Anal. Chem. 1986, 58, 1835-1838
1835
Relationship between Gas Chromatographic Retention of Polychlorinated Dibenzofurans and Calculated Molecular Surface Area W. J. Dunn III* and Michael Koehler Department of Medicinal Chemistry and Pharmacognosy, College of Pharmacy, University of Illinois at Chicago, Chicago, Illinois 60612 I).
L. Stalling and T. R. Schwartz
Columbia National Fisheries Research Laboratory, US.Fish and Wildlife Service, Route 1, Columbia, Missouri 65201
I n the analysis of the composition of organic residues in environmental samples, complex mixtures are generally observed. Complex residues, such as toxaphene, can contain as many as 500-600 components. The major analytical technique used for characterizing such mixtures is GWMS. Isomer identlkation Is one of the most Important objectives of the gas chromatographic analysis of such mlxtures. Identification of constttuents in such complex mixtures is based, In part, on the relathre retenth time ur retenth M e x of the component. A number of attempts have been made to develop predlcthre relatlonshlps between retention data and the structure of an anaiyte. I n thls report the relationship between the relative retention t h e of polychlorinated dibenzofurans and molecular structure is explored using molecular modeling techniques.
A number of recent reports have appeared concerning the prediction of the retention behavior in capillary gas chromatography systems of components of complex mixtures detected in environmental samples. Examples of the types of compounds considered are low-molecular-weightorganics (1-5), polycyclic aromatic hydrocarbons (61, and polychlorinated biphenyls (7)and dibenzofurans (PCDF’s) (8,9). In these reports, parameters such as the square of the mean radius of the solute, connectivity indexes, electronic charges, and hydrogen bonding terms were used in efforts to derive relationships between chemical structure and retention behavior. While such approaches have some utility, it has not been possible to develop relationships between parameters derived solely from chemical structure and retention behavior. Recently, Mazer and co-workers (8) have reported the synthesis of the 38 possible isomers of TCDF’s along with their relative retention times on the capillary columns SP-2330 and SE-54. In a subsequent paper (9),retention indexes of 115 of the possible 135 PCDFs were reported. An elegant tabular treatment of the retention data showed the specific ring effects on retention could be detected in the data (9). Two regression models were derived, with indicator variables for specific substitution patterns, for the prediction of PCDF retention behavior. One model contained 10 independent variables while the other contained 18. In this report, relationships between the calculated size of an anal* and ita gas chromatographicretention are explored. The surface area, which is a measure of the size of a compound (IO,211, can be calculated in a number of ways. Here the van der Walls surface area (11)of a compound was used as the size-related parameter. The van der Waals surface was calculated by using molecular modeling techniques (21). This parameter has been shown to be related to a number of 0003-2700/86/0358-1835$01.50/0
measured physicochemical properties, such as aqueous solubilities (11) of low-molecular-weight organic solutes.
