2124
Anal. Cham. 1984, 56, 2124-2128
Flame Ionization Detector Response Factors for Compound Classes in Quantitative Analysis of Complex Organic Mixtures . Y. Tong and F. W. Karasek* Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
FID response factors for 59 organic compounds were determined on GC using a WCOT column and cool on-column Injection. Observed responses gave average response factors characteristic of each compound class. On the basis of multiple determinations, the mean response factor of 15 alkanes was determined to be 14.53 area counts/ng with a relative standard deviation of 1.4% and that of 22 polycyclic aromatic hydrocarbon standards was 13.47 area counts/ng with a relative standard deviation of 5.7%. Limited studies on oxy-PAH and nltro-PAH Indicate that these compounds also exhibit characteristic FID response factors. These experimental results facilitate the quantitative analysis of multicomponents In a complex organic mixture by reducing demand on the number of standard compounds.
Studies
on
understood (16). However, two important features of the FID are generally accepted: the FID response is proportional to the carbon number of the hydrocarbons, often called the equal-per-carbon response, and the FID response of substituted hydrocarbons is always less than that of the parent
hydrocarbon (15, 17). On the basis of these observations several attempts have been made to develop a system called the “effective” carbon number, in which the FID responses of heteroatom groups were quantitatively related to the equal-per-carbon response (18,19). The quantitative characteristics of the FID response of some compound classes such as paraffin hydrocarbons, perfluoroalkanes, alcohols, carboxylic acids, and steroids have been studied (20-23). The results obtained in these earlier studies can be used as a general guideline to roughly estimate the FID response behavior of different compound classes but are not widely used in quantitative GC analysis for several reasons. In most of these studies, the FID response per mole of compound (molar response) or the relative molar response were used (18-21,23). The individual compounds of a given homologous series, because they have different numbers of carbon atoms, yielded different molar response values, although certain relationships existed among them. In some cases, response per unit weight of compounds was used, but the compound class which was subject to study, such as perfluoroalkanes, did not exhibit unique FID responses (22). Consequently, the practical approach for quantification of mixtures containing such homologues involved direct determination or indirect calculation of the molar response for individual compounds. An important factor which must be considered in the study of FID response factors is the performance of the GC instrument. In most earlier studies, packed columns and vaporization injection techniques were used. Accuracy and precision of peak areas were limited by the resolution offered by packed columns. Bleeding of the stationary phase of the column commonly led to broadening and tailing of peaks and poor base lines. Discrimination against both low and high boiling point compounds occurs in the vaporization injection technique. These occurrences adversely affect the accurate evaluation of FID response factors for many compounds. Lao determined FID response factors for a number of PAH standards on a packed column using the vaporization injection technique (24). The various PAH standards were found to exhibit different FID response per unit weight of the compounds. The relative difference in response per unit weight between the lowest molecular weight (biphenyl) and highest molecular weight compounds (coronene) studied by these workers was observed to be as high as 100% (24). The results of other studies showed even greater variation among the response factors for PAH and results were often conflicting
environmental pollution usually involve the
analysis of extremely complex mixtures of organic compounds. A large number of organic compounds have been found in extracts of airborne particulate matter, diesel engine exhaust particulates, and fly ash from municipal waste incinerators (1-7). Most of the available information about the components in these samples is qualitative, although some individual components have been studied quantitatively.
Quantitative data are important for the determination of the environmental impact of organic pollutants. However, reliable quantification of the multicomponents of complex environmental samples is often difficult to achieve owing to a number of variables. Two of the most common problems in such a study are the interferences among the components and the unavailability of a sufficient number of pure standard compounds. The interferences among the numerous components in a complex mixture can be minimized or eliminated by use of preseparation or cleanup procedures. Among various separation procedures, high-performance liquid chromatography (HPLC) has definite advantages which have led to its increasing use in the analysis of environmental samples (2,4-8). High separation efficiency, good reproducibility, and recovery are more easily obtained in HPLC separation. The complexity of the mixture can be effectively reduced by separating the components into different HPLC fractions. This HPLC technique has been applied for prefractionation of samples in the analysis of diesel exhaust particulates, fly ash from municipal incinerators, and airborne particulate matter (2, 6, 9,10). Several hundred organic components in diesel exhaust and fly ash extract were successfully sorted into classes of hydrocarbons, polycyclic aromatic hydrocarbons (PAH), oxygenated PAH (oxy-PAH), and polar compounds. A number of these compounds have been identified or tentatively identified by using GC/MS and GC analysis (2, 9, 10). For the simultaneous quantification of complex mixtures of organic compounds, gas chromatography with flame ionization detection (GC/FID) is a powerful tool. Several mechanisms have been proposed to explain the relative response of the FID for different organic compounds (11-15). Many factors of the basic theory of the FID are still not well 0003-2700/84/0356-2124$01.50/0
(25).
