Isotherms chromatographic method

Chromatographic methods are widely used for the study of both physisorption and chemisorption. In its simplest form the technique consists of passing a pulse of the adsorbate through a column of the adsorbent and measuring the retention time and registering the elution curve. Measurement of the variation in the retention time as a function of temperature permits the evaluation of the enthalpy of adsorption, and analysis of the shape of the elution curve provides information about the adsorption isotherm. 

An alternative way of determining kH is by a gas chromatographic method. This is the generally preferred approach at higher temperatures, where the isotherm curvature is reduced. A novel perturbation chromatographic technique was adopted by Denayer and Baron (1997) in their recent study of the adsorption of a range of normal... 

Several experimental techniques have been developed for the investigation of the mass transport in porous catalysts. Most of them have been employed to determine the effective diffusivities in binary gas mixtures and at isothermal conditions. In some investigations, the experimental data are treated with the more refined dusty gas model (DGM) and its modifications. The diffusion cell and gas chromatographic methods are the most widely used when investigating mass transport in porous catalysts and for the measurement of the effective diffusivities. These methods, with examples of their application in simple situations, are briefly outlined in the following discussion. A review on the methods for experimental evaluation of the effective diffusivity by Haynes [1] and a comprehensive description of the diffusion cell method by Park and Do [2] contain many useful details and additional information. 

The experimental method used in TEOM for diffusion measurements in zeolites is similar to the uptake and chromatographic methods (i.e., a step change or a pulse injection in the feed is made and the response curve is recorded). It is recommended to operate with dilute systems and low zeolite loadings. For an isothermal system when the uptake rate is influenced by intracrystalline diffusion, with only a small concentration gradient in the adsorbed phase (constant diffusivity), solutions of the transient diffusion equation for various geometries have been given (ii). Adsorption and diffusion of o-xylene, / -xylene, and toluene in HZSM-5 were found to be described well by a one-dimensional model for diffusion in a slab geometry, represented by Eq. (7) (72) ... 

The adsorption equilibria were measured using a gravimetric method and were expressed as isotherms. A chromatographic method was used to get the initial slope of the isotherms. In the simulation, GCMC method was used to calculate amounts adsorbed for various conditions. When the experiment result and simulation result of chloroform are compared, the simulation for the acid site model was most agreement with chromatographic data and baratron data. The simulation result of tetrachloroethylene with three models corresponded mostly for the non-polar molecule, and above all the acid site model was the closest to the experiment result. Therefore, to get better coincidence between experimental data and simulation, it was found to be necessary to account for aluminum rather than silanol nest. 

Economy and Lin [340] investigated the phenol adsorption characteristics of high-surface-area activated carbon fibers Fig. 17b shows that their correlation was not nearly as good, but the role of surface chemistry was not invoked. Additional evidence that the agreement shown in Fig. 17a is more often the exception [439] than the rule is contained in the study of Dondi et al. [342], who u.sed a chromatographic method to determine low-concentration phenol adsorption isotherms on four different carbons, as well as in many other investigations (see, for example. Refs. 356, 382, 384, and 436). 

In addition to appropriate isotherm behavior and displacer concentration, other factors are important in determining the effectiveness of a displacement chromatographic method. Highly efficient columns and fast-mass-transfer kinetics are necessary to achieve sharp boundaries between the adjacent solute bands in the isotachie train. Diffuse boundaries mean significant regions of overlap between adjacent solute bands, and thus a low recovery of purified material. 

One of the most important applications of frontal chromatography is the determination of equilibrium adsorption isotherms. It was introduced for this purpose by Shay and Szekely and by James and Phillips [4,5], The simplicity as well as the accuracy and precision of this method are reasons why the method is so popular today and why it is often preferred over other chromatographic methods e.g., elution by characteristic points (ECP) or frontal analysis by characteristic points (FACP) [6,7]. Frontal chromatography as a tool... 

The first section of this thesis deals with development and validation of analytical biotechnological methods for qualitative and quantitative analysis (papers I-II). The second section (papers III- VI) concerns the issue of isotherm parameters determination for preparative purposes. More particularly, this section deals with the validated characterization of phase systems through the determination of isotherm parameters and computer simulations (paper III) and with the development and validation of methods to determinate adsorption isotherm parameters directly from component mixtures (papers IV-VI). The two sections together have one important feature in common the development and validation of chromatographic methods for analytical and preparative purposes (papers I-VI). The intention of this summary is to give readers who are new in the area a general introduction to the fields described above. For a more detailed discussion, see papers I-VI. 

The dynamic methods are based on direct chromatography and are popular because they are faster and easier to automate. Four direct chromatographic methods that are available for determination of adsorption isotherms are frontal analysis (FA) [13, 109] frontal analysis by characteristic points (FACP) [109], elution by characteristic points (ECP) [109] and the perturbation peak (PP) method [118-121], The FACP and ECP methods have... 

There are, so far, only two reports on the determination of isotherm data from mixtures containing more than two components. In both cases, the FA method was used for ternary mixtures [135, 136], There are no reports on the experimental determination of adsorption isotherms for quaternary mixtures using any chromatographic method. The competitive quaternary adsorption isotherm parameters could be very valuable for the separations of the four isomers of a compound with two chiral centers, or for the preparative separation of two compounds in the presence of one or two impurities. 