EXPERIMENTAL SECTION The GC retention data of the PCDFs, obtained in three liquid-phase systems, were taken from the literature and are given in Table I. The data are those of Hale et al. (8)on a J&W DB-5 column with retention determined relative to co-injected normal hydrocarbon standards. The data in Table I, from the 913-2330 and SE-54 columns, are from Mazur et al. (9). The nomenclature for the PCDFs is that used in these reports (8, 9). The van der Waals surface area was calculated by use of the method of Hermann (20)with a program provided by Pearlman (11)and adapted in this laboratory to run on a DEC VAX 11/750. Structures were input assuming standard bond lengths and angles; conformations were optimized with MM2 (12)in the software package, CHEMLAB I1 (13). The calculations were carried out on the VAX 11/750 of the Research Resources Center of the University of Illinois at Chicago. RESULTS AND DISCUSSION The van der Waals surface of a molecule is that surface at one van der Waals radius from the nuclei of the solute. The van der Waals surface of 2378 TCDF is shown in Figure 1. The van der Waals surface areas for the PCDF’s vary considerably with the 1-chlorodibenzofuran having the smallest van der Waals surface area and octachlorinated isomer having the greatest. A plot of the retention indexes against surface area for the PCDF’s is given in Figure 2. The retention data here were obtained from the J&W DB-5 column. This plot shows that there is an increase in retention as a function of chlorine number. The clustering by chlorine number is interesting and shows that resolution on this column is complete with regard to this parameter. One thing is obvious from the plot and that is that with surface area as an independent variable and retention as the dependent variable, there are only eight types of PCDF’s. An analysis of variance shows that 96.9% of the variance in the retention data is accounted for by the surface area (SA) of the PCDF’s. A model, eq 1,developed with this variable can be used to distinguish one isomer group from the other. retention time = -924.62 (*54.06) 13.21 (*0.22) SA
+
(1) s = 0.98 An examination of the structure of the isomer clusters in Figure 2 is interesting and leads to further development of the relationship between surface area and retention. Figure 3 is a plot of the van der Waals surface for the TCDF’s vs. relative retention on the J&W DB-5 column. The tetrachloro compounds show three distinct groupings in this plot with members of each group having a common chlorine positional feature. The largest group (group 1)has a chlorine at positions
n = 115 r2 = 0.969
0 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58,NO. 8, JULY 1986
1838
Flgure 1. van der Waals surface of 2378 TCDF with van der Waals radius, r , shown for the carbon-chlorine bond.
I
7
I i
2604
I
CALCULATED SURFACE AREA
Flgure 3. Plot of the van der Waals surface vs. relative retention for the TCDF's on the DB-5 column. Data are from Table I with the compound label Indicated in Table I. n = 0-2. 260 114
2MD.
U6D.
x
I
0
32540.
z
160
E
I i I90
200
210
220
230
240
2SO
2M
270
Z S M
210
I O
52520. Y r
CALCULATED SURFACE AREA
Flgure 2. Plot of the retention index (RI) PCDF's on the DE5 column vs. van der Waals surface from Table I. The numbers represent the
number of chlorine atoms in the isomer.
1 or 9; one (group 2) has chlorines at 1 and 9; and the other (group 3) has no chlorines at these positions. The only exception to this is 17, 1234 TCDF, the only isomer with substituents on only one ring. It appears in the group with chlorine at both 1 and 9. The retention data for these three groups can be modeled with surface area as shown in eq 2-4. retention timel = 9778.99 (f2497) - 30.54 (&lo)SA (2) n = 19 r2 = 0.70 s = 14.96 retention times = 11362.28 (f4169) - 37.17 (f17)SA (3)
n = 10 r2 = 0.76 s = 20.30 retention time, = 12854.45 (f4330) - 42.51 (f17) SA (4) n = 9 r2 = 0.83 s = 15.12 The relationship holds for other chromatography systems as shown for group 1 TCDF's and the retention data from SP-2330 and SE-54. This is shown in eq 5 and 6. The 1234 isomer behaves as it does in the other chromatography system and is not included with this. These models are significant at the 95% level of confidence. re1 retention timesP-23so= 20.57 (f7.2) - 0.081 (f0.029) SA (5) n = 19 r2 = 0.67 s = 0.04 re1 retention timesE.54 = 11.91 (f3.2) - 0.045 (*0.013) SA (6) n = 19 r2 = 0.75 s = 0.02
