Environ. Sci. Technol. 1988,22,1362-1364
NOTES Vapor Pressures of Chlorinated Dioxins and Dibenzofurans Brian D. Eitzer and Ronald A. Hites" School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405
rn The subcooled liquid vapor pressures of five polychlorinated dibenzo-p-dioxins (PCDD) and nine polychlorinated dibenzofurans (PCDF) were determined by a gas chromatographic (GC) method. The experimentally determined vapor pressures are highly correlated to published GC retention indexes. By use of this correlation, vapor pressures were calculated for all chlorinated dioxins and dibenzofurans; these predicted vapor pressures correlate with vapor pressures determined or predicted by other methods. Introduction It is now known that polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) can be transported through the atmosphere to remote locations (I). However, predicting the details of this transport process or understanding reactions taking place in the atmosphere (if any) is limited by our lack of knowledge of important physical properties of PCDD/F. One such property is their vapor pressure. Clearly, a compound's vapor pressure will affect its partitioning between the vapor and particulate-bound phases, both of which occur in the atmosphere. This partitioning, in turn, affects depositional processes and atmospheric photodegradation rates. For these reasons, we have measured the vapor pressures of selected chlorinated dioxins and dibenzofurans. These data were then correlated to gas chromatographic retention indexes so that the vapor pressures of all individual congeners could be determined. The vapor pressures of compounds of low volatility are normally determined by either gas saturation (2-5) or effusion (2,5,6) methods. These methods require pure compounds in measurable quantities, and thus, they are not applicable to costly and toxic compounds such as the chlorinated dioxins and dibenzofurans. Gas chromatography (GC) is an alternate method for measuring vapor pressures (7-10). This method can be used for mixtures of compounds at low concentrations, thus reducing the problems stated above. It is based on the use of a nonpolar stationary phase and isothermal conditions such that the compound's GC retention time is related directly to its vapor pressure. The GC method has been used to study PCBs (7,8), herbicide esters (9),and organophosphorus pesticides (10). Experimental Section The standards used for this study were 1,2,3,4-tetrachlorodibenzo-p-dioxin, octachlorodibenzo-p-dioxin(OCDD) (Foxboro/Analabs, North Haven, CT), 1,2,3,7,8pentachlorodibenzo-p-dioxin,1,2,3,4,7,8-hexachlorodibenzo-p-dioxin (KOR Isotopes, Cambridge, MA), 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin, 2,3,7,8-tetrachlorodibenzofuran, 1,2,3,7,8-pentachlorodibenzofuran, 2,3,4,7,8-pentachlorodibenzofuran, 1,2,3,4,7,8-hexachloro1362 Environ. Sci. Technoi., Voi. 22, No. 11,
1988
Table I. Subcooled Liquid (Below 110 "C) and Liquid Vapor Pressures of p,p'-DDT as a Function of Temperature temp, "C
vp, Torr
temp, "C
vp, Torr
25 90 100 110
0.00000258 0.00225 0.005 18 0.0114
120 130 140 150
0.0243 0.0497 0.0984 0.189
dibenzofuran, 1,2,3,6,7,8-hexachlorodibenzofuran, 1,2,3,7,8,9-hexachlorodibenzofuran,2,3,4,6,7,8-hexachlorodibenzofuran,1,2,3,4,6,7,8-heptachlorodibenzofuran, (Cambridge Isotope 1,2,3,4,7,8,9-heptachlorodibenzofuran Laboratories, Woburn, MA), and p,p'-DDT (EPA, Research Triangle Park, NC). Samples were analyzed on a Carlo-Erba Fractovap series 4160 GC with a 0.75-m, J&W DB-5,0.25-pm film thickness, capillary column and with an electron-capture detector. The GC was operated isothermally at 10 "C intervals from 90 to 150 "C; the helium flow was set by maintaining the column head pressure at 0.35 kg/cm2. Retention times were determined on an HP 3390 A integrator. All sample runs were made in duplicate; average retention times were used for calculations. Sample runs consisted of the entire set of dioxins or dibenzofurans plus the p,p'-DDT standard. The runs on the dibenzofurans had a pair of unresolved peaks (1,2,3,4,7,8 and 1,2,3,6,7,8; 