Relevance of 1,2,5,6,9,10-hexabromocyclododecane diastereomer structure on partitioning properties,column-retention and clean-up procedures

To optimize clean-up procedures for the analysis of α-, β-, and γ-hexabromocyclododecanes (HBCD) in environmental and biological extracts, their retention behavior on silica gel and florisil was investigated using diverse mobile-phase solvents and accounting for matrix effects. The β-diastereomer, relative to the α- and γ-diastereomers, is substantially retained on both florisil and silica gel regardless of the solvent used. The β-diastereomer is therefore prone to undergo selective loss during clean-up. This sequence is counterintuitive to sequences based on reverse-phase chromatography with a C₁₈-column, in which the α- (and not the β-) isomer is eluted first when using a polar solvent. There has been some discrepancy regarding the structures of these diastereomers in the literature, with structures based on X-ray crystallography only becoming recently available. Based on these X-ray crystal structures, physical–chemical properties (the octanol–water partitioning constant, the Henry's law constant, subcooled liquid vapour pressures and subcooled liquid water solubilities) of the HBCD diastereomers were estimated using the quantum-chemistry based software COSMOtherm, and were found to differ from previously calculated values using different structures (e.g. log K_aw for α-, β-, and γ-HBCD are here estimated to be −8.3, −9.3 and −8.2 respectively). Hypothesis relating differences in structure to physical–chemical properties and retention sequences are presented. The extra retention of the β-diastereomer on silica gel and florisil is likely because it can form both greater specific (i.e. polar) and non-specific (i.e. non-polar) interactions with surfaces than the other diastereomers. Non-specific interactions can also account for the counter-intuitive elution orders with C₁₈-reverse-phase chromatography. These results indicate that care should be taken when isolating HBCDs and other molecular diastereomers from environmental and biological samples, and that reported concentrations of β-HBCD in the literature may be negatively biased.