Their predictive utility can be illustrated by fitting the 2378 isomer to eq 2, 3, and 4. These models give predicted retentions of 2212,2153, and 2324, respectively. This correctly suggests that the compound does not have the 1 or 9 chloro substitution pattern, and its residual is within the standard error of estimate for eq 2. This narrows the possibilities to one of four TCDF's, 2347,2378,2348, or 2346. This is a highly significant result considering the fact that it was obtained by using only one variable, the van der Waals surface area. The subclasses are observed in the other isomer clusters as shown in Figure 4 for the pentachloro PCDF's. Models for these groups can also be developed as previously discussed. However, these examples illustrate the utility of the approach. The surface areas published in Table I could be used by others to derive models specific for their chromatography systems. An explanation for the observation of the three subclasses cannot be definitively stated on the basis of this study. It may be that the three groups interact with different types of sites in the liquid phase or on the support. Another possibifty is that the interactions of the members of the three groups could be due to different orientations. For example, 1368 (no. 49 in Figure 3) and 2367 (no. 82) have essentially the same surface areas but much different retention times. Also, Figure 3 suggests that retention time does not appear to be a function
ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986
1837
Table I. Surface Areas (A') and Retention Time Data for the PCDF's
compd
congener
1
1
2 3 4 5 6 7 8 9 10
3
11
12 13 14 15 16 17 18 19 20 21 22
23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68
2
4 13 14 17 18 24 16 37 27 12
28 36 26 46 23 34 19 137 138 136 249 134 147 124 148 146 247 248 246 239 127 128 123 349 139 126 237 238 347 348 236 346 149 234 129 1368 1468 2 468 1347 1378 1247 1348 1346 1248 1246 1367 1379 1268 1478 1467 2 368 2 467 1369 1237 1238
van der Waals surface area 198.04 200.15 200.17 199.93 214.40 214.09 214.33 214.28 216.39 214.05 216.60 216.62 212.94 216.56 216.44 216.39 216.29 215.77 215.46 211.84 230.89 230.72 230.55 230.43 229.63 230.54 229.11 230.37 229.98 232.81 232.82 232.68 229.90 229.42 229.34 228.11 229.60 228.23 229.10 232.15 232.15 231.92 231.86 231.98 231.73 227.80 230.72 226.58 246.89 246.49 249.08 245.93 246.34 245.55 245.95 245.68 245.48 245.25 245.94 243.70 245.48 246.00 245.66 248.41 248.23 244.20 244.56 244.51
DB-5
retention SP-2330'
SE54*
1739 1749 1749 1760 1884 1913 1910 1925 1912 NR" 1930 1930 1934 1935 1944 1946 1953 1939 1959 1975 2057 2070 2072 2082 2088 2086 2085 2100 2094 2099 2097 2101
2288
2297 2305 2296 2294 2307
congener
69 70
1234 1236 2 349 1469 1278 1349 1267 2 347 1279 1249 2 348 2 246 2 378 2 367 3 467 1269 1239 1289 13468 12 468 23 479 13478 13467 12 368 12478 12 467 13479 23 469 12 479 23 468 13469 12347 12469 12348 12 246 12 378 12 367 12 379 23 489 13489 23 478 23 467 12489 12 369 12 349 12 389 123468 134678 124678 134679 124679 124689 123478 123467 123678 123479 123679 123689 123469 234 678 123789 123489 1234678 1234679 1234 689 1234789 12 346 789
242.71 244.26 244.79 243.73 244.93 243.20 244.63 247.08 243.08 242.54 247.13 246.97 247.73 247.47 247.25 242.53 241.58 241.41 262.14 261.61 261.14 261.56 261.19 260.61 261.10 260.71 259.65 260.79 259.07 263.38 259.21 259.16 258.54 259.08 258.83 260.04 259.75 258.18 259.70 258.03 262.71 262.47 257.30 257.63 256.18 256.55 275.22 276.37 275.83 274.70 274.00 273.27 274.71 274.33 274.87 272.63 273.11 272.46 272.12 277.65 271.63 270.93 289.57 287.67 286.84 285.99 300.44
71
72 73 74 75 76 77 78 79 80 81
82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100
101 102 103 104 105 106 107 108 109 110
2111
2109 2129 2113 2125 2124 2125 2134 2132 2150 2151 2141 2152 2151 2148 NR" 2227 2242 2254 2257 2263 2264 2276 2262 2274 2264 2272 2273 2281 2290
compd
van der Waals surface area
111 112
0.550 0.640 0.763 0.630 0.620 0.647 0.666 0.687 0.690 0.707 0.652 0.620 0.707 0.717 0.769 0.867 0.919 0.720 0.713 0.763
