34 feature article -- Multidimensional LC www.sepscience.com
Presentation of the data: In LCxLC,
only one detector is used and the
chromatogram obtained actually
consists of several second dimension
chromatograms. This is not a
feasible way to interpret results and,
therefore, data are converted to a
contour or colour plot (Figure 5).
Set-up of the modulation valve:
LC�LC systems can be easily
constructed using conventional
commercial high-pressure liquid
chromatographic setups equipped
with extra pump(s) and column(s)
and an automated switching
valve. In practice, there are several
instrumental setups for LC�LC, but
inmostcases8-portor10-port
switching valves have been used.
Best results are obtained when the
loop volume is clearly larger than
the volume of the fraction being
collected. For example, if the fraction
volume is 100 �L, an appropriate
loop size is 125 to 150 �L.
There are two approaches that
should be tested first, namely
symmetrical and asymmetrical
configuration, as shown in Figure 6.
Bothuseeitheran8-portor10-port
valve.
configuration � the flow direction
is the same for both loops. (b)
Asymmetrical configuration in which
each loop is flushed in a different
direction into the second dimension.
L1 = loop 1; L2 = loop 2; LC = from
the first dimension pump; P = from
theseconddimensionpump;SEC=
to the second-dimension column.
The symmetrical configuration
allows flushing of the fractions to
the second dimensions in the same
direction in which they were loaded,
thus making both positions identical.
There are also more complex
solutions to be used. A modification
of the sampling loop interface is the
use of two guard columns instead
of loops. The effluent from the
first column is alternately trapped
and sampled onto the secondary
columns through a guard column
interface. When one guard column
traps the eluate, the other injects
the previously trapped components
onto a secondary column. This cycle
is repeated throughout the analysis.
The guard column is of the same
material as the second-dimension
column. A similar approach, utilizing
18solid-phaseextraction(SPE)
columns, has also been developed.
In this system, the use of several
SPEcolumnsallowedlongertimes
for second-dimension separation
without compromises in sampling
frequency.
It is also possible to use two second-
dimension columns connected in
parallel or even use two modulator
valves with two second-dimension
columns. Also a stopped-flow
approach utilizing two 6-port valves
has been developed.
LCxLC-MS
The same MS systems that are used
in conventional one-dimensional
LC-MS can be used also in LC�LC-MS.
However, there are a few additional
points that must be considered in
the combination of LC�LC with MS:
1.Because the second-dimension
eluent must be compatible with
MS some LC�LC combinations are
not suitable with MS detection;
for example, ion-exchange
chromatography is not the best
option for the second dimension
separation.
2.As the second-dimension
separation in LC�LC is typically
Figure 4
Figure 4: Continuous and valve-switching concepts in LCxLC.

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