Anal. Chem. 1984, 56, 1335-1338
partment, University of Florida, for support during a sabbatical leave. The Finnigan MS/MS at the University of Florida was purchased with funds from the National Science Foundation. Although this work was supported, in part, by
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the U S . E.P.A., it has not been subjected to the Agency's required peer and administrative review and therefore does not necessarily reflect the view of the Agency and no official endorsement should be inferred.
Identification of Partially Hydrogenated Nitrogen-Containing Polycyclic Aromatic Hydrocarbons in Coal Liquids by Tandem Mass Spectrometry K. V. Wood* EnginelFuel Laboratory, Institute of Interdisciplinary Engineering, Chemistry Building, Purdue University, West Lafayette, Indiana 47907 C. E. Schmidt
US.Department
of
Energy, Pittsburgh Energy Technology Center, P.O. Box 10940, Pittsburgh, Pennsylvania 15236
R. 0.Cooks
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 B. D. Batts
School of Chemistry, McQuarie university, North Ryde, New South Wales 2113, Australia
A nitrogen-base extract of a coal iiquld has been examined for partially hydrogenated nitrogen-containing polynuclear aromatic hydrocarbons (PAH) uslng mass spectrometry/mass spectrometry. Neutral loss scans coupled with trifluoroacetyl derlvatlzatlon of the sample allowed dlfferentiatlon of the partially hydrogenated components from other nitrogen-containing species present. Four dlfferent partially hydrogenated nitrogen-containing PAH structures were detected, Including tetrahydroqulnolines, tetrahydroazaacenaphthenes, and tetrahydrobenzoquinolines. Some of the other nitrogen-containlng constituents which were identifled include anlllnes, aminonaphthalenes, amlnophenanthrenes/amlnoanthracenes, quinolines, and carbazoles. Parent scanning was used to show that benzyiamlno containing components were not present at a detectable level.
In recent y e m , considerable effort has been directed toward the characterization of coal-derived liquids (1-6). While a large range of compound types have been studied, the principal focus has been on those constituents containing heteroatoms, particularly nitrogen (7-13). Interest in the nitrogen compounds in coal liquids is largely the result of environmental and processing concerns. It is now realized that much of the mutagenicity associated with coal liquids resides in nitrogenous fractions (14-16). Considerable expenditures of hydrogen (17,18) are required, in upgrading coal liquids, to produce a fuel that can serve as a direct substitute for a petroleum-based product. With this in mind, a primary aim of this work was the identification of those nitrogen-containing species important in the hydroprocessing of coal liquids. Model compound studies (19) have shown that heterocyclic nitrogen removal occurs through the formation of hydrogenated intermediates. In the denitrogenation of quinoline, for example, 1,2,3,4-tetrahydroquinoline is formed upon hydrogenation of the heteroaromatic ring, the 0003-2700/84/0356-1335$01.50/0
kinetically preferred reduction pathway. Nitrogen removal then proceeds through opening of the hydrogenated ring followed by elimination of ammonia. This paper reports the results of a search for hydrogenated intermediates in a nitrogen-base fraction from a coal liquid, using mass spectrometry/mass spectrometry (MS/MS) (20-23) coupled with trifluoroacetyl derivatization. Special emphasis is placed on identification of partially hydrogenated heterocyclic compounds bearing secondary nitrogen atoms. EXPERIMENTAL SECTION The MS/MS results were obtained with a Finnigan-MAT triple-stage quadrupole mass spectrometer. This system, which has been described previously (24, consists of three coaxial quadrupole rod assemblies. The first and third quadrupoles are conventional quadrupole mass analyzers. The second quadrupole is used in the rf-only configuration as a focusing collision cell for the collision-induced dissociation (CID) process applied in MS/MS. The first quadrupole selects an ion of given mass-tocharge ratio from a complex mixture. The selected ion, which has a specified axial kinetic energy (20 eV for this study), passes into the collision cell, which contains a gas at a specified pressure, and undergoes fragmentation as a result of CID. The third quadrupole is scanned to mass analyze the set of fragment ions produced in the collision process. The coal liquid used was produced by the Pittsburgh Energy Technology