Page 38 - Multidimensional Chromatography

28 Multidimensional Chromatography

2.4 TRANSFER OF WATER-CONTAINING SOLVENT MIXTURES

A large number of samples to be analysed by gas chromatography have an aqueous
matrix. Figure 2.1 shows that for the analysis of reversed-phase eluents both direct
and indirect methods have been used. The direct injection of water-containing sol-
vent mixtures in capillary GC would be attractive for accelerating an analysis but has
many difficulties. The main drawbacks are as follows: the high boiling point and
molar enthalpy of vaporising, thus requiring extensive solvent evaporation; due to its
small molecular weight, water forms a very large volume of vapour per volume of
liquid (about six time more than hexane), and elimination of this vapour volume via
the column is tedious and time-consuming; condensed water destroys the deactiva-
tion of the precolumn due to the hydrolysis of siloxane bonds; the extremely high
surface tension of water does not allow it to wet deactivated capillary surfaces,
which thus makes water a poor solvent for the formation of the solvent film that is
essential for solute trapping.

2.4.1 DIRECT INJECTION BY USING A RETENTION GAP

On-column injection of large volumes of aqueous samples has achieved considerable
attention in the field of on-line reversed phase LC–GC. The main problem in direct
introduction of water, as mentioned above, is the poor wettability of the uncoated
precolumns. In 1989, Grob and Li (20) tested several fused silica and glass pre-
columns deactivated by using different methods and concluded that the transfer of
aqueous solvents by retention gap techniques was not achievable because it was
impossible to find a precolumn that at the same time was both water-wettable and
inert. This problem was tentatively solved by using an organic solvent with a higher
boiling point than that of water. In fact, these same authors (21) investigated the wet-
tability of phenyl- and cyanosilylated precolumns with mixtures of organic solvents
and water, by using the retention gap technique for transferring water and mixtures
of water with organic solvents. Their results demonstrated that, depending on the
organic solvent being used, mixtures of such solvents with high concentrations of
water still wet the precolumns (e.g. 70% water and 30% 1-propanol), although water
did not evaporate together with the organic component. Azeotropically boiling mix-
tures, e.g. 28% water and 72% of 1-propanol, demonstrated that wettability of such
precolumns is possible and thus allows the introduction of water by the retention gap
technique.

2.4.2 DIRECT INJECTION BY USING CONCURRENT
SOLVENT EVAPORATION

As mentioned above, concurrent solvent evaporation does not need good wettability
of the solvent used for the LC–GC transfer. However, due to the large amount of
vapour released and the high temperatures needed for concurrent solvent evaporation