Environ. Sci. Technol. 2004, 38, 2288
Response to Comment on “High-Resolution Gas Chromatography Retention Data as a Basis for the Estimation of Kow Values Using PCB Congeners as Secondary Standards” We thank Fenner et al. (1) for their comments concerning our paper to calculate Kow data of semivolatile halogenated compounds based on a correlation with GC retention data. Their comments might help avoid possible misinterpretations of our paper (2). In short, Fenner et al. (1) argue that our correlations per se do not consider the H-donor/H-acceptor interactions of eq 3 in ref 1 to correctly correlate the GC retention factor k and the Kow value of a compound. In their minds, our approach is thus in general invalid. Our (GC) Kow correlation approach is based on the experimental finding that in the log Kow > 4.5 range for halogenated compounds the molecular interaction with a stationary phase in gas/liquid chromatography expressed as the retention factor k and the distribution between water and octanol apparently have a common molecular base within a given range of accuracy (2). Such a correlation is not surprising for nonpolar compounds, which we discussed in ref 2, when one realizes that adjusted GC retention data using the gas/liquid mode in GC summarizes the molecular interaction of the considered compound with the molecules of the stationary phase in the liquid state in a cavity formation in all its complexity. The retention factor thus reflects the molecular interaction in solution seen as Coulomb-type forces, ranging in a very general sense from van der Waals, to dipole-dipole and Lewis acid-base type interactions. In more formal terms, retention of a molecule in GC is the sum of its polarizability volume R, its ionization energy I, its electronaffinity F, and the square of its dipolemoment µ (3). A measure of the complex interactions in the stationary phase of gas-liquid GC is given in simple time units. We agree that our statement in our original paper that “our semi-experimental approach in approximating physicochemical data ... and log Kow can be extended to further classes of compounds because normalized GC retention data are easily available”, can be misinterpreted. However, our Kow (GC) approach is not proposed as “the” full-scale Kow determination method as Fenner et al. tried to disprove (1). Figure 1 of ref 1 is meant to indicate a systematic deviation between log Kow (HPLC) and log Kow (GC) for the PCDEs. Table 1 summarizes the mean deviation between log Kow (HPLC) (3) and log Kow (GC) for the congeners of PCDEs. The mean deviations have no systematic trend for the degree of chlorination. They are in the range of experimental determinations of Kow by the slurry method (5). Additional material can be obtained from the authors. Figure 2 of ref 1 does not disprove the feasibility our Kow (GC) approach as claimed by Fenner et al., but it only indicates that GC retention factor k/Kow correlations have to be carefully
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TABLE 1. Polychlorinated Diphenyl Ether (PCDE): Mean Absolute Deviation between Log Kow (HPLC) (4) and Log Kow (GC)a log Kow (HPLC) log Kow (GC)
mean deviation
range of deviation
no. of congeners
(Cl3-PCDE) (Cl4-PCDE) (Cl5-PCDE) (Cl6-PCDE) (Cl7-PCDE) (Cl8-PCDE)
(0.16 (0.22 (0.23 (0.16 (0.13 (0.19
0.00-0.69 0.00-0.62 0.02-0.59 0.08-0.37 0.03-0.25 0.11-0.27
n ) 10 n ) 19 n ) 16 n)7 n)5 n)2
a Log K ow (GC) is based on correlation using PCBs for constants a and C of eq 3 in ref 2 and using retention factors k obtained on the stationary phases Sil5-C18, Sil8, and DB1701.
applied and cannot be randomly selected. Only four values of a total of 102 in Figure 2 of ref 1 cover the range of log Kow > 4. Fenner et al. used the data of Figure 2 to totally disqualify our Kow (GC) approach, although they agree that it works for nonpolar compounds and for groups of homologues. Our results start with log Kow values > 4.5. We believe that our approach is a valuable and experimentally solidly founded procedure, for the hundreds of nonpolar or semipolar persistent compounds found in the environment that often are groups of congeners.
Literature Cited (1) Fenner, K.; Roth, Ch.; Goss, K.-U.; Schwarzenbach, R. P. Environ. Sci. Technol. 2004, 38, 2286-2287. (2) Hackenberg, R.; Schu ¨ tz, A.; Ballschmiter, K. Environ. Sci. Technol. 2003, 37, 2274-2279. (3) Ong, V.; Hites, R. Anal. Chem. 1991, 63, 2829-2834. (4) Kurz, J.; Ballschmiter, K. Chemosphere 1999, 38, 573-586. (5) Braekevelt, E.; Tittelmier, S.; Tomy, G. Chemosphere 2003, 51, 563-567.
Karlheinz Ballschmiter* Department of Analytical and Environmental Chemistry University of Ulm Albert-Einstein-Allee 11 898081 Ulm, Germany
Stefan Ellinger Department of Polymer Science University of Ulm Albert-Einstein-Allee 11 898081 Ulm, Germany
Rudolf Hackenberg Federal Office of Consumer Protection and Food Safety Diedersdorfer Weg 1 D-12277 Berlin-Marienfelde, Germany ES040329A
10.1021/es040329a CCC: $27.50
2004 American Chemical Society Published on Web 03/05/2004
