1610
Anal. Chem. 1984, 56, 1610-1615
would be upset until it reached its original level of sensitivity. To keep errors at a minimum during this time period, the chromatograph’s calibration is checked before analysis of a sample. Syringe contaminationproblems c a w d by previous q d y s i s of high parts per million samples gave false peaks during blanks and samples. Disassembling the syringe and w&hing each part separately with acetone and concentrated HC1 usually removed any contamination. The most frequent problem was day to day instability in the NPD. Minor upsets would affect sensitivity and the response factors. Checking the calibration before a series of runs proved effective in resolving this problem.
Wilson, Roberta Wentz, and Carol Yates for their constant assistance and guidance in the development of this method. Registry No. NMP, 872-50-4; H20, 7732-18-5. LITERATURE CITED (1) Yakovleva, T. P.; Vall, E. I. Vopr. Tekhnol. Ylavlivanja ferefab, Produktov Koksovanlya 1979, (8), 83-87. (2) Frick, Darryl A. J . Ll9. Chromatogr. l983, 6 , 445-524. (3) Belder, T. B.; Kuz’mlnskaya, M. D. Mefody Anal. Konfrolya Kach. Plod. Khlm. fromsfi. 1978, (e), 1-3. (4) Masalova, L. S.; Kedrina, N. N.; Sstavrati, V. I . Khlm. fromsf., Ser.: Mef@‘Anal. Konholye Kach. Rod. Khlm. fromsfi. 1980, (6), 21-24. (5) Ned, J. W., Standard 0 11 Co. (Ind.), Napervllle, IL, unpubllshed work, 14 June 1982. (6) Mosescu, Nicolae; Stejaru, Deea; Dalmutchi, George Rev. Chlm. (Bucharest) 1977, 28 (3), 272-274. (7) Mlskovich, J. J., Standard 011 Co. (Ind.), Napervllle, IL, personal communlcatlon, 20 June 1983.
ACKNOWLEDGMENT The author thanks Terry Adams, Shelly Arispe, Jerry Phillips, Robert Smead, Rick Solan, Patti Stasik, Regina
RECEIVED for review March 1, 1984. Accepted April 5, 1984.
Structural Characterization of Polycyclic Aromatic Compounds by Combined Gas Chromatography/Mass Spectrometry and Gas Chromatography/Fourier Transform Infrared Spectrometry Kin
S.Chiu a n d Klaus Biemann*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Krishnaswamy Krishnan a n d Steve L. Hill
Digilab, Division of Bio-Rad, Cambridge, Massachusetts 02139
The complernentarlty of the mass spectral and Infrared spectral data for the structural characterlzatlon of polycycllc aromatic compounds (PAC) Is demonstrated by the parallel analyses of two PAC mlxtures by gas chromatography/mass spectrometry and gas chromatography/Fourler transform lnfrared spectrometry. Whlle the electron impact mass spectral data provide unlque lnformatlon regardlng the molecular welghts of the compounds of Interest, the Infrared data often afford unamblguous dtfferentlatlon between the Isomers. Speclflc examples are presented In which the comblnatlon of the two techniques resulted In Identifications which could not have been possible using elther technlque alone.
