1,3,1
0 ,3
0 -Cl 4 BPE increased in concentration, while the 2,3,2
0 ,3
0 -Cl 4 BPE decreased.
Basically, the shift in the isomeric proportions towards the lower reaches of the river
may result from a discrimination of the 2,3,2
0 ,3
0 -Cl 4 BPE by physical or biochemical
processes. A decision about this alternative was expected by an investigation of the
enantiomeric composition over the entire length of the river, since 2,3,2
0 ,3
0 -Cl 4 BPE
is a chiral compound. A change of the enantiomeric ratios would be indicative of
enzymatic microbial degradation, while retention of a racemic composition would
point to a non-enzymatic process. As earlier noted already, appropriate chiral
selectors in cGC for solving such problems often are modified cyclodextrin phases.
However, the separation potential of a cyclodextrin-type chiral stationary phase is
still difficult to predict and, therefore, needs to be tested empirically. In this case,
several cyclodextrin derivatives had to be evaluated with respect to their selectivity
towards the chiral bis(chloropropyl) ethers with mixtures of standard compounds of
different degrees of chlorination. Experienced analytical chemists will confirm that
similar time-consuming tests need to precede the determination of the enantiomer
signature as a part of the quality control procedure, when analytical methods for
emerging chiral environmental pollutants are developed.
As an example for the evaluation of the CSP selectivity and determination of the
enantiomer-specific elution order in the optimised cGC, separation of a potential of
some cyclodextrin phase is illustrated for chloro-bis(propyl) ethers in the herediscussed report (Franke et al. 1998).
A satisfactory enantiomeric separation of 1,2
0 -Cl 2 BPE and 1,3,2
0 -Cl 3 BPE (the
digits indicate the position of the chlorine atoms; this simplified nomenclature is
unambiguous) is achieved with octakis(3-O-butyryl-2,6-di-O-n-pentyl)-γ-cyclodextrin (Lipodex E). The stereoisomers of the tetrachloro compounds, however, are not
resolved on this phase. Heptakis(6-O-tertbutyldimethylsilyl-2,3-di-O-methyl)-βcyclodextrin resolves 1,3,2
0 -Cl 3 BPE and the tetrachloro compounds 1,3,2
0 ,30 -Cl 4 BPE and 2,3,2
0 ,3-Cl 4 BPE with the (S,S)-enantiomer of 2,3,2
0 ,3
0 -Cl 4 BPE and
the (+)-enantiomer of 1,3,2
0 ,3
0 -Cl 4 BPE co-eluting. Similarly, separations can be
achieved with the heptakis(6-O-tertbutyldimethylsilyl-2-O-methyl-3-O-n-pentyl)β-cyclodextrin column as long as a slower temperature program is applied.
In addition, 1,2
0 ,3
0 -Cl 3 BPE is completely separated into four stereoisomers. No
separations of haloethers were observed on the column with the mixed phase of
heptakis(2,6-di-O-methyl-3-O-n-pentyl)-β-cyclodextrin and heptakis(6-O-methyl2,3-di-O-n-pentyl)-β-cyclodextrin. None of the investigated cyclodextrin phases
was able to resolve all compounds of interest. But since Franke et al. had known
from earlier investigations that the contamination of the Elbe river with tetrachloro
compounds was more severe than with tri- and dichloro homologues, they focused
on the investigation of the tetrachloro bis(propyl) ethers, and as a consequence, they
chose the heptakis(6-O-tertbutyldimethylsilyl-2,3-di-O-methyl)-β-cyclodextrin column which separated these homologues well. The assignment of the order of elution
of the enantiomers of 2,3,2
0 ,3
0 -Cl 4 BPE was achieved by enantioselective synthesis
of the 2(R,2
0 R/S)-stereoisomer and the subsequent cGC analysis of the achieved
enantiomer. The determination of the absolute configuration of one of the two
stereogenic centres is sufficient, since the mesoform (note: 2R,2
0 S ¼ 2S, 2
0 R) can
126
8 Enantiomer-Specific Fate and Behaviour of Chiral Contaminants
0 ,3
0 -Cl 4 BPE increased in concentration, while the 2,3,2
0 ,3
0 -Cl 4 BPE decreased.
