Anal. Chem. 2002, 74, 3476-3483
Estimating Octanol-Air Partition Coefficients of Nonpolar Semivolatile Organic Compounds from Gas Chromatographic Retention Times Frank Wania,*,†,‡ Ying Duan Lei,†,§ and Tom Harner⊥
Division of Physical Sciences, University of Toronto at Scarborough, 1265 Military Trail, Toronto, Ontario, Canada M1C 1A4, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6, Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, Canada M5S 3E5, and Meteorological Service of Canada, 4905 Dufferin Street, Downsview, Ontario, Canada M3H 5T4
Relative gas chromatographic retention times on a nonpolar stationary phase can be used to determine the octanol-air partition coefficient (KOA) and the energy of phase transfer between octanol and the gas phase (∆OAU) for semivolatile, nonpolar organic compounds. The only prerequisites are knowledge of the temperature-dependent KOA of a standard reference compound and directly measured KOA values at one temperature for a sufficient number of calibration compounds. It is shown that the technique is capable of predicting the KOA of polychlorinated benzenes, biphenyls and naphthalenes as well as polybrominated diphenyl ethers within the environmentally relevant temperature range with an average deviation from directly measured values of 9, the KOA-GC tends to be smaller than the KOA-lit. Figure 2 plots the data from Table 2, that is, regresses the KOA-GC against KOA-lit for the compounds for which p,p′-DDT served as a reference. The plot confirms the strong correlation between GC-derived KOA-GC and the KOA values from the literature.
log KOA-lit ) (1.105 ( 0.032)‚log KOA-GC -
Figure 1. Linear relationships between the logarithm of the ratio of the retention times of four chlorinated naphthalenes to that of the standard reference compound p,p′-DDT at five to seven temperatures and the logarithm of the KOA value of p,p′-DDT at these temperatures as calculated from eq 13. The KOA-GC of the chlorinated naphthalenes can be obtained from the slope and intercept of these regressions.
derived KOA-GC data. With the help of that equation, the KOA-GC values can be corrected for the systematic deviations from directly measured KOA values. Calibrated KOA-calc values were obtained by substituting log KOA-calc for log KOA-lit in eq 15 and are also listed in Table 2. The standard deviation given for the KOA-cal values in that Table were derived from the standard error of the slope and intercept of the calibration eq 15 and the standard deviation of the KOA-GC values by error propagation. It is clear that the error, which is on the order of 5%, is dominated by the uncertainty of the coefficients in the calibration equation rather than by the uncertainty in the determination of KOA-GC. In the case of the CBzs, two sets of KOA-GC were derived on the basis of the use of the reference compounds C-18 and HCBz (Table 3). The KOA-GC values derived using HCBz as a reference are consistently higher than those derived using C-18 by ∼0.6 log units, showing that the value of KOA-GC depends