Page 242 - Multidimensional Chromatography

236 Multidimensional Chromatography

2 5
(b)

(a)
4
8
6
3
7
1

5 10 15
Retention time (min)
Figure 10.10 Liquid chromatogram (a) of sunflower oil and gas chromatogram (b) obtained
after transfer of the indicated fraction (3416 l of methanol–water eluent (78 : 22). Peak iden-
tification is as follows: (1) -tocopherol; (2) -tocopherol; (3) campesterol; (4) stigmasterol;
7
7
5
(5) -sitosterol; (6) -avenasterol; (7) -stigmasterol; (8) -avenasterol. LC conditions:
50 4.6 mm i.d. column, slurry packed with 10 m silica; flow rate, 2000 l/min; UV detec-
tion, 205 nm. GC conditions: 5% phenyldimethylpolysiloxane column (25 m 0.25 mm
i.d.; 0.25 m film thickness); carrier gas, He; column temperature, from 130 °C to 265 °C at
20 °C/min, and then to 300 °C at 3 °C/min. LC–GC transfer: the discharge of the large volume
of vapour resulting from the aqueous eluent during LC–GC transfer is promoted by removing
the GC column end from the PTV body; after completion of the transfer step, temperature
programming of the GC column was started, and the PTV injector was heated at
14 °C
sec to 350 °C. Reprinted from Journal of Agricultural and Food Chemistry, 46,F. J.
Señoráns et al., ‘Simplex optimization of the direct analysis of free sterols in sunflower oil by
on-line coupled reversed phase liquid chromatography–gas chromatography’, pp. 1022–1026,
copyright 1998, with permission from the American Chemical Society.

developed for the determination of free sterols in edible oils by using a pro-
grammable temperature vaporiser (PTV) (41, 51–53). Figure 10.10 shows the
LC–GC separation of the sterol fraction of a sunflower oil obtained by using this
system.
Many applications involving the study of the composition of essential oils are
based on the use of the on-column interface and retention gap techniques because of
the high volatility of the components to be analysed.
In the case of citrus essential oils, LC pre-fractionation can be used to obtain
more homogeneous chemical classes of compounds for analysis by GC without any
problems of overlapping peaks.
Mondello et al. (54) have developed some applications of on-line HPLC–HRGC
and HPLC–HRGC/MS in the analysis of citrus essential oils. In particular, they used
LC–GC to determine the enantiomeric ratios of monoterpene alcohols in lemon,
mandarin, bitter orange and sweet orange oils. LC–GC/MS was used to study the
composition of the most common citrus peel, citrus leaf (petitgrain) and flower
(neroli) oils. The oils were separated into two fractions, i.e. mono- and sesquiterpene