During the last few years the development of WCOT GC (also called high-resolution GC) has resulted in improved quality GC analysis. Also, the introduction of cool on-column injection into WCOT GC has minimized, or completely eliminated, the discrimination against both low and high boiling point compounds during sample injection (26, 27). The ©
1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 12, OCTOBER 1984
precision and accuracy of quantitative GC have consequently been greatly improved. We applied WCOT GC/FID with cool on-column injection to the analysis of several classes of compounds and found that members of each of the classes exhibited an FID response factor, expressed in response per unit weight of compound, unique to class rather than to individual compounds. Response factors of 59 pure standards were determined. The standard deviations in the mean response factor obtained for 22 PAH and 16 alkane standards were 5.7% and 1.7%, respectively. The implication of this finding is that the number of pure standards required for the quantitative analysis of individual components of a complex organic mixture can be drastically reduced. Separation of complex mixtures into various compound classes by suitable fractionation techniques such as HPLC, followed by quantification through application of a response factor unique to each class fraction, is now a feasible approach and greatly simplifies these analyses. The application of this method for the quantification of organic compounds associated with diesel exhaust particulate matter is demonstrated elsewhere (9, 28).
EXPERIMENTAL SECTION Standard Compounds. All standard compounds used in this
are commercially available. The normal alkane standards purchased from Polyscience Co. (Niles, IL). Their purities Eire greater than 98%. Standard solutions were prepEired at three different concentration levels of 1000, 100, and 10 ng/µ , respectively. Other standard compounds were from Aldrich Chemical Co. (Montreal, Quebec, Canada) or Chem. Service Inc. (West Chester, PA) and their purities were 95-99%. Concentrations of these standards were prepared in a range from 20 ng/µ to 100 ng/µ for the different compound classes. High-Resolution GC Analysis. Determination of the FID response of standard compounds was done on a Hewlett-Packard HP 5880A gas chromatograph equipped with FID and cool oncolumn injector for WCOT columns. A 30 m X 0.32 mm i.d. Durabond DB-5 fused silica capillary column (J&W Scientific Inc., Rancho Cordova, CA) was used. The injector temperature was less than 50 °C, and FID temperature was at 350 °C. The flow rate of helium carrier gas, hydrogen, and air for FID at room temperature were 3, 30, and 480 mL/min, respectively. The modified injection technique called “four segment injection” was used for on-column injection of small sample volumes on the WCOT column at room temperature (8). The syringe filling sequence for the injection technique was pure solvent (ca. 0.8 µ ), Eiir space (ca. 0.8 µ ), desired volume of sample solution (variable), and air space for emptying the syringe needle (variable). After the reading of sample solution was accurately taken, the syringe bar was pushed all the way in to ensure complete sample loading. The volume of sample injected ranged from 0.8 µ to a maximum of 1.2 µ . At least three replicate injections were made for each sample. The FID response factors (RF) of compounds in this study were calculated as
study were
RF (area counts/ng)
=
peak area counts of compd -, quantity of compd injected ,—r (ng)
-——
.
.
