Development of a Gas Chromatographic-Ultraviolet Absorption Spectrometric Method for Monitoring Petroleum Pitch Volatiles in the Environment R. A. Greinke and 1. C. Lewis Union Carbide Corporation, Parma Technical Center, Carbon Products Division, 12900 Snow Road, Parma, Ohio 44 130
A procedure employing the techniques of gas chromatography (GC) and ultraviolet absorptlon spectroscopy (UV) has been developed for the specific analysis of volatiles from petroleum pitch. These techniques have been used In conjunction with mass spectrometry to ldentlfy nine of the major volatile compounds in petroleum pitch. In contrast to the major compounds present in coal tar pitch volatiles, many of the identified compounds In petroleum pitch volatiles were methyl substituted aromatlcs, and Included: methylchrysene, dlmethylchrysene, methylphenanthrene and methylpyrene. These compounds together with five other identified aromatic hydrocarbons were used in a combined GC-UV method for monitoring petroleum pitch volatiles. This analytical method, which showed a detectlon limit of approximately 0.5 pg per compound, was tested by analysis of commercial petroleum pitches.
As a result of current environmental awareness, considerable effort has been directed towards developing analytical procedures for monitoring polynuclear aromatics in the environment (1-4). Most of the methods have been applied t o measurements of coke oven, coal, and coal t a r volatiles. T h e common procedure has involved t h e use of t h e benzene solubles method in which the organic effluent is trapped on a filter and the amount soluble in benzene is quantitatively determined (5, 6). T h e major shortcoming of this technique is its lack of specificity for any of the volatile organic components ( 2 , 7). A more recent and improved procedure for coal tar pitch volatiles uses a combination of gas chromatography (GC) and ultraviolet absorption spectroscopy (UV) t o quantitatively analyze for eight specific compounds in t h e effluents (8). This GC-UV procedure was modified ( I , 2 ) t o include fourteen compounds, most of which are major compounds in coal tar volatiles. T h e wide industrial usage of petroleum pitch (9) (petroleum pitch is a n aromatic residue produced by thermal cracking or oxidation of petroleum refined material and is principally used in the manufacture of carbon electrodes) emphasizes the importance of developing a procedure for measuring specific compounds in petroleum pitch volatiles. T h e only current procedure recommended for measuring petroleum pitch volatiles is the nonspecific benzene soluble method ( 5 , 6 , I O ) . T h e GC-UV method developed for measuring coal tar pitch volatiles ( I , 2, 7) determines specific major components in the volatiles. Because of differences in chemical composition ( 1 1 ) , t h e use of a similar procedure for petroleum pitch requires the identification of the major compounds in the petroleum pitch volatiles. This paper describes a general analytical procedure for specifically determining petroleum pitch volatiles. Major components of t h e volatiles have been identified by mass spectrometry and absorption spectroscopy. A GC-UV method was then devised for quantitatively determining t h e major compounds.
EXPERIMENTAL Mass Spectrometry of Petroleum Pitch Volatiles. Samples of a commercial petroleum pitch and the volatiles collected from the distillation of this petroleum pitch were subjected to analysis by mass spectrometry. The samples were introduced into the ionization chamber of a AEI-MS-12 mass spectrometer by means of an indirectly heated probe. The mass spectra were obtained at ionizing voltages ranging from 10 to 70 eV. The spectra were recorded at various intensity levels to identify the major peaks. Although the mass spectra for petroleum pitch volatiles are complex, molecular peaks which are predominant and common to most materials can readily be ascertained. Purification of Polynuclear Compounds. The reference polynuclear compounds and internal standards were obtained from a variety of supply houses. All compounds were purified by column liquid chromatography by using 80-200 mesh chromatographic grade alumina. Each purified compound was at least 99% pure as verified by gas chromatographic analysis. Pitch Volatile Samples. The pitch volatile samples were obtained from laboratory distillations of commercial petroleum pitch. Gas Chromatography. An F&M-810 gas chromatograph, equipped with a flame ionization detector, was modified with a trapping assembly in a manner similar to that reported previously (7). A 10-ft X %-inch 0.d. stainless steel column, packed with 3% by weight Dexsil 300 GC on Chromosorb G (AW, DMCS Treated, 80-100 mesh), was employed for separations. The chromatograph was programmed at 6 OC/min. from 175 to 350 OC. The stainless steel injection port liner was kept at 340 "C, since injection port temperatures above 340 OC could cause some of the polynuclear compounds to decompose. To establish that the high injection port temperatures were not causing decomposition, a known weight of 3,4-benzopyrene, a component found unstable at high injection port temperatures, was periodically injected into the chromatograph, trapped, and quantified by UV. The effluent from the column was passed through a 10/1 splitter. With this arrangement, 9% of the effluent went t o the flame ionization detector and 91% was available for trapping and subsequent UV measurement. Ultraviolet Spectroscopy. A Cary Model 14 spectrophotometer was used for measuring the UV spectra of trapped polynuclear compounds. The compounds in the trap were washed into the UV cell with spectrograde cyclohexane.
