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 alpha-, beta-, and gamma-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 beta-diastereomer, relative to the alpha- and gamma-diastereomers, is substantially retained on both florisil and silica gel regardless of the solvent used. The beta-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 alpha- (and not the beta-) 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 alpha-, beta-, and gamma-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 beta-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 C(18)-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 beta-HBCD in the literature may be negatively biased.
Mariussen
Espen
E
Norwegian Defence Research Establishment (FFI), P.O. Box 25, N-2027 Kjeller, Norway. espen.mariussen@ffi.no
Haukås
Marianne
M
Arp
Hans Peter H
HP
Goss
Kai-Uwe
KU
Borgen
Anders
A
Sandanger
Torkjel M
TM
D005591:Chemical Fractionation; D002851:Chromatography, High Pressure Liquid / Q000379:methods*; D056148:Chromatography, Reverse-Phase / Q000379:methods*; D006842:Hydrocarbons, Brominated / Q000737:chemistry* / Q000302:isolation & purification*; D017633:Magnesium Silicates / Q000737:chemistry; D012822:Silicon Dioxide / Q000737:chemistry; D013237:Stereoisomerism
D006842:Hydrocarbons, Brominated; D017633:Magnesium Silicates; C005013:Florisil; C089796:hexabromocyclododecane; D012822:Silicon Dioxide