Page 238 - Multidimensional Chromatography

232 Multidimensional Chromatography

SW column using methanol:water (1 : 9) containing 0.1 M KH 2 PO 4 and 0.01 M
1-heptanesulfonic acid for elution, and a MicroPak MCH-10 RP column, eluted with
a methanol/water gradient solvent system.
On-column concentration of vitamins in food matrices offers several advantages,
i.e. reducing sample preparation times, and increasing detection sensitivity through
the use of higher sample loading on the exclusion chromatography (EC) column.
Using an analogous method, Apffel et al. (33) carried out the analysis of sugars in a
candy formulation. These same authors also analysed sugars in molasses by using a
heart-cutting technique and normal-phase HPLC in the second dimension, as shown
in Figure 10.8. In this case, the solvents for the two columns are non-compatible, and
the volume of solvent that can be passed from the EC column to the normal-phase
column is thus greatly restricted.
Johnson et al. (34) coupled SEC in the non-aqueous mode (Micropak TSK gel
eluted with tetrahydrofuran) to a gradient RP LC system using acetonitrile/water for
the determination of malathion in tomato plants and lemonin in grapefruit peel.
Gel permeation chromatography (GPC)/normal-phase HPLC was used by
Brown-Thomas et al. (35) to determine fat-soluble vitamins in standard reference
material (SRM) samples of a fortified coconut oil (SRM 1563) and a cod liver oil
(SRM 1588). The on-line GPC/normal-phase procedure eliminated the long and
laborious extraction procedure of isolating vitamins from the oil matrix. In fact, the
GPC step permits the elimination of the lipid materials prior to the HPLC analysis.
The HPLC columns used for the vitamin determinations were a 10 m
polystyrene/divinylbenzene gel column and a semipreparative aminocyano column,
with hexane, methylene chloride and methyl tert-butyl ether being employed as
solvent.
Figure 10.9 shows the chromatograms of fortified coconut oil obtained by using
(a) normal-phase HPLC and (b) GPC/normal-phase HPLC. As can be seen from
these figures, chemical interferences due to lipid material in the oil were eliminated
by using the MD system that was used for quantitative analysis of all of the com-
pounds, except DL- -tocopheryl acetate, where the latter was co-eluted with a
trigliceride compound and needed further separation.
Reversed-phase HPLC employing UV detection has been used for the analysis of
polar pesticides in aqueous samples, although the sensitivity is usually insufficient
for trace analysis experiments. The selectivity can be improved by using more selec-
tive electrochemical or fluorescence detectors, suitable for a restricted number of
samples, or an additional sample preparation step can be used. A major advantage
can be gained using on-line sample enrichment by large-volume injection in combi-
nation with column-switching techniques. Hogendoorn and co-workers (36–38)
have used this technique for the analysis of single or groups of polar pesticides in
water samples. These authors used two C18 columns with high separation power,
increasing the selectivity by applying the cutting technique and the sensitivity by
using large-volume injections. Chloroallyl alcohol (CAAL), bentazone, isoproturon,
metamitron, pentachlorophenol, and other polar pesticides were determined, with
the detection limits being at levels of 0.1 g/l.