Changes in the flow-rates of sample (10 ml) and
eluent (using an air stream as carrier) scarcely
affected the chromatographic signal over the
range 1–6 ml min−1; a sample and eluent flowrate
of 5 and 3 ml min−1, respectively, were
selected. In order to apply the proposed method
to plant material by extraction with aqueous acid
solutions and as the enrichment factor is directly
related to the sample volume, the breakthrough
volume was evaluated. For this purpose, different
volumes of aqueous standards at pH 3.0 containing
500 ng of each herbicide were completely
aspirated into the SPE system. Sample volumes
up to 20 ml can be used with negligible changes in
the chromatographic signals.