Relative retention volume

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). 

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). 

The simplest mode of IGC is the infinite dilution mode , effected when the adsorbing species is present at very low concentration in a non-adsorbing carrier gas. Under such conditions, the adsorption may be assumed to be sub-monolayer, and if one assumes in addition that the surface is energetically homogeneous with respect to the adsorption (often an acceptable assumption for dispersion-force-only adsorbates), the isotherm will be linear (Henry s Law), i.e. the amount adsorbed will be linearly dependent on the partial saturation of the gas. The proportionality factor is the adsorption equilibrium constant, which is the ratio of the volume of gas adsorbed per unit area of solid to its relative saturation in the carrier. The quantity measured experimentally is the relative retention volume, Vn, for a gas sample injected into the column. It is the volume of carrier gas required to completely elute the sample, relative to the amount required to elute a non-adsorbing probe, i.e. 

Fig. 17. A schematic of the alkane line obtained by inverse gas chromatography (IGC) measurements. The relative retention volume of carrier gas required to elute a series of alkane probe gases is plotted against the molar area of the probe times the. square root of its surface tension. The slope of the plot is yielding the dispersion component of the surface energy of...

The column is calibrated using proteins of known molar mass. The relative retention volumes 0.0 and 1.0 are defined by the elution of Blue Dextran (molecular weight 2 000 000) and sulfosalicylic acid (molecular weight 218), respectively. 

With 0.5M sodium hydroxide as eluent, Sephadex G-50 effects fractionation in the molar mass range 1000-15000 dalton and can be used for a period of 3-4 weeks with a single calibration carried out with proteins and polypeptides of known molar mass, as revealed by Figure 10. Relative retention volumes 0.0 and 1.0 are defined with Blue Dextran and phenol, respectively. 

Fractionation on Sephadex G-25 using 0.5M sodium hydroxide as eluent causes the low molar mass lignin components in black liquor to elute in the relative retention volume range 0.3-1.3 with partial separation from each other, as shown in Figure 11. 

It should be noted that in this relative retention volume range elution does not necessarily take place in order of decreasing molecular size, because, as seen from Figure 12, functional groups may have a greater effect on elution behavior than molecular size (Forss, K. Talka, E., The Finnish Pulp and Paper Research Institute, unpublished results). 

Fraction I, which consists mainly of low molar mass compounds, also contains a small amount of high molar mass lignin derivatives eluting with relative retention volumes of 0-0.1. These derivatives are polar and some may be bound to carbohydrates, or otherwise they would have been eluted by RPC along with the hydrophobic fractions II-IV. 

Equilibria may be expressed in terms of activity, instead of concentrations. The expression that relates the relative retention volume to the association constant is given by the following equation ... 

It is likely that a relative retention volume that is ratioed to a standard would be more reproducible. Such has been found to be true a is a relative retention ratio it was defined in Chapter 1, where it was called the column selectivity or a separation factor ... 

Fig. 3.3. Relationship between log (relative retention volumes) on Apiezon M and Reoplex 400 for various fatty acid esters at 197°C. (Reproduced from J. Chromatogr., 2 (1959) 552, by courtesy of A.T. James.)...

Fig. 3.4. Relationship between log (relative retention volumes) of a range of alkanes on octadecene-1 (OD-1) and dimethylsulpholane (DMS). (Reproduced from ref. 24 by courtesy of the Swiss Chemists Association)...

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. 

Thiazolyl pyridine, gas-liquid chromatography of, relative retention volume, 358... 

Liebman et al. (/ 7) detected the glass transition of poly(vinyl chloride) stationary phases by recording the separation of cisitrans isomers as a function of temperature. It was found that no separation could be achieved in the vicinity of Tg while separation was possible both above and below Tg. Similar discontinuities in the plots of relative retention volumes were reported by Yatmunoto et id. 18) for poly carbonate stationary phases of differing molecular weights. 

TABLE 16. Relative retention volumes for several alkyl/aryl and perfluorinated organoarsines at various operating conditions" ... 