RESULTS AND DISCUSSION Constitution of Petroleum Pitch Volatiles. Table I presents some typical analytical data for coal t a r pitch and petroleum pitch volatiles. T h e volatiles were collected from t h e distillation of commercial pitches. Both materials are highly aromatic; however, NMR analysis shows that the petroleum pitch volatile components contain considerably more aliphatic protons than coal t a r pitch volatiles. Examination of the N M R spectra shows t h a t most of t h e aliphatic hydrogen in the petroleum pitch is present in the form of alkyl sidechains on aromatic rings. Analysis of Petroleum Pitch Volatiles by Mass Spectrometry. Both 'petroleum pitch volatiles and the parent petroleum pitch were analyzed by mass spectrometry. In agreement with the average molecular weight data (Table I), mass spectra of petroleum pitch volatiles exhibited peaks predominantly in t h e 200-400 molecular weight range. Essentially all of t h e major peaks in the volatiles
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t
1 Eu
40
35
30
25
40
35
30
25
20 TIME IN MINUTES
I5
10
20
I5
IO
71°C
IU U l l l l I T C e
5
Figure 1. Gas chromatograms of petroleum pitch volatiles ( a )and coal tar pitch volatiles (b) Table I. Analytical D a t a for Coal Tar Pitch and Petroleum Pitch Volatiles
%'cc
%H Atomic C/H %S
O/oN
7co Av mol wt
% Aromatic H (NMR)
Coal tar pitch
Petroleum pitch
volatiles
volatiles
93.6 5.3 1.48 0.4 0.4 1.o 300 82
92.2 5.9 1.30 2 .o 0.1 0.4 300 53
were also identified as major components in the mass spectra of the parent petroleum pitches. Although petroleum pitch and petroleum pitch volatiles can contain large numbers of compounds ranging up to molecular weights of 600 or greater, the components in the lower molecular weight range can most effectively be analyzed by GC. Furthermore, this low molecular weight fraction contains the individual compounds which are present in greatest amounts in petroleum pitch. Since these low molecular weight compounds appear to be common constituents of residual materials obtained from cracking of petroleum distillates, they are an effective indicator for monitoring petroleum pitch volatiles. The data in Table I1 summarize the predominant low molecular weight masses identified by mass spectral analysis. Possible structural designations for each of the masses are also given in the Table. Most of the species in Table I1 are assigned to aromatic 2152
Table 11. Major Low Molecular Weight Species in Petroleum Pitch Volatiles from Mass Spectrometry Mass
178 192 202 216 228 230 24 2 252 256 266 2 a0
Possible structural designation
Phenanthrene, Anthracene Methylphenanthrene, Methylanthracene Pyrene Met hy lpyr e ne C hr ysene , Benzanthracene Dimethylpyrene Methylchrysene, Methylbenzanthracene Benzopyrene, Benzofluoranthene Dimethylc hr ysene , Dimethylbenz anthr acene Methylbenzopyrene Dimethylbenzopyrene
hydrocarbons substituted by methyl groups. This result differs from that for coal tar volatiles where most of the components in this molecular weight range are unsubstituted aromatics. Gas Chromatographic Analysis of Petroleum P i t c h Volatiles. Figure l a presents a gas chromotogram for the volatiles obtained from a commercial petroleum pitch. For comparison, Figure l b shows a chromatogram of a coal tar pitch volatile sample. The major compounds in the coal tar sample which were identified in previous work ( 1 ) are indicated on the chromatogram. Several major coal tar pitch compounds, fluoranthene, and the benzofluoranthenes are barely detectable in petroleum pitch volatiles. Gas chromatographic peaks in the petroleum pitch volatile sample that are common with the major coal tar pitch volatiles compounds are indicated in