1,2,3,7,8,9and 2,3,4,6,7,8);for the calculations, it wm assumed that the vapor pressures were the same for both. Results The choice of p,p'-DDT as the reference standard was dictated by the need of a compound that would chromatograph in a manner similar to PCDD/F, be readily detected by the electron-capture detector, and have vapor pressures known at the experimental temperatures. Average vapor pressures of p,p'-DDT at 25 "C and at the expeirmental temperatures were determined by regression of the data presented in references 11-14 and are presented in Table I. These workers used either an effusion (11) or gas saturation (12-14) method to measure the solidphase vapor pressure of p,p'-DDT from 20 to 100 OC. However, the GC method gives the subcooled liquid vapor pressure (defined as the liquid vapor pressure extrapolated below the melting point) (8). To properly apply the GC method, it was necessary to convert the literature-based solid vapor pressures to subcooled liquid vapor pressures by the equation developed by MacKay et al. (15). In ( P l / P s )= 6.8(Tm- T ) / T (1) where Pl is the subcooled liquid vapor pressure, P, is the solid vapor pressure, T, is the melting temperature of the compound, T is the ambient temperature, and 6.8 is an empirical constant related to the entropy of fusion. The
0013-936X/88/0922-1362$01.50/0
0 1988 American Chemical Society
Table 11. Measured Gas Chromatographic Retention Times (in Minutes)" temp, "C
p,p'-DDT
1234
12378
dioxins 123478
90 100 110 120 130 140 150
19.0 9.13 4.50 2.37 1.32 0.76 0.48
14.1 7.00 3.61 1.97 1.13 0.67 0.44
42.23 19.82 9.73 5.05 2.72 1.54 0.92
110.5 48.86 23.10 11.45 5.95 3.22 1.83
temp, "C 90 100 110 120 130 140 150
2378
12378
23478
19.05 9.08 4.51 2.37 1.31 0.75 0.46
12.45 6.22 3.23 1.76 1.02 0.60 0.38
27.96 13.50 6.81 3.60 2.00 1.15 0.69
1234678
OCDD
280.45 120.86 55.18 26.20 13.07 6.89 3.78
654.0 269.6 118.5 55.48 22.66 13.58 7.25
furans 123478 and 123678 123789 and 234678 37.44 17.62 8.68 4.51 2.46 1.40 0.82
75.9 34.91 16.86 8.49 4.53 2.51 1.45
1234678
1234789
100.6 45.90 21.76 10.83 5.69 3.07 1.73
180.0 80.22 37.17 18.15 9.35 5.00 2.78
" Average of duplicate measurements. Table 111. Regression Parameters and Experimental PaC compd dioxins 1234 12378 123478 1234678 OCDD dibenzofurans 2378 12378 23478 123478 and 123678 123789 and 234678 1234678 1234789
C"
L1/L2"
Pcc(25 "C), Torr
r2
0.011 -0.627 -1.223 -1.865 -2.388
0.952 1.030 1.090 1.138 1.191
7.83 X 1.31 X 2.97 X 7.68 X lo4 2.08 X
0.99 0.90 0.98 0.99 0.96
0.098 -0.436 -0.566 -1.084 -1.229 -1.661 -1.903
0.947 0.993 1.019 1.051 1.074 1.099 1.119
9.21 X 2.73 X 1.63 X 6.07 X 10" 3.74 X 1.68 X 10" 9.79 x 104
0.99 O.5Ob 0.88 0.96 0.97 0.99 0.99
2,900 2,800 -
2,700 -
" See eq 4. *Removal of the 150 Oc data point improves 1.2 to 0.95 but only causes a 5% change in PO,-. melting point of p,p'-DDT is 109 "C (8). The relevant equations for determining vapor pressure by the GC method (hereafter called PGc)have been developed by Hamilton (9). At a constant temperature, the vapor pressures of a test and of a reference compound (subscripts 1and 2, respectively) are related by the ratio of their latent heats of vaporization: In PI = (L1/L2)In Pz
+C
(2)
where L is the latent heat of vaporization and C is a constant. These vapor pressures are also related to their GC retention times: In PI = In P2 - In (tr,l/tr,z)
(3)
where t, is the retention time. Combining eq 2 and 3 and rearranging yields In
(tr,I/tr,z)
RI 3,000 r
= (1 - L1/Lz) In Pz - C
(4)
Therefore, a plot of In (tr,l/t,,z)versus In Pz would have a slope of (1- L1/L2) and an intercept of (4). Equation 2 can then be used to determine the vapor pressure of the test compound at any temperature given the vapor pressure of the reference compound at that temperature. Table I1 gives the GC retention time data measured in this study. From these data, the parameters in eq 4 were calculated by regression; Table I11 shows these parameters
2,600 -
2,500 -
2,400
t
2,300 I -9
I
-8.5
I
-0
h
I
I
I
-7.5
-7
-6.5
S
I
-6
-5.5
Log Pgc Flgure 1. Plot of calculated retention index versus experimental log PQC.