Polycyclic aromatic compounds (PAC) have become an important class of environmental pollutants due to their widespread occurrence in fossil fuel combustion products and because of their mutagenic and carcinogenic potentials. In view of the structure dependence of the biological activities exhibited by these compounds, it is very important that analytical methodologies be developed to facilitate the positive identification of the individual compounds. Although gas chromatography/mass spectrometry (GC/ MS) has been extensively used for the identification of the PAC, there are limitations to the effectiveness of the mass spectrometer as a gas chromatographic detector for the positive characterization of these compounds. Because of their aromaticity, the electron impact ionization mass spectra of the PAC show predominently the molecular ion but very 0003-2700/64/0356-1610$01.50/0
limited fragmentation. The lack of structurally specific cleavages greatly hampers the differentiation of the structural isomers and even certain isobars from the mass spectral information alone. However, it is very important to positively identify these aromatic compounds because their mutagenicities may vary widely. In some cases, this can be accomplished by using the gas chromatographic retention behavior of the compound if it is known for the particular gas chromatographic column or if an authentic sample is available for calibration. A limiting requirement is the necessity to know the retention behavior of all the isomers and that they differ sufficiently to be clearly differentiated. Needless to say, the larger the molecules and the more highly substituted they are, the more isomers are possible, and soon the point is reached where it is not feasible to acquire all the necessary reference compounds. In order to achieve a higher level of confidence in the identifications of the PAC, it is necessary to use complementary techniques which are sensitive to the position of substitution and functional groups. We have therefore investigated the utility of gas-phase infrared spectrometry in conjunction with gas chromatography. Mamantov et al. (1) and Tokousbalides et al. (2)have already developed methodologies using matrix isolation Fourier transform infrared spectrometry (MI-FTIR) to directly identify PAC in simple mixtures. However, the efficiency of MI-FTIR is limited by the level of complexity of the samples, and a more promising approach involves prior separation of the components of the mixture before detection by the FTIR (3,4). With sensitivity in the submicrogram levels, an FTIR spectrophotometer can 0 1984 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 9,AUGUST 1984
be readily interfaced to the gas chromatograph and provide additional information which could complement the mass spectral data for complex mixture analysis (5-8). Recent improvements in light pipe designs and minimization of interfacing dead volumes have resulted in much en. hanced response of the GC/FTIR system (9). This increase in sensitivity is especially significant since detectability at lower concentrations is compatible with capillary column gas chromatographic separation, and the system does not have to compromise for sensitivity by using a higher capacity but less efficient packed column. In addition, the advent of powerful and relatively inexpensive computers not only makes possible the efficient acquisition and interpretation of data, it also allows the accumulation and compilation of a large body of reference vapor phase infrared spectra in a "library", which together with efficient retrieval algorithms and search routines further contributed to recent successes of GC/FTIR. The advantage of IR is that it can readily differentiate isomeric and isobaric compounds because of finite differences in their vibrational spectra. However, it is not very sensitive to the overall molecular size, e.g., molecular weight, for which the mass spectrometer is well suited. Thus, the combination of the mass spectral and infrared spectral data logically complements each other, and when used together for gas chromatographic analysis of complex mixtures, will greatly improve the conclusive characterization of all the components (10,11). This paper reports the results obtained from the combined GC/MS and GC/FTIR identification of a large number of polycyclic aromatic compounds. The unique complementarity of the two approaches is evaluated by using a relatively simple mixture of known components and then applied to a more complex sample generated by combustion of bituminous coal. EXPERIMENTAL SECTION Gas Chromatography/Mass Spectrometry. Gas chromatography/mass spectrometry was performed on a Finnigan MAT 212 double focusing mass spectrometer interfaced to a Varian 3700 gas chromatograph fitted with an on-column injector. The mass spectrometer was operated at an electron energy of 70 eV, and the ion source temperature was maintained at 200 OC. The system was set to scan from rn/z 45 to 500 every 3 s at a resolution of about 800. The data thus generated were stored and processed on-line by a Finnigan MAT SS-200 Data System. A 15 m X 0.32 mm i.d. (0.25 pm film thicknes) DB-5 fused silica capillary column (J & W Scientific) was used with helium as the carrier gas. The column head pressure was set at 10 psi to maintain a flow rate of about 1.5 mL/min (measured at 45 OC). For the analysis of the known mixture, the GC oven was initially set at 45 OC and programmed up at 5 "C/min until the last component has eluted. For the analysis of the coal combustion extract, because of the presence of volatile compounds, the oven temperature was initially maintained at 40 OC for 3 min after sample introduction. It was then programmed at 3 OC/min to 240 OC and then 8 OC/min until it reached 300 "C and maintained at that temperature for 10 min. Gas Chromatography/Fourier Transform Infrared Spectrometry. GC/FTIR was performed on a Hewlett-Packard 5880A GC interfaced to a Digilab FTS-20 FTIR spectrometer equipped with the Digilab GCIR accessory. The accessory consists of a gold-coated light pipe (whose dimensions are matched for the elution volume of the capillary gas chromatographic peaks), the associated optics, and a high-sensitivity narrow-band mercury-cadmium-telluride detector. In the GC/FTIR experiments, a 15 m X 0.32 mm i.d. (1pm film thickness) DB-5 fused silica column (J & W Scientific) was used for the separation. The flow rate of the helium carrier gas in the column was about 1.5 mL/min. The gas chromatographic effluent was directed to the GCIR light pipe by fused silica capillary transfer lines through a makeup gas fitting. To preserve chromatographic resoluton and to maintain a positive pressure on the light pipe, enough helium makeup gas was added to the column effluent to bring the total flow rate through the light pipe to around 2 mL/min. The eluents from the light pipe were then directed to the flame ionization detector of the gas chromatograph.
