Page 39 - Multidimensional Chromatography

Coupled HPLC with HRGC 29

of water, this technique is limited to high-boiling analytes. As an illustration of this,
the determination of atrazine in tap water is shown in Figure 2.9 (22). This method is
based on enrichment of the atrazine from 10 ml of water on a small LC column
packed with silica-C18, and desorption with methanol–water (60 :40) 5%
1-propanol to the GC column by concurrent eluent evaporation using a loop-type
interface. Both the retaining precolumn and the separation column were coated with
Carbowax 20 M which had a very high retention for atrazine. This was necessary to
obtain the high elution temperature required for atrazine. In fact, when transferring
the LC fraction at 112°C the atrazine peak was perfectly shaped only when eluted at
about 250°C.

2.4.3 DIRECT INJECTION BY USING CONCURRENT SOLVENT
EVAPORATION WITH A CO-SOLVENT

A partial solution to the problem of producing sharp peaks at low elution tempera-
tures is to add a small amount of a higher-boiling co-solvent to the main solvent. As
suggested by Grob and Muller (23, 24), butoxyethanol can be used as a suitable co-
solvent for aqueous mixtures in such cases.

2.4.4 DIRECT INTRODUCTION OF WATER VIA A VAPORIZER
CHAMBER/PRECOLUMN SOLVENT SPLIT/GAS
DISCHARGE INTERFACE

Recently, the direct introduction of water-containing eluents via a vaporizer cham-
ber/precolumn solvent split/gas discharge interface has been reported (25, 26). Water
and water-containing eluents were driven into a vaporizer chamber at 300°C by the
LC pump (Figure 2.10). This high temperature permitted evaporation of water at a
rate up to around 200 l/min. The vapours were then removed through a retaining
precolumn and a early vapour exit, driven by the flow of carrier gas (discharge). The
vaporizing chamber consisted of a 1 mm id glass tube, packed with a 2 cm plug of
Carbofrit and internally coated with polyimide. Solvent/solute separation occurred
in the retaining precolumn, and special attention was given to the oven temperature
during the transfer, being held close to the temperature at which recondensation
occurrs (the dew point). This method was successfully applied to the determination
of phthalates in drinking water. Figure 2.11(a) shows a liquid chromatogram,
obtained on a column packed with C-18 (5 m) bonded silica (1 cm 3 mm i.d.) of
a water sample spiked with dibutyl phthalate (DBP) and diethylhexyl phthalate
(DEHP). After sample enrichment, 10 ml of the fraction was transferred to the gas
chromatograph, driven by the LC eluent (water/methanol 15:85) and by reducing
the flow rate to 100 l/min. The LC–GC–MS(EI) chromatogram of the treated water
containing 55 and 40 ng/l of DBP and DHEP, respectively is shown in Figure
2.11(b).