0.846 0.870 0.882 0.895 0.900 0.897 0.909 0.902 0.915 0.903 0.900
0.909 0.925 0.929 0.935 0.947 0.961 0.944 0.943 0.962
113 114 115 116 117 118 119 120 121 122
123 124 125 126 127 128 129 130 131 132 133 135 136
DB-5 2310 2307 2308 2314 2322 2325 2329 2337 2341 2335 2340 2339 2338 2354 2362 2364 2369 2406 NR" NR" 2467 2469 2469 NR" NR" 2465 2473 2476 2479 2495 NR" 2495 2497 2508 2496 2507 2540 NR" 2521 NR" 2551 2555 2559 2546 NR" 2593 2650 NR" NR" NR" NR" 2686 2708 2706 NR" 2720 NRa NR" NR" 2748 NR" NR" 2898 2913 2922 2986 3147
retention SP-2330b
SE-54*
0.758 0.772 0.754 0.861 0.807 0.800 0.847 0.963 0.850 0.867
0.968 0.963 0.966 0.966 0.979 0.991 0.991 1.004 1.002
1.011
1.035
1.004 1.004
1.000
1.000
1.050 1.135 0.998 0.965 1.210
1.026 1.037 1.038 1.051 1.103
1.000
" Data not reported. Only TCDF retentions were reported. of substitution adjacent to the ether linkage as has been proposed (9). In the group with no substituents at positions 1 and 9,3467 (no. 83) and 2468 (no. 51) have both positions
adjacent to the ether linkage substituted. They represent the extremes in retention for this group. This is seen with the other groups also. Deviation from planarity does not appear
1838
ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1988
to be a factor as all of the TCDF's are found to be planar by molecular modeling methods used here. One outstanding feature of the relationships between surface area and retention time for the TCDFs is their inverse nature. The increase in retention of the PCDFs with the number of chlorine atoms (Figure 2) suggests that the PCDF isomer groups are chromatographicallyseparated on the basis of solubility in the liquid phase. This is observed for homologous series (14);retention increases as the carbon number increases. As the number of carbon atoms increases in such a series there will be an increase in surface area and retention. The difference in retention behavior as a function of size for the isomer groups compared to those of the PCDF congeners is difficult to explain but could result from the importance of the different steps involved in the solution of dibenzofurans in the liquid phase. There are two steps involved in the solution of a compound in the liquid phase: (1)formation of a cavity in the liquid phase and (2) placing the compound in the cavity. Cavity formation is an endothermic process, the smaller the cavity required the more favorable and more soluble the compound. Therefore, if cavity formation determines retention, smaller compounds will have greater retention. The second step, placing the compound in the cavity, is exothermic. If it is more exothermic than the first step is endothermic, the solution process will be controlled by this step. The larger the compound, the more soluble it will be in the liquid phase and the more it is retained. For the TCDF's, since the smaller compounds have greater retention, the first step appears to determine retention, while differences in retention of isomer groups appear to be determined by the overall solubility process. A number of reports have recently appeared in which relationships have been explored between molecular features and various physicochemical processes such as relative retention and solubility ( 4 , 5, 8, 10). One feature that these studies have in common is the use of multivariable regression. Such studies are computer intensive with the strategy being to input a chemical structure and have the computer generate large numbers of parameters. These parameters are then used as independent variables in an effort to predict a physicochemical property of a group of compounds. Models derived in this way can obscure the interesting and, perhaps, most useful aspects of the relationship studied. This can sometimes be avoided by merely plotting the data to examine the results graphically. With regard to retention behavior, even within the class of TCDF's, there are apparently three subclasses of isomers. By the use of multivariable regression methods, a "general" model could be derived that would allow the three subgroups to be treated as one. This would require the generation of "variables" to account for the differences in the three groups, but such a model would contain no more information than the relationships shown in Figures 2 and 3.
