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Figure 2 Retention indices on squalane (IS) as a function of Tc for
isothermal GC at 27°C, 49°C, 67°C, and 86°C 2.
TMP Trimethylpentane ECP Ethylcyclopentane
MCH Methylcyclohexane DMH Dimethylhexane
Retention Cross-over Phenomenon
inGas Chromatography- Can the
Mystery be Revealed? Part 1
Continued from page 2
Therefore, the dependence of the retention time upon column temperature is
usually expressed graphically as the log of the retention parameter (net retention
time t´R or retention factor k´ or retention index I) vs. Tc or 1/Tc, where Tc is
the absolute column temperature. In many cases, the plots are linear over the
temperature range employed and, furthermore, the lines are approximately
parallel to each other indicating that there is little change in selectivity by
changing the column temperature in isothermal mode. This is valid for chem-
ically similar compounds. But closer inspection reveals that some lines diverge
slightly in their slope and even cross each other (Figure 2).2 The practical
implication is coelution of the two compounds at the temperature where the
lines intersect. By further changing the column temperature the compounds
are again separated but in reverse elution order. As mentioned above, this kind
of behavior is often experienced when compounds of different chemical
nature are analyzed on moderate to highly polar stationary phases.
But not only compounds with different functional groups will behave this
way! In the next issue of the Restek Advantage, you will see examples of
aliphatic versus cyclic compounds or cyclic compounds differing in their ring
number, and the cross-over effect on non-polar columns.
References:
1 Mehran M. et al., HRC, 14 (1991) 745 – 750.
2 Hively, R.A. and R.E. Hinton, J. Gas Chromatogr. 6 (1968) 203 – 217.
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more to reveal!
See the next issue of the Restek Advantage for Part 2 of
Retention Cross-over Phenomenon in Gas Chromatography–
Can the Mystery be revealed?
IS