1611
Table I. Components of the Known Mixture and Results of the IR Computer Search
peak 1 2
3 4 5
6 7
8 9 10 11 12
13 14 15
16 17
18 19
20 0
compound
results of IR computer search
N-methylaniline 5-ethyl-rn-cresol 2,6-dimethylaniline naphthalene isoquinoline %methylnaphthalene 2-methylquinoline 1-methylnaphthalene 2-ethylnaphthalene 1-ethylnaphthalene 2,6-dimethylnaphthalene 1,4-dimethylnaphthalene 2,3-dimethylnaphthalenea 1,4-dimethylnaphthalene 1-cyanonaphthalene 1-cyanonaphthalene dibenzofuran dibenzofuran 2-nitronaphthalene 2-nitronaphthalene 1-indanone 9-fluorenone" dibenzothiophene dibenzothiophene phenanthrene phenanthrene carbazole carbazole
N-methylaniline 2,3-dimethylphenola 2,6-dimethylaniline naphthalene isoquinoline 2-methylnaphthalene 2-methylquinoline 1-methylnaphthalene 2-ethylnaphthalene 1-ethylnaphthalene 2,6-dimethylnaphthalene l&dimethylnaphthalene
Spectrum not in library.
Using the standard Digilab GCIR software (12),real time GCIR spectra were recorded from 4000 to 700 cm-' every 0.8 s as the gas chromatographic fractions transversed down the light pipe. FTIR data points were collected every 4 cm-' to give a 8 cm-' spectral resolution. To determine the exact frequency of absorption of the spectrum of interest, the software allowed expansion of the specified spectralregion. If more than one spectrum was recorded during the passage of a particular gas chromatographic fraction through the light pipe, these could be coadded to yield the final IR spectrum of this fraction. After the GCIR spectra of all the fractions were recorded, they were searched against the Sadtler vapor phase library containing 6800 entries. Mixture of Known Polycyclic Aromatic Compounds. A standard solution in toluene of 20 authentic compounds that have been identified in various combustion extra& was prepared. The components (second column of Table I) ranged from PAC such as naphthalene and its alkyl homologues to phenanthrene and heteroaromatic systems such as isoquinoline, carbazole, dibenzofuran, and dibenzothiophene to oxygen- and nitrogensubstituted aromatics of higher polarity. All of these standards were obtained from Chem Service (West Chester, PA, high purity grade) except 2-methylquinoline and 1-and 2-ethylnaphthalenes (ICN Pharmaceutical, Plainview, NY) and 1-cyanonaphthalene and 2-nitronaphthalene (Aldrich, Milwaukee, WI, technical grade). They were used without further purification. The correlation of the chromatographicpeaks from the GC/MS and GC/FTIR was straightforward. Both instruments used the same liquid phase and the elution sequence is therefore the same. Since the mixture is well resolved, the correlation of the spectra corresponding to the same component is not difficult. Coal Combustion Extract. The sample of coal combustion product was generated from a 700-kW experimental fluidized-bed combustor (Energy Laboratory, M.I.T.) utilizing bituminous coal as fuel. Conditionsfor sample collection and workup procedures have been presented elsewhere (13). Even in this case of the gas chromatograms from the GC/MS and the GC/FTIR experiments, the peak patterns were sufficiently alike to correlate the corresponding peaks even though the peak heights often differ in the two detection modes.
RESULTS AND DISCUSSION The mass spectrometric total ion plot of the known mixture is shown in Figure 1. Table I lists the components of the mixture and the results of a computer search of the IR spectrum recorded for the effluent of the gas chromatograph at the position indicated by the numbers in Figure 1. I t is of interest to note that of the 20 compounds present in the mixture, the correct identity was listed as the best IR match
ANALYTICAL CHEMISTRY, VOL, 56, NO, 9, AUGUST 1884
1612
R
8 A I
i
JfI
“I -.s ,/ ’
.J
SCANS
mcH,
z
3 4 W
$tL,!,,,, a
~
50
100
I,I ,,1,
j~l,
-CH3
- C2H5
x‘ 1
Figure 1. GC/MS total ion plot of known mixture. The numbered peaks are listed in Table I. GC conditlons: 15-m DB-5 fused silica capillary column, temperature programmed from 45 O C to 250 OC at 5 OC/mln.
