Double-disk solid-phase extraction: Simultaneous cleanup and trace enrichment of herbicides and metabolites from environmental samples

Phenylurea and triazine herbicides, including some metabolites, were isolat ed from water and soil extracts by solid-phase extraction using a layered s ystem of two extraction disks, a method called double-disk solid-phase extr action. The first disk consisted of strong anion exchange (SAX) of 10-mu m styrene divinylbenzene (SDB) particles embedded in Teflon, and the second d isk was a C-18 disk of 10-mu m particles also embedded in Tenon. A volume o f 500 mt of water or aqueous soil extract is passed through the layered sys tem with the SAX disk first. The purpose of the SAX disk is to remove the h umic and fulvic acids from the water or aqueous soil extract by ion exchang e through their carboxyl groups. Even during methanol elution of herbicides , the humic substances remain bound to the SAX disk with >85% retention. El ution with methanol results in more than 90% recovery of the herbicides fro m the layered extraction disks. Removal of the humic and fulvic acids resul ts in greater sensitivity for diode array detection quantitation (0.05 mu g /L for herbicides) by substantially reducing the absorbance of the humic pe ak on the LC chromatogram. The herbicides adsorb to the SAX disk either thr ough hydrogen bonding to the anion-exchange sites or by hydrophobic interac tion with the SDB surface of the anion-exchange disk. The method was tested for the analysis of natural water samples from the Mississippi Embayment, a cotton-growing area of the southeastern United States.