on the standard reference compound. This had also been observed with the GCbased determination of vapor pressure.22,23 A comparison shows that the KOA-GC for the CBzs derived with C-18 as reference are much closer to the KOA-lit values in ref 8 than the KOA-GC derived with HCBz as a reference. Whereas the latter are ∼0.1 log units lower, the former are 0.5 log units higher than the directly measured values. The respective calibration relationships are depicted in Figure 3 and read
HCBz log KOA-lit ) (1.056 ( 0.081)‚log KOA-GC (0.84 ( 0.48) n ) 5, r2 ) 0.983 (16) C-18 log KOA-lit ) (1.050 ( 0.077)‚log KOA-GC -
2
(0.86 ( 0.29) n ) 26, r ) 0.980 (15)
(0.15 ( 0.41) n ) 5, r2 ) 0.984 (17)
This relationship can be interpreted as a calibration of the GC-
The KOA-calc values obtained with these two equations are virtually Analytical Chemistry, Vol. 74, No. 14, July 15, 2002
3479
Table 2. Uncalibrated (KOA-GC) and Calibrated (KOA-Cal) Octanol-Air Partition Coefficients for Various Halogenated Aromatic Compounds at 25 °C Determined from Gas Chromatographic Retention Times Using p,p′-DDT as a Reference, Octanol-Air Partition Coefficients from the Literature (KOA-lit), and Energies of Phase Transfer, ∆OAU, Obtained from the Literature and Gas Chromatographic Retention Times ∆OAU, kJ‚mol-1 compd
log
KOA-liti
log KOA-GC
log KOA-cal
4-monoCB 4,4′-diCB 2,4,5-triCB 2,3,4,5-tetraCB 2,2′,5,5′-tetraCB 3,3′,4,4′-tetraCB 2,2′,4,5,5′-pentaCB 2,2′,4,4′,5,5′-hexaCB
6.93 ( 0.02 7.77 ( 0.01 8.03 ( 0.01 8.75 ( 0.01 8.44 ( 0.02 9.19 ( 0.01 9.05 ( 0.00 9.65 ( 0.01
Chlorobiphenyls 6.80 ( 0.37 6.77,a 6.82,c 6.85,h 6.79*f 7.73 ( 0.39 7.66,a 7.65*f 8.01 ( 0.39 7.95,a 7.80*f 8.80 ( 0.41 8.73,a 8.64*f 8.47 ( 0.40 8.22,d 8.27*f 9.29 ( 0.42 9.70,c 9.68*f 9.14 ( 0.41 9.06,c 8.78,d 9.06*f 9.80 ( 0.43 9.73,c 9.37,d 9.75*f
1,2-diCN 1,4-diCN 1,2,3-triCN 1,2,3,4-tetraCN 1,2,3,5-tetraCN 1,2,4,7-tetraCN 1,2,3,4,6-pentaCN 1,2,3,5,7-pentaCN 1,2,3,5,8-pentaCN 1,2,3,4,6,7-hexaCN 1,2,3,5,6,7-hexaCN 1,2,3,5,7,8-hexaCN 1,2,4,5,6,8-hexaCN 1,2,3,4,5,6,7-heptaCN octaCN
7.01 ( 0.03 6.91 ( 0.01 7.72 ( 0.00 8.29 ( 0.00 8.28 ( 0.00 8.14 ( 0.01 8.85 ( 0.01 8.76 ( 0.01 9.01 ( 0.00 9.45 ( 0.00 9.45 ( 0.00 9.53 ( 0.01 9.55 ( 0.00 10.18 ( 0.00 10.78 ( 0.01
Chloronaphthalenes 6.89 ( 0.37 6.76*g 6.78 ( 0.37 6.92b 7.66 ( 0.38 7.62*g 8.30 ( 0.40 8.33*g 8.29 ( 0.40 8.26*g 8.13 ( 0.39 8.08b 8.92 ( 0.42 8.90b 8.82 ( 0.41 8.72b 9.10 ( 0.41 9.12b 9.58 ( 0.42 9.69b 9.58 ( 0.42 9.69b 9.67 ( 0.42 9.82b 9.69 ( 0.42 9.88b 10.38 ( 0.44 10.63*g 11.05 ( 0.45 11.48*g
2-monoBDE 3-monoBDE 2,4-diBDE 3,4-diBDE 2,6-diBDE 4,4′-diBDE 2,4′-diBDE 3,4′-diBDE 2,3,4-triBDE 2,4,6-triBDE 3,3′,4-triBDE 2,4′,6-triBDE 3,4,4′-triBDE 2,3′,4,6-tetraBDE 2,4,4′,6-tetraDE 2,2′,4,4′-tetraBDE 2,3′,4,4′-tetraBDE 3,3′,4,4′-tetraBDE 2,2′,4,4′,5-pentaBDE 2,2′,3,3′,4-pentaDE 2,3′,4,4′,6-pentaDE 2,2′,4,4′,5,5′-hexaBDE
7.34 ( 0.01 7.44 ( 0.01 8.36 ( 0.01 8.52 ( 0.00 8.13 ( 0.01 8.60 ( 0.01 8.45 ( 0.00 8.54 ( 0.01 9.37 ( 0.01 8.94 ( 0.01 9.48 ( 0.00 9.18 ( 0.00 9.54 ( 0.00 10.04 ( 0.02 9.95 ( 0.01 10.14 ( 0.01 10.28 ( 0.02 10.46 ( 0.02 10.99 ( 0.01 10.86 ( 0.01 11.20 ( 0.01 11.78 ( 0.02
