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. 1962,5 4 , 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, 6 3 , 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
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
for proton exchange to occur. The species can then undergo up to n proton exchanges where n equals the number of unsubstituted positions on the benzene ring, i.e., n equals the number of aromatic protons. For example, a disubstituted benzene undergoing ionization and maximum proton exchange with ROD reagent gas will show an (M 6) mass peak. In the present study, the phenomenon of aromatic proton exchange on alkylbenzenes is exploited in order to develop methodology which allows the number of alkyl-substituted positions on a benzene ring to be determined. The use of reagent gases D20,CH30D, and CH3CH20Dwas investigated in order to determine their ability to exchange deuterium with aromatic hydrogens and to provide an acceptably low background of mass spectral peaks. This method allows rapid and routine changeover to and from electron impact mode and works well with complex samples. The method is applied to the characterization of alkylbenzene isomers in diesel exhaust.
+
EXPERIMENTAL SECTION A Hewlett-Packard Model 5985B gas chromatograph/mass spectrometer equipped with a dual electron impact/chemical ionization source was used to obtain all of the mass spectra reported. Deuteriomethanol (KOR Isotopes, Cambridge, MA), deuterioethanol (Aldrich Chemical Co., Milwaukee, WI), and D z O (Stohler Isotope Chemicals, Waltham, MA) were all 199% isotopically pure and used as received. The reagents were introduced directly into the source through 1/16 in. 0.d. stainless steel tubing running through the heated transfer block collinear with the chromatographic column. Source pressure and ionizing voltage were optimized for each reagent gas by introducing m-diisopropylbenzene at a constant rate into the source through the direct insertion probe and maximizing the intensity of the (M + 6) ion that is formed from the addition of D+ and the exchange of all four aromatic hydrogens for deuterium. The source temperature was also optimized for each reagent by varying it from 100 to 200 OC and observing the effect on chemical ionization induced proton exchange on 150-ng samples of m-diisopropylbenzene injected into the chromatographic column. All gas chromatographic separations were performed with a Hewlett-Packard Model 5840 gas chromatograph (GC/MS) or a Model 5730 (FID) equipped with a 60 m X 0.32 mm i.d., SE-54 fused-silica capillary column (J and W Scientific,Rancho Cordova, CA). Electron impact spectra were collected at 70 eV and a source temperature of 200 "C. Samples of diesel exhaust were collected from the exhaust stream of a diesel tractor into 100-mg Tenax-GC traps by using a personal air sampling pump (Du Pont Model P4000) calibrated to draw 100 mL/min. The collected species were thermally desorbed at 220 OC for 10 min and swept with helium directly into the chromatographic column for cryogenic trapping at -50 OC. The column was then heated to 0 OC and the sample species were eluted at a temperature programming rate of 8 OC/min. Detailed sample collection and analysis methods have been reported elsewhere (14).
RESULTS AND DISCUSSION
+
Maximum intensity of the (M 6) ion for m-diisopropylbenzene with each reagent gas occurred at a source pressure of approximately 0.2 torr. This pressure of reagent gas was used for all following experiments. Source temperature had only a small effect on the extent of proton exchange with each reagent gas when varied from 100 to 200 OC. Since one of the goals of this method was to allow rapid conversion from standard E1 conditions to chemical ionization-proton exchange (CIPE) conditions, the standard E1 source temperature of 200 OC was used for all further work. Electron voltage was also optimized and was approximately 50 eV for each reagent gas. Under the described conditions, routine changeover from E1 to CIPE conditions and back can easily be accomplished in less than 5 min. Even though chemical ionization of m-diisopropylbenzene readily occurs when D 2 0 is used as the reagent gas, very little proton exchange was observed at any of the conditions de-
695
Table I. Relative Abundance of Reagent Ion Clusters Formed from CHaOD and CH3CH20D cluster CH3OD2' (CH30D)2Dt - D20 (CH30D)2Dt (CH,OD),D' - DIO (CH,OD),D+ (CH30D)dD' - D2O CH3CH20D2' (CH&H20D)2D+ (CH&H20D)2D+ (CH&H20D)3D+ (CH&H20D)3D+ (CH3CH20D)dD'