,
,
RESULTS AND DISCUSSION The response factors of each of 16 normal alkanes
were
determined by GC/FID. These compounds cover a wide range of boiling points. The response factors (RF) were determined based on a total of six injections at four different concentration levels: 500 ng, 100 ng, 10 ng, and 4 ng. Table I lists these response factors. The number of injections for each concentration level is indicated in this table. The small standard deviation in multiple injections reflects the high precision of the injection technique. It has been found that fast injection of a relatively large volume of solution usually results in diffusion of part of the injected sample from the column back into the injection chamber. The “four segment injection”
Table I. FID Response Factors (RF, Normal Alkane Standards formula
mol wt
C14H30 C15H32
198 212 226 240 254 268 282 296 310 324 338 352 366 394 422 450
c16h34
CnH36 Ci8H38 c19h40 C2()H42 C21H44 C22H46
c23h48 C24H50 C25H52 C26H54 C28H58 C8oH62
032 ßß
area
2125
counts/ng) of
RF determined at 100 ng injected (std dev)° 14.50 14.54 14.68 14.49 14.25 14.49 14.64 14.81 14.17 14.51 14.44 14.53 14.62 14.47 14.02 14.07
·
av
RF (std dev)6
14.68 14.71 14.82 14.60 14.35 14.61 14.73 14.55 14.20 14.51 14.45 14.57 14.68 14.60 14.21 13.13
(0.11) (0.08) (0.10) (0.06) (0.11) (0.08) (0.08) (0.07) (0.03) (0.04) (0.03) (0.04) (0.06) (0.09) (0.10) (0.10)
(0.20) (0.19) (0.20) (0.16) (0.20) (0.18) (0.19) (0.44) (0.16) (0.22) (0.27) (0.29) (0.28) (0.14) (0.08) (0.06)
Based on three injections. 6 Based on a total of six injections at three concentration levels: one injection at 500 ng, three injections at 100 ng, two injections at 10 ng. 0
Table II Average Response Factor of at Different Concentration Levels av
RF of
16
Alkane Standards
16
alkanes, area
injection amt, ng
counts/ng
std dev°
500 100
14.44 14.46 14.64 13.67
0.23 0.21 0.33 0.39
10 4
Overall Average RF of Standards overall av RF6 std dev6 rel std dev, %
16
Alkane 14.53
0.20 1.4
Calculated from RF of 16 alkanes, each of which was determined by a total of six injections at three concentration levels (see note b in Table I). 0
technique provides a means of injecting a small volume of sample with high accuracy. The data in Table II shows that good linearity of response was observed for the concentration range determined except for a slight decrease of the RF at the 4 ng concentration level. The concentration range determined is the most commonly used for quantitative analysis in WCOT column GC. An overall average RF of 14.53 with relative standard deviation of 1.4% was obtained for alkane standards based on a total of six injections for each alkane. The alkane compounds listed in Table I will not have the same response factor if expressed in terms of molar response as was done in earlier studies. However, they have almost identical weight response factors in terms of area counts/ng. This constancy of the response factors among alkanes agrees with the observation that the FID response is proportional to the carbon number of the hydrocarbons. Table III lists the response factor obtained for 22 commercially available PAH standards. These PAH standards were studied at two concentration levels with duplicate injections at each level. An average RF of 13.47 with relative standard deviation of 5.7% was obtained. In one proposed mechanism of FID operation, it was postulated that every hydrocarbon is broken down to the same distribution of single-carbon species before ionization takes place (12, 15). Then, the charged ions are collected at an electrode, thus producing a signal. Although more steps would
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Table III. FID Response Factors (RF) of PAH Standards0 compound
naphthalene 2-methylnaphthalene 2,6-dimethylnaphthalene 2,3-dimethylnaphthalene acenaphthene fluorene 9,10-dihydroanthracene phenanthrene anthracene 2-methylanthracene 9-methylanthracene 1,2,6,7-tetrahydropyrene fluoranthrene pyrene 1,2-benzofluorene 1,1-binaphthyl chrysene
triphenylene 7,12-dimethylbenz [o] -anthracene benzo[e]pyrene benzo[o]pyrene dibenzanthracene
mol wt
formula
128 142 156 156 154 166 180 178 178 192 192 206 202 202 216 254 228 228 256 252 252 278
CjoHg CnH10 c12h12 c12h12 c12h10 c13h10 Ci4H12 c14h10 c14h10 c15h12 Ci«H12 CieH14 CleH10 c16h10 c17h12 C20H14 C18H12
Ci8H12 C2oH16
C20Hi2 c20h12 c22h14
RF,
area
counts/ng
14.02 13.75 13.62 13.63 14.16 13.66 13.60 13.77 13.85 13.41 14.28 11.86 13.51 13.72 12.60 13.92 12.88 14.09 12.86 12.80 14.81 11.54
std dev6
rel std dev, %
0.65 0.58 0.58 0.11 0.60 0.58 0.57 0.60 0.23 0.52 0.63 0.92 0.54 0.54 0.25 0.50 0.49 0.17 0.16 0.47 1.27 0.49
4.6 4.2 4.2 0.7 4.3 4.2 4.2 4.3 1.7 3.8 4.4 7.7 4.0 4.0 2.0 2.0 3.6 1.2 1.3
4.2 8.6 4.2