Bromodiphenyl Ethers 7.24 ( 0.38 7.36 ( 0.38 8.37 ( 0.40 8.55 ( 0.40 8.12 ( 0.39 8.64 ( 0.40 8.47 ( 0.40 8.57 ( 0.40 9.49 ( 0.42 9.02 ( 0.41 9.61 ( 0.42 9.28 ( 0.42 9.68 ( 0.42 10.23 ( 0.44 10.13 ( 0.43 10.34 ( 0.44 10.53e 10.49 ( 0.44 10.82e 10.70 ( 0.45 10.87e 11.28 ( 0.46 11.32e 11.14 ( 0.46 11.52 ( 0.46 12.15 ( 0.48 11.82e
literaturei
this study
75.9,a 66.4,c 66.7*f 72.6,a 75.9*f 72.6,a 84.3*f 66.3,a 96.6*f 83.1,d 74.4*f 73.3,c 79.7*f 73.5,c 83.5,d 74.4*f 89.9,c 84.8,d 79.7*f
54.9 ( 2.9 66.2 ( 3.4 69.3 ( 3.6 76.9 ( 4.0 74.6 ( 3.9 81.4 ( 4.2 80.6 ( 4.2 86.5 ( 4.5
66.0*g 62.2b 73.2*g 79.5*g 79.1*g 76.6b 88.6b 84.1b 89.7b 95.8b 95.8b 94.0b 94.3b 99.5*g 106.6*g
58.9 ( 3.1 56.9 ( 2.9 66.1 ( 3.4 71.5 ( 3.7 71.6 ( 3.7 70.2 ( 3.6 77.1 ( 4.0 76.3 ( 3.9 78.6 ( 4.1 82.6 ( 4.3 82.6 ( 4.3 83.2 ( 4.3 83.5 ( 4.3 89.0 ( 4.6 94.3 ( 4.9
97.0e 106.8e 94.5e 91.1e 98.2e
61.9 ( 3.2 63.6 ( 3.3 73.6 ( 3.8 75.3 ( 3.9 71.3 ( 3.7 76.1 ( 3.9 74.6 ( 3.8 75.6 ( 3.9 83.2 ( 4.3 79.2 ( 4.1 84.5 ( 4.4 81.4 ( 4.2 84.9 ( 4.4 89.2 ( 4.6 88.3 ( 4.6 90.1 ( 4.7 91.6 ( 4.8 93.4 ( 4.8 98.5 ( 5.1 97.2 ( 5.0 100.0 ( 5.2 105.7 ( 5.5
a Ref 8. b Ref 10. c Ref 9. d Ref 14. e Ref 12. f Derived from data reported in ref 19. g Ref 20. h Ref 13. i Those values with an asterisk (*) were determined indirectly by interpolating regression using gas chromatographic capacity factors.
identical (Table 3). The relatively high estimated standard errors of the KOA-calc for the CBzs of 10 to 14% are due to the small number of compounds used in the derivation of eqs 16 and 17, and not due to a poor relationship between KOA-GC and KOA-lit, as can be seen in Figure 3 and is evident from the high r2 values. The large discrepancy between KOA-GC obtained with HCBz as a reference and KOA-lit is somewhat surprising considering the smaller deviations observed for KOA-GC values obtained with C-18 and p,p′-DDT as a reference. This suggests that the condition of eq 3 is not fulfilled well for HCBz and the CBzs; i.e., the ratio of the activity coefficients of the not fully chlorinated CBzs and HCBz 3480 Analytical Chemistry, Vol. 74, No. 14, July 15, 2002
in pure octanol does not equal the ratio of the activity coefficients in the DB-1 stationary phase. Whereas that may be appear peculiar, the octanol-air partition coefficient as well as the subcooled liquid vapor pressure of HCBz does, indeed, seem to be unusual when compared to the other CBzs. Simple linear regressions between size-related molecular descriptors, such as molar mass or the number of chlorine substituents, and log KOA and log PL for the chlorinated benzenes suggests that the PL for HCBz is unusally low and the KOA for HCBz unusually high by the same extent. For that reason, the KOA-lit for HCBz was also not included in the regression eq 17. We therefore believe that
