Net retention time

Net retention time (volume) 5 HMR, bonded phases (LC) 336 NMR, interface (LC) 998 Noise (detecto 86... 

While retention time is used for peak identification, it is dependent on the flow rate, the column dimension, and other parameters. A more fundamental term that measures the degree of retention of the analyte is the capacity factor or retention factor (k ), calculated by normalizing the net retention time (% > retention time minus the void time) by the void time. The capacity factor measures how many times the analyte is retained relative to an unretained component. ... 

Efficiency or plate count (N)—an assessment of column performance. N should be fairly constant for a particular column and can be calculated from the retention time and the peak widths. Selectivity (a)—the ratio of retention k ) of two adjacent peaks. Sample capacity— the maximum mass of sample that can be loaded on the column without destroying peak resolution. Capacity factor k )—a measure of solute retention obtained by dividing the net retention time by the void time. 

Rohrschneider [205,210] has developed a scheme for the characterization of stationary phases for gas chromatography. The scheme is based on the retention index (/). The retention index is a dimensionless retention parameter, designed to be independent of flow rate, column dimensions and phase ratio. The retention index of a solute is defined as 100 times the number of carbon atoms in a hypothetical n-alkane, which shows the same net retention time as that solute. This definition is illustrated in figure 2.2. By plotting the logarithm of the net retention time against the number of carbon atoms in n-alkanes, a straight line is obtained. The net retention time for a solute may then be located on the vertical axis, and the retention index found on a horizontal scale, which represents 100 times the scale for na... 

Figure 2.2 Illustration of the definition of the retention index in GC. ncis the number of carbon atoms in n-alkanes, / the retention index and t R the net retention time. 

Figure 5.13 Curves relating the isocratic composition (p ) to the net retention time under gradient conditions for various values of the isocratic capacity factor. Curves calculated on the basis of eqns.(3.45) and (3.46). Linear gradient 0 — 100% methanol in water. /0= 125 s. Figure taken from ref. [536]. Reprinted with permission. 

Laub [544] has suggested the use of 1/ Nne as a criterion. Noyes [547] suggested that for optimization on a given column log f2/ i (where retention times and not net retention times are used) might relate more easily to Rs than does amin. However, Smin [546] may be obtained just as easily from a chromatogram and this quantity is exactly proportional to Rs, as long as the plate count (N) is constant. 

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. 

There are several reports in the literature that measure binary adsorption equilibria using gas chromatography [4,S,6]. In GC techniques the adsorbent is equilibrated with a continuous flow of carrier gas (gas 1). Then a pulse of gas 2 is injected at the column inlet. A peak of the gas 2 is eluted at the exit of the column after some time. Net retention time (or volume) is calculated from the first moment of the peak after correcting for void volume (by measuring the retention time of a non-adsorbing species). If the carrier gas is inert (i.e. helium) the net retention time is related to the pure component Henry s constant. Typical binary measurements reported so r use a mixture of the two gases as carrier and introduce a small perturbation in composition. The net retention volume is related to the thermodynamic properties by [4]... 

The success of the method prompted the design of an Automatic Molecular Probe apparatus for the collection of retention data 82). At a preset cycle time a mixture of solute and noninteracting marker was iiqected into the carrier gas and the output of the GC detector was fed to an electronic peak detector. The temperature in the oven was programmed linearly and recorded with a thermocouple. Upon conversion into digital form, a printout of net retention time and temperature was obtained. Retention diagrams have been obtained with this apparatus for high and low density polyethylene 82, 83). 

Figure 8 shows the effect of the alkyl chain length on the retention of organobromoar-sines. A plot of the logarithm of the net retention time of these compounds vs the boiling point of each component gives a nearly linear relationship (Figure 9). However, in the same conditions methyldibromoarsine could not be resolved from methylbutylbromo-arsine.

FIGURE 9. Plot of logarithm of net retention time vs boiling point (2) dimethylbromoarsine, (3) methylethylbromoarsine, (4) methylbutylbromo-arsine. Reprinted with permission from Reference 151. Copyright (1962) American Chemical Society... 

Measurement Procedure. IGC measurements were started after the thermal and flow equilibrium in the column were stable (2 to 3 h). To facilitate rapid vaporization of the probe (0.01 yL), the injector temperature was kept 30°C above the boiling point of the probe. Measurements were made at five carrier gas flow rates. The retention volumes of six injections for each probe and twenty injections of the marker (H2) at a given flow rate were averaged. The values obtained were extrapolated to zero flow rate to eliminate the flow rate dependence of the retention data. The net retention time (tR) is defined as the time difference between the first statistical moment of the solvent peak and that of the marker gas. Thus, tR was calculated by an on-line computer statistical peak analysis rather than the retention time at the peak maximum (tp,maY). This eliminated inaccuracies arising from slight peak asymmetry, which occurs even for inert and well-coated supports. The specific retention volumes (Vg°) derived from tR and tR max differed by as much as 5% for small retention times and slightly skewed peaks (11,12). 

In Equation la, W is the weight of the stationary phase and tN is the net retention time defined as... 

If za is substituted by the IC column length /c, the equation yields the net retention time in the column. We again stress that we discuss the internal chromatograms rather than the elution curves, unless stated otherwise. It means that now the total retention time equals the duration of the experiment. The latter is preset by the experimenter, thus characterizing experimental conditions rather than the results. Therefore, we need to know how za depends on rather than vice versa, and a more logical form of Eq. 4.1 would be ... 

Suppose that the TC column temperature profile is linear, that is Tz = 7s - gz and dz = -dTz/g Then the net retention time equals ... 

The value of 9t is the ratio of the times spent by nonadsorbable and adsorbable species to pass an isothermal column. The net retention time is Z- (from Eq. 2.9) multiplied by ra. In a smooth open cylindrical column of the length w x 1 s, we have Zz = %dcioum/4Q = umjdc.U leads to ... 

Thus, the required value of Aads H can be obtained from the slope of the appropriate linear regression though, in principle, from but two experimental values of the constant. The same result is also provided by simply differentiating the net retention time or the retention volume, because Eq. 5.6 shows that ka Qt. For example,... 

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