9T
R
- C8H6
Z I
u -
,
,
, ,
150
J
j,~,,!,,,,,
50
,~ ,
,I
100
, ,
L 150
1000
1000
*ow
1000
reo0
3000
2om
Imo
Figure 3. Authentic vapor-phase infrared spectra of 1- and 2-substitued naphthalenes.
I
I
3000
2000 W AVENUMBERS
1000
3000
2000
1000
WAVENUMBERS
Figure 2. Mass spectra and infrared spectra of 2- and l-methylnaphthalenes (peaks 0 and 8 of Figure 1). The spectral region from 2450 to 2300 cm-‘ In the I R was not reported due to C02background.
for 17 of them. It turned out that the spectra of the remaining three components were not in the vapor phase FTIR library and the search therefore retrieved the next closest match. It should be noted that this match was always of the same structural type as the compound actually present. The ability of the infrared data to differentiate isomers varying in the position of alkyl substituents is clearly demonstrated by the correct identifications of the two methylnaphthalenes and ethylnaphthalenes, respectively, and of certain dimethylnaphthalenes. As an illustration, both the mass spectra and infrared spectra of isomeric methylnaphthalenes (peaks 6 and 8 of Figure 1)are shown in Figure 2. As previously discussed, the mass spectra of the 1-and 2-methyl isomers are very similar. They both show prominent molecular ion M’, and fragment ions at m / z 141 (M - H)’ and m/z 115 (M - C2H3)+.Although the mass spectra did not allow differentiation of the isomers, substantial differences were observed in their vapor-phase infrared spectra. At the region of 700-900 cm-l, the aromatic C-H out of plane bendings exhibited distinctly different absorptions between the 1- and 2-substitutions which could be correlated to the number of adjacent hydrogen atoms on the ring (14). Thus, in 1-methylnaphthalene, a closely spaced doublet is observed in this region; the one at lower wavenumber (770 cm-l) corresponds to the four hydrogen atoms on the unsubstituted ring, and the band at higher wavenumber (790 cm-l) is due to the presence of the three adjacent hydrogen atoms on the other ring. In the spectrum of 2-methylnaphthalene, three
bands are observed instead. The one at lowest wavenumber (740 cm-l) corresponds to the four hydrogen atoms on the unsubstituted ring, the band at 810 cm-l is due to the two adjacent hydrogen atoms on the substituted ring, and the band at highest wavenumber (830 cm-l) is due to the isolated hydrogen atom. This difference was also observed in the 1- and 2-ethylnaphthalenes. Furthermore, inspection of the vapor phase infrared spectra of authentic phenyl-, cyano-, and nitro-substituted naphthalenes indicated that they exhibited similar behavior (Figure 3). It suggests the predominance of these aromatic C-H bending vibrations which are relatively independent from steric or electronic effects. Furthermore, this coupling of the out of plane bending of ring hydrogen to adjacent hydrogen atoms can also be used to interpret the subsitution pattern in more condensed ring system (15). Thus, it is not necessary to have the proper isomer for comparison as long as one has an analogue with a substituent in the proper position on the same ring system. It is of interest to note that while the GC/MS differentiation of the 1-and 2-methylnaphthalenesshould present no problem because they are very well resolved by gas chromatography, this is not the case with the ethylnaphthalenes. The latter have a very similar retention behavior (difference of 0.5 retention units (16)) and are very difficult to separate on the DB-5 column. This is illustrated in Figure 1 in which the two isomers were observed to elute as one peak in the total ion plot. In this instance, the infrared data would be indispensible for the unambiguous distinction of the two isomers since the GCIR data reduction software can be effectively used to enhance chromatographic resolution by displaying the absorption at wavelength characteristic for each of the compound vs. time (17). The mutually complementary nature of the infrared information and mass spectral data was further examined by the analysis of a complex sample generated from a fluid-
ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUGUST 1984
1613
SCANS Figure 4. GC/MS total ion plot of coal combustion products. GC condltlons: 15-m D E 5 fused sllica capillary column, column temperature initially at 40 O C for 3 min, then programmed up at 3 'Clmin untll 240 O C . and then at 8 OClrnin till 300 O C .