“Average RF of 22 PAH standards (calculated from averaging RF of 22 PAH standards), 13.47; standard deviation 0.77; Relative standard 6 deviation, 5.7%. Based on a total of four injections at two concentration levels (two injection at the 100 ng level and two injections at the 20 ng level). be required for the decomposition of higher molecular weight hydrocarbons, the overall rate of decomposition would still be fast enough to ensure their complete decomposition in the time available (12,15). If this postulate can be applied to the PAH, the different PAH compounds having similar ratios of constitutional elements should have quite identical response factors in terms of response per unit weight of compounds. We defined carbon content (C-content) as the weight of carbon per nanogram of standard compound injected, calculated by dividing the weight of total carbon atoms in a molecule by molecular weight. The C-content values of some typical PAH and their alkyl-substituted derivatives are given as follows; biphenyl (0.94), decamethylbiphenyl (0.90), naphthalene (0.94) fluorene (0.94), dimethylbenz[o]anthracene (0.94), picene (0.95), and coronene (0.96). Factors considered for selection of the these compounds were number of rings, degree of alkyl substitution, degree of structural condensation, and suitability to GC analysis in terms of reasonable elution temperature and peak shape. These given compounds show that the values of their C-content decrease with increasing degree of alkyl substitution and with decreasing degree of condensation. However, there is almost no difference in the C-content between fluorene and dimethylbenz[o]anthracene and only 6% between the two extreme cases, decamethylbiphenyl and corcnene. If the postulate regarding the FID mechanism of hydrocarbons is fundamentally correct, the response factors of fluorene and dimethylbenz [a] anthracene should be very similar but not differ as high as 65% as was previously ob-
served (24).
From the data in Table I and Table III, PAH compounds exhibit slightly lower FID response than alkanes. Structural condensation can be expected to be one of the factors to contribute to the lower response per carbon of PAH compounds. At present, we cannot deduce the contribution of structural condensation to the observed difference between the FID response of alkanes and PAH solely based on the data obtained in this study. However, the results obtained do support the theory that the C-content predominately governs the FID response behavior of PAH. Since different PAH compounds have very similar C-content, they should have very similar FID response factors (response/ weight). Furthermore, it should be possible to experimentally quantify different
compounds of each group of hydrocarbons, such as alkanes, alkenes, and PAH, by GC/FID using their own unique characteristic FID response factors in terms of area counts/ng. The relative standard deviation of average RF of 22 PAH standards was slightly larger than that of alkane standards from the data in Tables II and III. The relatively large difference in the C-contents and structures among the PAH standards compared with those factors among alkanes, and the larger impurity range (95% to 99%) of PAH standards used may be part of the reason causing a larger standard deviation of average RF of PAH. Additionally, some unexpected values of RF, such as large difference in RF of benzo[e]pyrene and benzo[o]pyrene and low RF for tetrahydropyrene and dibenzanthracene, can be seen in Table III. However, the differing values of RF cannot be explained simply by the one or two reasons mentioned above. Table IV lists the response factors of some heteroatom containing PAH compounds. The number of injections for different compounds is listed in the table. Peak broadening and tailing of nitro-PAH compounds resulted in relatively larger standard deviation in the mean response factors than was found for the alkanes. Figure 1 shows the chromatograms of alkane, PAH, and nitro-PAH compounds. Best precision was obtained in the determination of alkanes which also gave the best chromatographic peak shape. Eight oxy-PAH and four nitro-PAH compounds studied showed characteristic response factors. An average response factor of 11.90 area counts/ng with relative standard deviation of 7.6% was obtained for oxy-PAH and for nitro-PAH the average response factor was 9.83 area counts/ng with a relative standard deviation of 4.7 %. These data for heteroatom containing PAH standards are very limited. We must emphasize that the use of average response factors assumes that the chemical species does not decompose during analysis. This is a potential problem which can be partially alleviated by observation of peak shapes in WCOT analysis. The average response factors of alkanes, PAH, and heteroatom containing PAH are summarized in Table V. On the basis of these data, three factors are considered to contribute to the FID response of these organic compounds. These are the C-content, structural factors, and a heteroatom factor. The data in Table V show that the FID response of
ANALYTICAL CHEMISTRY, VOL. 56, NO. 12, OCTOBER 1984
·
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Table IV. FID Response Factors (RF) of Heteroatom Containing PAH Standards RF
formula
compound
mol wt
9-fluorenone enthrone 2-fluorenecarboxaldehyde phenanthrene-9carboxaldehyde xanthone anthraquinone phenanthrenequinone benz[a ] anthracene-7,12dione
180 194 194 206
CisHeO
196 208 208 258
CisHgOg
std dev"
rel std dev, %
12.97 11.64 11.64 12.03
0.67 0.47 0.47 0.27
5.2 4.1 4.1 1.7
11.88 12.13 10.04 12.85
0.42 0.28 0.22 0.36
3.5 2.3 2.2 2.8
11.84
0.14
1.2
10.13 10.26 9.66 9.25 7.04
0.42 0.37 0.14 0.79 0.64
4.2 3.6 1.4 8.5 9.0
area
counts/ng
Oxy-PAH Ci4H10O
c14h10o C15H10O
c14h8o2
c14ha
CisHkA S-PAH
dibenzothiophene
184
1-nitronaphthalene 2-nitrobiphenyl 2-nitrofluorene 9-nitroanthracene 2,7-dinitrofluorene
173 199 211 223 256
c12h8s
Nitro-PAH Ci0H7NO2
c12h9no2 Ci3H9N02 c14h9no2 c13h8n204
“For oxy-PAH, based on three injections at one concentration level (100 ng); for others, based centration levels (two injections at the 100 ng level and two injections at the 20 ng level).