Basically, the shift in the isomeric proportions towards the lower reaches of the river
may result from a discrimination of the 2,3,2
0 ,3
0 -Cl 4 BPE by physical or biochemical
processes. A decision about this alternative was expected by an investigation of the
enantiomeric composition over the entire length of the river, since 2,3,2
0 ,3
0 -Cl 4 BPE
is a chiral compound. A change of the enantiomeric ratios would be indicative of
enzymatic microbial degradation, while retention of a racemic composition would
point to a non-enzymatic process. As earlier noted already, appropriate chiral
selectors in cGC for solving such problems often are modified cyclodextrin phases.
However, the separation potential of a cyclodextrin-type chiral stationary phase is
still difficult to predict and, therefore, needs to be tested empirically. In this case,
several cyclodextrin derivatives had to be evaluated with respect to their selectivity
towards the chiral bis(chloropropyl) ethers with mixtures of standard compounds of
different degrees of chlorination. Experienced analytical chemists will confirm that
similar time-consuming tests need to precede the determination of the enantiomer
signature as a part of the quality control procedure, when analytical methods for
emerging chiral environmental pollutants are developed.
As an example for the evaluation of the CSP selectivity and determination of the
enantiomer-specific elution order in the optimised cGC, separation of a potential of
some cyclodextrin phase is illustrated for chloro-bis(propyl) ethers in the herediscussed report (Franke et al. 1998).
A satisfactory enantiomeric separation of 1,2
0 -Cl 2 BPE and 1,3,2
0 -Cl 3 BPE (the
digits indicate the position of the chlorine atoms; this simplified nomenclature is
unambiguous) is achieved with octakis(3-O-butyryl-2,6-di-O-n-pentyl)-γ-cyclodextrin (Lipodex E). The stereoisomers of the tetrachloro compounds, however, are not
resolved on this phase. Heptakis(6-O-tertbutyldimethylsilyl-2,3-di-O-methyl)-βcyclodextrin resolves 1,3,2
0 -Cl 3 BPE and the tetrachloro compounds 1,3,2
0 ,30 -Cl 4 BPE and 2,3,2
0 ,3-Cl 4 BPE with the (S,S)-enantiomer of 2,3,2
0 ,3
0 -Cl 4 BPE and
the (+)-enantiomer of 1,3,2
0 ,3
0 -Cl 4 BPE co-eluting. Similarly, separations can be
achieved with the heptakis(6-O-tertbutyldimethylsilyl-2-O-methyl-3-O-n-pentyl)β-cyclodextrin column as long as a slower temperature program is applied.
In addition, 1,2
0 ,3
0 -Cl 3 BPE is completely separated into four stereoisomers. No
separations of haloethers were observed on the column with the mixed phase of
heptakis(2,6-di-O-methyl-3-O-n-pentyl)-β-cyclodextrin and heptakis(6-O-methyl2,3-di-O-n-pentyl)-β-cyclodextrin. None of the investigated cyclodextrin phases
was able to resolve all compounds of interest. But since Franke et al. had known
from earlier investigations that the contamination of the Elbe river with tetrachloro
compounds was more severe than with tri- and dichloro homologues, they focused
on the investigation of the tetrachloro bis(propyl) ethers, and as a consequence, they
chose the heptakis(6-O-tertbutyldimethylsilyl-2,3-di-O-methyl)-β-cyclodextrin column which separated these homologues well. The assignment of the order of elution
of the enantiomers of 2,3,2
0 ,3
0 -Cl 4 BPE was achieved by enantioselective synthesis
of the 2(R,2
0 R/S)-stereoisomer and the subsequent cGC analysis of the achieved
enantiomer. The determination of the absolute configuration of one of the two
stereogenic centres is sufficient, since the mesoform (note: 2R,2
0 S ¼ 2S, 2
0 R) can
126
8 Enantiomer-Specific Fate and Behaviour of Chiral Contaminants