Other common and characteristic MS/MS fragments were C4ClO -(di-chloro, bromodichloro and dibromochloro HCDs) and C4BrO -(tribromo, dibromo and dibromochloro HCDs). These MS fragmentation patterns match those found by Pan et al. , who synthetised, isolated and characterised tribromo-HCD. Given that there were measured mass spectral data available, this substance was reported at Level 2a here as a result. Its analogue, 2,2,4-trichloro-5-hydroxycyclopent-4-ene-1,3-dione was reported at Level 2b here on the basis of MS/MS fragmentation pattern closely resembling the one of trichloro-HCD. The other six halogenated HCDs were reported at Level 3 because multiple structural isomers and enantiomers were possible for each one of them. No toxicity data could be found in literature for these substances either, but all halogenated HCDs structures reported in this paper at all levels of confidence were predicted to be mutagenic by the EPA TEST software. Commercial reference materials were not available, but indicates that further investigations are needed to assess their toxicity and exposure via drinking water.