Table 3. Uncalibrated (KOA-GC) and Calibrated (KOA-cal) Octanol-Air Partition Coefficients for the Chlorinated Benzenes at 25 °C Determined from Gas Chromatographic Retention Times Using Either Hexachlorobenzene (HCBz) or n-Octadecane (C-18) as a Reference, Octanol-Air Partition Coefficients from the Literature (KOA-lit), and Energies of Phase Transfer ∆OAU Obtained from the Literature and Gas Chromatographic Retention Times compd
c
log KOA-GC
∆OAU, kJ‚mol-1
log KOA-lita
log KOA-cal
reference
HCBz
C-18
HCBz
C-18
1,2-diCBz 1,3-diCBz 1,4-diCBz 1,2,3-triCBz 1,3,5-triCBz 1,2,4-triCBz 1,2,3,4-tetraCBz 1,2,4,5-tetraCBz 1,2,3,5-tetraCBz pentaCBz hexaCBz
4.96 ( 0.02 4.82 ( 0.00 4.83 ( 0.00 5.58 ( 0.01 5.31 ( 0.00 5.46 ( 0.00 6.22 ( 0.00 6.07 ( 0.01 6.06 ( 0.00 6.75 ( 0.01
4.33 ( 0.00 4.23 ( 0.01 4.25 ( 0.01 4.96 ( 0.00 4.69 ( 0.00 4.84 ( 0.00 5.60 ( 0.00 5.45 ( 0.00 5.44 ( 0.01 6.13 ( 0.00 6.77 ( 0.02
4.40 ( 0.63 4.25 ( 0.62 4.27 ( 0.62 5.06 ( 0.66 4.77 ( 0.64 4.93 ( 0.65 5.73 ( 0.70 5.57 ( 0.69 5.56 ( 0.69 6.29 ( 0.73
4.39 ( 0.53 4.29 ( 0.52 4.32 ( 0.52 5.06 ( 0.56 4.77 ( 0.54 4.93 ( 0.55 5.73 ( 0.59 5.57 ( 0.58 5.56 ( 0.58 6.29 ( 0.62 (6.95 ( 0.66)
literature
HCBz
C-18
4.33b
49.3b
4.13*c 4.18*c 5.16b 4.64*c 4.84*c 5.61b 5.65b 5.53*c 6.28b 7.38d
57.0*c 55.6*c 55.7b 68.6*c 62.1*c 62.3b 60.8b 66.3*c 71.3b 55.8d
29.0 ( 1.3 25.9 ( 0.9 26.0 ( 0.9 35.7 ( 1.3 32.6 ( 1.2 34.3 ( 1.3 44.1 ( 1.6 42.3 ( 1.6 42.2 ( 1.5 49.7 ( 1.8
28.2 ( 1.7 27.1 ( 1.7 27.9 ( 1.7 35.7 ( 2.1 32.3 ( 1.9 34.1 ( 2.0 43.9 ( 2.6 42.2 ( 2.5 42.0 ( 2.5 49.6 ( 2.9 55.6 ( 3.3
a Those values with an asterisk (*) were determined indirectly by interpolating regression using gas chromatographic capacity factor. b ref 8. ref 20. d ref 13.
Figure 2. Relationship between the KOA-GC values at 25 °C derived from gas chromatographic retention times relative to p,p′-DDT and directly measured KOA values for polychlorinated biphenyls and naphthalenes, as well as polybrominated diphenyl ethers. The solid line represents a 1:1 relationship; the broken line is the linear fit through the data points.
for HCBz, the interactions with the DB-1 phase do not resemble those with the octanol or pure liquid phase, suggesting that HCBz is not a highly suitable reference compound in the presented method. However, it should be stressed that after calibration, the calculations relying on reference compound HCBz gave the same final KOA values as those using the reference compound C-18, highlighting the usefulness and strength of the calibration procedure. Comparison of KOA-GC with KOA Values Determined by a Multicolumn Regression Method. For some of the compounds in Table 2, Zhang et al.19 and Su et al.20 used multiple linear regression analysis between previously measured KOA values and capacity factors on multiple gas chromatographic columns of different polarity to interpolate KOA values for structurally related substances. The KOA-lit* values obtained with this multicolumn regression are included in Tables 2 and 3 and are identified by
Figure 3. Relationship between the KOA-GC values at 25 °C derived from gas chromatographic retention times relative to hexachlorobenzene and n-octadecane and directly measured KOA values for chlorinated benzenes.