2 +
4w
a -m
50
11 I
MI2
3000
2000
3000
io00
WAVENUMBERS
Flgure 5. Mass spectrum and infrared spectrum of peak 8 of Figure 4.
ized-bed combustor utilizing bituminous coal as fuel. The complexity of the sample is clearly demonstrated by its total ion plot which is shown in Figure 4. A complete GC/MS analysis of this extract has been presented elsewhere and will not be discussed in detail here (13). However, we wish to show some data demonstrating the unique complementarity of the mass spectral and infrared spectral information with particular emphasis on certain compounds which have significant chemical or biological importance. An example is peak 8 whose mass spectrum shows major ions a t m/z 116 and 115. Subsequent inspection of the EPA-NIH mass spectral library (18) indicated that the observed spectrum is practically indistinguishable from those of indene, phenylpropadiene, phenylpropyne, and isomers of methylphenylacetylene. Since these compounds are not commercially available, the definitive identification of this component is not feasible from the GC/MS data unless all of these substituted benzenes are synthesized to determine their retention behavior. However, the ability to differentiate these four classes of compounds is important in elucidating mechanistic pathways for PAC formation because it has been postulated that the unsaturated side chains play an important role in the pyrosynthesis of larger PAC moieties in the flame (19). This uncertainty was resolved by inspection of the corresponding infrared spectrum shown in Figure 5. The absence of absorptions at 21W2250 cm-l and 1950-2000 cm-* corresponding to the acetylenic C=C and asymmetric allenic -C-C=Cstretching frequencies ruled out the presence of
2000
1000
WAV ENUMEERS
Flgure 6. Mass spectrum and Infrared spectrum of peak 2 of Figure 4.
propadienyl, propynyl, or acetylenic groups, thus eliminating the possibility of the substituted benzenes. In fact, the computer search listed indene as the best match for the FTIR spectrum. In this extract, compounds with unsaturated side chains have also been identified from the complementary GC/MS and GC/FTIR data. Thus, a strong acetylenic EC-H stretching absorption at 3330 cm-l and a relatively weaker C s C stretch near 2110 cm-' together with a mass spectrum which shows a parent ion at m / 2 102 (Figure 6) indicated that peak 2 is phenylacetylene. Similarly, styrene (peak 3) was also identified and differentiated from isomeric cyclooctatetraene. Another example where the infrared data could be highly informative is demonstrated by the latter's ability in distinguishing 9-fluorenone (peak 36),l-phenalenone (peak 44),and benzo[c]cinnoline. These compounds exhibited very similar mass spectra and have been reported in various environmental matrices (20,21).However, they have very different biological activities. While 9-fluorenone and benzo[c]cinnoline were biologically inactive in bacterial assays, 1-phenalenone shows considerable mutagenicity. Thus, it is important to be able to positively differentiate them. Leary et al. (21) used highresolution mass spectrometry, gas chromatography/mass spectrometry, and high-pressure liquid chromatography with continuous UV scanning capability for the differentiation of these three compounds in effluents of oil burners. Inspection of the GC/MS and GC/FTIR data indicated that this was also feasible using these two techniques. Figure 7 shows both
ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUQUST 1984
1614
50
100
150
+, 50
ZOO
M/Z
100
150
200
100
50
150
200
MI.?
t-
'
1 3000 3000
2000 WAVENUMBERS
1000
3000
2000 WAVENUMBERS
1000
Flgure 7. Mass spectra and infrared spectra of peak 36 (left) and peak 44 (right) of Figure 4.
