692
Anal. Chem. 1985, 57, 692-694
(25) Henis, N. B. H.; Busch, K. L.; Bursey, M. M. Inorg. Chim. Acta 1981, 53, L31. (26) Stocki, D.; Budzikiewicz, H. Org. Mass Spectrom. 1982, 17, 376. (27) Deinzer, M.; Griffin, D.; Miller, T.; Lamberton, J.; Freeman, P.; Jonas, V. Homed. Mass Spectrom. 1982, 9 , 85. (28) Rappe, C.; Buser, H. R.; Stalling, D. L.; Smith, L. M.;Dougherty, R. C. Nature (London) 1981, 292, 524. (29) West, R. Pure Appl. Chem. 1971, 28, 379. (30) Dougherty, R. C.; Bergner, A.; Levonowich,.P.; Roberts, J. D. Adv. Mass Spectrom. Biochem. Med. 1976, 1 , 181. (31) Dougherty, R. C., personal communication, Florida State University, 1983.
(32) Hass, J. R.; Friesen, M. D.; Busch, K. L.; Bursey, M. M. "American Society for Mass Spectrometry, 26th Annual Conference on Mass Spectrometry and Allied Topics, St. Louis, MO, (1978) Abstracts, p. 390.
RECEIVED for review September 17,1984. Accepted November 13, 1984. The work was supported by the U.S. Department of Energy (Grant No. 80EV-10449) and the U.S. Environmental Protection Agency (Grant No. R808865).
Combination of Chemical Reduction and Tandem Mass Spectrometry for the Characterization of Sulfur-Containing Fuel Constituents Karl V. Wood* EnginelFuels Laboratory, Chemistry Building, Purdue University, West Lafayette, Indiana 47907 R. Graham Cooks, James A. Laugal, and Robert A. Benkeser Department of Chemistry, Purdue University, West Lafayette, Indiana 47907
Tandem mass spectrometry has been combined with a caicium/mixed amines reduction system to characterize an SRC-I I middle distillate fraction for sulfur-containing polynuclear aromatlc hydrocarbons. Parent scans, which characterize a complex mixture for ail components which fragment to common structural moleties, were used to identify aikyibenzothlophenes and dibenzothiophenes as well as alkyibenrothiophene sulfones.
A chemical reduction scheme has been applied to a middle distillate of an SRC-I1coal-derived liquid in order to facilitate characterization of sulfur-containing constituents by tandem mass spectrometry (MS/MS) (1-4). The identification of sulfur-containing polynuclear aromatic hydrocarbons (PNAs) in coal and liquids derived from coal (5-7) is of particular interest because of the environmental effect these constituents have after combustion (8). However, these compounds have proven difficult to identfy because of their low relative concentration and characteristic mass overlap with hydrocarbon components (9). The approach used here is an alternative to the rather complex separation schemes which have been used to address these problems by concentrating sulfur-containing PNAs prior to GC/MS (6, 7, 9). EXPERIMENTAL SECTION A Finnigan (Model 4500) triple quadrupole mass spectrometer was used to characterize the reduction products (10, 11). This system consists of three coaxially arranged quadrupole rod assemblies. The first and third quadrupoles are conventional mass analyzers and the second quadrupole is used as a focusing collision cell. Samples were admitted into the mass spectrometer with the direct insertion probe. Negative ion chemical ionization (NICI) was employed with isobutane as the reagent gas (0.4 torr). Argon was used as the collision gas at a gauge pressure of 2.0 mtorr. The collision energy was set at 20 eV. The chemical reduction utilizes calcium metal (10-12 g) in a mixed amine solvent system (12) consisting of ethylenediamine and methylamine or n-butylamine. The calcium-mixed amine
reducing system seems superior to the lithium-ethylenediamine system in the reduction of polynuclear organic sulfur compounds. The lithium-ethylenediamine system has been reported to reduce dibenzothiophene to a plethora of products (13). The reduction of SRC-I1middle distillate which follows is typical of the general procedure. A 500-mL dry round bottom flask was fitted with a single Hershberg stirrer and an ethylene glycol cooled (-26 OC) Friedrichs condenser. The system was flushed with methylamine vapor and the flask then filled with 75 mL of condensed methylamine gas. Calcium metal shot (12.0 g), white sand (24 g), SRC-I1 middle distillate (3.3 g), and 75 mL of dry ethylenediamine were also added. The brown mixture was stirred for 24 h, after which time the methylamine was allowed to evaporate. Technical diethyl ether (100 mL) was added to the flask immersed in ice, and hydrolysis of the gray product was effected with a solution of NHlCl(27 g in 100 mL of water). The aqueous layer was separated, made acidic with concentrated HCl, and then extracted several times with diethyl ether. The ether extracts of the base-soluble layer were combined and dried over anhydrous sodium sulfate. The original ether layer (base insoluble fraction) was washed with aqueous HC1 followed by aqueous NaHC03and then also dried over anhydrous sodium sulfate. The solvent volume of both fractions was reduced by means of a vacuum jacketed distillation column to yield a brownish base-insoluble liquid fraction (1.6 g) and a brownish base-soluble liquid fraction (2.0 9) (14). The middle distillate SRC-I1sample was obtained from Charles E. Schmidt of the Pittsburgh Energy Technology Center. The sodium salts of benzenesulfinate and p-toluenesulfinate, as well as the other reagents, were obtained commercially and used without further purification.