Center's 400 Ib/day coal liquefaction unit. West Virginia Ireland Mine coal was liquefied at 2000 psig H, in a fixed-bed reactor with no added catalyst at a temperature of 450 "C. The nitrogen bases were separated from a particulate-free methylene chloride soluble dialyzate (25) by using a cation exchange column chromatography method previously described (26). The trifluoroacetyl derivatives were formed by reacting trifluoroacetyl chloride with a solution of the nitrogen base fraction in dry methylene chloride. The trifluoroacetyl chloride was introduced at -78 "C. The mixture was then allowed to warm to room temperature, and excess pyridine was added to scavenge hydrogen chloride evolved in the reaction. After complete reaction the solution was allowed to reach room temperature and nitrogen was gently bubbled through the mixture to remove excess trifluoroacetyl chloride. The pyridine was removed by washing with 0 1984 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. m
a, JULY
1984
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I
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2T2
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$I 244
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E ,
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~
Chemical Ionization Nitrogen Bases
Flgure 1. Isobutane chemical ionization mass spectra of nitrogen-base extract, derivatized and underivatized.
a 50% saturated sodium bicarbonate solution followed by filtration. The derivatized amino-substituted polycyclic aromatic hydrocarbons were separated from the azaarenes by passing the mixture through the cation exchange column by using the same procedure as in the generation of the nitrogen base fraction. The same method was also used to derivatize authentic 1,2,3,4tetrahydroquinoline and 9-aminophenanthrene, but subsequent separation by the cation exchange column was not necessary. The sample was admitted into the mass spectrometer via the direct insertion probe and volatilized by heating the probe to 200 O C . The sample was ionized by isobutane chemical ionization at an ion source pressure of 0.60 torr. The mass spectrometer ion source temperature was maintained at 250 O C throughout the course of the analyses. Argon was used as the collision gas in the MS/MS experiments and was maintained at a gauge pressure of 2.0 mtorr. All reagents used in this study were obtained commercially and used without further purification.
RESULTS AND DISCUSSION In this investigation of a complex fuel mixture, separation into two main chemical classes was effected by chemical derivatization. Thereafter, all separations and identifications were made by tandem mass spectrometry. The nitrogencontaining constituents that are expected to form trifluoroacetyl derivatives are those having a relatively basic nitrogen bearing an exchangeable hydrogen. For example, primary amines, such as aniline, and partially hydrogenated nitrogen heterocycles, such as l72,3,4-tetrahydroquino1ine, should undergo derivatization under the reaction conditions used. However, compounds with no active hydrogen, such as pyridine, and weakly basic compounds, such as carbazole, should not form derivatives. The chemical ionization mass spectrum of the underivatized nitrogen-base fraction is compared in Figure 1with the spectrum of the derivatized nitrogen-base fraction. (Sample introduction is by evaporation from a heated probe; higher mass components can be examined with higher probe temperatures.) Derivatization greatly simplifies the mass spectrum. Two predominant homologous series in the mass spectrum of the derivatized bases, m / z 190, 204, 218, 232, 246 and 260 and m / z 230, 244, 258, 272, 286, and 300, can be seen in Figure 1. After allowing for the mass of the trifluoroacetyl derivative the parent compounds have molecular weights of 107, 121, 135, 149, and 163 and 133, 147, 161, 175, 189, and 203. Both series, while present in the underivatized sample, are not nearly as prominent as in the spectrum of the derivatized sample. Evidence supporting the identification of individual compounds which give rise to these series, as well as the other compounds that appear in less intense series of ions, can now be considered. Structural identifications are based on daughter spectra as illustrated in Figure 2. Here the MS/MS spectra of m / z 134 and its derivatized counterpart, m / z 230, are compared with the
230
106 ~
Tetrahydroquinoline standord
i Tetrahydroquinoline
j ;1
derlvative
1
!