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Table 111. Comparison of the Ultraviolet p-Bands for Phenanthrene a n d Methylphenanthrene Isomers with Unknown No. 1 Compounds
Phenanthrene 1- Methylphenanthrene 2-Methylphenanthrene 3- Methylphenanthrene Unknown No. 1
p-Bands, nm
293, 300, 295, 297, 296,
281, 280, 283, 205, 283,
274 277 277 277 277
i
1
380 340
300 ZGO
220
1
1
1
1
380 340 M O 260 220
NANOMETERS
Figure 2. Ultraviolet absorption spectra of trapped Unknown (curve A ) and 3-rnethylphenanthrene (curve B)
No. 1
Figure l a . The compounds indicated in Figure l a were identified by spiking the petroleum pitch volatile sample with the model compounds. The identification was confirmed by trapping the compounds as they eluted from the chromatograph, followed by subsequent ultraviolet spectroscopy evaluations. The mass spectrometry data (Table 11) also support the presence of these compounds. Several of the major peaks in the chromatogram of the petroleum pitch volatiles are absent in the chromatogram for the coal tar pitch volatiles. The identification of those major compounds present in petroleum pitch volatiles and not present in coal tar pitch volatiles would be useful, since these compounds could also be used in the analytical procedure to specifically characterize petroleum pitch volatiles. Because of the lack of resolution for the compounds that elute after the benzopyrenes, no attempt was made to identify these higher boiling compounds and include them in the analytical method for characterizing petroleum pitch volatiles. The very low boiling compounds, such as phenanthrene and those compounds containing less than three rings, were also excluded from the analysis. These materials readily sublime a t room temperature and would readily be lost from the environmental sample prior to analysis. For completing the characterization of the major low molecular weight compounds in petroleum pitch volatiles, we focused on the identification of five compounds that eluted between phenanthrene and 3,4-benzopyrene. These unknown compounds are indicated in Figure la. Unknown No. 1 eluted immediately after phenanthrene. Mass spectrometric data indicated the presence of a component with a molecular weight of 192 (Table 11).The most reasonable assignment for unknown No. 1 was, therefore, either methylphenathrene or methylanthracene. Three of the isomeric methylphenanthrenes (1, 2, and 3) were obtained and were individually added to a portion of the petroleum pitch volatiles. Each of the three isomers enhanced the chromatographic peak labeled as Unknown No. 1. Unknown No. 1 was subsequently trapped and the resulting UV spectrum (Figure 2) confirmed the presence of methylphenanthrene and i;he absence of methylanthracene. Examination of the p-bands (12) present in the ultraviolet spectra for the methylphenanthrene model compounds with those p-bands present in the spectrum of Unknown No. 1 yielded some additional information pertaining to the structural assignment of Unknown No. 1. Table I11 compares the wavelengths of the p-bands for
