Figure 4.25
Headspace sample injection system (positioned in a heated oven).Comparison of Retention Times
Provided that operating conditions remain constant and are reproducible, the retention times of the
components of a sample can be compared directly with those of known materials and synthetic
mixtures. An unfamiliar peak can sometimes be identified by 'spiking' a sample with a pure substance
whose presence is suspected. An increase in the size of the unknown peak is good evidence for it being
the substance added. As two materials may have the same retention time for a given stationary phase,
this method is not infallible. It is advisable, therefore, to run unknowns on two different stationary
phases.
In cases where a mixture has a large number of components, or pure standards are not available,
published retention data must be consulted. The uncorrected retention time, tR (p. 86), is not suitable for
this purpose because it cannot be compared with data from different columns and instruments. Valid
comparisons can be made using relative retention data which are dependent only on column
temperature and type of stationary phase. An adjusted retention time, t'R, is first obtained by subtracting
from tR the time required to elute a non-retained substance such as air (Figure 4.26)
Thus,
where tair denotes the time taken to elute the air peak. The term tair is related to the volume of the
injection port, column void space and detector, known as the dead volume VDV, by the expression tair =
VDV/F where F is the volumetric flow rate. Values of t'R are obtained for the sample components to be
identified and also for a standard substance chromatographed under identical conditions, preferably by
adding it to the sample. The relative adjusted retention time, αx, for component x is given by