Correspondence Comment on “High-Resolution Gas Chromatography Retention Data as Basis for the Estimation of Kow Values Using PCB Congeners as Secondary Standards” In a recent paper published in this journal, Hackenberg et al. (1) present an approach to determine octanol-water partition coefficients (Kow) from capillary gas chromatographic (GC) retention data. They claim that this approach is especially useful because it can be applied to substances in the nanogram range and with unknown structure. Based on the Collander equation, they deduce that log Kow can be linearly related to the retention factor k () tR/tM - 1; with tR being the retention time of the compound and tM being the dead time in the chromatographic system) obtained from GC measurements:
log Kow ) a log k + C
(1)
The regression constants a and C are to be derived from known Kow and GC retention data of secondary standards, in this case polychlorinated biphenyls (PCBs). They suggest that this semi-empirical approach can be extended to estimate the Kow of classes of compounds other than PCBs and that it is useful because normalized GC data are easily available. We think that these suggestions are misleading because of the following: (a) There are only few cases where this approach will give correct results. In these cases, however, GC measurements are not needed at all because the same information can be deduced more simply from the molecular structure of the compounds; (b) In many cases the suggested approach will produce systematically wrong data. Thus, the suggestions of the authors are confusing because they do not point out the limitations of the approach. We would like to give two illustrations of our above statements. First, for polychlorinated naphthalenes (PCNs) as an example, we find a correlation between their molecular mass (Mr) and log Kow values that is superior to the one obtained with the method suggested in ref 1 (r2 ) 0.935 for correlation between log Kow (HPLC) and log Kow (calculated from GC data)) and was obtained with considerably less effort:
log Kow (HPLC) ) -66.8 + 58.2Mr (g/mol)
(2)
r2 ) 0.971, n ) 19 Second, Figure 1 contains a plot of consistently measured GC retention data plotted against the respective log Kow values for 102 compounds with very different structures and functional moieties (i.e., different capabilities for molecular interactions with the solvents). Obviously, there is no single linear relationship between GC retention data and log Kow over all classes of compounds. The main reason for the finding in Figure 1 is that Collander-type equations (i.e., oneparameter linear free-energy relationships (LFERs)) are not applicable if the two two-phase systems, in which partitioning is measured, differ in their solvent-solute interactions. Instead, polyparameter LFERs, which considerall important 2286
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FIGURE 1. Kovats retention index (proportional to the logarithm of the normalized retention factors in ref 1) measured in a GC system with a zonyl column (4) plotted against log Kow data obtained from ref 5 for a diverse set of chemicals (n ) 102). molecular interactions, need to be used in such a situation (2):
log Ki12 ) a12CAivdWi + b12Vi + d12HAi + e12HDi + c12 (3) The relevant interactions are (i) molecular-size dependent cavity formation in the solvent (Vi) and (ii) interactions between solute and surrounding solvent molecules (i.e., H-donor/H-acceptor interactions (HAi, HDi) and van der Waals interactions (CAivdWi)). The coefficients a12, b12, d12, e12, and c12 describe the differences in the two solvents, 1 and 2, with respect to each type of interaction. A comparison of the two systems examined and related to each other in ref 1 shows that GC retention times are a measure of the transition between the gas phase (without any interactions) and the rather apolar stationary phases (with dominant van der Waals interactions). Kow data, however, describe the transfer between octanol and H2O, which are both bipolar phases that are capable of H-bonding interactions as both acceptor and donor. If solutes with an H-bonding capability are investigated, it can thus not be expected that GC data, which do not represent these interactions, are a good descriptor (i.e., linearly correlated with log Kow data). Specifically, even the more polar GC phases used in ref 1 have only H-acceptor but no H-donor capabilities (3). They can thus not reflect the strong influence that differences in the H-donor capabilities of the two solvents octanol and water have on solute partitioning between them (3). As discussed in ref 2, one-parameter LFERs only work in very specific cases: (i) If substance properties are such that only vdW interactions and no polar interactions are possible or (ii) if the polar interactions are the same for a group of compounds, which is the case within a class of homologues. Case i applies to the PCBs, PCNs, DDE, and HCB investigated in ref 1. All of them are purely apolar compounds, for which only vdW interactions determine their partitioning behavior. Since these are roughly proportional to the size of the molecule, a Collander-type correlation between GC reten10.1021/es035227j CCC: $27.50
2004 American Chemical Society Published on Web 03/05/2004
line (see Figure 2). This systematic deviation stems from the fact that H-bonds are present in the octanol-water system but not in the GC system and that they are stronger in the water phase than in the octanol phase. Thus, the log Kow values measured with HPLC for H-acceptor-type substances are lower (i.e., more substance is in water) than those extrapolated via GC data. The same reasoning also holds for the three PBDEs, which were identified by Hackenberg et al. (1) as outliers without further explanation. In conclusion, one-parameter LFERs are only safely applicable within a group of homologues. The claim that the method developed by Hackenberg et al. (1) can be generally applied to other compounds is thus wrong and bound to produce erroneous Kow data. We therefore think that the use of GC retention data to estimate Kow data should not be further promoted.
Literature Cited FIGURE 2. Log Kow data calculated from GC retention data in ref 1 plotted against log Kow data measured by HPLC for the class of PCDEs. The black line indicates the correlation derived in ref 1, while the dotted line indicates the ideal 1:1 relationship between measured and estimated Kow data. tion data and log Kow can be expected to work. On the same grounds, however, and because molecular size is also roughly proportional to molecular mass (Mr), a correlation between Mr and log Kow as given in eq 2 is expected to work similarly well. The other two compound classes investigated in ref 1, the PCDEs and PBDEs, contain an ether moiety and can therefore act as H-acceptors. According to case ii, the correlation developed with PCBs as standards should not be applicable to them. Surprisingly, at a first glance, the GC retention data also seem to reflect the partitioning of the ethers sufficiently well. If, however, the data pairs log Kow (HPLC) and log Kow (calculated from GC data) as reported in ref 1 are plotted against each other, it can be seen that most log Kow values calculated with GC data lie systematically above the ideal 1:1
(1) Hackenberg, R.; Schutz, A.; Ballschmiter, K. High-resolution gas chromatography retention data as basis for the estimation of Kow values using PCB congeners as secondary standards. Environ. Sci. Technol. 2003, 37 (10), 2274-2279. (2) Goss, K. U.; Schwarzenbach, R. P. Linear free energy relationships used to evaluate equilibrium partitioning of organic compounds. Environ. Sci. Technol. 2001, 35 (1), 1-9. (3) Abraham, M. H.; Poole, C. F.; Poole, S. K. Classification of stationary phases and other materials by gas chromatography. J. Chromatogr. A 1999, 842 (1-2), 79-114. (4) Patte, F.; Etcheto, M.; Laffort, P. Solubility Factors for 240 Solutes and 207 Stationary Phases in Gas-Liquid Chromatography. Anal. Chem. 1982, 54 (13), 2239-2247. (5) 5. Abraham, M. H.; Chada, H. S.; Whiting, G. S.; Mitchell, R. C. Hydrogen-Bonding. 32. An Analysis of Water-Octanol and Water-Alkane Partitioning and the Delta-Log-P Parameter of Seiler. J. Pharm. Sci. 1994, 83 (8), 1085-1100.
Kathrin Fenner,* Christine Roth, Kai-Uwe Goss, and Rene´ P. Schwarzenbach Swiss Federal Institute of Environmental Science and Technology (EAWAG) 8600 Diibendorf, Switzerland ES035227J
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