the mass spectra and infrared spectra of 9-fluorenone and 1-phenalenone. Under electron ionization, both compounds show loss of 28 amu which is consistent with the facile loss of C=O moiety resulting in the resonanced stabilized cation (m/z 152). However, because of the angular strain in a five-membered ring, the carbonyl of 9-fluorenone absorbed a t 1736 cm-', in contrast to that of 1-phenalenone which absorbed at 1662 cm-l. Similar values have also been reported for the gas-phase spectrum of 9-fluorenone (9) and liquidphase spectrum of 1-phenalenone (22). It is thus expected that the carbonyl absorption could be used to differentiate these two isomers as well as their akyl homologues. Moreover, this absorption can further distinguish these compounds from benzo[c]cinnoline (which has a mass spectrum very similar to that of 9-fluorenone and 1-phenalenone because the -N= N- moiety can be lost as N,)which does not have a carbonyl group. In addition to its ability to differentiate various isomeric compounds, the FTIR data could also be used with the mass spectral information to improve confidence in the identification of unknown components. During the GC/MS analyses of coal combustion extracts, we have frequently encountered several minor peaks (peaks 54-57) whose mass spectra were similar and show parent ions M+ a t m/z 218 and fragment ions at m / z 189 (M - 29)+. These spectra were searched against the EPA-NIH mass spectral library containing about 39 000 reference spectra. The search failed to retrieve compounds of reasonable identity because their spectra are apparently not part of the library. However, visual inspection of the data indicated that for these peaks of interest, the ratio of the abundance of,the two ions, M+ and (M - 29)+, was similar to that of dibenzofuran (peak 29). In addition, the difference of 50 amu between the unknowns and dibenzofuran is consistent with the addition of a benzo ring to the latter. Thus from the mass spectral data, it was initially concluded that these copounds are isomers of benzonaphthofuran. However, from the low-resolution mass spectral data, we could not eliminate the possibility of a C2-substituted cyclopenta[deflphenanthrene or a C3-substitutedethynylacenaphthylene which have the same nominal mass. The uncertainties were resolved by the corresponding FTIR data. Although these peaks were minor components in the mixture (peak 54 is less than 0.1% of the total extract), reasonable infrared spectra were obtained for these peaks which show a sharp absorption band near 1200 cm-' similar to the asymmetric C-0-C stretching frequency of dibenzofuran (see Figure 8). It should be noted that dibenzofuran was listed as the best match for the FTIR spectrum of peak 54 indicating a similar structure,
2000
1000
3000
WAVENUMBERS
2000
1000
WAVENUMBERS
Figwe 8. Mass spectra and infrared spectra of peak 29 (left) and peak 54 (right) of Figure 4.
3000
2000 W AVENUMBERS
1000
3000
2000
1000
WAVENUMBERS
Figure 9. Mass spectra and infrared spectra of peak 34 (left) and peak 35 (right) of Figure 4.
thus confirming the identification as an isomer of benzonaphthofuran. Of come, the differentiation between C16H100, M+ 218.0732, and Cl7Hl4,M+218.1096, can also be made with a mass spectrometer with resolution of 1:6000 or better. The strong bands near 1200 cm-l have also been used to confirm that peaks 34 and 35 were isomeric methyldibenzofurans and to differentiate them from biphenylcarboxyaldehyde and xanthene which have similar mass spectra. In addition, the difference in the C-0-C stretching frequency of the methyldibenzofurans in liquid phase IR (23)could further be used to differentiate the isomers. Thus, with absorption at 1188 cm-l, peak 34 resembled that of the 1- or 4-isomers,whereas C-O-C absorption at 1200 cm-' identified peak 35 as the 2-isomer. This is also corroborated by the pattern of the aromatic C-H bending frequencies as shown in Figure 9. In peak 34, a singlet near 740 cm-l with a possible shoulder on the higher wavenumber side is consistent with either 1- or 4-substitution. In the 2-substituted isomer, however, the aromatic C-H bands are more complex due to the three different hydrogen couplings. In instances where the vapor-phase spectra of the peaks of interest are present in the FTIR library, the components were readily identified from the library search routine. Thus, besides the components discussed above, mass spectrometrically equivalent phenanthrene (peak 38) and anthracene (peak 39) were differentiated and identified from their aromatic C-H bendings. Also, ethylbenzene (peak l ) , benzaldehyde (peak 4), phenol (peak 7), naphthalene (peak 13),
ANALYTICAL CHEMISTRY, VOL.
