The applicability of trace enrichment and separation of microcontaminants on a single 10×2 mm I.D. high-pressure-packed liquid chromatography (LC) column, combined on-line with an atmospheric pressure chemical ionization (APCI)–ion-trap tandem mass spectrometry (MS–MS) instrument was studied for the target analysis of herbicides in river water. The total analysis time was 15–20 min. With the on-line short-column LC–MS–MS method, detection limits of 0.1–1 μg/l can be achieved using only 4 ml of river water. However, the results obtained for a mixture of six triazines were considerably better than those for a mixture of eight phenylureas. An attempt is made to explain this difference on the basis of various processes that occur within the ion trap and the measurement procedure itself.