Flgure 2. MS/MS identification of tetrahydroquinoline in nitrogen-base extract by comparing (1) m Iz 134 of underivatized sample with m lz 134 of protonated 1,2,3,4-tetrahydroqulnoline and (2)m l z 230 of derivatized sample with mlz 230 of protonated trifluoroacetyl derivative of 1,2,3,4-tetrahydroquinoline (collislon energy 20 eV, argon collision pressure 2.0 mtorr).
corresponding spectra of authentic 1,2,3,4-tetrahydroquinoline and trifluoroacetyl-1,2,3,4-tetrahydroquinoline. The elimination of a fragment of 28 mass units (probably ethylene) characterizes both the derivatized and the underivatized sample. In both cases, the authentic compounds give daughter spectra with fewer peaks, indicating the probable presence of minor isobaric or isomeric constituents at the same nominal mass in the mixture. However, the major species present at this mass in the nitrogen-base sample is tetrahydroquinoline. White et al. (27) have also identified tetrahydroquinoline in a coal-derived product. An MS/MS spectrum can be used for identification of individual components in complex mixtures by comparison with spectra of authentic compounds, as just shown. Alternative MS/MS scanning techniques allow one to obtain additional information from a given complex mixture by characterizing the mixture for those components having a chemical moiety in common. One such scanning technique is the neutral loss scan. In neutral loss scanning, quadrupole 1 and quadrupole 3 are scanned simultaneously with a mass offset corresponding to the loss of a stable neutral molecule occurring as a result of the CID process. In this manner, the entire complex mixture can be characterized for those constituents which can eliminate a particular neutral fragment. One application of neutral loss scans made in this study was differentiation of partially hydrogenated nitrogen heterocycles from primary amino PAH compounds in the derivatized nitrogen-base fraction. The MS/MS daughter spectra of an authentic trifluoroacetyl-derivatized primary amino PAH and an authentic trifluoroacetyl-derivatized (and partially hydrogenated) nitrogen heterocycle show one characteristic difference in fragmentation. This difference can be seen in Figure 3, which compares the daughter spectra of protonated trifluoroacetyl-1,2,3,4-tetrahydroquinoline, mf z 230 (intense loss of 28 mass units), and protonated trifluoroacetyl-9-aminophenanthrene, m / z 290 (no loss of 28 mass units and intense loss of both 97 and 113 mass units). The observed difference lies in the intensity of the fragment ion produced upon loss of ethylene, undoubtedly the result of opening of the hydrogenated ring followed by dissociation. The fragment ion resulting from ethylene loss is very abundant in the case of partially hydrogenated nitrogen heterocycles and absent or very weak for primary amino PAHs. This difference can be exploited by obtaining neutral loss 28-amu scans to differentiate these two classes of compounds. The neutral loss 28-amu spectrum of the derivatized bases sample
ANALYTICAL CHEMISTRY, VOL. 56, NO. 8, JULY 1984
Electron Impact
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W V
9 Aminophenonthrene derivative
2 z
/93
z
1337
i,2,3,4 Tetrohydroquinoiine derivative 230
3
m
a W
2
5
/33 /54 I
T 1
M/Z
M/Z
Flgure 3. Comparison of the protonated CIMSlMS spectra of the trifluoroacetyl derivative of 9-aminophenanthrene, m / z 290, and 1,2,3,4-tetrahydroquinoline, m / z 230. The 9-aminophenanthrene does not show the loss of a 28 mass unit fragment (collision energy 20 eV, argon collision pressure 2.0 mtorr). 244 I
I
Nitrogen Bases1 derivatized 1
nnn
Flgure 4. Neutral loss 28 amu spectrum of the derivatized nitrogenbase sample (collision energy 20 eV, argon collision pressure 2.0 mtorr).
