Page 315 - Multidimensional Chromatography

Industrial and Polymer Applications 305

phases can be modified to effectively clean up unwanted components in sophisti-
cated matrices. This, in turn, reduces the number of components actually present in
the final analytical step (GC) by allowing heart-cut fractions of the selected peaks of
interest. HPLC can effectively separate compounds based on chemical classes and
can be used to enrich very dilute samples. The preseparation of components by
HPLC prior to introduction into the GC system also increases the sensitivity. This
can be achieved because of the efficient transfer of the whole HPLC fraction contain-
ing the compounds of interest and the efficient removal of interfering materials that
may suppress the detection limits obtained by analysis during the GC step. The pre-
treatment of samples by HPLC offers three main advantages: preseparation into
chemical classes by their different polarities, cleanup of the components of interest
from a ‘dirty’ matrix, and sample enrichment (9).
Normally, most LC–GC applications use normal phase LC as the first separation
method. Therefore, those low-boiling, non-polar solvents which are normally used in
normal phase LC, plus small LC fraction volumes, are recommended. In addition,
the volitization of the eluent which is being transferred is another important consid-
eration. The volume of the transferred LC eluent may pose a problem due to column
and detector overload and loss of resolution of the transferred fractions due to sol-
vent overlap of the desired peaks. For these reasons, low eluent flow rates and rela-
tively short LC retention times are normally used. The introduction of large volumes
has an effect on the efficiency of the solute resolution and therefore the inlet temper-
ature must be maintained so that it is close to the LC eluent boiling point. Ensuring
the proper temperature avoids overload of the GC detector and column and thus pre-
vents distorted, broad and even split peaks. These effects have been minimized by
the introduction of retention gaps prior to the GC column of uncoated, deactivated
capillary columns that allow the focusing and concentration of the solutes and have
low retention capabilities. Solvent evaporation can be allowed in these retention gaps
by two techniques, namely concurrent solvent evaporation (the GC initial tempera-
ture is higher than the boiling point of the eluent) and partial concurrent solvent
evaporation (the GC initial temperature is lower than the boiling point of the eluent).

12.4 LC–GC APPLICATIONS

12.4.1 NORMAL PHASE LC–GC APPLICATIONS

The methods of analysis of polymer additives and chemicals, such as hydrocarbons,
alcohols, etc., are not only restricted to the field of polymer chemistry but can also be
applied for the analysis of such materials in the field of food chemistry. In addition,
the analysis of polyaromatic hydrocarbons in edible oils has been of extreme impor-
tance. Polymeric packaging materials that are intended for food-contact use may
contain certain additives that can migrate into the food products which are actually
packaged in such products. The amounts of the additives that are permitted to
migrate into food samples are controlled by government agencies in order to show