Triasulfuron forms one of the group of sulfonylurea herbicides. These are used widely for controlling weeds as they are effective at very low application rates. This effectiveness is responsible for the crop losses due to persistence of trace amounts of the herbicides (< or = 100 pg/g) in the soil. The numerous immunoassays described have been constrained by the fact that the soil extract contains co extractants which interfere in the assays, so much so, that these assays are useless at low levels of herbicide. We describe here the preparation and application of an immunoaffinity column which binds specifically to triasulfuron, thus cleaning up the soil extract. The experiment design is such that this also leads to concentration of the triasulfuron, making it easier to assay reliably using ELISA. Six different soil types were used to validate this procedure. In most cases, the herbicide content could be detected at 100 pg/g (critical phytotoxic herbicide level in soil) with a variation of +/- 20% in the readings.