Table I. Major Hydrogenated Nitrogen Heterocyclic Compounds by MS/MS Neutral Loss Scanning neutral loss 28 amu mol wt derivatized corresponding bases base 230, 244, 258, 272, 286 256, 270, 284, 298 280, 294, 308, 322 330, 344, 358
133, 147, 161, 175
tetrahydroquinolines
159,173, 187, 201 183, 197, 211, 225 233, 247, 261
tetrahydroazaacenaphthenesl
tetrahydroazabiphenyls tetrahydrobenzoquinolines
tetrahydrodibenzoquinolines
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-
-
-L
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structure. The molecular weight of this series is consistent with a tetrahydrodibenzoquinoline type structure. It should be pointed out that differentiation of the partially hydrogenated compounds using the neutral loss 28-amu scan is only applicable to the first two members of a series because the probability of ethylene loss greatly increases if there are two or more alkyl carbons in a derivatized primary amino PAH. Further structural information could be gleaned from both the derivatized and the underivatized samples by taking MS/MS spectra of the intense peaks which were not identified as tetrahydro nitrogen-containing compounds. Some of the major primary amino compounds which could be identified in the derivatized sample by their distinctive fragmentations (see Figure 3) are anilines (190 77 (loo%), 190 92 (60%) and 204 91 (loo%), 204 106 (30%)),aminonaphthalenes (240 127 (loo%),240 143 (30%)and 254 141 (loo%), 254 157 (30%)),aminoacenaphthenes/aminobiphenyls (266 153 (60%),266 169 (100%) and 280 167 (loo%), 280 183 (55%)), and aminoanthracenes/aminophenanthrenes (290 177 (70%),290 193 (100%) and 304 191 (loo%), 207 (70%)). Some of the major compound types 304 identified through their failure to undergo derivatization are pyridines (see discussion in following section), quinolines (130, 144, ...), carbazoles (168,182),benzoquinolines (180,194), and azapyrenes/azafluoranthenes(204,218). The MS/MS spectra of the first member of each of these structural types is typified by the lack of any significant fragmentation, indicative of a stable aromatic compound. For each of these compounds, the principal fragment ion results from loss of H., with relative fragment ion intensities being compound dependent. The principal fragment ions in the MS/MS spectra of the methyl analogue of each of these structural types are due to loss of either CHs or CH4. For example, the MS/MS spectrum of m/z 144 (144 128 (5%))in the underivatized nitrogen bases sample is consistent with the MS/MS spectra of authentic methylquinolines (7). Correspondingly, MS/MS spectra of m / z 182 (182 167 (85%)) and m / z 194 (194 179 (5%)) agree with the MS/MS spectra of authentic methylcarbazole and methylbenzoquinoline, respectively. The character of the MS/MS spectrum of m/z 218 (218 203 (8%))is similar to the above described methyl analogues and suggests a methylazapyrene/methylazafluoranthene type structure. A question of particular interest in samples of the type under investigation is the nature of the primary amino group attached to the polycyclic aromatic hydrocarbon; that is, is the amino group attached directly to the aromatic ring or is it attached via methylene groups as in a benzylamino structure? This question was addressed by using another type of MS/MS scan, the parent scan. In this scanning mode, the third quadrupole is set to pass a given fragment ion, and the first quadrupole is set to scan the mass range of interest. This scanning mode characterizes the mixture for those constituents which produce a given fragment ion upon CID. The benzylamine type of structure would be expected to fragment to
-- --
possible structure
-
Flgure 5. Electron ionization parent scan of m l z 30 in the underivatized nitrogen-base sample (collision energy 20 eV, argon collision pressure 20 mtorr).
