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Table II. Concentration of Ethalfluralin and Trifluralin Measured with a GC-MS-NCI and a GC-ECD0 j"vs concentration, rate, after ng/g of oven-dry soil kg/ha treatment GC-MS-NCI GC-ECD Ethalfluralin 1.0 199 46 50 1.5 199 124 129 1.0 555 12 12 1.5 555 16 23 standard error of mean for methods of analysis = 2.6 Trifluralin 1.0 199 122 126 1.5 199 206 192 1.0 555 57 65 1.5 555 89 85 standard error of mean for methods of analysis =4.80 Differences in concentration of trifluralin and ethalfluralin due to rates and sampling dates were significant and differences between GC-MS-NCI and GC-ECD were not significant (P = 0.05) by analysis of variance.(Lethbridge Research Station, unpublished data). Thus, the GC-MS-NCI can be used to confirm levels of ethalfluralin and trifluralin residues that are toxic to plants.The ability to analyze trifluralin and ethalfluralin with the GC-MS-NCI indicates that this system has potential for rapidly confirming structures and concentrations of several pesticides with a positive electron affinity in soil extracts. ACKNOWLEDGMENTWe thank Drs.