Az állófázisok geometriai változatosságának hatása a csúcsalakra folyadékkromatográfiában

A kromatográfiában elválasztások során a minta alkotóelemei diszkrét zónákként válnak el egymástól, ezek a sávok azonban az oszlopon való vándorlás során egyre jobban kiszélesednek. Az elválasztás eredménye azon múlik, hogy az egyes összetevők sávjai elég keskenyek maradnak-e, hogy így elkerüljük azok átfedését. Ezáltal a kromatográfia egyik célja a csúcsszélesedés minimalizálása. Ennek elérése érdekében szükséges a csúcsok kialakulásához és szélesedéséhez járuló folyamatok mélyebb megértése. A kromatográfiás elválasztások hatékonyságát döntően befolyásolják az oszlop és a mintamolekulák fizikai paraméterei, mint például a pórusméret és annak eloszlása, valamint a mintamolekulák mérete és méreteloszlása. A molekulák méretének pórusmérethez viszonyított arányáról ad tájékoztatást a méretkizárásos kromatográfia, mely lehetővé teszi, hogy tisztább képet kapjunk az eloszlások elválasztásra gyakorolt hatásairól. Az ideális méretkizárás során ugyanis nincs kölcsönhatás a mintamolekulák és az állófázis között, az elválasztást pusztán a molekulák és a pórusok méretének viszonya szabja meg. Ahhoz azonban, hogy figyelembe tudjuk venni az eloszlások csúcsalakra és elválasztásra gyakorolt hatásait, megfelelő modellre van szükségünk. A kromatográfia sztochasztikus elmélete az elválasztási folyamatot molekuláris szinten írja le, így célszerűvé válik, hogy beépítsük abba a pórusméret-eloszlást és a molekulaméreteloszlást. Ezáltal egy valósághoz közelebbi képet kaphatunk az elválasztásokról

The correctness of van 't Hoff plots in chiral and achiral chromatography

Abstract van 't Hoff plots (logarithm of the retention factor, ln k, vs. the reciprocal of absolute temperature, 1/T) are commonly used in chromatographic studies to characterize the retention mechanisms based on the determined enthalpy (ΔH∘) and entropy (ΔS∘) change of analyte adsorption. In reversed phase liquid chromatography, the thermodynamic parameters could help to understand the retention mechanism. In chiral chromatography, however, the conclusions drawn based on van 't Hoff plots can be deceptive because several different types of adsorption sites are present on the surface of stationary phase. The influence of heterogeneity, however, cannot be studied experimentally. In this study, we employed two reversed phase columns with different retention mechanisms to show that by serially coupling the columns, the obtained thermodynamic parameters are not related to the results obtained on the respective individual columns. Furthermore, our results show that the experimental conditions – such as flow-rate or choice of instrument – will strongly influence the calculated enthalpy and entropy values

Inverse Size-Exclusion Chromatography

The most preferred method to separate sample molecules based on their size relative to the pore size is size exclusion chromatography (SEC) (also referred to as gelpermeation, gel-filtration, molecular sieve, or simply gel chromatography) because using a strong solvent, there will be no interaction between the solute molecules and the stationary phase. The inverse version of SEC, inverse size-exclusion chromatography (ISEC) was also described in the middle of the 1970s [1,2], where the pore sizes were determined in the knowledge of the molecule size. Some sources attributed the first description of the ISEC technology incorrectly to Ogston [3] or to Aggerbrandt [4]. There is a plethora of information on the theoretical and experimental aspects of SEC and ISEC and also a number of great reviews have been published. The porous structure of the chromatographic particles is of great complexity and that has a number of consequences during the separation process. The physicochemical properties of high-performance liquid chromatography (HPLC) stationary phases play an important role on column performance and efficiency. The proper characterization of the pore structure and the pore size distribution (PSD) is relevant, because the mass transfer across the particles is greatly affected by the nature of the pores. SEC allows getting a more accurate picture of the impact of the distributions (pore size distribution and polydispersity) on the separation efficiency from ISEC measurements using a proper model. In this chapter, we summarize the most important theories and the newest applications regarding ISEC

Solvent minimization in two-dimensional liquid chromatography

An algorithm was developed for the minimization of consumption of organic solvent in comprehensive two-dimensional liquid chromatography (2DLC). It was shown that one can reach higher peak capacities only by using more eluent. The equilibration volume of the second dimension, however, did not affect the solvent consumption significantly. Calculations confirmed that the same target peak capacity could be achieved by consuming significantly different volume of organic modifier depending on the number of fractions analyzed in the second dimension suggesting that 2D separations can be optimized for eluent consumption. It was shown that minimization of eluent usage requires the use of small and high efficient columns in the second dimension. A simple equation was derived for the calculation of the optimal number of collected fractions from the first dimension that allowed the minimization of eluent usage, cost and environmental impact of comprehensive 2DLC separations

Polydispersity in size-exclusion chromatography: a stochastic approach

Abstract We investigate the impact of polydispersity of the sample molecules on the separation process and on the efficiency of size-exclusion chromatography. Polydispersity was integrated into the molecular (stochastic) model of chromatography; the characteristic function, the band profile and the most important moments of the elution profiles were calculated for several kind of pore structures. We investigated the parameters affected by polydispersity on the separation for a number of pore shapes. Our results demonstrate that even a small distribution in the molecular size (i.e. polydispersity) can contribute substantially to the total width of the chromatographic peak. The pure effect of polydispersity can only be investigated via mathematical modeling, because its contribution to an experimental chromatogram cannot be separated from other band-broadening effects

Effect of particle size distribution on the separation efficiency in liquid chromatography

In this work, the influence of the width of particle size distribution (PSD) on chromatographic efficiency is studied. The PSD is described by lognormal distribution. A theoretical framework is developed in order to calculate heights equivalent to a theoretical plate in case of different PSDs. Our calculations demonstrate and verify that wide particle size distributions have significant effect on the separation efficiency of molecules. The differences of fully porous and core-shell phases regarding the influence of width of PSD are presented and discussed. The efficiencies of bimodal phases were also calculated. The results showed that these packings do not have any advantage over unimodal phases