Net retention volume

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... 

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 ... 

When the relationship between the distribution coefficient of a solute and solvent composition, or the corrected retention volume and solvent composition, was evaluated for aqueous solvent mixtures, it was found that the simple relationship identified by Purnell and Laub and Katz et al. no longer applied. The suspected cause for the failure was the strong association between the solvent and water. As a consequence, the mixture was not binary in nature but, in fact, a ternary system. An aqueous solution of methanol, for example, contained methanol, water and methanol associated with water. It follows that the prediction of the net distribution coefficient or net retention volume for a ternary system would require the use of three distribution coefficients one representing the distribution of the solute between the stationary phase and water, one representing that between the stationary phase and methanol and one between the stationary phase and the methanol/water associate. Unfortunately, as the relative amount of association varies with the initial... 

The net retention volume and the specific retention volume, defined in Table 1.1, are important parameters for determining physicochemical constants from gas chromatographic data [9,10,32]. The free energy, enthalpy, and. entropy of nixing or solution, and the infinite dilution solute activity coefficients can be determined from retention measurements. Measurements are usually made at infinite dilution (Henry s law region) in which the value of the activity coefficient (also the gas-liquid partition coefficient) can be assumed to have a constant value. At infinite dilution the solute molecules are not sufficiently close to exert any mutual attractions, and the environment of each may be considered to consist entirely of solvent molecules. The activity... 

Specific Retention Volume V, Net retention volUM at 0 C for unit weight of stationary phase V, 273 W, T. 

It is a great deal of work to actually determine a true equilibrium constant and most chemical separation methods speak in terms of values which are proportional to the actual equilibrium constant. At constant flow, the time that a given type of molecule is retained is related to the time for the void volume to pass after the sample is placed in a column or on a plate with the addition of the time for the net retention volume. If the flow remains constant, the temperature of the separation remains constant and no stationary phase is gained or lost, one can attempt qualitative identification using retention times. It is more reasonable to calculate the ratio of net retention volume to the void volume and call the result partition factor or capacity factor, k. ... 

Retention volumes of monosubstituted benzenes, benzoic acid, phenols, and anilines have been measured in RPLC [76]. Buffered acetonitrile/water and tetrahydrofuran/water eluents were used with an octadecylsilica adsorbent. From the net retention volumes, a substituent interaction effect was calculated and described with the linear free energy relationship developed by Taft. The data was interpreted in terms of hydrogen bonding between the solutes and the eluent. 

The competition model of Snyder assumes that the adso tion surface is completely covered by adsorbed eluent molecules forming a mono-layer. When solute molecules are adsorbed they displace olvent molecules. Due to the size differences one or more eluent mofecules are displaced by the solute molecules. The adsorbent surface is ijissumed to be homogeneous and each molecule tends to interact totally wRh the surface, i.e., it is adsorbed flatwise. Thus, the adsorbent surface area that the molecules require can be calculated from their molecular dimensions. Neglecting the interactions between solute and eluent molecules in the liquid and the adsorbed phase, the retention of an adsorbed molecule (expressed as net retention volume per unit weight of adsorbent K) can be related to the properties of the stationary phase, the eluent, and the sample by... 

Lately the most frequently used technique for the determination of thermodynamic and acid/base characteristics is inverse gas chromatography [30,73-75]. In IGC the unknown filler or fiber surface is characterized by compounds, usually solvents, of known properties. IGC measurements can be carried out in two different ways. In the most often applied linear, or ideal, IGC infinite concentrations of n-alkane are injected into the column containing the filler to be characterized. The net retention volume (V ) can be calculated by ... 

Net retention volume. VN- The adjusted retention volume multiplied by the pressure gradient correction factor. 

Specific retention volume. Vg. The net retention volume per gram of stationary phase corrected to 0°C. 

Since a finite pressure drop exists over the length of the column, the adjusted retention volume is often multiplied by the pressure gradient correction factor, j, to give a net retention volume, VN. 

The specific retention volume, Vg, the net retention volume per unit mass may be given by... 

Generally, an approximate heat of vaporization can be extracted from the slope of a plot of net retention volume versus the reciprocal of the column temperature. Table 11.5 lists some compounds whose vapor pressures have been determined by GC. 

The net retention volume is affected by temperature according to the following equation ... 