RESULTS AND DISCUSSION Calcium has been shown to be an effective metal in reducing aromatic substrates (12). In the case of aromatic heterocycles containing a thiophene moiety, the thiophene ring is opened by calcium through reductive cleavage of the carbon-sulfur bond (14). For example, carbon-sulfur bond cleavage in benzothiophene yields o-ethylthiophenol. In basic media, the reduced benzothiophene exists as a thiophenolate anion which resists further reduction. The unsaturated substituent present
0003-2700/85/0357-0692$01.50/00 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
SULFUR REDUCTIONS
137 A
Parent Spectrum 151
W 0
693
M/Z 122SRCII -Reduced
Z
8 Z
3
m
a
5 W
#
-3 Ca H0 '
NO REDUCTION
EDA/BA
Figure 1. Major reduction products of dibenzothiophene, benzothiophene, and thiophenoi. (Ca is calcium, EDA ls ethylenediamine, MA is methylamine, and BA is n-butylamine.) MS/MS
I
Figure 3. Negative ion chemical ionization parent scan of m / z 122 of the reduced SRC-I I middle distillate: argon collision gas pressure, 2.0 mtorr; collision energy, 20 eV.
M / Z 191
n
W 0
z
8
3
m 4 W
122
33
109
1
,,
M/Z
after carbon-sulfur bond cleavage is reduced rapidly to yield the alkylated thiophenol. Figure 1 shows the principal reduction products and reaction yields for benzothiophene, dibenzothiophene, and thiophenol. Figure 2 is the NICI daughter spectrum of the conjugate anion, mlz 191, of the reduction product of dibenzothiophene. Of particular interest is the ion at mlz 122, which corresponds to the negative thiatropylium radical anion, C7H6S--. This ion is important because it is also the principal fragment ion in the daughter spectrum of the reduction product of benzothiophene. Therefore, it might be possible to characterize the reduction products of both alkylbenzothiophenes and dibenzothiophenes by a parent scan of m / z 122. This scan characterizes a complex mixture for all components which, upon collision-induced dissociation, fragment to a common structural moiety, in this case the negative thiatropylium radical anion, C7H6S--. Figure 3-is a NICI parent scan of m / z 122, which gives evidence for the reduction products of Co-C, alkylbenzothiophenes (mlz 137,151,165,179, and 193) and dibenzothiophene (mlz 191) in the reduced middle distillate fraction of a SRC-I1 coal-derived liquid. This confirms the value of parent spectra, combined with the calcium/mixed amine reduction system, in characterizing thiophene derivatives in coal liquids. Individual components suggested by the parent spectrum were confirmed by daughter spectra. Figure 4 is the comparison of the NICI daughter spectrum of m / z 137 from the reduced middle distillate fraction of a SRC-I1 coal-derived liquid with the NICI daughter spectrum of the (M - HI- ion (mlz 137) of o-ethylthiophenol. The similarity of the two spectra confirms the presence of the reduced benzothiophene in the reduced SRC-I1 coal-derived liquid sample. Under chemical reduction conditions similar to those used in the benzothiophene study (15),sulfones are reduced to the ring-opened sulfinic acids. These species can be identified
Figure 4. Comparison of the negative ion chemical ionization daughter spectrum of m l z 137 of the reduced SRC-I1 middle distillate with the negative ion chemical ionization daughter spectrum of the conjugate anion, m / z 137, of o-ethylthiophenoi: argon collision gas pressure, 2.0 mtorr; collision energy, 20 eV.