phenanthrene, three of the methyl isomers of phenanthrene, and Unknown No. 1. It is known that methyl substitutents do not alter the character of the UV spectra of polynuclear aromatic hydrocarbons but merely cause a shift in p-band absorption to longer wavelengths (13).This shift is observed in Table 111. Based on the data in Table 111, Unknown No. 1 may be composed of either 2-methylphenanthrene or 3-methylphenanthrene or a mixture of both. The presence of significant amounts of 1-methylphenanthrene is unlikely. The 4 and 9 methyl isomers of phenanthrene were not available; hence, the possible presence or absence of these isomers in the volatiles was not ascertained. Unknowns No. 2 and 3 eluted after pyrene. The peaks corresponding to these components were not completely resolved. The UV spectrum obtained for the sample traps from these peaks was similar to that of pyrene. Since the mass spectra data indicated the presence of methylpyrene in the pitch volatiles, a sample was spiked with both 1methylpyrene and 2-methylpyrene. The Unknown No. 2 chromatographic peak was enhanced by 2-methylpyrene and the Unknown No. 3 chromatographic peak was enhanced by 1-methylpyrene. The longest wavelength pbands in the UV spectra for 1-methylpyrene and 2-methylpyrene occur at 344 nm and 338 nm, respectively. The UV spectrum of the composite trap of Unknown No. 2 and 3 contained a doublet of peaks a t 344 and 338 nm. The 4methylpyrene isomer was not available; however, the long wavelength p-band of this compound has previously been measured a t 338 nm. Hence, we conclude that the petroleum pitch volatiles definitely contain 1-methylpyrene. A second methylpyrene component consisting of either 2methyl or 4-methylpyrene is also present. Unknown No. 4, eluting after the 1,2-benzanthracenechrysene peak, was the largest peak appearing on the chromatogram between phenanthrene and 3,4-benzopyrene. Possible identifications from the mass spectra data in Table I1 are methylchrysene or methylbenzanthracene, both having molecular weights of 242. The sample of pitch volatiles was spiked with 7-methyl-1,2-benzanthracene; however, this compound eluted between Unknowns No. 4 and 5 . The compound 3-methylchrysene did elute a t the same retention time as Unknown No. 4. Figure 3 shows the UV absorption spectra for 3-methylchrysene (curve A ) , the sample trap from Unknown No. 4 (curve B ) ,and 7-methyl-l,2-benzanthracene(curve C). The major absorption bands for the unknown at 271 and 261 nm were similar to the absorption bands in 3-methylchrysene (curve A ) . The minor absorption bands for Unknown No. 4 at 291 nm and 278 nm were similar to the absorption bands in 7-methyl-1,2-benzanthracene (curve C). After Unknown No. 4 was trapped, mass spectrometric analysis gave molecular weights of 242 (major component) and 258 (trace component). The 242 molecular weight also supports the presence of methylchrysene (the major 242 molecular weight component) and methyl-1,2-benzanthracene(the minor 242 molecular weight component). Because only one
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Table IV. Conditions for Ultraviolet Determination of Petroleum Pitch Volatile Compounds Trap No.
Compound
UV absorptivit),
Peak
Baseline or back-
m l l u g cm
wavelength, nm
ground wavelength, nm
1 o-Terphenyl (standard) 0.055 23 5 249 Methylphenanthrenes 0.24" 2 2 54 272 Pyrene 0.25 3 336 32 7-3 4 5 Methylpyrenes 4 0.18b 2 84 276 Chrysene 5 0.53 267 260-275 5 1,2-Benzanthracene 2 89 0.40 297 6 Methylchrysenes 0.38c 271 2 62-2 7 8 Dimethylc hrysenes 7 272 0.3@ 263-279 332 0.16 8 1,2-Benzopyrene 325-338 3,4- Benzopyrene 8 0.11 3 84 398 a 2-Methylphenanthrene. 1-Methylpyrene. 3-Methylchrysene. Used 3-Methylchrysene since a dimethylchrysene isomer was not available.