There is a fundamental relationship described in chromatographic theory between the retention volume of a elution peak and the mid-point of a breakthrough curve achieved by operating the column under frontal analysis conditions (41 ). In the Henry s Law region of the adsorption isotherm, the net retention volume and its measurement can be used to describe the variation of sorbate breakthrough volume as illustrated in Figure 8. Utilizing the experimental apparatus described in the last section, retention volumes were measured as a function of pressure at 40°C (T =... 

J. Takacs, M. Rockenbauer and I. Olacsi, Determination of the relationship between retention index and column temperature in gas chromatography through the temperature-dependence of the net retention volume, 7. Chromatogr., 42, 19-28 (1969). 

According to this equation, we can expect to obtain a straight line if we plot In (k/ T) versus 1 /T. This somewhat odd-looking relationship corresponds to the more common procedure of plotting the logarithm of the specific retention volume (Vg) against the reciprocal temperature. Vg is defined as the net retention volume at standard temperature (0 °C) and per unit weight of stationary phase ... 

Figure 3.5 Variation of retention (net retention volume per unit surface area of stationary phase) with the temperature in GSC. Stationary phase GTCB (carbon). Solutes o/p-xylene (1), m-xylene (2), n-octane (3), ethylbenzene (4), toluene (5), n-heptane (6), methylcyclohexane (7), exo-5-methyl-norbornene (8), endo-5-methylnorbornene (9), norbornane (10) and norbomene (11). Figure taken from ref. [309], Reprinted with permission. 

Net retention volume, 63, 75 Noise (detector), 93 Normal distribution, 12-14 Normal phase LC (NPLC), 158, 162 Number of theoretical plates, 14-16... 

Figure 6.3. Plot of carbon number versus log net retention volume for a homologous series.

Qualitative analysis is enhanced if data are acquired on more than one system. For example, in GC it is fairly common and easy to run a sample on each of two columns that are chosen to be widely different in their polarities. The results can be plotted as net retention volumes or as Kovats index values on either linear or log scales as shown in Figure 6.5. In either case, straight lines result for homologous series, thus aiding qualitative identifications. The principle is simple the more data, the more reliable the analysis. 

Verification of this relationship in GC is provided by Figure 8.19, in which the log of the net retention volume is plotted versus 1 IT in accordance with Eq. (11). The slope of each line is proportional to that analyte s enthalpy of vaporization, and the fact that straight lines are obtained indicates that the enthalpy is constant as assumed. 

VN is called the net retention volume. Since an analogous situation does not exist in LC, it is possible that some confusion may arise over the use of different terms. Thus, we originally used the equation... 

Net retention volume is the one term that would be expected to be the most reproducible, but it is not good enough to be tabulated. In the early days of GC, the specific retention volume Vg was also defined for use in qualitative analysis, but it was not found to be useful. 

The equilibrium thermodynamic functions describing the retention process are essentially related to the net retention volume. For example, the intermolecular adsorption of Gibbs free energy, —AGa, for one mole of solute vapor from a reference gaseous state with the partial pressure, Po, to a reference adsorbed state with the equilibrium spread pressure (or two-dimensional pressure), no, is given as [115]... 

Since the intermolecular adsorption of Gibbs free energy, —AGa, in Eq. (48), is independent of the specific surface area and weight of the solid in the column, except for the net retention volume, Fn, in the inverse GC at infinite dilution. Therefore — AGa can be rewritten... 

Plot of the natural logarithm of the net retention volume (or the adsorption free energy of the probes used [61]) against the number of carbon atoms of linear n-alkanes gives a slope from which, incremental adsorption free energy of a methylene group, —AGa(CH2), is determined, as illustrated in Fig. 11. 

For the access of the adsorption isotherm, the volumes injected are in very small quantities less than 0.1 /jL of gaseous probes for the infinite dilution, and in the range of 0.1 fjL to about 10 fiL of liquid probes for the finite dilution up to the saturation of the adsorbate (or, up to the increase of net retention time when the quantity adsorbed increased) in the chromatography, on account of the sensitivity of detector. In the chromatographic approach, the peak maxima method [115] is generally used to determine the net retention volumes, which are corrected by the compressibility, temperature as well as flow rate, as shown in Fig. 13. 

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