Mobile phase parameters, definition

A general approach to the problem of identification, should more definitive detectors not be available, is to change the chromatographic system , which in the case of HPLC is usually the mobile phase, and redetermine the retention parameter. The change obtained is often more characteristic of a single analyte than is the capacity factor with either of the mobile phases. 

In the Mint model, we have to take into account the following considerations (i) the initial filtration coefficient Xq, which is a parameter, presents a constant value after time and position (ii) the detachment coefficient, which is another constant parameter (iii) the quantity of the suspension treated by deep filtration depends on the quantity of the deposited solid in the bed this dependency is the result of the definition of the filtration coefficient (iv) the start of the deep bed filtration is not accompanied by an increase in the filtration efficiency. These considerations stress the inconsistencies of the Mint model 1. valid especially when the saturation with retained microparticles of the fixed bed is slow 2. unfeasible to explain the situations where the detachment depends on the retained solid concentration and /or on the flowing velocity 3. unfeasible when the velocity of the mobile phase inside the filtration bed, varies with time this occurrence is due to the solid deposition in the bed or to an increasing pressure when the filtration occurs with constant flow rate. Here below we come back to the development of the stochastic model for the deep filtration process. 

In gas chromatography the analyte partitioning between mobile gas phase and stationary liquid phase is a real retention mechanism also, phase parameters, such as volume, thickness, internal diameter, and so on, are well known and easily determined. In liquid chromatography, however, the correct definition of the mobile-phase volume has been a subject of continuous debate in the last 30 years [13-16]. The assumption that the retardation factor, i /, which is a quantitative ratio, could be considered as the fraction of time that components spend in the mobile phase is not obvious either. 

Figure 7.1 A two-component chromatogram, showing the definitions of various parameters. The peak at corresponds to an unretained (or mobile phase) component...

Among the parameters related to the retention time, the distribution constant K — defined for a compound as the ratio between its concentrations in the stationary phase and in the mobile phase — depends only on analyte, stationary phase, and temperature, and can be used to compare retention data. K can be calculated from chromatographic measures from its definition. 

For any particular column, y tat and Vntoi, are fixed values (at least for the duration of a particular analysis), so k is directly related to K (distribution coefficient). Ihis makes it a useful parameter to measure, but in this form it is not any easier to measure than K itself. In order to get a definition of k that allows us to make practical measurements, we need to consider /as the fraction of time spent in the mobile phase. It follows (with a bit of thought) that/is also the fraction of the solute present in the mobile phase at any one time. If the total amount of solute is n + ob> then... 

The chapter has examined advantages of TLC, definitions, structure, occurrence, function, sample preparation, sorbents, mobile phases, usual modes of development, and detection procedures for lipids. Although a discussion of 2-D lipid analysis has been provided, mention was not made of the newer technique of multiphase TLC, in which components are separated in two different directions according to different parameters, e.g., conventional silica gel in one direction and reversed phase in the other direction. Ritchie and Jee (143) have used this technique for the analysis of triacylglycerols. 

Section 2.3 describes various methods for the characterization and classification of mobile and stationary phases for chromatography. In section 2.3.1 the solubility parameter is introduced as a quantitative definition of the word polarity . Section 2.3.2 describes the characterization of GC stationary phases according to Rohrschneider and section 2.3.3 the classification of LC solvents according to Snyder. The applicability of the different methods is summarized in section 2.3.4. 

So far the solution of the mass-balance equation for models with a single dominating process (partitioning or adsorption) was discussed in Sections 2.8 and 2.9. In both cases the solutions have similar form, with the difference in the definition of the parameters (volumes of the mobile and stationary phases in the case of partitioning total volume of the liquid phase and adsorbent surface area in the case of adsorption model). 

A concise review of the relative order, mobility, density, and possible types of phase transitions of one-component systems is presented by the schematic of Fig. 2.115, along with the dictionary definition of the word transition. This schematic is discussed in Sect. 2.5 in connection with an initial description of phases and their transitions. More details of the structure and properties of crystals, mesophases, and amorphous phases are given in Chap. 5. Some characteristics of the three types of mesophases are given in Fig. 2.107. Quantitative information on the thermodynamic parameters of the transitions between the condensed phases is summarized in Fig. 2.103 and described in more detail in Sect. 5.5. The dilute phases in Fig. 2.115, the gases, are of lesser interest for the present description, although the ideal gas law in Figs. 2.8 and... 

The apparent diffusivity describes the approach of a system response to equilibrium. The larger is this parameter, the sooner the system approaches equilibrium. A system having high mobility in both free and adsorbed phases does not necessarily mean that it would approach equilibrium quickly. It does also depend on the quantity which can be accommodated by the solid at equilibrium. The speed to approach equilibrium depends on the two factors - mobility and capacity - in the form of ratio as reflected in the definition of the apparent diffusivity. Let us demonstrate this with the following two systems. 

It is also clear that activity of a filler should be related to any definite property of material. It was proposed to introduce the concept of structural, kinetic, and thermod3uiamic activity of fillers. Structural activity of a filler is its abihty to change the polymer structure on molecular and submolecular level (crystallinity degree, size and shape of submolecular domains, and their distribution, crosslink density for network pol3rmers, etc.). Kinetic activity of a filler means the ability to change molecular mobility of macromolecides in contact with a solid surface and affect in such a way the relaxation and viscoelastic properties. Finally, thermodynamic activity is a filler s ability to influence the state of thermodynamic equilibrium, phase state, and thermodynamic parameters of filled polymers — especially important for filled poljmier blends (see Chapter 7). 

---