Page 90 - Multidimensional Chromatography

82 Multidimensional Chromatography

the dispersion/diffusion processes in each column. For a discussion on two-dimen-
sional separations, with zone formation, considerations of resolution, and expres-
sions for dispersion in each dimension, the reader is directed to the treatment by
Giddings (12). If single dimension capacity is calculated by how many peaks of
basewidth x ( 4 )s can be fitted into a total available elution time of y s, then
approximately y/x peaks can be separated. For GC GC, we can approximate the
capacity by calculating how many peak ‘areas’ can be fitted into the total available
area. Taking a peak as having a 15 s basewidth in dimension 1, and an average of 200
ms in the second dimension with a modulation 4 s, then for a 60 min (3600 s) analy-
sis we should have a capacity of (3600/15) (4/0.2) 4800 peaks. This statisti-
cally available capacity probably cannot all be used in an analysis, but more
importantly it does increase dramatically the potential separation power, and many
more compounds can be separated, hence achieving the goal of MDGC analysis. To
properly implement the comprehensive GC GC method, zone compression is
used (an option not available in the TLC experiment or liquid mobile phase separa-
tions), which in combination with fast second-dimension analysis gives very much
improved sensitivity (13, 14) with improvements of 50-fold or better being reported.

4.3 INTRODUCTION TO MODULATION TECHNOLOGY

There are essentially two procedures which can deliver the requirements of modula-
tion between the two columns as defined above–these are the thermal sweeper/mod-
ulator of Phillips, and the cryogenic modulator of Marriott. A third option which has
been reported is the valve/diaphragm modulator of Synovec and co-workers (15).
The latter is capable of producing very narrow peak pulses to a second column, but it
does this by a rapidly switching diphragm valve, which momentarily diverts a slice
of the peak entering the valve to a second column. This may be as small as a 10 ms
zone from each 1s portion of a peak, although the rest of the solute is vented. Whilst
the chromatogram presentation seems to be analogous to that of the other compre-
hensive gas chromatography technologies, this diaphragm method does not increase
the mass sensitivity of detection, and there is no solute compression. The first two
options will be discussed further below.

4.3.1 THERMAL SWEEPER

The thermal sweeper is a commercial product licensed to Zoex Corporation,
Lincoln, NA, USA (16). The sweeper incorporates a slotted heater (operated at about
100 °C above the oven temperature) which passes over the capillary column (nor-
mally an intermediate thicker film column is used in this region as an accumulator
zone). Figure 4.3 is a schematic diagram of how the instrumental arrangement may
be considered. The greater temperature of the rotating sweeper forces the solute
which has been retained in the phase in the accumulator section to be volatilized out
of the phase into the carrier gas stream, and then bunched up and brought forward