--
is shown in Figure 4. A major series of ions, m / z 230, 244, 258, ...,corresponds to the tetrahydroquinoline series previously identified. Further identifications, by neutral loss scanning, of components present in the derivatized nitrogen bases afe given in Table I. The ion series m / z 256, 270, ... fragments similarly to derivatized tetrahydroquinolines, the most intense fragment ion in the MS/MS spectra being loss of 28 mass units, probably ethylene; Le., 256 228 (70%), 270 242 (50%). (The percentages given in parentheses are the relative fragment ion abundances obtained from the MS/MS spectrum.) This behavior is consistent with derivatized tetrahydroazaacenaphthenes/tetrahydroazabiphenyls. The ion series m/z 280,294, ... when individually mass selected and dissociated, showed fragment ions related to both derivatized tetrahydroazaarenes (intense loss of 28 mass units) and derivatized amines (intense loss of both 97 and 113 mass units). MS/MS spectra of the suspected tetrahydroazaarenes (280 252 (50%) and 294 266 (25%)) are indicative of tetrahydrobenzoquinolines. The derivatized amines are indicated by an ion series (beginning 14 mass units below m / z 280) 266,280, .... The resulting MS/MS spectra (266 153 (60%),266 169 (loo%),and 280 167 (loo%),280 183 (55%)) are consistent with aminoacenaphthenes J aminobiphenyls. A fourth rather weak ion series, m / z 330,344, ... also gave substantial fragment losses of 28 mass units (330 302 (30%),344 316 (60%))suggesting a tetrahydroazaarene
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 8, JULY 1984
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MS/MS 107
28 f
/z
50
rnh
100
50
106
I06 I
30
77 Ill,
100
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I
I
130
,
I
I
Figure 6. Comparison of the EIMSlMS spectra of mlz 107 in the underivatized nitrogen-base sample with benzylamine, o-toluidine, and N-methylaniline (colllsion energy 20 eV, argon collislon pressure 2.0 mtorr).
CHZNHZ', m / z 30. For this reason electron ionization was used and a parent scan (see Figure 5) for the m/z 30 ion obtained to characterize the underivatized nitrogen-base sample for the presence of the benzylamino moiety. A significant number of constituents fragmented to yield the m / z 30 ion, suggesting the possibility that some of the primary amino moieties identified are not directly attached to the aromatic ring but rather are bound via the benzyl functionality. To confirm this, a daughter ion MS/MS spectrum was obtained for one of the constituents detected in the parent scan corresponding to m / z 107 (Figure 6). Mass 107 was selected because it corresponds to the molecular weight of benzylamine itself allowing a comparison to be made with the daughter spectrum of authentic benzylamine (Figure 6). Figure 6 also includes the daughter ion spectra of o-toluidine and N-methylaniline. As expected, the benzylamine ion has a large m/z 30 fragment ion corresponding to the methyleniminium ion. However, the daughter ion spectrum of the nitrogen-base fraction did not show an abundant m/z 30 ion. Daughter ion spectra of the other abundant ions observed in the parent scan were also obtained to ascertain if they produced an intense fragment ion a t m / z 30 indicative of the benzylamino functionality. No abundant m/z 30 fragment ions were detected indicating that amino substituents are directly bonded to the polycyclic hydrocarbon skeleton. Particular interest attaches to the intense ions at even masses in the scan of parents of m / z 30. First indications suggest that these ions correspond to molecular ions containing either no nitrogen or an even number of nitrogen atoms. In fact, since this parent scan was obtained via electron ionization, these even mass ions arise in a two-step process from the ion formed by the loss of H. from the molecular ion of the nitrogen-containing species in the ion source (28). (This was verified by comparison of the MS/MS spectra of both the M+. and (M - H)+ ions of the authentic nitrogen-containing species in Figure 6.) Therefore, the parent scan, while intended to test for the identification of the benzylamino functionality, which produces an intense fragment ion at mlz 30, resulted in the detection of other nitrogen-containing compounds that
