Retention time, adjusted definition

As mentioned in Section 11.8.4, the parameters that are most important for a qualitative analysis using most GC detectors are retention time, tR adjusted retention time, t R and selectivity, a. Their definitions were graphically presented in Figures 11.16 and 11.17. Under a given set of conditions (the nature of the stationary phase, the column temperature, the carrier flow rate, the column length and diameter, and the instrument dead volume), the retention time is a particular value for each component. It changes... 

The definition of retention times, hold-up time, retention time, and adjusted retention time, have been given previously (paragraph 1.2). 

Note the use of acmal rather than adjusted retention times. Although not as accurate or conforming to the operational definition of RI as defined in Eq. (12.1), use of Eq. (12.2) with retention times acquired over the linear ramp portion provides a useful approximate value. Essentially the RI values of unknowns are calculated by linear interpolation between the values of the flanking alkanes. If one had the PDMS phase RI values for the three dimethyinaphthalenes illustrated in Fig. 12.11, one could probably assign identity to these compounds, whose mass spectra in GC-MS would be indistinguishable. To be certain, however, one would need to have RI values for aU such isomers (there are many more than these three). If there were some others whose RI values were very close to those observed in that chromatogram, it would be dangerous to rely on RI to chnch the identification. At best it would tell which were the most likely candidates, and then the confirmation standards could be selected. If RIs are within less than one unit of one another. 

Unfortunately retention times are not on their own sufficiently definitive to characterize compounds. Slight differences in carrier gas flow rate, column temperature and column condition can alter retention times. Such differences are usually more pronounced when the same column is used in different gas chromatographs or with different columns (same packing) in the same gas chromatograph, and for this reason retention data are most frequently reported as relative retention times or retention indices. The relative retention time of a solute is the ratio of the adjusted retention time of the solute to that of a selected reference compound and in many of the chromatograms shown elsewhere in the book (see also the Appendix), n-tetracosane is used as the... 

When a flocculated feed is added to a filter tank, there is a definite time lag before this material reaches the surface of the filter medium. Since this lag time is not known at the time of testing, a lag time of 8 to 10 minutes should be allowed before starting the first leaf test on a flocculated shiny. Two, or perhaps three, tests can be run before the elapsed time exceeds the probable retention time in the full-scale filter tank. With knowledge of the elapsed time after flocculation and data relating to the rate of degradation, the rates obtained on the leaf test runs can be adjusted to some constant lag time consistent with the anticipated full-sc e design. 

Retention index. I. A number relating the adjusted retention volume of a component A to the adjusted retention volumes of normal paraffins. Each n-paraffin is arbitrarily allotted, by definition, an index of one hundred times its carbon number. The index number of component A is obtained by logarithmic interpolation. 

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