Table I. Daughter Spectra of (M - H)-Derived from Two Sulfinate Salts and the First Three Members of the Benzothiophene Sulfones from Reduced SRC-11" (M - H)ion 141-
155169183197-
sample sodium benzenesulfinate sodium p-toluenesulfinate reduced SRC-I1 reduced SRC-I1 reduced SRC-I1
relative abundance loss of (M-H)SO2 SO, 80 100
100 85
35, 38
100 100 100
15 15 15
45 45
33
"Ion intensities are given in percent relative abundance. All major ions are included. Collision energy was 20 eV; argon collision gas pressure was ca. 2 mtorr. by major fragment ions in the NICI daughter spectra of the corresponding (M - H)-ions due to loss of SO2and formation of SO,. For example, benzenesulfinate and p-toluenesulfinate show these ions as the only significant fragments (see Table I). These results indicate that a NICI parent scan of m / z 64 (which could be Sz- in addition to SO,) could provide information on the presence of sulfones in the original sample. Figure 5 is a NICI parent scan of 64- from the reduced coal-derived liquid. A prominent series can be seen at mlz 169, 183, indicating the presence of alkylbenzothiophene sulfones, as confirmed by the daughter spectra (Table I). A second, much weaker series m / z 121, 135, ... is also present in Figure 5, indicating the possible presence of alkylthiophene
...
Anal. Chem. 1985, 57, 694-698
694 Parent Spectrum M/Z 64SRC II Reduced
1
w
2
t
$1
and other results of these analyses will be presented in a subsequent publication. Registry No. C,H,$-., 94203-57-3;dibenzothiophene,132-65-0; benzothiophene, 95-15-8; thiophenol, 108-98-5; o-cyclohexylthiophenol conjugate anion, 94203-55-1; o-ethylthiophenol conjugate anion, 94203-56-2; o-ethylthiophenol, 4500-58-7.
I
169
’r
LITERATURE CITED (1) McLafferty, F. W. Science 1981,274, 280. (2) Cooks, R. G.; Busch, K. L. J. Chem. Educ. 1982,5 9 , 926. (3) Yost, R. A.; Fetterolf, D. Mass Spectrom. Rev. 1983,2 , 1-45. (4) Hunt, D. F.; Shabanowitz, J. Anal. Chem. 1962,54, 574. (5) Czogalla. C. D.; Broberg, F. Sulfur Rep, 1983,3 . (6) Lee, M. L.; Willey, C.; Castle, R. N.; White, C. M. I n “Polynuclear Aromatic Hydrocarbons: Chemistry and Biological Effects”; BJorseth, A,,
100
150
250
M/Z
Figure 5. Negative ion chemical ionization parent scan of m / z 64 of the reduced SRC-I1 middle distillate: argon collision gas pressure, 2.0 mtorr; colllsion energy, 20 eV.
