Page 22 - Multidimensional Chromatography

Introduction 11
system is much more probable than in a linear system because two displacements
similar to that of another component are much less likely than for a single displace-
ment.
The coupling of chromatographic techniques is clearly attractive for the analysis
of complex mixtures, and numerous combinations have been proposed and devel-
oped (Figure 1.6). Truly comprehensive two-dimensional hyphenation is generally
achieved by frequent sampling from the first column into the second, with a very
rapid analysis. The interface is crucial here, and is designed so that components sep-
arated in the first dimension are not allowed to recombine; a variety of multiport
valving arrangements have been used, but transfer between columns is most efficient
if some kind of modulation is employed. The best example so far is the thermally
modulated injection of very small samples from a primary GC column into a second
GC column (33, 34).
More commonly, a fraction, based on chemical type, molecular weight or volatil-
ity, is ‘heart-cut’ from the eluent of the primary column and introduced into a sec-
ondary column for more detailed analysis. If the same mobile phase is used in both
dimensions, fractions may be diverted by means of pressure changes–an approach
first used in 1968 in GC-GC by Deans (35), and applied by Davies et al. in
SFC–SFC (36). If the mobile phases are different, valves are employed, and special

Figure 1.4 Two-dimensional plot of HPLC (log I L ) and GC (log I G ) retention indexes:
(1) naphthalene; (2) 2-methylnaphthalene; (3) 2,3-dimethylnaphthalene; (4) 2,3,6-trimethyl-
naphthalene; (5) biphenyl; (6) fluorene; (7) dibenzothiophen; (8) phenanthrene; (9) 2-
methylphenanthrene; (10) 3,6-dimethylphenanthrene; (11) benzo[a]fluorene; (12) chrysene
(data replotted from reference (31)).