Despite the limitations mentioned above, the chromatographic method is easy to handle and generates adsorption isotherm data quickly for a large number of combinations of polymers and gases or vapors. These data are important when membrane gas separation and pervaporation data are interpreted, and polymer materials are chosen for the preparation of membranes used for the above separation processes. 

Whereas conventional chromatographic methods manipulate the surface energetics of sorbents to separate fluid mixtures, inverse gas chromatography uses known properties of fluids to characterize surface properties of solids. Specifically, Lewis acids and bases are used, in this study, as probes to deduce the nature and extent of solid/gas attraction from the shape of chromatograms, which are transformed adsorption isotherms. IGC can determine the specific surface (m2/g) of the substrate, whether the surface is acidic, basic, amphoteric, or neutral, and whether the surface is homogeneous or heterogeneous. 

Modified Frontal Chromatographic Method for Water Sorption Isotherms of Biological Macromolecules... 

The primary use of isotherm data measurements carried out on single-component elution profiles or breakthrough curves is the determination of the single-component adsorption isotherms. This could also be done directly, by conventional static methods. However, these methods are slow and less accurate than chromatographic methods, which, for these reasons, have become very popular. Five direct chromatographic methods are available for this purpose frontal analysis (FA) [132,133], frontal analysis by characteristic point (FACP) [134], elution by characteristic point (ECP) [134,135], pulse methods e.g., elution on a plateau or step and pulse method) [136], and the retention time method (RTM) [137]. 

Figure 3.37 Typical experimental chromatograms obtained in the determination of equilibrium isotherms by chromatographic methods, (a) Frontal analysis staircase. FACP on the diffuse rear boundary after the last frontal step, (b) ECP. Data recorded with an HP 1090 (Hewlett-Packard, Palo Alto, CA) liquid chromatograph. Reproduced with permission from S. Golshan-Shirazi and G. Guiochon, Anal Chem., 60 (1988) 2630 (Figs. 1 and 2), ( )1988 American Chemical Society.

Figure 3.38 Typical chromatograms obtained in the determination of equilibrium isotherms by pulse chromatographic methods, (a) Injection on a concentration plateau, and response of a selective detector for a tracer, (b) Response of a nonselective detector for the tracer injection made in (a), (c) Individual profiles of the labeled tracer (gray line) and the unlabeled component (black line).

Finally, the validity of the chromatographic methods for the determination of isotherms is based on the assumption that phase equilibrium is reached rapidly during the experiment. The rate constant for phase equilibration must be large enough for the experimental results to be independent of the mobile phase velocity. When carrying out FA, FACP, or ECP measurements on proteins that tend to equilibrate slowly, it is advisable to check the influence of the flow velocity on the isotherms (Figures 3.15 and 3.44 [38]). 

This equation assumes that the contribution of the probe vapour to the gas flow rate across the column is negligible and can be considered as ideal, and that the contribution of the injection band and diffusion processes along the column play a minor role. One of the main advantages of the chromatographic method, especially when applying the Rudzinski-Jagiello s method, is the fact that this method provides directly the first derivative of the isotherm and therefore, in a very direct way the DFCA. 

A chromatographic method has been used to measure single component sorption isotherms of O, N / CH, ... 

Another possible cause for such a behavior may not be due to the non-ideality of the system but may be rooted in the nature of the experimental technique and the method of data analysis. The binary sorption equilibria (total amount adsorbed and the composition of the adsorbed phase) are not measured directly in the chromatographic method but are deduced from integration of the slopes of the isotherms at different gas phase compositions. Such an integration may give rise to a significant cumulative error which, in turn, may be responsible for underpredicting the binary... 

Shcherbakova, Petrova, and co-workers (258-263) studied the chemical modification of silica for applications in gas chromatography (for example, in the adsorption separation of hydrocarbons by gas chromatographic methods). The adsorption properties are investigated as a function of the degree of surface modification with ClSi(CH3)3. A number of silica samples were chemically modified so that they would have the desired adsorption properties (17). Chemical modification is an effective means of changing the shape of adsorption isotherms. 

A method was developed [41] for the analysis of natural penicillins after methylation. Previously only paper chromatographic methods for the analysis of penicillins were known [42, 43]. Penicillins were extracted with diethyl ether from aqueous solutions at pH 2 and esterified with a small excess of diazomethane at 0—2°C. The solution of methyl esters was evaporated to dryness and the residue was dissolved in acetone. Samples of 1 —3 fxl were analysed on a glass column (150 cm x 4 mm I.D.) packed with 3% OV-17 on Gas-Chrom Q (100-120 mesh) with a carrier gas (nitrogen) flow-rate of 60ml/min. After lOmin isothermal at 180°C the temperature was increased at 4 C/min for 10 min and then kept constant until the end of the analysis. All of the penicillins analysed under these conditions were separated within 45 min methylpenicillin,... 

The only gas chromatographic method used for the measurement of diffusion coefficients of gases on solid surfaces is the RF-GC technique validating a recent mathematical analysis, also permitting the estimation of adsorption and desorption rate constants, local adsorbed concentrations, local isotherms, local monolayer capacities, and energy distribution functions." The RF-GC technique has been successfully applied for the time-resolved determination of surface diffusion coefficients for physically adsorbed or chemisorbed species of O2, CO, and CO2 on heterogeneous surfaces of Pt/Rh catalysts supported on Si02." All calculations for the... 

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