have low-intensity m / z 30 fragment ions. This result emphasizes the care which must be exercised in interpreting parent scans where sequential fragmentation can occur. The fragment ions observed in the MS/MS spectrum of m / z 107 from the nitrogen-base sample most closely reflect those observed in an MS/MS spectrum of a C,-alkyl pyridine isomer 107 -* 106 (76%), 92 (68%), 79 (24%), 77 (23%), 66 (48%), 65 (94%). A second component at m/z 107, as well as the probability of higher molecular weight ions fragmenting to m / z 107 in the ion source as a result of electron ionization, would further account for the observed MS/MS spectrum. This study has provided evidence of the presence in a coal liquid of a number of tetrahydro compounds containing secondary nitrogens. This is consistent with the suggestion that such compounds are intermediates in reductive denitrogenation. These results were obtained by application of tandem mass spectrometry using all three conventional scanning modes: daughter, parent, and neutral loss. LITERATURE CITED 55,232-241. M. M.; Hurtubise, R. J.; Silver, H. F. Anal. Chem. 1983, Boduszynski, Boduszynski, M. M.; Hurtubise, R. J.; Silver, H. F. Anal. Chem. 1983, 55,225-231. Romanowskl, T.; Funcke, W.; Grossman, I.; Konig, J.; Balfanz, E. Anal. Chem. 1983, 55, 1030-1033. White, C. M.; Li, N. C. Anal. Chem. 1982, 54, 1570-1572. Swansiger, J. T.; Best, H. T.; Donner, D. A.; Youngiess, T. L. Anal. &em. 1982, 54,2576. Bcduszynskl, M. M.; Hurtublse, R. J.; Silver, H. F. Anal. Chem. 1982, 54,375-381. Clupek, J. D.; Zakett, D.; Cooks, R. G.; Wood, K. V. Anal. Chem. 1982, 54,2215-2219. Felice, L. J. Anal. Chem. 1982, 54,869-872. Later, D. W.; Lee, M. L.; Wilson, B. W. Anal. Chem. 1982, 5 4 , 117- 123. Tomkins, B. A.; Ho, C. H. Anal. Chem. 1982, 54,91-96. Later, D. W.; Lee, M. L.; Bartle, K. D.; Kong, R. C.; Vassilaros, D. L. Anal. Chem. 1981, 53, 1612,. Novotny, M.; Kump, R.; Merll, F.; Todd, L. J. Anal. Chem. 1980, 52, 401. Pandier, W. W.; Chepien, M. Fuel 1879, 58,775. Wilson, B. W.; Pelroy, R.; Cresto, J. T. Mutat. Res. 1980, 79, 193-202. Guerln, M. R.; Ho, C. H.; Rao, T. K.; Clark, B. R.; Epler, J. L. Environ. Res. 1080, 23,42-53. Haugen, D. A.; Peak, M. J.; Suhrbler, K. M.; Stamoudis, V. C. Anal. Chem. 1982, 54,32-37. Katzer, J. R.; Sivasubramanian, R. Catal. Rev. 1979, 2, 155-208. Cocchetto, J. F.; Satterfieid, C. N. Ind. Eng. Chem. Process Des. Dev. 1978, 15 (2), 272-277. Fiinn, R. A.; Larson, 0. A.; Beuther, H. Hydrocarbon Process. Pet. Refiner 1963, 42, 129-132. Ciupek, J. D.; Cooks, R. G.; Wood, K. V.; Ferguson, C. R. Fuel 1983, 62, 829. Cooks, R. G.; Giish, G. L. Chem. Eng. News 1981, 59 (48), 40. McLafferty, F. W. Sclence 1981, 214,280. Kondrat, R. W.; Cooks, R. G. Anal. Chem. 1978, 50,81A. Siayback, J. R. B.; Story, M. S. Ind. Res./Dev. I981 (Feb), 129. Flnseth, D. H.; Prozucek. 8. J.; Koppenaai, D. W. Fuel 1982, 61, 1155-1159. Finseth, D. H.; Przybylski, 2. T.; Schmidt, C. E. Fuel 1982, 61, 1174-1 176. White, C. M.; Schultz, J. L.; Schweighardt, F. J. Fuel Process. Techno/. 1978, 1 (3), 209~215. Heller, S. R.; Milne, G. W. A. "EPA/NIH Mass Spectral Data Base"; U S . Government Prihting Office: Washlngton, DC, 1978; Vol. 1.
RECEIVED for review September 30, 1983. Accepted March 1, 1984. This research was supported with funds from the Department of Energy (DE-F622-81PC40780). Reference in this report to any specific commercial product, process, or service is to facilitaw understanding and does not necessarily imply its endorsement or favoring by the United States Department of Energy.
