Retention Indices A Generalization of Relative Rt Information

In the last paragraph of Section 12.8.2, we saw in the case of xylene isomers that even the great compound fingerprinting capability of full scan MS detection could not distinguish 

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. 

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