- D2O - DzO - D2O
mass
re1 intens, %
35 48 68
29 28
100
81
19
101
18
114
0.2
49 76 96 123 143 170
9.1 6.2
100 9.3 46 0.2
scribed above. Since proton exchange with D 2 0 remained minimal when other alkylbenzene isomers (e.g., 1,3,5-triethylbenzene and n-hexylbenzene) were used as sample species, further work was limited to CH30D and CH3CH20D reagent gases. Both CH30D and CH3CH20Dreagent gases were successful at producing mass spectra showing ionization and complete proton exchange with the test species m-diisopropylbenzene, 1,3,5-triethylbenzene, and n-hexylbenzene. However, CH3CHzOD is less desirable as a reagent gas because it produces significant background ions in the mass region of interest. Mass and relative intensity data for reagent ion clusters generated from CH30D and CH3CH20Dare shown in Table I. The ion clusters are in the general form of (ROD),D+ and (ROD),D+ minus DzO. When a mass range of 10-400 is scanned, both reagent gases give a similar background total ion current, and a similar distribution of reagent ion clusters. But, because of its higher molecular weight, CH3CHz0D produces a high background current a t masses as high as m/z 143 (and its 13Cisotope peak at m/z 144), making the analysis of low concentrations of C2, C3, and C4 alkylbenzene isomers very difficult. In contrast, CH30D produces no significant background ions above m/z 101 (and its 13Cisotope m/z 102). Since the identification of alkylbenzene isomers using CIPE does not depend upon fragmentation patterns, the lower mass regions do not have to be monitored, and all alkylbenzene isomers containing two or more alkyl carbon atoms can be analyzed without interference from reagent ion background when CH30D is used for the reagent gas. As shown in Figure 1,data from chemical ionization-proton exchange mass spectrometry can be used to determine the number of unsubstituted aromatic carbons in alkylbenzene isomers. Mass spectra generated with CH30D reagent gas of the eight isomers of C3 alkylbenzenes show that trimethyl, methylethyl, and propyl isomers can easily be identified. CIPE mass spectra for several alkylbenzene isomers are given in Table 11. The number of unsubstituted positions on the benzene ring ( X )can be determined by the formula X = M , - M I - 2 where MI is the molecular weight of the alkylbenzene isomer, M z is the mass representing maximum exchange of the aromatic protons, and the number 2 is subtracted to account for the D+ added to the alkylbenzene isomer during the chemical ionization step. For example, the number of unsubstituted positions on a trimethylbenzene would be X = 125 - 120 - 2 = 3 (Table 11). All of the standard alkylbenzene compounds that have been studied give reproducible CIPE mass spectra which can easily be predicted based on the number of aromatic hydrogens available for exchange with deuterium from the reagent gas. The predictability of the spectra given in Table I1 demonstrates that standard compounds do not necessarily need to be available and analyzed in order to allow classification of
696
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
Table 11. CIPE Mass Spectra of Alkylbenzene Isomers with CH30D Reagent Gas % re1 abundancen
mol wt
M
(Md-2)
(M+3)
(M+4)
(M+5)
(M+6)
o-xylene m-xylene p-xylene ethylbenzene
106
25 27 27 35
14 28 12 40
21
40
30 57 26 71
49 100 44
100 39
100
54
34
propylbenzene isopropylbenzene o-ethyltoluene rn-ethyltoluene p-ethyltoluene 1,2,3-trimethylbenzene 1,2,4-trimethylbenzene 1,3,5-trimethylbenzene
120
23 18 17 24 8 29 29
13 13 16 55 14 62 48 100
49 49 52 100 49 100 100 15
67 67 100 35 100
100 100
23 68 20 77 66 72
30 30 32 83 28 99 86 39
o-diethylbenzene m-diethylbenzene p-diethylbenzene
134
26 54 7
35 67
46
70 100
100
81
92
compound
18
1,2,3,44etramethylbenzene 1,2,3,54etramethylbenzene 1,2,4,54etramethylbenzene 1-methyl-4-isopropylbenzene
19 26 25 25 22 23
100 41 26
12
pentamethylbenzene n-pentylbenzene
148
n-hexylbenzene
162
m-diisopropylbenzene p-diisopropylbenzene 1,3,5-triethylbenzene hexamethylbenzene
9
100
21
11
23 21 24
12 90
18
56 20 21
6
100
36
43
100
100 58 57 38
18 96 19 100 56
91
100
38 19
29
44
69
100
19 100 84 57
28 97 89 19
42 81 93 4
66 30 100
100
11
66 100 100
8
(M+7)
“All CIPE mass spectra were generated as described in the text from approximately 150-ng samples injected into the chromatographic column of the GC/MS. Ion abundance5 are not correct for the contribution of the 13C isotope.
