Page 57 - Multidimensional Chromatography

48 Multidimensional Chromatography

applied to moderately complex samples such as gasoline, they still suffer from
1
incomplete resolution of the mixture. The analysts most common approach
to improve resolution is generally a modification of single-column physical parame-
ters, increase the length, decrease the column internal diameter, or a combination of
the two. For many complex sample analyses, however, changes such as these offer
only slight improvements in resolving typical target compounds, which may well be
isomeric or enantiomeric in nature. It is a well-known theory that a doubling in col-
umn length results in only a √2N increase in the number of theoretical plates. What
is more often required is not simply greater numbers of theoretical plates on the
same column, but complementary selectivity, achieved by using a serially coupled
secondary separation. The degree to which a multidimensional GC separation pro-
duces enhancement in peak capacity can be related to the degree of orthogonality
between the stationary phase selectivities in each dimension. For any given applica-
tion, therefore, single-column methods may be described as being reliant on column
efficiency, whereas-two dimensional system depend on stationary phase selectivities.
Application of two-dimensional GC to increase the speed of analysis was pio-
neered initially by industrial and process applications, which required on-line high-
speed analysis of only a single or very limited number of target analytes. In this
mode, the primary GC column has taken on a role similar to the primary column in
LC–GC, in that it is used more for sample pre-fractionation than high resolution
separation. Through the use of pre-columns and backflushing, many applications
have taken advantage of the power of two-dimensional GC, to allow the rapid anal-
ysis of relatively volatile components in a matrix of higher-molecular-weight
species.

3.2 PRACTICAL TWO-DIMENSIONAL GAS CHROMATOGRAPHY

In many respects, the coupling of GC columns is well suited since experimentally
there are few limitations and all analytes may be considered miscible. There are,
however, a very wide variety of modes in which columns may be utilized in what
may be described as a two-dimensional manner. What is common to all processes is
that segments or bands of eluent from a first separation are directed into a secondary
column of differing stationary phase selectivity. The key differences of the method
lie in the mechanisms by which the outflow from the primary column is interfaced to
the secondary column or columns.
In most two-dimensional GC applications reported from the late 1960s until the
early 1990s, coupling was via the transfer of limited numbers of discrete fractions of
eluent from one standard capillary column to a secondary one. In early work, this
focused on using packed columns for at least one of the dimensions, although cur-
rently most new techniques report the coupling of two or more capillary columns.
This mode is often described in literature as ‘heart-cut’, or linear in nature. Since
both columns have roughly equivalent peak capacities, the time taken for the analy-
sis by each dimension is significant with respect to the other. That is to say that only