and the experimentally determined PGC at 25 "C. This experiment has only determined the vapor pressures of a selected few of the 210 different congeners of these compounds. However, since the method is based on GC retention times, recently published retention indexes of dioxins (16) and dibenzofurans (17) on the same type of column can be used to determine the vapor pressure of the other congeners. Figure 1 is a plot of the retention index versus log PGC at 25 "C for each of the congeners studied here. Regression of these data (r2 = 0.99) yields the following equation for calculating the vapor pressure from the GC retention index (RI): log PGC(Torr) = (404- RI)/319 (5) It is useful to compare the results produced by this experimental method with the limited amount of vapor pressure data on dioxins and dibenzofurans that have been previously published. Schroy et a1 (18) reported a 30 "C solid vapor pressure for 2,3,7,8-tetrachlorodibenzo-p-dioxin (2378-TCDD) of 3.5 X lo4 Torr. When the vapor pressure Environ. Sci. Technol., Vol. 22, No. 11. 1988
1383
Fortunately, Rordorf gives two different vapor pressures for this congener, and the other value agrees much better. Registry No. p,p '-DDT, 50-29-3;1,2,3,4-tetrachlorodibenzo-
/ /''
p-dioxin, 30746-58-8;1,2,3,7,8-pentachlorodibenzo-p-dioxin,
40321-76-4;1,2,3,4,7,8-hexachlorodibenzo-p-dioxin, 39227-28-6; 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin, 35822-46-9;octachlorodibenzo-p-dioxin,3268-87-9; 2,3,7,&tetrachlorodibenzofuran, 51207-31-9;1,2,3,7,8-pentachlorodibenzofuran,57117-41-6; 2,3,4,7,8-pentachlorodibenzofuran, 57117-31-4; 1,2,3,4,7,8-hexachlorodibenzofuran, 70648-26-9;1,2,3,6,7,8-hexachlorodibenzofuran, 57117-44-9; 1,2,3,7,8,9-hexachlorodibenzofuran, 72918-21-9; 2,3,4,6,7,8-hexachlorodibenzofuran, 60851-34-5;1,2,3,4,6,7,8heptachlorodibenzofuran, 67562-39-4;1,2,3,4,7,8,9-heptachlorodibenzofuran, 55673-89-7.
Literature Cited Czuczwa, J. M.; McVeety, B. D.; Hites, R. A. Science
(Washington, D.C.) 1984,226,568-569. Spencer, W. F.;Cliath, M. M. Residue Rev. 1983,85,57-71. Sonnefeld, W. J.; Zoller, W. H.; May, W. E. Anal. Chem.
,-
-1OL-10
-9
I
I
I
I
-8
-7
-6
-5
I
-4
I
I
-3
-2
I
I
-1
0
Log New Figure 2. Log-log plot of solid-phase vapor pressure from Rordorf's data (79)versus this work. The regression line (solld) and the y = x line (dashed) are shown.
calculated by our experiment is converted to a solid vapor Torr, pressure at 25 "C by eq 1, the result is 1.0 X which compares quite favorably given the lower temperature. The only other reported data are that of Rordorf (19,20),who measured, by the gas saturation method, the solid vapor pressures for 10 dioxin and 4 dibenzofuran congeners and then used these data to predict the vapor pressures of an additional 15 dioxin and 51 dibenzofuran congeners. These data have been compared to the current work by predicting the vapor pressure of each congener Rordorf studied with eq 5 and then converting them to solid-phase vapor pressures by eq 1with the melting point data given by Rordorf. Figure 2 shows a log-log plot of this comparison with the regression line and the y = x line. Although the results are well correlated (r2 = 0.97), the paired t test shows that the data are different. The slope of the regression line is 1.21 f 0.03, which indicates that the more chlorinated congeners show a greater vapor pressure by this GC method than predicted by Rordorf and the less chlorinated congeners show a smaller vapor pressure than predicted by Rordorf. The worst outlier on this regression line is a point that represents 2378-TCDD.
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Envlron. Sci. Technol., Vol. 22, No. 11, 1988
1983,55,275-280. Fed. Regist. 1980,45, 77345-77348. OECD Guidelines for Testing of Chemicals;Paris, 1981; Sections 104-105. Murray, J. J.; Pottie, R. F.; Pupp, C. Can. J. Chem. 1974, 52,557-563. Westcott, J. W.; Bidleman, T. F. J. Chromatogr. 1981,210, 331-336. Bidleman, T. F. Anal. Chem. 1984,56, 2490-2496. Hamilton, D. J. J. Chromatogr. 1980,195,75-83. Kim, Y.-H.; Woodrow, J. E.; Seiber, J. N. J. Chromatog. 1984,314,37-53. Balson, E. W. Trans. Faraday SOC.1947,43,54-60. Dickinson, W. Trans. Faraday Soc. 1956,52,31-35. Spencer, W. F.;Cliath, M. M. J.Agri. Food. Chem. 1972, 20,645-649. Rothman, A. M. J. Agric. Food. Chem. 1980,28,1225-1228. MacKay, D.; Bobra, A.; Chan, D. W.; Shiu, W. Y. Enuiron. Sci. Technol. 1982,16,645-649. Donnelly, J. R.; Munslow, W. D.; Mitchum, R. K.; Sovocool, G. W. J. Chromatog. 1987,392,51-63. Hale, M. D.; Hileman, F. D.; Mazer, T.; Shell, T. L.; Noble, R. W. Anal. Chem. 1985,57,640-648. Schroy, J. M.; Hileman, F. D.; Cheng, S. C. Chemosphere
1985,14, 877-886. Rordorf, B. F. Chemosphere 1986,15,1325-1332. Rordorf, B. F.Proceedings of the 7th International Sym-
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Received for review March 29,1988.Accepted June 7,1988. We thank the U.S. EPA for support (Grant CR812588).
