Page 155 - Multidimensional Chromatography

Coupled Supercritical Fluid and Chromatographic Techniques 147

6.8 FROM MULTIDIMENSIONAL TO UNIFIED
CHROMATOGRAPHY PASSING THROUGH SUPERCRITICAL FLUIDS

In 1965, Giddings proposed that there are no theoretical boundaries between GC, LC
and SFC, and that such distinctions are arbitrary, artificial and counter-productive
(61). This concept was extended by Ishii et al. who proposed the concept of Unified
Chromatography in 1988 (62). According to these authors, it is possible to demon-
strate different-mode separations, i.e. liquid chromatography (LC), supercritical
fluid chromatography (SFC) and gas chromatography (GC) by using a single chro-
matographic system (62). Still according to Ishii and co-workers, the separation
mode could be selected by changing the pressure in the column and the column tem-
perature (62). Since then a selected number of papers have appeared in the literature
covering this subject (for a review of Unified Chromatography, the interested reader
should consult references 63–65). Most of the work carried out so far is in fact
related to multidimensional chromatography using different chromatographic
modes, and not on Unified Chromatography as originally described by Ishii and co-
workers. In this respect, several reports have dealt with techniques such as GC–GC,
LC–LC, LC–GC, SFC–GC, and so on. Using such an approach, either the total or a
fraction of the analytes exiting the first column is transferred (switched) to a second
column for a second ‘dimension’ of analysis. The major conceptual difference
between this approach and the Unified Chromatography concept is that in the latter
system the sample is introduced into the first analytical unit (e.g GC) and part of it is
then eluted from the column by using a ‘proper’ mobile phase. Following this and
without changing the column, the other portion of the sample which is still retained
on this column is then eluted in a different chromatographic mode (e.g. SFC), either
with or without changing the mobile phase. Since both approaches, i.e. Multi-
dimensional Chromatography and Unified Chromatography, use equivalent symbols
to express the hyphenated character of both (e.g. GC–SFC), there is often confusion
about their fundamental principles. In short, Multidimensional Chromatography
uses a column-switching approach, with the analyte being transferred from one to
the other column, while Unified Chromatography uses the same column for all of the
chromatographic modes, without any analyte transfer. Since in both cases the tech-
niques are coupled on-line they are both considered to be hyphenated systems.
Supercritical fluid chromatography plays a very important role in the Unified
Chromatography approach. Most of the (albeit, rather few) papers published on
Unified Chromatography use SFC as a ‘bridge’between the two limiting extremes of
the mobile phase, represented by a gas (in GC) and a liquid (in LC). This subject will
not be covered any further here since it is discussed in detail elsewhere in this pre-
sent volume (see Chapter 7).

REFERENCES

1. C. Cagniard de la Tour, Ann. Chim. Phys. 21: 27 (1822).
2. J. B. Hannay and J. Hogarth, On the solubility of solids in gases’, Proc. R. Soc. (London)
29:324–326 (1879).