Resources Center of the University of Illinois at Chicago for the surface area calculations. Registry No. 1, 84761-86-4; 2, 25074-67-3; 3, 51230-49-0;4, 74992-96-4; 5, 94538-00-8; 6, 94538-01-9; 7, 94538-02-0; 8, 81638-37-1; 9, 24478-74-8; 10, 74992-97-5; 11, 58802-21-4; 12, 74992-98-6; 13, 64126-85-8; 14, 5409-83-6; 15, 74918-40-4; 16, 60390-27-4; 17, 64560-13-0; 18, 64126-86-9; 19, 94570-83-9; 20, 70648-14-5; 21, 64560-16-3; 22, 76621-12-0; 23, 83704-39-6; 24, 82911-59-9; 25, 82911-61-3; 26, 83704-41-0; 27, 24478-73-7; 28, 64560-14-1; 29, 82911-60-2; 30, 83704-42-1; 31, 54589-71-8; 32, 58802-14-5; 33, 58802-18-9; 34, 83704-37-4; 35, 83704-34-1; 36, 83636-47-9; 37, 83704-46-5; 38, 83704-40-9; 39, 64560-15-2; 40, 58802-17-8; 41, 57117-32-5; 42, 83704-44-3; 43, 83704-45-4; 44, 57117-33-6; 45, 83704-43-2; 46, 70648-13-4; 47, 57117-34-7; 48, 83704-38-5; 49, 71998-72-6; 50, 71998-72-6; 51, 58802-19-0; 52, 70648-16-7; 53, 57117-35-8; 54, 83719-40-8; 55, 92341-04-3; 56, 83704-27-2; 57, 64126-87-0; 58, 71998-73-7; 59, 57117-36-9; 60, 64560-17-4; 61, 83710-07-0; 62, 83704-29-4; 63, 66794-59-0; 64, 57117-37-0; 65, 57117-38-1; 66, 83690-98-6; 67, 83704-22-7; 68, 62615-08-1; 69, 24478-72-6; 70, 83704-21-6; 71, 83704-33-0; 72, 70648-19-0; 73, 58802-20-3; 74, 83704-28-3; 75, 83704-25-0; 76, 83704-31-8; 77, 83704-26-1; 78, 83704-24-9; 79, 83704-32-9; 80, 83704-30-7; 81, 51207-31-9; 82, 57117-39-2; 83, 57117-40-5; 84, 70648-18-9; 85, 83704-23-8; 86, 70648-22-5; 87, 83704-55-6; 88, 69698-57-3; 89, 70648-21-4; 90, 58802-16-7; 91, 83704-36-3; 92, 83704-51-2; 93, 58802-15-6; 94, 58802-15-6; 95, 70648-20-3; 96, 83704-35-2; 97, 71998-74-8; 98, 67481-22-5; 99, 70648-15-6; 100, 83704-48-7;101,70648-24-7;102,67517-48-0; 103,83704-47-6;104, 57117-41-6; 105,57117-42-7;106,83704-53-4;107,69433-00-7; 108, 70~2-82-1;109,57ii7-31-4; 110,57117-43-8; 111,70648-23-6;112, 83704-52-3; 113,83704-49-8; 114,83704-54-5; 115,69698-60-8; 116, 71998-75-9; 117,67562-40-7;118,92341-05-4;119,75627-02-0;120, 69698-59-5; 121,70648-26-9;122,79060-60-9;123,57117-44-9; 124, 91538-84-0; 125,92341-06-5;126,75198-38-8; 127,91538-83-9; 128, 60851-34-5; 129,72918-21-9; 130,92341-07-6;131,67562-39-4;132, 70648-25-8; 133, 69698-58-4; 135, 55673-89-7; 136, 1010-77-1.
ACKNOWLEDGMENT We acknowledge the use of the VAX 11/750 of the Research
RECEIVED for review December 23,1985. Accepted March 5, 1986.
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