Retention volumes

The total volmne of the mobile phase (Vj.) required to elute an analyte is the sum of the volume of the mobile phase (V ) and the volume of the mobile phase which flows while the analyte is held immobile (VgR ), or the tendency of an analyte to sorb on a defined amount of stationary phase. 

In a more practical manner, the value of Vj. can be obtained from the chromatogram since 

The distance of the peak maxima from the injection point expressed in time units is called retention time (r ), and it serves as an identifier for the given analyte on that particular system. Retention time is probably the most widely used descriptor of the analyte behavior, and it is the most easily measurable parameter. However, even though it is easily measurable, it is the least universal parameter. 

Analyte retention time is dependent on the mobile phase flow rate the faster the flow rate, the smaller the analyte retention time. It is also dependent on the flow rate stability. The product of the analyte retention time and the mobile-phase flow rate is the retention volume (Vr). Analyte retention volume 

Application of IGC to study the properties of a solid is based on the assumption that the adsorbate equilibrium conditions are achieved between the mobile and stationary phases. Thus, chromatogram should be symmetric and the maximum of the peak must not depend on the amount of the injected adsorbate. Moreover, as the amount of adsorbate is very small, the concentration of the adsorbate in the gas phase is minimal and the adsorption process is conditioned by the real adsorbate-adsorbent interactions. Under these conditions, the retention volume—key parameter in IGC-of the solute depends on its partition between the stationary and mobile phase, and is an indication of the interaction strength between the solute molecule and the metal/adsorbent surface. The specific retention volume, Vg, in cm /g, is given as  

In both cases, the James-Martin factor for the correction of gas compressibility under pressure difference between column inlet, pi, and column outlet, po, is introduced  

In the case of perfect symmetric peaks, the retention time can be determined directly from the peak maximum method, which is the simplest and most common. The peak maximum method is useful for determination of retention time if the skewness ratio is 0.7-1.3 [15]. The skewness ratio is defined as the ratio of tangent slope to the peak leading part and tangent slope to the peak tailing part whereas both tangents are drawn in the inflexion points. In such cases the skewness ratio is our of this interval, Ir is obtained from the first-order moment method or the Conder and Young method. Between these two methods, Conder and Young is recommended [16]  

The retention volume is related to the surface area and surface energy that is, the higher the surface area and energy, the higher the retention time, and therefore, retention volume. 

Moreover, the Vn and the slope of adsorption isotherm are related by Eq. (16.13) for small adsorbate injections, where conditions of infinite dilution are achieved  

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The yield of each of these fractions will depend on their retention volume which in turn wiil depend on the adsorbent selected and the eluting force of the solvents. 

On nonpolar columns, the compounds of a homologous series separate as a function of their boiling points, and linear relationships have been established between the logarithms of the retention volumes and the number of carbon atoms in the 2-, 4-, and 5-positions (see Fig. III-l). 

A comparison of the molar volumes of 2-, 4-, and 5-alkylthiazoles with their relative retention volumes shows that these values also vary in the same direction (see Fig. III-2). 

Kg. lU-2. Comparison between molar volumes and retention volumes for 1-. 4-. and S-alkylthiazoles. 

The accurate determination of relative retention volumes and Kovats indices is of great utility to the analyst, for besides being tools of identification, they can also be related to thermodynamic properties of solutions (measurements of vapor pressure and heats of vaporization on nonpolar columns) and activity coefficients on polar columns by simple relationships (179). 

Elution volume, exclusion chromatography Flow rate, column Gas/liquid volume ratio Inner column volume Interstitial (outer) volume Kovats retention indices Matrix volume Net retention volume Obstruction factor Packing uniformity factor Particle diameter Partition coefficient Partition ratio Peak asymmetry factor Peak resolution Plate height Plate number Porosity, column Pressure, column inlet Presure, column outlet Pressure drop... 

Recovery factor Reduced column length Reduced plate height Reduced velocity Relative retention ratio Retardation factor d Retention time Retention volume Selectivity coefficient Separation factor... 

