Page 362 - Multidimensional Chromatography

Multidimensional Chromatography in Environmental Analysis 353

shows the chromatogram of a surface water spiked at 2 gl 1 of atrazine, and its
metabolites, obtained under the conditions shown in Table 13.1.
Another interesting group of pesticides contains gluphosinate, glyphosate and
aminomethylphosphonic acid (AMPA). Gluphosinate and glyphosate are widely
used as non-selective contact herbicides, while aminomethylphosphonic acid is the
main metabolite of glyphosate. All are very polar and detection by LC thus requires a
derivatization step to enhance their fluorescence since they do not exhibit UV
absorption. The method (28) includes precolumn derivatization with a 9-fluorenyl-
methylchloroformate (FMOC-Cl) reagent, followed by LC–LC separation. The
experimental conditions are shown in Table 13.1.
Since there is a high percentage of acetonitrile in the derivatization solution, the
latter must be diluted in order to decrease the retention of the derivative in the first
column and so that the derivatives can be separated from the excess reagent. The
1
three compounds can be determined at a level of 1 g l , with the sample through-
put being at least 40 samples per day.
When AMPA or gluphosinate are determined alone, the sensitivity is higher
because a higher dilution is not required. For glyphosate, when the transfer volume
is precisely adjusted to 280 l for the FMOC-glyphosate-containing fraction, a limit
of detection of about 0.2 g l 1 can be reached (28).
Chlorophenoxy acids are relatively polar pesticides which are usually determined
by LC because volatile derivatives have to be prepared for GC analysis. This group
of herbicides can be detected by multiresidue methods combined with automated
procedures for sample clean-up, although selectivity and sensitivity can be enhanced
by coupled-column chromatographic techniques (52). The experimental conditions
for such analyses are shown in Table 13.1.
The first attempts employing two C 18 columns showed that the selectivity was not
high enough, although this improved when the first column was substituted by a
5 m GFF II internal surface reversed-phase material. This is known as a restricted-
access-material (RAM) column which, since it restricts some compounds because
of their size and includes reversed-phase interaction and ionic exchange, is very
useful for analysing herbicides in samples with high contents of humic and fulvic
acids (54).
Figure 13.11 shows the chromatogram obtained for a surface water sample spiked
1
with various chorophenoxy acids at a level of 0.5 g l , under the same conditions
as previously and after enrichment on a C 18 column and clean-up on silica SPE
cartridges.
With dicamba, a more polar chlorobenzoic acid herbicide, a gradient step is
needed to elute all of the compounds in one chromatographic run. Depending on the
buffer and selectivity of the detector, the baseline can be severely disturbed. If this
happens, a step-gradient elution is recommended (52), and in this way the method
can detect all of the compounds at very low levels.
Another interesting application of LC–LC is the determination of low-molecular-
mass carbonyl compounds in air. Carbonyl compounds, such as aldehydes and
ketones, are now being given more and more attention, both as pollutants and as