1
271
-20
3
w o NANOMETERS
'
: I 4 02 0
,Bo
2?o
Figure 3. Ultraviolet absorption spectra of 3-methylchrysene (curve A), trapped Unknown No. 4 (curve B), and 7-methyl-l,2-benzanthracene (curve C)
isomer of methylchrysene and methyl-1,2-benzanthracene was available, further characterization of the isomers of these two compounds present in the pitch volatiles was not possible. Because the retention time of 7-methyl-1,2 benzanthracene differed from Unknown No. 4, this isomer is not present in detectable amounts in the petroleum pitch volatiles. Unknown No. 5 eluted after the methylchrysenemethyl-l,2-benzanthracenepeak (Unknown No. 4). The UV spectrum of Unknown No. 5 closely resembled that of Unknown No. 4 (Figure 3, curve B ) . After it was trapped, mass spectrometric data for Unknown No. 5 indicated molecular weights of 256 (major component) and 276 (trace component). A molecular species with mass 256 has been identified in Table I1 as either dimethylchrysene or dimethylbenzanthracene. Because the UV spectrum of Unknown No. 5 was nearly identical to Unknown No. 4, the most likely identification of Unknown No. 5 is dimethylchrysene (the major 256 molecular weight component) and dimethyl-1,2-benzanthracene (the minor 256 molecular weight component). Model compounds could not be obtained to verify this identification. Analytical P r o c e d u r e f o r Determination of Petroleu m P i t c h Volatile Compounds. The aforementioned results have been incorporated into an analytical procedure for determining petroleum pitch volatiles in the environment. The GC-UV procedure developed for coal tar pitch volatiles (2, 8) has been modified to incorporate the new analytical results on the constitution of petroleum pitch 2154
volatiles. The nine aromatic hydrocarbon components of petroleum pitch volatiles which are listed in Table IV are determined by using this procedure. Table IV also lists the UV wavelengths used to measure the net absorptivity for each of the nine compounds in a sample. The listed compounds include four methyl substituted and five unsubstituted hydrocarbons. Although some of the methyl isomers have been identified, specific identification is not required for the success of the method. The procedure has been simplified by selecting a UV wavelength which is common to all isomers of a particular methyl substituted compound. The method is outlined as follows: Either a weighed sample of pitch volatiles is dissolved in benzene, or a silver membrane filter, used to collect the petroleum pitch volatiles from the atmosphere, is extracted with cyclohexane or benzene in a soxhlet extractor. The internal standard, o terphenyl (40 to 80 pg), is added and the extract is evaporated to dryness a t room temperature. (Recovery studies on the most volatile compound, methylphenanthrene showed that it does not sublime under these conditions.) Just prior to injection into the chromatograph, a second internal standard, bibenzyl (40 to 80 pg), is added to the sample. (The second internal standard is employed for the analytical calculations if any interference is observed in the UV spectrum obtained for quantifying the internal standard, o-terphenyl. Our experience in analyzing pitch volatiles by gas chromatography has demonstrated the need for using two internal standards.) For quantifying bibenzyl, the area of its GC peak was measured. Bibenzyl was an excellent backup internal standard, since compounds present in petroleum pitch volatiles that would elute near bibenzyl usually sublime from the sample or the silver membrane filter prior to analysis. Since bibenzyl readily sublimes, it must be added to the sample just prior to injection into the chromatograph. The addition of bibenzyl after sample concentration may yield low values for the analyzed compounds, if some of the sample was lost during concentration. Owing to this latter shortcoming, o-terphenyl, which does not sublime at room temperatures, should be used for the calculations whenever possible. If necessary, the sample is diluted to 40 to 80 pl with toluene and a 10 to 15-pl portion is injected into the chromatograph. The desired compounds (Table IV) are trapped in a manner previously described (8). The trapped compounds are washed from the stainless steel traps into UV cells with cyclohexane to a volume of 3.0 ml. After UV spectra are obtained, the trapped compounds are quantified (8). Table IV lists the UV wavelengths employed for quantifying all of the trapped polynuclear compounds. With the aid of a computer for the calculations, approximately 2.5 labor hours are required for the complete analy-
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Table VI. Results of GC-UV Analysis for Six Production Lots of Commercial Petroleum Pitch