benzo[b]thiophene (peak 14), 2-methylnaphthalene (peak 181, 1-methylnaphthalene (peak 20), biphenyl (peak 21), acenaphthylene (peak 26), and dibenzothiophene (peak 37) were correctly retrieved from the FTIR spectral library. Although only a few examples are discussed here to demonstrate the unique complementarityof the mass spectral and infrared spectral data, it is conceivable that many other situations exist where either technique alone would have provide inconclusive results. An example is the characterization of cyano-substituted PAC in combustion products (24). Because of the similarities in the mass spectra of the cyano derivatives with their isomeric azaarenes,positive differentiation between these two classes of compounds is not feasible with conventional mass spectral data alone. However, the additional infrared information should afford unambiguous distinction of these compounds from the differences in the characteristic absorption bands, e.g., CEN stretching frequency of cyanosubstituted PAC near 2250 cm-l. Thus, 1-cyanonaphthalene was correctly identified in the mixture of known compounds (Table I). Another area where the complementary FTIR data would be highly informative is the identification of various aromatic amines. The information regarding the nature of these amines is necessary in the characterization of synthetic coal liquids because the primary amines have been associated with the observed mutagenicities of certain liquefied coals (25). Although the mass spectral information could not conclusively differentiate the amino-substituted PAC from isomeric azaarenes, the infrared data unequivocally distinguish them from their N-H stretching (3500-3400 cm-l) and bending frequencies (near 1600 cm-l). Thus, from the GC/MS and GC/FTIR data, we could identify the major components in the basic fraction of solvent refined coal liquids as carbazole and its alkyl homologues and differentiate them from the isomeric aminoacenaphthylene, its alkyl homologues, and aminofluorene and its alkyl homologues (26). In analogy to the ability to differentiate various substituted naphthalenes, the FTIR data should also be capable of identifying position of alkyl substitution in more condensed ring compounds from the correlation of their aromatic C-H out of plane bending frequenciesand adjacent hydrogens (15). The positive identification of the various positional isomers will be of great importance since certain alkylated PAC have recently been found to be more mutagenic than their unsubstituted homologues (27). In addition, it was demonstrated that in these compounds, the mutagenic potential is highly dependent on the position of substitution (28). At present, the greatest limitation of the FTIR is its relatively low sensitivity when compared to other gas chromatographic detectors such as the mass spectrometer which is a t least 1, if not 2 orders of magnitude more sensitive, or the flame ionization detector which is 2 or 3 orders of magnitude more sensitive. Although this is especially critical for the polycyclic aromatic compounds which are relatively poor infrared absorbers, our data indicated that with the use of a thick film bonded phase capillary column, the desired quantity (50-500 ng per component) of materials could be injected without appreciable deterioration of chromatographic resolution. In addition, the FTIR detector, which is nondestructive, allows interfacing of ancillary detectors downstream. One example is the interfacing of a mass spectrometer directly
56, NO. 9,
AUGUST 1984
1615
after the GC/FTIR system (8, 29, 30). Registry No. N-Methylaniline, 100-61-8; 5-ethyl-rn-cresol, 698-71-5; 2,6-dimethylaniline, 87-62-7; naphthalene, 91-20-3; isoquinoline, 119-65-3; 2-methylnaphthalene, 91-57-6; 2methylquinoline, 91-63-4; 1-methylnaphthalene, 90-12-0; 2ethylnaphthalene,939-27-5;1-ethylnaphthalene,1127-76-0;2,6dimethylnaphthalene,581-42-0; 1,4-dimethylnaphthalene,57158-4; 2,3-dimethylnaphthalene,581-40-8; 1-cyanonaphthalene, 86-53-3; dibenzofuran, 132-64-9;2-nitronaphthalene,581-89-5; 1-indanone,83-33-0;dibenzothiophene, 132-65-0;phenanthrene, 85-01-8; carbazole, 86-74-8.
LITERATURE CITED (1) Mamantov, G.; Wehry, E. L.; Kemmerer, R. R.; Hinton, E. R. Anal. Chem. 1977. 49, 86. (2) TokousbalMes, P.; Hinton, E. R.; Dlckinson, R. B.; Bllotta, P. V.; Wehry, E. L.; Mamantov, G. Anal. Chem. 1978, 50, 1189. (3) Reedy, G. T.; Bourne, S.; Cunnlngham, P. E. Anal. Chem. 1979, 57, 1535. (4) Hembree, D. M.; Garrison, A. A,; Crocombe, R. A.: Yokley, R. A.; Wehry, E. L.; Mamantov, G. Anal. Chem. 1981, 53, 1783. (5) Shafer, K. H.; Lucas, S. V.; Jakobsen, R. J. J. Chromatogr. Sci. 1979, 17, 464. (6) Griffiths, P. R. Appl. Spectrosc. 1977, 31, 497. (7) Wllkens, C. L.; Gllss, G. N.; Brlssey, G. M.; Steiner, S. Anal. Chem. 1981, 53, 113. (8) Crawford, R.; Hirschfeld, T.; Sanborn, R. Pittsburgh Conference on Analytical Chemistry and Applled Spectroscopy, Atlantic City, NJ,
.-- ..
