Page 72 - Multidimensional Chromatography

Multidimensional High Resolution Gas Chromatography 63

Two-dimensional gas chromatography plays an important role in such analysis, in
combination with a wide array of sample preparation, and preconcentration tech-
niques and injection devices (25–27). While preconcentration prior to the primary
column separation is the first step in obtaining sufficient target compounds, ‘on-
column’ subsequent refocusing at the midpoint between the dimensions can also be
used as a method of preconcentration. This can be achieved through either multiple
heart-cuts at the same primary column retention time or through zone compression,
thus leading to a narrower eluting band (and hence greater mass/unit time) emerging
from the second column into the detector.
Sample concentration, and hence enrichment, is certainly a key issue in this area
of analysis, since complementary information obtained from NMR or IR spectro-
scopic detection is often desirable in conjunction with mass spectrometry. Detection
methods such as these have far higher concentration thresholds than MS and obtain-
ing adequate quantities of material for detection becomes a significant challenge.
It is worth noting that while a significant number of papers on flavour and
fragrance analysis are published each year, this constitutes perhaps only a fraction
of the amount of research time spent on this area, and it is likely that the commercial
nature of such work has resulted in an under representation in the published litera-
ture. Much of the early two-dimensional GC analysis work on flavours and
fragrances focused on detecting product deterioration rather than on identifying
active fragrance components (28), while work by Nitz et al. (29) demonstrated the
use of multidimensional separations in combination with human odour assessment.
In this latter study, a two-dimensional GC technique was used to examine wheat
grain samples, with pre-fractionation, followed by separation, organoleptic assess-
ment and the final collection of fractions for further analysis. This final collection
phase, described as ‘micropreparative,’ yielding sufficient material for either further
GC analysis or spectroscopic measurement. The detection of a compound responsi-
ble for ‘off-odour’ in this type of product was found to be 2-methyllisoborneol, a
trace constituent in what was a highly complex mixture. Its co-elution with
higher-concentration species masked identification with single-dimension GC–MS
and organoleptic assessment methods.
The analysis of ‘more pleasant’ odours associated with, for example, fruits, plants
and extracted essential oils, is also an area that exploits the resolution possible with
two-dimensional GC. The analysis of fruit extracts and products by using two-dimen-
sional methods was first reported in the mid 1980s and much published work has fol-
lowed since then (30, 31). Figure 3.6 demonstrates a two dimensional separation of
orange oil extract (32), analysed on primary apolar and secondary Carbowax 20M
columns. Two-dimensional GC in combination with organoleptic assessment indi-
cated that co-elution of a minor concentration (but odour significant) compound with
-myrcene was occurring when the analysis was performed on a single column. The
analysis made use of a non-polar primary column in combination with a carbowax
secondary column, interfaced by using a Deans switch at the midpoint. The very short
cut to the secondary column resulted in no requirement for an intermediate trap, and
although the system was operated by using two ovens, the secondary oven was held
isothermally.