Table V. Results of GC-UV Analysis for a Petroleum P i t c h Volatile Analyzed Six Times Cornpound
Wt %
Compound
(1) Methylphenanthrenes (2) Pyrene (3) Methylpyrenes (4) Chrysene (5) 1 , 2 - Benzanthracene (6) Methylchrysenes 47) Dimethylchrysenes (8) 1 , 2 - Benzopyrene (9) 3,4- Benzopyrene (10) Total amount of the nine compounds
1.67 i 0.18 0.46 0.03 1.20 0.22 0.29 i 0.04 0.24 i 0.05 0.55 i 0.06 0.43 i 0.08 0.26 i 0.04 0.39 i 0.07 5.46 i 0.40
(1) Methylphenanthrenes (2) Pyrene (3) Methylpyrenes (4) Chrysene (5) 1,2-Benzanthracene (6) Methylchrysenes (7) Dimethylchrysenes (8) 1, &Benzopyrene (9) 3,4- Benzopyrene (10) Total amount of the nine compounds
* *
sis of a sample. The detectability limits of the trapped compounds are approximately 0.5 wg per sample. Statistical Analysis. T o obtain statistics relating to the precision of the method, a sample of petroleum pitch volatiles was analyzed six times for the nine components listed in Table IV. Table V presents the analytical results for each of the nine volatile components. The percent relative standard deviation for each compound varied from f6.296 for pyrene to f 2 1 % for 1,2-benzanthracene. T h e nine polynuclear compounds account for 5.5% of the pitch volatiles. T h e percent relative standard deviation for the total of the nine polynuclear compounds is f7.4%. We have also applied the method to the analysis of six different production lots of commercial petroleum pitch obtained from the same supplier over EL two-year period. The results are presented in Table VI. The percent relative standard deviation varied from f7'% for methylchrysene to &45% for 1,2benzanthracene. The nine polynuclear hydrocarbons account for approximately 2.5% of the petroleum pitch with a relative standard deviation of f 11%.The relatively high precision obtained from analysis of the different commercial pitch samples indicates that the amounts of these nine compounds in the commercial petroleum pitch are consistent from lot to lot. CONCLUSIONS T h e identification of nine of the major petroleum pitch components resulted in the development of a GC-UV procedure which can be used to analyze petroleum pitch volatiles. The five unsubstituted aromatic hydrocarbons are also found in coke oven emissions; however, the four methyl substituted hydrocarbons (not found in coke oven emissions) are specifically diagnostic for petroleum pitch, Extensive testing with different types of petroleum volatile samples should show whether the procedure can be classified as a general method for monitoring petroleum pitch
Wt
36
0.22 i 0.06 0.14 i 0.04 0.46 i 0.14 0.22 i 0.05 0.13 i 0.06 0.59 0.04 0.38 0.04 0.10 i 0.03 0.19 i 0.05 2.47 i 0.28
*
*
volatiles in the environment. Since data on the collection of these hydrocarbons from the atmosphere were not obtained, a study will be necessary to determine the efficiency of isolation of these hydrocarbons by standard filter collection methods. ACKNOWLEDGMENT The authors thank P. 0. Schissel, I. R. Ladd, and W. J. Lambdin for the mass spectrometric analysis, and L. H. O'Connor for the computer program used to quantify the polynuclear compounds. LITERATURE CITED (1) K. A. Schulte, D. J. Larson, R. W. Harnung. and J. V. Crable, "Report on Analytical Methods Used In a Coke Oven Effluent Study", National lnstitute for Occupational Safety and Health, HEW Publication No. 74-105, Cincinnati, Ohio, 1973. (2) K. A. Schulte, D. J . Larson, R. W. Harnung, and J. V. Crable, Am. lnd. Hyg. Assoc. J., 38, 131 (1975). (3) R. T. Richards, D. T. Donovan, and J. R. Hall, Am. lnd. Hyg. Assoc. J., 28, 590 (1967). (4) E. Sawlcki et al., Health Lab. Sci., 7, 45 (1970). (5) "Documentation of the Threshold Limit Values for Substance In Workroom Air", 3d ed., American Conference of Government Industrial Hygienists, Cincinnati, Ohio, 1971, p. 57. (6) PSM-1013 In Reference 1. (7) H. J . Selm, W. W. Hanneman, L. R. Barsotti, and T. J. Walker. Am. ind. Hyg. Assoc. J., 35, 718 (1974). (8) T. D. Searl, F. J. Cassidy. W. H. King, and R. A. Brown, Anal. Chem., 42, 954 (1970). (9) G. L. Ball, C. R. Gannon. and J . W. Newman, American Inst. of Mining, Metallurgical and Petroleum Engineers, Light Met., 127 (1972). (10) 29 CFR Section 1910. 936 as amended Nov. 21, 1972 (Fed. Reglst., 30, 23545, June 27, 1974). (11) L. F. King and W. D. Robertson, fuel, 47, 197 (1968). (12) E. Clar, "Polycyclic Hydrocarbons", Academic Press, London, 1964, Vol. I, p 50. (13) R. N. Jones, Chem. Rev.. 41, 353 (1947).
RECEIVEDfor review June 17, 1975. Accepted July 31, 1975. Presented at the 170th National Meeting of the American Chemical Society, Chicago, Ill., August 25-29, 1975.
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