Retention volume absolute

Absolute retention volume peak elution volume... 

Although, for most moderators, the surface of a stationary phase in LC can be considered stable at moderator concentrations above about 5%v/v, the results from the same experiments as those carried out by Purnell and his group could still be considered invalid and, at best, would not lead to unambiguous conclusions. Katz et al. [9] avoided this problem by examining liquid/liquid distribution systems using water as one phase and a series of immiscible solvent mixtures as the other and by measuring absolute distribution coefficients as opposed to retention volumes. 

Temperature programming was introduced in the early days of GC and is now a commonly practiced elution technique. It follows that the temperature programmer is an essential accessory to all contemporary gas chromatographs and also to many liquid chromatographs. The technique is used for the same reasons as flow programming, that is, to accelerate the elution rate of the late peaks that would otherwise take an inordinately long time to elute. The distribution coefficient of a solute is exponentially related to the reciprocal of the absolute temperature, and as the retention volume is directly related to the distribution coefficient, temperature will govern the elution rate of the solute. 

Scott and Beesley [2] measured the corrected retention volumes of the enantiomers of 4-benzyl-2-oxazolidinone employing hexane/ethanol mixtures as the mobile phase and correlated the corrected retention volume of each isomer to the reciprocal of the volume fraction of ethanol. The results they obtained at 25°C are shown in Figure 8. It is seen that the correlation is excellent and was equally so for four other temperatures that were examined. From the same experiments carried out at different absolute temperatures (T) and at different volume fractions of ethanol (c), the effect of temperature and mobile composition was identified using the equation for the free energy of distribution and the reciprocal relationship between the solvent composition and retention. 

Graph of Log Corrected Retention Volume against the Reciprocal of the Absolute Temperature... 

The significant intrinsic limitation of SEC is the dependence of retention volumes of polymer species on their molecular sizes in solution and thus only indirectly on their molar masses. As known (Sections 16.2.2 and 16.3.2), the size of macromolecnles dissolved in certain solvent depends not only on their molar masses but also on their chemical structure and physical architecture. Consequently, the Vr values of polymer species directly reflect their molar masses only for linear homopolymers and this holds only in absence of side effects within SEC column (Sections 16.4.1 and 16.4.2). In other words, macromolecnles of different molar masses, compositions and architectures may co-elute and in that case the molar mass values directly calculated from the SEC chromatograms would be wrong. This is schematically depicted in Figure 16.10. The problem of simultaneous effects of two or more molecular characteristics on the retention volumes of complex polymer systems is further amplifled by the detection problems (Section 16.9.1) the detector response may not reflect the actual sample concentration. This is the reason why the molar masses of complex polymers directly determined by SEC are only semi-quantitative, reflecting the tendencies rather than the absolute values. To obtain the quantitative molar mass data of complex polymer systems, the coupled (Section 16.5) and two (or multi-) dimensional (Section 16.7) polymer HPLC techniques must be engaged. 

Qualitatively, the influence of the eluent gradient on chromatographic separation according to functionality is shown in Fig. 24. The initial point on the sab axis roughly determines the absolute values of retention volumes at which the macromolecules are eluted, and the slope of eab vs. V curve the distance between the zones of different functionality. Unpublished experimental data obtained in gradient chromatography of PBTP in a binary heptane — tetrahydrofuran eluent fully support this conclusion. 

This is largely due to the fact that retention data depend on certain factors the effects of which are difficult to eliminate completely or control and which are normally neglected. These factors are the imperfections in the gas phase and the compressibility of the stationary phase (cf., the quantities vh v , zq and 0 in eqn. 1), the finite rate of equilibration of the solute, variations in the composition of the sorbent, spurious sorption of the solute, solubility of the carrier gas in the stationary phase, etc. Hence, even relative retention volumes and/or retention indices must depend to some extent on the kind, flow-rate and absolute pressure of the carrier gas, the load of the liquid stationary phase on the support, which production batch of the stationary phase has been used and the kind of support. The absolute column pressure will obviously vary with the column length and particle size of the support. Moreover, adjusted retention data are required in all instances, which renders it necessary to measure the dead retention time. This is a crucial step in obtaining accurate retention data and presents a problem per se. 