an asteriks. Figure 4 compares these values with the KOA-calc values obtained in this study (using eq 17 for the CBzs). This is a truly independent evaluation of the method, because the data from refs 19 and 20 had not been used in the derivation of the calibration eqs 15-17. The agreement is good, with deviations averaging at 0.14 log units. Somewhat larger deviations are observed at very high and low KOA values. Temperature Dependence of the KOA. Equation 10 suggests that it is also possible to estimate the temperature dependence of the KOA values from the slope of eq 9. The energies of octanolair transfer ∆OAU for all experimental chemicals estimated using eq 10 are listed in Tables 2 and 3. The standard deviations reported for these energies take into account the uncertainty of the factor (S + 1) as obtained from replicate ln(tR/tRref) vs ln KOAref regressions and the uncertainty of the energy of phase transfer of the standard reference compound ∆OAUref as estimated from the error of the slopes in eqs 11-14. It is the latter uncertainty that dominates the error of the estimated ∆OAU values. Analytical Chemistry, Vol. 74, No. 14, July 15, 2002
3481
Figure 4. Comparison between the KOA values at 25 °C determined in this study with KOA values derived by a GC-based interpolation method. The data set includes chlorinated benzenes, biphenyls, and naphthalenes. The solid line represents a 1:1 relationship; the broken line is the linear fit through the data points.
Figure 5. Comparison between energy of phase transfer between n-octanol and air ∆OAU determined in this study and ∆OAU values reported in the literature. The solid line represents a 1:1 relationship.
The energies of phase transfer allow the estimation of KOA as a function of temperature using
log KOA(T) ) log KOA(Tref) + ∆OAU/(R‚ln(10))‚(1/T - 1/Tref) (18) where Tref ) 298.15 K. Previous publications report parameters m and b for the equation.
log KOA ) m/T + b
(19)
These parameters correspond to m ) ∆OAU/(R‚ln(10)) and b ) log KOA(Tref) + ∆OAU/(R‚ln(10)‚Tref). Figure 5 compares the ∆OAU from this study with directly measured values reported previously.8-10,12,14 ∆OAU values for the same compound reported by different authors often show significant deviations (Table 2). An example is the energies of phase transfer for the PCBs reported by Harner and Bidleman9 and Ko¨mp and McLachlan.14 Even replicate experiments by the same authors have sometimes yielded significantly different values of ∆OAU, as illustrated by the case of 4-monochlorobiphenyl.8,9 Considering this uncertainty of the directly determined energies of phase transfer, the ∆OAU estimated from gas chromatographic retention times agree reasonably well with those reported in the literature. Most encouraging is the observation that the ∆OAU for 2,2′,4,5,5′-pentaCB and 2,2′,4,4′,5,5′-hexaCB fall between those reported in refs 9 and 14. Exceptions to the generally good agreement are the ∆OAU’s for the CBzs, which are not included in Figure 5. Although the ∆OAU values for the CBzs obtained with HCBz and C-18 as a reference compound agree exceptionally well with each other, they are ∼20 kJ/mol lower than those reported in ref 8. It can be quite deceptive to compare and evaluate ∆OAU or the temperature slopes m in isolation, because they are very dependent on the log KOA at the reference temperature or the 3482 Analytical Chemistry, Vol. 74, No. 14, July 15, 2002
Figure 6. Comparison between log KOA values at 5, 15, 25, 35, and 45 °C derived in this study and the respective log KOA values derived from relationships reported in previous studies.
parameter b. We thus evaluated the capability of the parameters determined in this study to predict KOA values within the environmental temperature range that are comparable to values measured as a function of temperature with the generator column technique. We thus calculated KOA values at 5, 15, 25, 35, and 45 °C using eq 18 and the parameters log KOA-calc and ∆OAU from Tables 2 and 3. We further did a similar calculation with the m’s and b’s reported in refs 8, 9, 10, 12, and 14 and plotted the results against each other in Figure 5. This graph shows that despite the relatively large discrepancies in the reported ∆OAU, the technique used in this study is capable of predicting the KOA of the investigated substances within the environmentally relevant temperature range with an average deviation from directly measured values of
