Chromatographic systems, equilibria

In static headspace sampling [301,302] the polymer is heated in a septum-capped vial for a time sufficient for the solid and vapour phases to reach equilibrium (typically 2 hours). The headspace is then sampled (either manually or automatically) for GC analysis, often followed by FID or NPD detection. Headspace sampling is a very effective method for maintaining a clean chromatographic system. Changing equilibrium temperature and time, and the volumes present in the headspace vial can influence the sensitivity of the static headspace system. SHS-GC-MS is capable of analysing volatile compounds in full scan with ppb level... 

Method of Moments The first step in the analysis of chromatographic systems is often a characterization of the column response to small pulse injections of a solute under trace conditions in the Henry s law limit. For such conditions, the statistical moments of the response peak are used to characterize the chromatographic behavior. Such an approach is generally preferable to other descriptions of peak properties which are specific to Gaussian behavior, since the statistical moments are directly correlated to equilibrium and dispersion parameters. Useful references are Schneider and Smith [AlChE J., 14, 762 (1968)], Suzuki and Smith [Chem. Eng. Set, 26, 221 (1971)], and Carbonell et al. [Chem. Eng. Set, 9, 115 (1975) 16, 221 (1978)]. 

The performance of a chromatographic system is generally evaluated by the time required for the elution of each solute retention time) and the width of the elution curve peak width), which represents the concentration profile of each solute in the effluent from a column. Although several models are used for evaluation, the equilibrium, stage, and rate models are discussed here. 

Another way of decreasing the retention volume of functional macromolecules can be found in raising the column temperature. In this case, it should be borne in mind that a change in temperature causes a number of changes in the chromatographic system, mainly associated with the shift in the equilibrium between the components of the adsorbed mobile phase and water, which results in a change in the eab of the mixture and the adsorbent activity. 

This definition is equivalent to the one implicitly assumed in Henry s law (section 3.1.1). An alternative way to arrive at eqn.(3.25) is to consider the thermodynamic potential (p). A condition for equilibrium is that fi should be equal in the two phases of a chromatographic system for each compound. Therefore, we can write for the solute... 

The thermodynamic equilibrium constants of all equilibria in the chromatographic system can be derived from basic thermodynamic considerations. When the charge status of the analyte and HR are the same, or if the analyte is neutral, ion-pairing equilibria do not apply. For the condition of equilibrium, it holds that the electrochemical potentials (ju) of species in the stationary and mobile phases are equal ... 

Column efficiency is mainly dependent on the kinetic factors of the chromatographic system such as molecular diffusion, mass-flow dynamics, properties of the column packing bed, flow rate, and so on. The smaller the particles and the more uniform their packing in the column, the higher the efficiency. The faster the flow rate, the less time analyte molecules have for diffusive band-broadening. At the same time, the faster the flow rate, the further analyte molecules are from the thermodynamic equilibrium with the stationary phase. This shows that there should be an optimum flow rate that allows achievement of an optimum efficiency for a given column. Detailed discussions of the... 

The partial derivative of the distribution function by the concentration is actually a full derivative since it is independent on the time and position of the analyte in the column given that the chromatographic system is at the equilibrium. Applying relationship (2-35), we get... 

A chromatographic system consisting of the binary eluent (e.g., acetonitrile-water) with liophilic salt added in low concentration (not more than lOOmM), along with basic analyte, is considered here. Adsorption behavior of acetonitrile in the column has been discussed in Section 2.12, and we assume that low concentrations of liophilic salt additives and injection of small amount of the analyte does not noticeably disturb its adsorption equilibrium. 

Each constant in the equation above represents single equilibrium process, which is assumed to be independent on other equilibria in the column. Equation (2-93) describes the retention of basic ionizable analytes in reversed-phase chromatographic system with binary eluents and liophilic counteranions added. Similar expression could be derived for the behavior of anionic analytes in the presence of liophilic countercation. 

Primary equilibrium in the chromatographic system is the analyte distribution between mobile phase and the surface of packing material. If the analyte could be present in the mobile or stationary phase in two or more different forms and there is an equilibria between these forms, this equilibria is usually called secondary. ... 

The enantiomeric separation with chiral mobile phases consists of the addition of an active compound in the mobile phase which is constantly pumped though the chromatographic system. The active ingredient contributes to a specific secondary chemical equilibrium, interacting with the enantiomers in the mobile phase as well as in the stationary phase, leading to the formation of diastereomeric complexes potentially in both phases. This affects the overall distribution of the analyte between the stationary phase and the mobile phase, affecting its retention and the overall enantiomeric separation. The rates of formation of the diastereomeric complexes should be similar to the diffusion rates to minimize excessive chemical contribution to the band-broadening. 

There are two main sources of drift, both due to non equilibrium conditions in the column and the detector. If the detector, column and mobile phase are not in thermal equilibrium, then serious drift will occur. This can be eliminated by careful temperature control of column and detector. Another and more common source of drift arises when the stationary phase and mobile phase have not been given sufficient time to come into equilibrium. This type of drift often occurs when changing the mobile phase composition and mobile phase should be pumped through the chromatographic system until a stable baseline is achieved. Trace impurities in the mobile phase can cause prolonged drift and longterm noise and so very pure solvents must be used for the mobile phase. Distilled in glass solvents may not necessarily be sufficiently pure to ensure drift-free detector operation. 

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