The differential (isosteric) adsorption heats Qa for negligible adsorbent surface occupation were derived from the linear dependence of log(Vm/T) against 1/T slope, where Vm is the specific retention volume (cm /g) [9]. Direct evaluation of the Vm values over the temperature range 100-150°C and calculation of the isosteric adsorption heats were performed employing hexane as the reference substance. For other adsorbates, the relative retention volumes Vrel = y orbate yhexane gj.g measured experimentally, allowing for the adsorption heat increments with respect to hexane to be estimated. 

Adsorption and separation processes involve also the active sites existing on the external surface of (CH3)4N - montmorillonite, their role being more important in the case of adsorption of isoparaffins and cyclohexane molecules. This is indicated by a significantly smaller differences between the specific retention volumes of iso- and cycloparaffins on natural and tetramethylammonium samples than the difference in Vm values characteristic for n-paraffines on the same adsorbents. Thus, the tetramethylammonium montmorillonite adsorbs n-alkanes selectively from the mixtures containing iso- and cycloparaffines, which is confirmed by the values of relative retention volumes for such hydrocarbon pairs as, for instance, n-heptane / 2,4- dimethylpentane. These can be easily calculated from the data presented in Table 5. 

From the experimental data [44], the selectivity of the adsorbents was calculated as the relative retention volume of the separated species. It can be seen in Table 9 that Cs-CPM adsorbent possesses better selectivity than the Ca-CPM one with respect to the separation of the isomers of xylene and ethylbenzene with like properties. The results enabled us to conclude that the separation of the isomers of m- and p-cresol up to the purity degree of 99% can be achieved using the columns with the theoretical plates number (TPN) not exceeding 200, and that for the o- and m-isomers - employing the packed intermediate efficiency columns with TPN of 2000-2500. To separate the same pairs of the species using the Ca-CPM, the capillary columns with the TPN of 20000 were reqnired. 

The retention volume of a compound, determined as shown in Fig. 1 (see p. 97), is a characteristic property when all of the conditions of the separation can be duplicated exactly. To avoid slight differences that may be caused by minor variations in the operating conditions, retention volumes are usually made relative to that of a standard compound which is assigned a value of unity. An unknown may therefore be identified by comparison of its retention volume with that of an authentic sample, both made relative to the same standard and obtained under the same conditions. The condition most difficult to duplicate is the column packing, and this situation makes it unlikely that relative retention volumes reported can be reproduced with numerical exactitude. However, the relative orders of a series of such values should be reproducible, provided that the same liquid phase is used. It should be emphasized that identification by the comparison of relative retention volumes does not obviate the need for confirming the identification by conventional methods. Wherever possible, the separated components should be collected from the effluent gas-stream and be identified by infrared spectroscopy and by the preparation of... 

Relative retention volumes and retention indices have been reported399 for silicon tetrachloride, methyltrichloro- and dimethyldichloro-silane, phosphorus trichloride and phosphorus oxychloride at 50 °C on stationary phases containing vaseline oil, poly-siloxane liquids VKZh-94, PFMS-2 and DS-701, fluorosilicone oil 169, dinonyl phthalate and dibutyl phthalate. The retention index for phosphorus oxychloride increased with increase in the polarity of the stationary phase that for silicon tetrachloride remained practically constant. 

Olefin B.p. Relative retention volume Equilibrium constant ... 

Relative retention volume V m retention volume corrected to take into account the volume of mobile phase in the system see retention volume. 

Retention volume, Fr volume of mobile phase required to elute a component. Corrected or relative retention volume Kr is a more accurate measure of the delay due to the retention forces by allowing for the volume of mobile phase in the system V. Usually a constant mobile phase flow-rate, Fq is maintained hence the more convenient term retention time is used. 

Solutions. For various organic compounds of Si, Ge, and Sn, including Ge(C4Hg)4, relative molar enthalpies and entropies of solution in nonpolar and polar stationary phases have been determined by GLC [46, 47] GLC retention volumes [22] and relative retention volumes [46] were reported. 

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