Retention Behavior. On a chromatogram the distance on the time axis from the point of sample injection to the peak of an eluted component is called the uncorrected retention time The corresponding retention volume is the product of retention time and flow rate, expressed as volume of mobile phase per unit time ... 

When the mobile phase is a gas, a compressibility factor j must be applied to the adjusted retention volume to give the net retention volume ... 

A chromatographic peak may be characterized in many ways, two of which are shown in Figure 12.7. The retention time, is the elapsed time from the introduction of the solute to the peak maximum. The retention time also can be measured indirectly as the volume of mobile phase eluting between the solute s introduction and the appearance of the solute s peak maximum. This is known as the retention volume, Vr. Dividing the retention volume by the mobile phase s flow rate, u, gives the retention time. 

The second important parameter is the chromatographic peak s width at the baseline, w. As shown in Figure 12.7, baseline width is determined by the intersection with the baseline of tangent lines drawn through the inflection points on either side of the chromatographic peak. Baseline width is measured in units of time or volume, depending on whether the retention time or retention volume is of interest. 

The size selectivity of a particular packing is not infinite, but is limited to a moderate range. All solutes significantly smaller than the pores move through the column s entire volume and elute simultaneously, with a retention volume, Vj, of... 

In between the inclusion limit and the exclusion limit, each solute spends an amount of time in the pore space proportional to its size. The retention volume for a solute is... 

Another important application is for the determination of formula weights. Calibration curves of log(formula weight) versus Vj are prepared between the exclusion limit and inclusion limit (figure 12.36). Since the retention volume is, to some... 

When a preparation of polyvinylpyridine of unknown formula weight was analyzed the retention volume was found to be 8.45. Report the average formula weight for the preparation. 

To use GPC for molecular weight determination, we must measure the volume of solvent that passes through the column before a polymer of particular molecular weight is eluted. This quantity is called the retention volume Vj. Figure 9.14 shows schematically the relationship between M and Vj it is an... 

Figure 9.14 Calibration curve for GPC as log M versus the retention volume Vj, showing how the location of the detector signal can be used to evaluate M. Also shown are the void volume Vy and the internal volume Vj in relation to Vj, and KVj as a fraction of Vj.

Progressively smaller molecules have access to successively larger fractions of the internal volume. Therefore, as Vj emerges, consecutive fractions of the polymer come with it. Thus we can write the retention volume for a particular molecule weight fraction as... 

Figure 9.17 Plot of log [i ]M versus retention volume for various polymers, showing how different systems are represented by a single calibration curve when data are represented in this manner. The polymers used include linear and branched polystyrene, poly(methyl methacrylate), poly(vinyl chloride), poly(phenyl siloxane), polybutadiene, and branched, block, and graft copolymers of styrene and methyl methacrylate. [From Z. Grubisec, P. Rempp, and H. Benoit, Polym. Lett. 5 753 (1967), used with permission of Wiley.]...

Hydiocaibon mistuies containing cyclohexane can be analyzed using gas chiomatogiaphy. Specific retention volumes for several stationary phases have... 

When fouling is absent, the optimum concentration is 0.37 C. If the permeate soHds are of primary value, it is usually preferable to diafilter at the minimum retentate volume to minimize permeate dilution. 

The dead point is the position of the peak maximum of an unretained solute. It is not the initial part of the dead volume peak as this represents a retarded portion of the peak that is caused by dispersion processes. The importance of employing the peak maximum for such measurements as dead volume and retention volume will be discussed in later chapters of the book that deal with peak dispersion. 

The retention volume (Vr) is the volume of mobile phase passed through the column between the injection point and the peak maximum. 

Each solute will also have a characteristic retention volume. 

Once the elution-curve equation is derived, and the nature of f(v) identified, then by differentiating f(v) and equating to zero, the position of the peak maximum can be determined and an expression for the retention volume (Vr) obtained. The expression for (Vr) will disclose those factors that control solute retention. 

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