Figure 4.26
Measurement of adjusted retention time, t'R.The value of αx is then compared with published values of known compounds for the appropriate
temperature and stationary phase used.
If a sample contains one or more members of a homologous series, identifications can be made using a
plot of log tR against the number of carbon atoms, previously prepared from standards. The plot, which
is valid for one temperature only, is linear and can be used for alkanes, alkenes, alcohols, aldehydes,
ketones, esters and ethers.
A universal system for qualitative identification was devised by Kováts some years ago. Based on the
linear relation between log t'R and the number of carbon atoms for a homologous series, Kováts selected
n-alkanes as standards for the following reasons:
(i) they cover a very wide range of boiling points
(ii) they are readily separated on virtually any column
(iii) they are chemically very stable and non-toxic
(iv) they are easily obtained and relatively cheap.
The Kováts retention index for each n-alkane is defined as 100 times the number of carbon atoms in the
chain at all temperatures and for any column, e.g. pentane is 500 and octane is 800. A plot of log t'R
against retention index for a series of n-alkanes is linear as shown in Figure 4.27. A substance with a
retention time between an adjacent pair of n-alkanes will have a retention index that can be determined
graphically or calculated by linear interpolation. For example, using the plot in Figure 4.27, the
graphical method for an unknown substance that elutes with a retention time a little more than halfway
between those of n-octane (C 8 ) and n-nonane (C 9 ), will have a retention index in the region of 860. The
possible identity of unknowns can be ascertained from tables (published or previously compiled in
house) or by using a computer database. Kováts indices are also used as a basis for comparing GC
stationary phases by computing a set of values from the retention times of a selected group of test
compounds on each phase.