The most accurate method for measuring dioxins and related compounds, such as polychlorinated biphenyls (PCBs), is isotope dilution?selected ion monitoring?mass spectrometry (ID-SIM-MS). To use the method correctly, however, analysts need to take various factors into account, such as proper spiking of the extraction and injection standards and establishing proper quality control approaches that are applicable to ID-SIM-MS. Below, renowned dioxins expert Yves Tondeur of SGS-Analytical Perspectives in Wilmington, North Carolina, answers questions about dioxin analysis that were raised during a recent web seminar.