Page 371 - Multidimensional Chromatography

362 Multidimensional Chromatography

more volatile analytes can be lost. A co-solvent (an organic solvent with a higher
boiling point than the transfer solvent) can be added before the transfer step to mini-
mize this effect (95).
A programmed-temperature vaporizer (PTV) has also been used as an interface
for introducing the LC fraction to the GC unit (84,96) and to desorb the analytes
retained in the SPE sorbent contained in the PTV liner. Water samples can then be
injected directly in to the PTV injector.

13.4.2 EXAMPLES OF LIQUID CHROMATOGRAPHY–GAS
CHROMATOGRAPHY APPLIED TO ENVIRONMENTAL ANALYSIS

One example of normal-phase liquid chromatography coupled to gas chromatogra-
phy is the determination of alkylated, oxygenated and nitrated polycyclic aromatic
compounds (PACs) in urban air particulate extracts (97). Since such extracts are very
complex, LC–GC is the best possible separation technique. A quartz microfibre fil-
ter retains the particulate material and supercritical fluid extraction (SFE) with CO 2
and a toluene modifier extracts the organic components from the dust particles. The
final extract is then dissolved in n-hexane and analysed by NPLC. The transfer at 100
l min 1 of different fractions to the GC system by an on-column interface enabled
many PACs to be detected by an ion-trap detector. A flame ionization detector (FID)
and a 350 l loop interface was used to quantify the identified compounds. The
experimental conditions employed are shown in Table 13.2.
Figure 13.16 shows the LC separation of this extract and the GC/FID chro-
matogram of the oxy-PAC fraction. The different oxy-PACs can be quantified with-
out interfering compounds with a non-selective detector such as an FID.
One of the first examples of the application of reverse-phase liquid chromatogra-
phy–gas chromatography for this type of analysis was applied to atrazine (98). This
method used a loop-type interface. The mobile phase was the most important param-
eter because retention in the LC column must be sufficient (there must be a high per-
centage of water), although a low percentage of water is only possible when the
loop-type interface is used to transfer the LC fraction. The authors solved this prob-
lem by using methanol/water (60:40) with 5% 1-propanol and a precolumn. The
experimental conditions employed are shown in Table 13.2.
An alternative way of eliminating water in the RPLC eluent is to introduce an
SPE trapping column after the LC column (88, 99). After a post-column addition of
water (to prevent breakthrough of the less retained compounds), the fraction that
elutes from the RPLC column is trapped on to a short-column which is usually
packed with polymeric sorbent. This system can use mobile phases containing salts,
buffers or ion-pair reagents which can not be introduced directly into the GC unit.
This system has been successfully applied, for example, to the analysis of polycyclic
aromatic hydrocarbons (PAHs) in water samples (99).
Another interface for RPLC–GC is the programmed-temperature-vaporization
(PTV) system, an interesting application of which is the determination of phthalates