Gas Chromatographic Enantiomer Separation of Atropisomeric PCBs Using Modified Cyclodextrins as Chiral Phases

Abstract

Columns containing different types of cyclodextrin derivatives have been evaluated for chiral gas chromatographic separation of atropisomeric PCBs, o,p´‐DDT and o,p´‐DDD. Separation was attempted on columns containing mixed chiral selectors, and the performance of two closely related selectors was also examined. The cyclodextrins were: permethylated‐β‐CD (PM‐β‐CD), heptakis(2,3‐di‐O‐methyl‐6‐O‐tert‐butyldimethylsilyl)‐β‐CD (2,3‐M‐6‐TBDMS‐β‐CD), heptakis(2,3‐di‐O‐methyl‐6‐O‐tert‐hexyldimethylsilyl)‐β‐CD (2,3‐M‐6‐THDMS‐β‐CD), and heptakis(2,3‐di‐O‐ethyl‐6‐O‐tert‐hexyldimethylsilyl)‐β‐cyclodextrin (2,3‐E‐6‐THDMS‐β‐CD). The cyclodextrins were dissolved in OV‐1701 or in a dimethylsiloxane/silarylene copolymer containing 5% phenyl in the backbone. The application of mixed chiral selectors led to improved separations, however; at most eleven PCB congeners were separated on a single column. Chiral resolution of o,p´‐DDD was achieved. The use of a dimethylsiloxane/silarylene copolymer as a matrix for the cyclodextrins is a promising approach. With such a matrix, blocking of the CD cavities by silicone substituent groups can be avoided, and a reasonable CD solubility can be provided. The selectivity of heptakis(2,3‐di‐O‐ethyl‐6‐O‐tert‐hexyldimethylsilyl)‐β‐CD and heptakis(2,3‐di‐O‐methyl‐6‐O‐tert‐hexyldimethylsilyl)‐β‐CD was quite different, the former selector could separate four congeners, while the latter separated ten congeners.

Assigning atropisomer elution orders using atropisomerically enriched polychlorinated biphenyl fractions generated by microsomal metabolism

“…Gas chromatographic separation of PCB atropisomers is possible on different chiral stationary phases. Over the years, considerable effort has been devoted to in-house synthesis of cyclodextrin-based gas chromatography columns and studies of the effect of cyclodextrin derivatization on separation of PCB atropisomers [6,7]. However, to this day no commercially available enantioselective gas chromatography column can resolve the atropisomers of all chiral PCBs [8], which represents a major challenge when studying the atropisomeric enrichment of PCBs in environmental samples.…”

Gas Chromatographic Enantiomer Separation of Polychlorinated Biphenyls (PCBs): Methods, Metabolisms, Enantiomeric Composition in Environmental Samples and their Interpretation

“…Another successful, albeit less frequently used, CSP for the enantioselective determination of PCB atropisomers is heptakis (2,3‐di‐ O ‐methyl‐6‐ O ‐ tert ‐hexyldimethylsilyl)‐β‐cyclodextrin . This CSP resolved eleven PCB atropisomers, and was also successfully applied for the determination of enantiomer fractions (explained in Section 3) of methylsulfonyl‐PCBs in environmental samples ,…”

“…GC enantiomer separations of polyhalogenated compounds, including PCB atropisomers, were generally performed with CSPs with modified cyclodextrins. The separation efficiency of PCB atropisomers on different CSPs has been summarized by Vetter and Schurig, Vetter, Wong and Garrison, Magnusson et al ., Bordajandi et al ., and Vetter and Bester . In brief, several CSPs were tested, but none of them had resolved the racemates of all 19 stable PCB atropisomers.…”

“…Magnusson et al . also prepared mixed CSP columns, with the idea that each part could resolve complimentary PCB atropisomers . However, this approach did not significantly improve the enantioselective separation of PCB atropisomers, compared with individual CSPs.…”

“…One reason could be that, in many cases, polysiloxane‐diluted β‐PMCD was used, which is less suited than chemically anchored β‐PMCD (Chirasil‐Dex CB). In this paper, it was also found that 6‐ O ‐ tert ‐butyldimethylsilyl‐2,3‐di‐ O ‐ethyl‐β‐cyclodextrin (β‐TBDE), instead of β‐TBDM, was less suited to the enantiomer separation of atropisomeric PCBs …”