manifold employed, depicted in Fig. 1,
operated in three steps. (1) A total of 10 ml of
standard solution or sample containing 2–1000 ng
ml−1 of phenylurea herbicides, in 10−3 mol l−1
HNO3, was passed through the sorbent column
(located in the loop of IV1) at a flow-rate of 5 ml
min−1; in this step, herbicides were retained and
the sample matrix sent to waste (W). (2) IV1 valve
was switched and the sorbent column dried for 3
min with an air stream introduced via the carrier
line of the IV2 valve; simultaneously, the loop of
IV2 was filled with eluent (ethyl acetate) by aspiration
from a syringe. (3) IV2 valve was switched
and 100 l of eluent injected into an air stream in
countercurrent to that of the sample at a flow-rate
of 3 ml min−1 and passed through the column to elute the herbicides. The eluate was collected in a
glass vial containing anhydrous sodium sulfate
and 1-l aliquot was manually injected into the
gas chromatograph. After each determination, the
sorbent column was washed with 2 ml of ethyl
acetate, carried out by an air stream, to remove
residual compounds from the matrix and then
conditioned with 1 ml of 10−3 mol l−1 HNO3.