ISO 18073:2004 specifies a method for the determination of tetra- to octa-chlorinated dibenzo- p -dioxins (PCDDs) and dibenzofurans (PCDFs) in waters and waste waters (containing less than 1 % by mass solids) using high-resolution gas chromatography/high-resolution mass spectrometry (HRGC/HRMS). The minimum levels (MLs) at which the PCDDs/PCDFs can currently be determined with no interferences present are specified. This method is ""performance based"". The analyst is permitted to modify the method to overcome interferences or lower the cost of measurements, provided that all performance criteria are met. The requirements for establishing method equivalency