IQAI
(9) Erickson, M. D.; Newton, D. L.; Pellizari, E. D.; Tomer, K. B. J. Chromafogr. Sci. 1979, 77, 449. (10) Kuehl, D. Proceedings of the 1981 Internatlonal Conference in Fourier Transform Infrared Spectroscopy, Columbus, SC, 1981. (11) Griffiths, P. R.; de Haseth, J. A.; Azarraga, L. V. Anal. Chem. 1983, 55, 1361A. (12) Krishnan, K.; Brown, R. H.; Siminoff, S. C.; Olson, M. L.; Kuehl, D. Am. Lab. (Fairfiekl, Conn.) 1981, March. (13) Chiu, K. S.;Walsh, P. M.; Beer, J. M.; Biemann, K. "Proceedings, Seventh International Symposlum of Polynuclear Aromatic Hydrocarbons": Battelle Press: Columbus, OH, 1982. (14) Colthup, N. B.; Daly, L. H.; Wlberley, S. E. I n "Introduction to Infrared and Raman Spectroscopy"; Academic Press: New York, 1975. (15) Cannon, C. G.; Sutherland, G. B. Spectrochim. Acta 1951, 4 , 373. (16) Lee, M. L.; Vasslloras, D. I.; Whlte, C. M.; Novotny, M. Anal. Chem. 1979, 51, 768. (17) Krlshnan, K.; Curbelo, R.; Chiha, P.; Moonan, R. C. J . Chromatogr. Sci. 1978, 77, 413. (18) Heller, S.R.; Milne, G. W. A. EPA/NIH Mass Spectral Data Base, Nati. Sfand. Ref. Data Ser. ( U . S . , Nafl. Bur. Stand.) 1978, NSRDS-NBS 63. (19) Crlttenden, B. D.; Long, R. Combust. Flame 1973, 20, 359. (20) Benolt, F. M.; Lebel, G. L.; Williams, D. T. Bull. Envlron. Contam. Toxicol. 1979, 23, 774. (21) Leary, J. L.; Lafleur, A. L.; Leber, H. L.; Biemann, K. Anal. Chem. 1983, 55, 758. (22) "The Aldrich Library of Infrared Spectra, Edition 111"; Aldrich Chemical Co.: Milwaukee, WI, 1981. (23) Trippett, S. J. Chem. SOC. 1957, 419. (24) Dubay, G. B.; Hltes, R. A. Environ. Sci. Techno/. 1978, 72, 965. (25) Wilson, B. W.; Pelroy, R.; Cresto, J. T. Mutat. Res. 1980, 79, 193. (26) Chiu, K. S.; Biemann, K.; Krlshnan, K.; HIII, S. Presented at the 31st Annual Conference on Mass Spectrometry and Allied Topics, Boston, MA, May 9-13, 1963. (27) Griest, W. H.; Tomkins, B. A.; Epler, J. L.; Roa, T. K. I n "Polynuclear Aromatio Hydrocarbons"; Jones, P. W., Leber, P., Eds.; Ann Arbor Science Publishers: Ann Arbor, MI, 1979. (28) Hecht, S. S.; Bondinell, W. E.; Hoffman, D. J. Natl. Cancer Inst. 1974, 53, 1121. (29) Wilkins, C. L.; Giss, G. N.; White, R. L.; Brissey, G. M.; Onyirioka, E. C. Anal. Chem. 1982, 54, 80. (30) Crawford, W. R.; Hirschfeld, T.; Sanborn, R. H.; Wong, C. M. Anal. Chem. 1982, 54, 817.
RECEIVED for review November 29,1983. Accepted April 11, 1984. This work was supported in part by grants from the National Institute of Environmental Health Sciences (5 P30 ES02109 and 2P01 ES0160).