on a
total of four injections at two
con-
Table V. Average FID Response Factors of Different Compound Classes av
of
no.
compound
C-
stds
class
content6
involved
alkane
0.85
PAH' oxy-PAH sulfur-PAH nitro-PAH dinitro-PAH
0.92-0.95 0.80-0.87
16 22 8
0.78
1
0.69-0.75
4
0.61
1
RF,
area
counts/ ng 14.53 13.47 11.90 11.84 9.83 7.04
rel std
std dev“
dev, %
0.20 0.77 0.90
1.4 5.7 7.6
0.46
4.7
Calculated from averaging the RF of members in corresponding 6 compound class. Nanograms of carbon per nanogram of standard ' And their injected. alkyl substituted derivative. “
the compounds tabulated is primarily related to the C-content. The substitution of a heteroatom on a hydrocarbon (including PAH) lowers the FID response of the parent compound. These observations are in agreement with the two generally accepted theories that the FID response is proportional to the carbon number of the hydrocarbons and that the FID response of substituted hydrocarbon is always less than that of the
parent hydrocarbon. The deviation caused by the slight difference in the Ccontent and degree of structural condensation among the constituent compounds of the various classes—alkanes, PAH, oxy-PAH, nitro-PAH—is relatively small compared with other experimental errors commonly involved in trace organic analysis of complex organic mixtures. Thus, for certain
1. Gas chromatograms of alkanes, PAH, and nitro-PAH standards. Chromatographic conditions were as follows: 30 m X 0.32 mm i.d. DB-5 fused silica capillary column; temperature at 80 °C for 1 min, programmed to 300 °C at a rate of 3 °C/min.; FID detector.
Figure
compound classes, the use of a single averaged response factor for the quantification of each group of organic compounds could be an acceptable approach for the quantitative analysis of complex mixtures. One should note, however, that there are exceptions. Table VI lists the response factors of some of chlorinated benzene
Table VI. FID Response Factors (RF) of Some Compounds Chlorinated Benzene and Phthalate Standards compound
mol wt
hexachlorobenzene pentachlorobenzene tetrachlorobenzene
282 248 214
dimethyl phthalate diethyl phthalate dibutyl phthalate dioctyl phthalate
194 222 278 390
formula C6C16
c6hci5 c6h2ci4
CioHkA C12Hi404 Ci6H2204 c24h3804
C-content6
RF,
area
counts/ng
std dev“
rel std dev, %
0.26 0.29 0.34
4.54 5.08 5.60
0.09 0.07 0.04
1.9 1.4 0.7
0.62 0.65 0.69 0.74
7.89 8.20 10.73 13.32
0.17 0.17 0.22 0.24
2.1 2.1
2.0 1.8
“Based on four injections at two concentration levels (two injections at the 100 ng level and two injections at the 20 ng level). 6Nanograms of carbon per nanogram of standard injected.
