Retention index window

Figure 13. Chromatogram of silylated ether extract (pH 5.5) of human urine prior to enzyme hydrolysis. (Chromatographic conditions same as Figure 4). Shaded area denotes retention time window during which the Olfax searched for the 9-THC-ll-oic acid/TMS contracted spectrum. Inset Auto-Assay analysis for THC panel of 8 compounds in which 240 ng (2.40 X 10 1 fig) of A9-THC-ll-oic acid was found with a Confidence Index of 62.

A summary of the retention indices of more than 80 alkyl nitrates has been reported. In general, the separation of the alkyl nitrates follows the order of boiling points in Table 19.8. The 1-n-alkyl nitrate is always the last member of each group to elute. Thus, it can be used as a window marker or retention index marker compound. The most abundant alkyl nitrates found in atmospheric samples are typically the secondary nitrates. For this reason, the 2-Cs and 2-Cio alkyl nitrates have also been suggested as marker compounds. ... 

Some standard one-dimensional GC methods use reference peaks to help recognize drift [23]. For more widely varying chromatographic conditions, retention times for targets can be related using a linear retenhon index (LRI) [24], in which retention times are referenced relative to the retention times of marker compounds. A common LRI scheme uses the n-alkanes as marker points with indices equal to 100 times the carbon number (foUowing the Kovats index [24]) then the indices for peaks between marker points are computed using piecewise hnear interpolation. If retention-time windows are defined relative to marker p>eaks that can be located, then any linear retention-time transformation observed in the marker jjeaks can be apphed to the windows used for chemical identification. 

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