Retention times time/carbon number plot

Mathematical methods for determining the gas holdup tine are based on the linearity of the plot of adjusted retention time against carbon number for a homologous series of compounds. Large errors in this case can arise from the anomalous behavior of early members of the homologous series (deviation from linearity in the above relationship). The accuracy with which the gas holdup time is determined by using only well retained members of a homologous series can be compromised by instability in the column temperature and carrier gas flow rate [353,357]. The most accurate estimates... 

In practice, the retention index is simply derived from a plot of the logarithm of the adjusted retention time versus carbon number times 100 (Figure 4.4). To obtain a retention index, the compound of interest and at least three hydrocarbon standards are injected onto the column. At least one of the hydrocarbons must elute before the compound of interest and at least one must elute after it. A plot of the logarithm of the adjusted retention time versus the Kovats index is constructed from the hydrocarbon data. The logarithm of the adjusted retention time of the unknown is calculated and the Kovats index determined from the curve (Figure 4.4). 

A plot of log retention time vs. carbon number for gas chromatography of a series of saturated fatty acid methyl esters. 

In Chapter 8 we will see that programmed temperature GC results in a regular, linear relationship between retention time and carbon number. Under those circumstances logs should not be used in Eq. (8) and in the retention index plot. The increase in temperature decreases the partition coefficients and effectively removes the logarithmic dependence of I. 

Plot of Log Retention Time versus Carbon Number... 

Plot a graph of logi0f against the molecular weight or alternatively the carbon number (i.e. the number of carbon atoms in the molecule) of the ketone. Estimate the retention time of a ketone containing six carbon atoms and check your result by a suitable injection. Finally examine the chromatographic behaviour of 3-methylbutan-2-one, 4-methylpentan-2-one and 5-methylhexan-2-one. 

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... 

A most important contribution to the above means of identification is the Kovats retention index system [28]. The Kovats retention index of a compound is 100 times the number of carbon atoms in a hypothetical n-alkane that would display in the given system the same retention as the compound in question. Hence, the retention index system essentially is also based on the regularities between the retention data and number of carbon atoms in homologous compounds. The concept of the Kovats retention index system is illustrated by the model in Fig. 3.7, which shows a plot of log A) values for homologous compounds of the type CH3(CH2) X and for n-alkanes against carbon number. It is apparent that the retention index of, e.g., C2H5X is 560, i.e., 7(C2HSX) = 200 +... 

Fig. 2 Semilogarithmic plot of organ retention half-times for PFCs, as a function of molecular weight. Open symbols indicate PFCs with lipophilic character F-nn E n and n represent the number of carbon atoms in the homologous series CnF2n+iCH=CHC F2n +i, i = iso for the other symbols see Table 1.

It has lung been known that within a homologous series, a plot of the logarithm of adjusted retention time r = r the number of carbon atoms is lin-... 

Figure 6 GCxGC 3 D-colour plots of the petroleum hydrocarbons at the site on day 40 (a) and day 50 (b) after the spill illustrating the extent of loss of the n-alkane envelope (n-alkanes are denoted by the carbon number at the top of the peaks) relative to the aromatic components of the petroleum hydrocarbons that appear behind the alkanes (i.e., at higher second-dimension retention times) [11].

It has long been known that within a homologous series, a plot of the logarithm of adjusted retention time (carbon atoms is linear, provided the lowest member of the series is excluded. Such a plot for C4 to C9 normal alkane standards is shown in Figure 27-18. Also indicated on the ordinate are log adjusted retention times for three compounds on the same column and at the same temperature. Their retention indexes are then obtained by multiplying the corresponding abscissa values by 100. Thus, the retention index for toluene is 749, and for benzene it is 644. 

ECL chromatographic retention behaviour of fatty acid methyl esters relative to the saturated homologues, which exhibit a straight line when the log of the retention times are plotted against the number of acyl carbons atoms. Used in the tentative identification of fatty acids by gas chromatography. 

Pollard et al have investigated the thermal decomposition of arylsilanes. They identified the pyrolysis products using retention time techniques. A graph of log retention time against carbon number indicated that the assignments were correct. Fig. 93 shows a plot of boiling point versus log corrected retention time (t R). 

Figure 5.2. Plot of the logarithms of the retention times of the methyl ester derivatives of a homologous series of saturated fatty acids against the number of carbons in the aliphatic chains (Equivalent Chain Lengths (ECL)), on a packed column of EGSS-X" (see footnote to Table 5.1 for further chromatographic details). The elution times of some unsaturated fatty acids are indicated.

---