Page 241 - Multidimensional Chromatography

Foods, Flavours and Fragrances Applications 235

10.4 MULTIDIMENSIONAL CHROMATOGRAPHY USING ON-LINE
COUPLED HIGH PERFORMANCE LIQUID CHROMATOGRAPHY
AND CAPILLARY (HIGH RESOLUTION) GAS CHROMATOGRAPHY
(HPLC–HRGC)

In coupled LC–GC, specific components or classes of components of complex mix-
tures are pre-fractionated by LC and are then transferred on-line to a GC system for
analytical separation. Because of the ease of collecting and handling liquids, off-line
LC–GC techniques are very popular, but they do present several disadvantages, e.g.
the numerous steps involved, long analysis times, possibility of contamination, etc.
The on-line coupled LC–GC techniques avoid all of these disadvantages, thus
allowing us to solve difficult analytical problems in a fully automated way.
Different transfer techniques and type of interfaces have been developed. Most of
the applications involve normal-phase HPLC conditions, although reversed-phase
coupled with capillary GC has also been reported.
On-line coupled LC–GC methods have been developed in food analysis for sev-
eral reasons, i.e. lower detection limits can be reached, the clean-up is more efficient,
and large numbers of samples can be analysed with a minimum of manual sample
preparation in shorter times.
Several applications involve the removal of large amounts of triglicerides, includ-
ing the determination of wax esters in olive oil (39), sterols and other minor compo-
nents in oils and fats (40, 41), PCBs in fish (42), lactones in food products (43, 44),
pesticides (45), and mineral oil products in food (46, 47). Grob et al. (47) studied the
capacity of silica gel HPLC columns for retaining fats, and concluded that the capac-
ity of such columns is proportional to their size, although the fractions of the vol-
umes that are then transferred to the GC system grow proportionally with the column
capacity. For these reasons, 2–3 mm i.d. LC columns are to be preferred for LC–GC
applications.
In 1996, Mondello et al. (48) published a review article on the applications of
HPLC–HRGC developed for food and water analysis over the period from 1986 to
1995. These authors cited 98 references, grouped by following a chronological order
and by the subject of the application, as follows:
• composition of edible oils and fats;
• composition of essential oils and flavour components;
• contamination of water and food products;
• mineral oil products in food.

For each application, the LC and GC conditions are listed, together with the type
of interface used, and some additional comments on the technique employed and the
detection method.
The analysis of sterols, sterols esters, erythrodiol and uvaol, and other minor
components of oils and fats, is usually carried out by normal-phase HPLC–HRGC
by using a loop-type interface and the concurrent eluent evaporation technique, as
reported in the papers cited by Mondello et al. (48) (up to 1995) and in more recent
papers (49, 50). More recently, reversed-phase LC–GC methods have been