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1984
compounds and phthalate esters. Significant variations are observed among the response factors determined for members of these two classes of compounds. The use of average response factors for quantitative analysis of such compounds is therefore not possible. This situation most likely applies to other compound classes such as perfluoroalkanes, alcohols, carboxylic acids, and steroids which were the subjects in earlier studies (20-23). It can be seen from the data on phthalates that the response factor of the highest molecular weight phthalate is very close to that of its hydrocarbon parent. For the low molecular weight compounds, the heteroatom exerts a considerable effect on the FID response of the compound compared with that of the parent hydrocarbon. With increasing molecular weight the influence of the heteroatom on the FID response becomes less significant, and the response factor approaches that of the parent compound. In this case, the response factor of the parent hydrocarbon may be used if the standard of the heteroatom-containing compound is not available. The RF value of a compound differs from instrument to instrument. Therefore, in the practical use of RF for quantitative analysis, the RF of standard compounds and samples must be determined on the same instrument and at the same operation conditions.
CONCLUSION The results obtained in the study of pure standard compounds show the feasibility of using averaged response factors for quantification of different compounds in certain classes by GC/FID analysis. The compound classes to which this is applicable include aliphatic hydrocarbons, PAH, and, with less certainty, oxy-PAH and mononitro-PAH. The use of average response factors in GC/FID analysis will minimize the demand on the number of standard compounds needed for quantitative analysis and facilitate the quantification of multiple components in a complex mixture. These results have been applied to quantify a number of polycyclic aromatic compounds in diesel exhaust particulate extracts, after the compounds in the mixture had been effectively classified by
HPLC fractionation
(9, 28).
anthrene, 85-01-8; anthracene, 120-12-7; 2-methylanthracene, 613-12-7; 9-methylanthracene, 779-02-2; 1,2,6,7-tetrahydropyrene, 57633-59-7; fluoranthrene, 206-44-0; pyrene, 129-00-0; 1,2benzofluorene, 238-84-6; 1,1-binaphthyl, 604-53-5; chrysene, 218-01-9; triphenylene, 217-59-4; 7,12-dimethylbenz[a]anthracene, 57-97-6; benzo[e]pyrene, 192-97-2; benzo[o]pyrene, 50-32-8; dibenzanthracene, 414-29-9; 9-fluorenone, 486-25-9; an throne, 9044-8; 2-fluorenecarboxaldehyde, 30084-90-3; phenanthrene-9carboxaldehyde, 4707-71-5; xanthone, 90-47-1; anthraquinone, 84-65-1; phenanthrenequinone, 84-11-7; benz[o]anthracene7,12-dione, 2498-66-0; dibenzothiophene, 132-65-0; 1-nitronaphthalene, 86-57-7; 2-nitrobiphenyl, 86-00-0; 2-nitrofluorene, 607-57-8; 9-nitroanthracene, 602-60-8; 2,7-dinitrofluorene, 540553-8; hexachlorobenzene, 118-74-1; pentachlorobenzene, 608-93-5;
tetrachlorobenzene, 12408-10-5; dimethyl phthalate, 131-11-3; diethyl phthalate, 84-66-2; dibutyl phthalate, 84-74-2; dioctyl phthalate, 117-81-7.
LITERATURE CITED (1) Lee, M. L.; Novotny, M.; Battle, K. D. Anal. Chem. 1976, 48, 1566. (2) Schuetzle, D.; Lee, F. S.-C.; Prater, T. J. Int. J. Environ. Anal. Chem. 1981, 9, 93. (3) Eiceman, G. A.; Clement, R. E.; Karasek, F. W. Anal. Chem. 1979, (4) (5) (6) (7) (8)
(9)
(10) (11) (12) (13) (14) (15) (16) (17)
(18)
ACKNOWLEDGMENT
(19) (20)
The authors are grateful to R. A. Moore for valuable discussions arid suggestions. Financial support from the Natural Sciences and Engineering Research Council of Canada is
(21) (22) (23) (24)
acknowledged. Registry No.
(25)
C14H30, 629-59-4; C16H32, 629-62-9; C16H34, 544-76-3; C17H36, 629-78-7; C18H38, 593-45-3; C19H40, 629-92-5; C20H42, 112-95-8; C21H44, 629-94-7; C22H46, 629-97-0; C23H48, 638-67-5; C24H60, 646-31-1; C25HB2, 629-99-2; C26HB4, 630-01-3; C28H5s, 630-02-4; C30H62, 638-68-6; C^H^, 544-85-4; naphthalene,
91-20-3; 2-methylnaphthalene, 91-57-6; 2,6-dimethylnaphthalene, 581-42-0; 2,3-dimethylnaphthalene, 581-40-8; acenaphthene, 8332-9; fluorene, 86-73-7; 9,10-dihydroanthracene, 613-31-0; phen-
(26) (27)
(28)
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Received for review February 16, 1984. Accepted May 21, 1984.
