Eluent kinetics

Optimum flowrates, resulting in high productivity and low eluent consumption, are estimated first for an ideal system , which means that kinetic and hydrodynamic dispersive effects are assumed to be negligible [46]. This procedure has recently been improved [57]. 

When sample components having ionizable groups are chromatographed the use of a background electrolyte and control of the eluent pH with an m>propridte buffer are mandatory. It is advisable to maintain a fairly high concentration of buffer in the medium in order to rapidly reestablish protonic equilibria and to thereby avoid peak sjditting or asymmetrical peaks due to slow kinetic processes. Acetic acid, phosphoric acid, and perchloric acid and their salts have been used for the control of pH. 

Before discussing particular carbon electrode materials, we should define the qualities on which a choice of material will be based. These are the criteria that matter the most to the user, and the importance of each will vary with the application. For example, a carbon electrode to be used for detecting eluents from a liquid chromatograph should have a low background current and long stability, whereas an electrode used for studying redox mechanisms should usually exhibit fast electron transfer kinetics. The criteria relevant to carbon electrodes are conveniently classified into four types. 

Frequently, IEC separations are carried out at elevated temperatures. This is because the kinetics of the ion-exchange process may be improved dramatically. The efficiency of the column may be increased by a factor of three if the temperature is increased from 30 to 70 °C [371], An additional advantage of an increased column temperature is a decrease in the eluent viscosity and hence a reduced pressure drop over the column. 

Results of efficiency enhancement studies have been controversial. Increasing the temperature lowers eluent viscosity and system back pressure, leading to the nse of (1) higher flow rates (shorter cycle times) [9], (2) longer columns, and (3) smaller particles that enhance efficiency in their own right. However, efficiency is also expected to increase because high column temperatures involve (1) faster adsorption-desorption kinetics, (2) enhanced diffusivity, (3) lower mass transfer resistance (C in the van Deemter Equation 6.4), and (4) flatter van Deemter curves. 

The kinetics of the ionic equilibration is also dependent on the analyte solvation. The greater the analyte solvation, the slower the equilibration kinetics. Solvation shell restricts the protonation or deprotonation of the analyte. Solvation is also influenced by the eluent ionic strength. With an increase of the concentration of ions in the analyte microenvironment, there is a corresponding decrease in the analyte solvation, thus increasing the ionic equilibration kinetics. The increase of the eluent ionic strength usually improves the analyte peak shape even if the mobile-phase pH is close to the analyte Ka. 

Ion association or ion-pairing reactions are most commonly studied for clathrochelate complexes exhibiting unique inertness. These reactions attract particular interest due to their marked effect on the kinetics and direction of the redox and photochemical reactions and on the characteristics of electrochemical processes. In certain cases, ion association reactions govern the catalytic activity of compounds. The ion-pairing ability of clathrochelates is utilized to resolve racemates into optical isomers (enantiomers) and to separate optically active anions using clathrochelates as chiral eluents. 

The kinetics of eluent migration will depend on the degree of saturation of the chamber. The eluent front runs faster in a saturated chamber than in an unsaturated one. The problem of the reproducibility of Rf values can be seriously affected by an unsaturated chamber. 

The composition of pore waters from contaminated cores 1 and 2 were used to initialize the model (Table 2). Concentrations represent leachate collected from the initial half pore volume of each core. Eluent specified in the transport simulations had the composition of uncontaminated ground water in Table 2. Reactions proposed to describe concentration changes for selected constituents within the cores are based on comparisons between eluent and leachate chemistry and analysis of selected constituents in the core samples. Equilibrium constants and kinetic rates for the reactions were adjusted to give the best fit to leachate concentrations from core 1. The same reactions, equilibrium constants, and kinetic rates were then tested by modeling the concentrations of constituents in leachate from core 2. This geochemical model will be used in the future to simulate evolution of contaminated ground water associated with the Area 4 landfill at the aquifer scale. 

The term system peak refers to signals that may not be attributed to solutes. System peaks are characteristic for ion chromatographic systems that have have no suppressor system when weak organic acids are used as the eluent. Despite numerous publications concerning this subject [72-76], system peaks were often the reason of misinterpretations. However, some facts about the thermodynamic and kinetic processes that occur within the separator column may be inferred [77,78] from their occurrence and help in understanding the chromatographic processes. 

Fig. 5-23. Separation of kinetically stable and unstable metal-cyanide complexes. - Separator column IonPac NS1 (10 pm) eluent 0.002 mol/L TBAOH + 0.001 mol/ L Na2C03 + 2 10 4 mol/L KCN / acetonitrile (70 30 v/v) flow rate 1 mL/min detection suppressed conductivity injection volume 50 pL solute concentrations 80 ppm KAg(CN)2, 40 ppm K2Ni(CN)4, 40 ppm K3Co(CN)6, and 80 ppm KAu(CN)2 (taken from [36]).

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