Page 28 - Multidimensional Chromatography

18 Multidimensional Chromatography

2.2 TRANSFER TECHNIQUES

This chapter will describe some of the direct transfer techniques used for both normal
phase and reversed phase eluents. An overview of the various techniques (direct and
indirect) for the analysis of water or water-containing eluents will also be given. In
fact the nature of the eluent greatly influences the choice of the transferring technique,
as will be explained in the discussion for each section. Figure 2.1 summarizes the
concepts of eluent evaporation (with some subclasses), thus allowing the transfer of
large LC fractions to the gas chromatograph.

2.2.1 RETENTION GAP TECHNIQUES

The retention gap method (1, 2) represents the best approach in the case of qualita-
tive and quantitative analysis of samples containing highly volatile compounds. The
key feature of this technique is the introduction of the sample into the GC unit at a
temperature below the boiling point of the LC eluent (corrected for the current inlet
pressure), (see Figure 2.2). This causes the sample vapour pressure to be below the
carrier gas inlet pressure, and has two consequences, as follows:

• volatile components are reconcentrated by the solvent effects, primarily sol-
vent trapping (3);
• the high-boiling compounds are spread by band broadening in space.

A layer of condensed eluent is built up ahead of the evaporation site which acts as
a thick layer of retaining stationary phase, thus blocking the further movement of all
but the most volatile compounds into the column. Solvent evaporation, therefore
proceeds from the rear towards the front of the sample layer (see Figure 2.2).
These effects refer to the reconcentration obtained with an uncoated inlet. In fact,
the term retention gap means a column inlet of a retention power lower than that of
the analytical column. This retention gap is placed in front of the analytical column,
thus allowing different reconcentration mechanisms to occur.
Together with this solvent effect, another effect, called phase soaking, occurs in
the retention gap technique: if a large volume of solvent vapour has saturated the car-
rier gas, the properties of the stationary phase can be altered by swelling (thicker
apparent film), a change in the viscosity or changed polarity. The consequence is that
the column shows an increased retention power, which can be used to better retain
the most volatile components.
During the evaporation process, band broadening in space spreads the high-
boiling compounds. Two retention gap effects can reconcentrate the bands, namely
phase-ratio focusing and cold trapping, generally known as stationary phase focus-
ing effects.
Phase ratio focusing is based on the higher migration speed of components
through the retention gap compared to that through the analytical column.
Reconcentration depends on the ratio between the retention power in the pre- and in