Herbicides.. like other pesticides, leave undesirable residues on the treated plants. To analyze them, the plant material is extracted and the herbicidal residues are separated from the bulk of the co-extractives of natural origin. about which too little is known. Quantitative determinations at the ppm level are best performed by gas chromatography with highly sensitive, specific detectors. In food control, where the history of the sample is unknown, the herbicide residues are identified and determined quantitatively together with thc residues of other pesticides.