sulfones. The SRC-I1 middle distillate sample had been stored in screw-top vials on a laboratory shelf for 2 years, suggesting that air oxidation may be the reason for the presence of the sulfones. The NICI parent scans of both m/z 64 and 122 provide useful information for characterizing a complex SRC-I1 coal-derived liquid for sulfur-containing constituents. At present, additional parent scans are being investigated for the identification of other series of sulfur-containing PNAs. Furthermore, the chemical reduction scheme has been used on a heavy distillate as well as four different coal samples with varying amounts of total sulfur and organic sulfur. The heavy distillate shows dibenzothiophene and its homologues; these
Dennis, A. J., Eds.; Battelle Press: Columbus, OH, 1980;pp 59-73. (7) Willey, C.; Iwao, M.; Castle, R. N.; Lee, M. L. And. Chem. 1981,53, 400-407. (8) Markuszewskl, R.; Mlller, L. J.; Straszhelm, W. E.; Fan, C. W.; Wheelock, T. D.; Greer, R. T. I n “New Aproaches in Coal Chemistry”; Blaustein, B. D., Bockrath, 6. C., Frledman S.,Eds.; American Chemical Society: Washington, DC, 1981; ACS Symposium Series 169. pp
401-415. (9) Kong, R. C.; Lee, M. L.; Iwao, M.; Tomlnaga, Y.; Pratap, R.; Thompson, R. D.; Castle, R. N. Fuel 1984, 63, 702-708. (IO) Yost, R. A.; Enke, C. G. Anal. Chem. 1979,57, 1251A. (11) Slayback, J. R. B.; Story, M. S. Ind. Res./Dev. I981 (Feb), 129. (12) Benkeser, R. A.; Belmonte, F. G.; Kang, J. J. Org. Chem. 1983,48, 2796. (13) Reggel, L.; Zahn, C.; Wender, I.; Raymond, R. Bull.-US., Bur. Mines 1085. No. 615. 37. (14) Laugal, J. A. Ph.D: dissertation, Purdue University, 1984. (15) Truce, W. E.; Tate, D. P.; Burdge, D. N. J. Am. Chem. SOC.1960, 82. 2872-2876.
RECEIVED for review October 15, 1984. Accepted November 19, 1984. This work was supported by the Department of Energy under Contracts DE-FG22-82-DC50803 (K.V.W. and R.G.C) and DE-AC02-81-ER10989 (R.A.B.).
Identifying Alkylbenzene Isomers with Chemical Ionization-Proton Exchange Mass Spectrometry Steven B. Hawthorne* and David J. Miller University of North Dakota Energy Research Center, Box 8213, University Station, Grand Forks, North Dakota 58202
Chemical lonlzatlon-proton exchange mass spectrometry (CIPE) allows the number of unsubstituted aromatic carbons In alkylbenzene Isomers to be determined. Only the aromatic hydrogens undergo exchange with deuterium when deuterated water, methanol, or ethanol Is used as the reagent gas. Chemical lonlzatlon with deuterated methanol glves an acceptable mass spectral background and allows the determlnation of the number of unsubstituted positions on the benzene ring yielding structural Information often unavailable from conventlonal electron Impact spectra. Structural Isomers such as propyl-, methylethyl-, and trlmethylbenzene can easily be identlfled. The comparison of CIPE spectra from standard compounds, which are often unavailable, Is not required to determine the number of unsubstltuted aromatic carbons in alkylbenzene Isomers. The method also allows ortho and para to be dlstlngulshed from meta disubstituted alkylbenzenes. Deuterlomethanol chemical lonlzatlon Is used to characterize alkylbenzenesIn a complex and relatively well studled sample, diesel exhaust.
Alkylbenzene isomers are major species in samples of industrial and environmental interest including coal-derived
liquids, petroleum products, and urban air. Unfortunately, the ability of conventional gas chromatography/mass spectrometry with electron impact ionization (EI) to differentiate among alkylbenzene isomers of the same molecular weight is limited. Electron impact mass spectra of these species are often indistinguishable, and appropriate standards which would allow identifications based on a comparison of chromatographic retention indexes and standard mass spectra are generally unavailable for larger molecular weight isomers (alkylbenzene isomers containing four or more alkyl carbon atoms). Some alkylbenzene isomers can be identified with the use of methane and isobutane chemical ionization (CI) and a study of the resultant fragmentation patterns (1-6), but such methods depend upon the availability of appropriate standards. Chemical ionization with deuterated reagents can be used to determine the number of protons bonded to heteroatoms in several classes of compounds including aromatic and aliphatic alcohols, carboxylic acids, amines, amides, and mercaptans (7-10). Deuterium exchange with aromatic protons has also been reported under certain conditions with D 2 0and CH3CH20Dreagent gases (11-13). Although the proton exchange mechanism is not well understood, ionization of the benzene ring by addition of H+ or D+ appears to be necessary
0003-2700/85/0357-0694$01.50/00 1985 American Chemlcal Society
