bidimensional Chromatography, HPLC 2 or spacial separations) and time×space separations (a new hybrid combination of a temporal and a spacial separation that we designed).

Final summarizing report on Grant DE-SC0001014 "Separation of Highly Complex Mixtures by Two-dimension Liquid Chromatography"

The goal of our research was a fundamental investigation of methods available for the coupling of two separate chromatographic separations that would considerably enhance the individual separation power of each of these two separations. This gain arises from the combination of two independent retention mechanisms, one of them separating the components that coelute on the other column, making possible the separation of many more compounds in a given time. The two separation mechanisms used must be very different. This is possible because many retention mechanisms are available, using different kinds of molecular interactions, hydrophobic or hydrophilic interactions, polar interactions, hydrogen bonding, complex formation, ionic interactions, steric exclusion. Two methods can be used, allowing separations to be performed in space (spreading the bands of sample components on a plate covered with stationary phase layer) or in time (eluting the sample components through a column and detecting the bands leaving the column). Both offer a wide variety of possible combinations and were studied

FINAL SUMMARIZING REPORT on Grant DE-SC0001014 "Separation of Highly Complex Mixtures by Two-dimension Liquid Chromatography"

The goal of our research was a fundamental investigation of methods available for the coupling of two separate chromatographic separations that would considerably enhance the individual separation power of each of these two separations. This gain arises from the combination of two independent retention mechanisms, one of them separating the components that coelute on the other column, making possible the separation of many more compounds in a given time. The two separation mechanisms used must be very different. This is possible because many retention mechanisms are available, using different kinds of molecular interactions, hydrophobic or hydrophilic interactions, polar interactions, hydrogen bonding, complex formation, ionic interactions, steric exclusion. Two methods can be used, allowing separations to be performed in space (spreading the bands of sample components on a plate covered with stationary phase layer) or in time (eluting the sample components through a column and detecting the bands leaving the column). Both offer a wide variety of possible combinations and were studied

High Precision Measurements in Gas Chromatography. Systematic Errors on the Determination of Retention Times

A comprehensive study of the sources of systematic errors in the measurements of retention times has been made. These sources include measurement errors, which are related to the problems of time measurements and are usually small and easy to correct and instrumental errors which are more difficult to correct and eventually limit the accuracy of the measurements. The main sources of systematic, instrumental errors are the retention of the inert peak (compounds which are easy to detect are not completely inert) and the effect of carrier gas volumes outside the column. These two factors are easy to correct in principle, but the random errors made in the determination of the parameters necessary to Calculate these corrections are the main limitations to the present accuracy of retention times measurements

N° 42. — Transformation des signaux finis dans une colonne chromatographique

Une théorie des colonnes échangeuses d’ions est déduite d’une théorie générale des processus chromatographiques publiée antérieurement.On montre d’abord quelles sont les propriétés générales de la propagation dans une colonne échangeuse d’ions, puis on applique les résultats théoriques obtenus à une élution frontale et à l’élution d’un pic. On constate un bon accord phénoménologique avec les résultats publiés dans la littérature.L’avantage de cette théorie est de permettre de calculer à priori les signaux cliromatographiques

N° 20. — Étude théorique et expérimentale de la cinétique de décomposition thermique du n-hexadécane, de son mécanisme et de la composition du mélange des produits obtenus

L’analyse qualitative et quantitative des produits de pyrolyse du n-hexadécane par chromatographie en phase gazeuse à température programmée permet une étude précise de la cinétique de cette réaction. Les variations de la vitesse de la réaction et de la composition des produits de pyrolyse avec la température et la pression initiale que l’on observe sont en excellent accord avec les résultats calculés sur la base des théories classiques de Rice-Hehzfeld-Kossiakoff si l’on suppose que tous les radicaux primaires sont des porteurs de chaîne efficaces et que les radicaux primaires lourds sont, par transfert interne 1-4 ou 1-5 d’atomes d’hydrogène, en équilibre thermodynamique constant avec les radicaux secondaires. Un excellent accord numérique est obtenu entre la théorie et l’expérience si l’on utilise comme valeurs numériques des constantes de vitesse les valeurs dérivées de celles mesurées par d’autres auteurs pour des réactions analogues subies par des radicaux légers

Heterogeneity of the Adsorption Mechanism of Low Molecular Weight Compounds in Reversed-Phase Liquid Chromatography

The retention mechanism in RPLC mode was investigated based on the acquisition of adsorption isotherm data by frontal analysis measurements and their modeling. This work is a review of the results of four years of adsorption data measurements. The data were acquired on a wide variety of brands of C18-silica columns (from Akzo Nobel, Bishoff, Hypersil, Merck, Phenomenex, Supelco, Vydac, and Waters) with several low molecular weight compounds such as phenol (94 g/mol), caffeine (194 g/mol), tryptophan (204 g/mol), sodium 2-naphthalenesulfonate (235 g/mol), and propranololium chloride (295 g/mol). The mobile phase was a mixture of methanol and water at variable composition. The adsorption isotherms were all convex upward (langmuirian), and the degree of heterogeneity of the adsorption system was determined from the calculation of the adsorption energy distribution using the expectation−maximization method. The adsorption isotherm parameters (number of types of adsorption sites, surface concentration of each type of site, and difference between the adsorption energies Ei − Ej on sites i and j), obtained from the mathematical fit of the adsorption data to the appropriate multi-Langmuir adsorption isotherm model, were analyzed and compared. The results allow the drawing of general conclusions regarding the relationships between the size of the analyte and the adsorption properties (saturation capacities, adsorption energies) characterizing the retention mechanism in RPLC mode for neutral, anionic, and cationic compounds

Analysis of Surface Diffusion Phenomena in Reversed-Phase Liquid Chromatography

Surface diffusion data obtained for a reversed-phase liquid chromatographic system (octadecylsilyl silica gel and a 70/30 (v/v) methanol/water solution) were analyzed in relation to their molecular diffusivity (Dm). The adsorbate−adsorbent interactions between the studied compounds and the stationary phase were taken into account. The surface diffusion coefficient (Ds) depends on the mobile-phase composition, especially on the nature and concentration of the organic modifiers. Differences between the values of Ds measured under various conditions stem probably from differences in Dm. It also seems that Ds tends toward Dm with decreasing retention factor. The surface diffusion mechanism was assumed to be a restricted molecular diffusion in a potential field of adsorption. A restriction energy for this diffusion (Er) was introduced to correlate Ds with Dm. The ratio of Er to the isosteric heat of adsorption (−Qst) was found to be nearly constant, irrespective of the retention factor, with an average value of 0.32 for our phase system. An estimation procedure for Ds using the enthalpy−entropy compensation effect for the adsorption equilibrium is proposed. From the adsorption equilibrium constant at 298 K only, Ds could be estimated at different temperatures with an error less than 50%