Page 60 - Multidimensional Chromatography

Multidimensional High Resolution Gas Chromatography 51

long column lengths used. Practically, the determination of a species of interest
may take an extremely long period of time if single-column efficiency is the only
tool used for its chromatographic isolation. The use of a pair of coupled columns
may often result in the equivalent peak capacity being generated in a substan-
tially shorter period of time. This, of course, is only really a useful option if the
analysis required is focused on only a limited number of analytes, but does offer
very significant time reduction possibilities if the methodology is optimized.
• Sample capacity. In common with the speed of analysis, the isolation of an ana-
lyte in a complex mixture using only a single dimension often results in the
application of narrow internal diameter capillary columns. While it is possible
when using large-volume sample techniques to introduce relatively large quanti-
ties of material on to capillary columns (5, 6), a limit may be reached when the
target analyte is in the present of an excess complex matrix. In these instances,
overloading of the column with interference material may well occur. Since the
sample capacity q of a capillary column is related to the internal diameter by the
following relationship:
5 1
q C d (1 k)(Lh) 2 (3.1)
2
m c
where C m – is the solubility in the mobile phase, d c is the internal column diameter, k
is the capacity factor, L is the column length and h is the height equivalent of a theo-
retical plate. There is clearly a very rapid decrease in available sample capacity when
column internal diameters are reduced. This may be effectively overcome by either
fractionation of unwanted matrix material or the generation of equivalent peak
capacities using two coupled GC columns with wider internal diameters.

3.2.2 TWO-DIMENSIONAL GC APPARATUS

For all three of the examples of two-dimensional gas chromatography configurations
shown in Figure 3.1, an interfacing unit is required between the primary and sec-
ondary columns. The types of instrumentation used for this purpose range from rela-
tively simple manually operated valves, to more complex but flexible computer
pressure and flow control systems. What is not indicated on the figure, but is, of
course, a further operational parameter is that the two columns used may be operated
at the same or independent temperatures, through the use of either single or multiple
GC ovens. In what may be considered the simplest case, both GC columns are con-
tained within a single GC oven, with a mechanical valve used to direct flow from the
primary to the secondary column at the appropriate moment during the primary
separation. This mode of operation highlights a major limiting factor in two-
dimensional gas chromatography–that peak widths introduced to the second column
from the first will critically limit the peak capacity of the second column. This arises
since the peak width eluting from the primary column must be less than the peak
width resulting from second column unless a refocusing or zone compression is
performed. Peak widths of 10 s may be typical of eluting peaks in capillary GC, but, of