Extraction time is an important parameter that could
influence the extraction performance. In this study, 200 mL
deionized water spiked with 5.0
μ
g L
−
1
each of the herbicides
(pH 6.5) was used to study the effect of the different
extraction time (20, 30, 40 and 60 min) on the extraction
efficiency. As shown in Fig.2, the results indicated that the
peak areas increased as the extraction time increased up to 40
min, and then remained almost unchanged after that. This
means that the adsorption equilibrium was reached at about 40
min. Hence, extraction time of 40 min was selected.