The variation of the retention volume with temperature is such that a plot of the logarithm of the retention volume versus reciprocal of the absolute temperature, called a retenticm di am, is linear. The slope of this straight line is related to the enthalpy of the process, solution in the stationary phase (GLQ or adsorption on the surface (GSC), with. 

The retention time can be taken as the product of the distance on the chart between the dead point and the peak maximum and the chart speed, using appropriate units. As in the case of the retention time, it can be more accurately measured with a stopwatch. Again, the most accurate method of measuring F for a non-compressible mobile phase, although considered antiquated, is to attach an accurate burette to the detector exit and measure the retention volume in volume units. This is an absolute method of measurement and... 

If the retention volumes of some constituents of the sample are too large, the dual mode is used. The retained constituents are eluted in the reversed mode. It is absolutely certain that no part of the sample can be trapped inside the CCC apparatus. 

The accuracy of the measurements was equal to 0.001 min for the retention time, 0.1 °C for the column temperature, 20 Pa for the atmospheric pressure, and 100 Pa for the pressure drop. Therefore, net retention volumes were known with a precision of about 5%. Consequently, the absolute error for free energy of adsorption and for the specific interaction parameter are estimated to be 0.1 and 0.2 kj/mol, respectively. 

R values for given sample and solvent on adsorbents 1 and 2 equivalent retention volume or A" values (milliliters per gram) for standardizing single-activity scale adsorbents values for a standard sample-solvent combination solvent molecule absolute temperature... 

Not only variations in the pressure at constant temperature influence column-to-column retention data the role of the column hold-up volume as well as the mass of stationary phase present in the column is also important. The net retention volume caleulated from the adjusted retention volume corrects for the column hold-up volume (see Table 1.2). The specific retention volume corrects for the different amount of stationary phase present in individual colunms by referencing the net retention volume to unit mass of stationary phase. Further correction to a standard temperature of 0°C is discouraged [16-19]. Such calculations to a standard temperature significantly distort the actual relationship between the retention volumes measured at different temperatures. Specific retention volumes exhibit less variability between laboratories than other absolute measures of retention. They are not sufficiently accurate for solute identification purposes, however, owing to the accumulation of multiple experimental errors in their determination. Relative retention measurements, such as the retention index scale (section 2.4.4) are generally used for this purpose. The specific retention volume is commonly used in the determination of physicochemical properties by gas chromatography (see section 1.4.2). 

There are certain conditions that must be fulfilled if Eqs. (2.2), (2.3) and (2.4) are to be used to calculate partition coefficients. The basic assumption is that the individual retention mechanisms are independent and additive. This will be true for conditions where the infinite dilution and zero surface coverage approximations apply or, alternatively, at a constant concentration with respect to the ratio of sample size to amount of liquid phase. The infinite dilution and zero surface coverage approximations will apply to small samples where the linearity of the various adsorption and partition isotherms is unperturbed and solute-solute interactions are negligible. The constancy of the solute retention volume with variation of the sample size for low sample amounts and the propagation of symmetrical peaks is a reasonable indication that the above conditions have been met. For asymmetric peaks, however, the constant concentration method must be employed if reliable gas-liquid partition coefficients are to be obtained [191]. It is difficult to state absolutely the conditions for which contributions to retention from the structured liquid phase layer can be neglected. This will occur for some minimum phase loading that depends on the support surface area, the liquid phase... 

The curves relating the corrected retention volume of the two isomers to the reciprocal of the absolute temperature is shown in figure 10.10. It is seen that the reversal occurs at about 120°C. In many examples, the temperature at which the reversal of the elution order occurs, may be out of practical range. Unfortunately, this was true for this particular example due to the thermal instability of the stationary phase above ca... 

Often, size exclusion chromatograms (SEC) (compare section 11.7, Size Exclusion Chromatography) of polymers under study are expressed as differential representations of molar mass dispersity. The chromatographic retention volumes are directly transformed into the molar masses. This approach renders useful immediate information about tendencies of molar mass evolution in the course of building or decomposition polyreactions but the absolute values of molar mass can be only rarely extracted from it. As a rale, polystyrene calibrations are applied for molar mass calculation so that one deals with the polystyrene equivalent molar masses, not with the absolute values. The resulting dispersity (distribution) functions may be heavily skewed because the linear part of the calibration dependence for the polymer under study may exhibit well different slope compared with the polystyrene calibration, which was employed for the transformation of retention volumes into molar masses. 

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