&
-I
m / z 120
130 120
bv
1111
130 1 2 0
Flgure 1. Chemical ionization-proton exchange mass spectra of C3 alkylbenzene isomers with CHBOD reagent gas.
sample species based on their CIPE spectra. This is particularly important for the analysis of complex hydrocarbon samples where appropriate standard compounds and sufficient time €or the generation of standard spectra are often unavailable. The relative intensity of the CIPE mass spectral peaks also yields information concerning the position of alkyl substituents on the benzene ring. All of the ortho- and para-disubstituted alkylbenzenes that have been studied (Table 11) show the greatest spectral intensity at the mass representing exchange
+
of all aromatic protons (Le., the (M 6) peak). In contrast, all of the meta-disubstituted alkylbenzenes show an intensity of about 30% to 40% at the (M 6) peak. This general trend continues with more highly substituted benzenes (e.g., compare 1,2,3- to 1,3,54rimethylbenzene, and 1,2,3,4- to 1,2,3,5-tetramethylbenzene), and may, in part, be a result of reduced exchange at sterically hindered aromatic carbons. However, the directive influences of the attached alkyl groups ( I I , 1 2 ) may also determine the rate of proton exchange a t any one position. For example, both 1,2,4-trimethylbenzene and 1,2,4,5-tetramethylbenzenehave sterically hindered positions but do not show reduced proton exchange. Such exceptions make the assignment of some positional isomers difficult, unless CIPE spectra of standard compounds are available. At present, ortho- and para-disubstituted benzenes can be distinguished from meta isomers without the use of standard compounds. The results in Table I1 demonstrate that, when appropriate standards are available, CIPE spectra can also be used to identify some other positional isomers (e.g., the three tetramethylbenzenes). The use of the CIPE technique to identify alkylbenzene isomers in a diesel exhaust sample is shown in Figures 2 and 3 and Table 111. Initial classification of alkylbenzene isomers by molecular weight was based on a study of the M and (M 2) masses generated by the CIPE experiment and confirmed by E1 analysis. Major proportions of the exhaust sample consisted of alkanes (as identified by GC/MS with E1 ionization, Figure 2), and many of the components were poorly resolved even with high-resolution capillary gas chromatography. Even so, the simple and highly characteristic nature of alkylbenzene CIPE spectra made it possible to assign the number of unsubstituted aromatic carbons for over 75 isomers of C3 to C7 alkylbenzenes (Figure 3 and Table 111). Since this method does not depend upon the study of fragmentation patterns, identifications could be made even in the presence of overlapping spectra resulting from poorly resolved species.
+
+
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
697
Table 111. Chemical Ionization-Proton Exchange Determination of Aromatic Substitution for Alkylbenzene Isomers Found in Diesel Exhaust mlz
(Figure 3) 122
136
5
10
20
15
no. of aromatic hydrocarbons
Flgure 2. Gas chromatographic separation of organic species found in diesel exhaust. Diesel exhaust samples were collected into Tenax traps and analyzed as described in the text.
150
Pr Me-Et (Meh
1. 2
5
BU
9, 10, 15
4
Me-Pr or (Et)2
3
(MB)Z-Et (Me)* Pe Me-Bu or Et-Pr Me-(Et), or (Me)z-Pr (Me)3-Et
11 (4, 12 (0-PI, 13 (m), 14 (m), 17 (0-p), 18 (0-P) 16, 19, 20, 21, 22
5
5 4
3 2 1
164
5 4
3
i'"
2
24
I
peak no. (Figure 3)
4 3
2
Retention Time (minutes)
possible alkyl substituentso
1
(Meh Hx Me-Pe, Et-Bu, or (Prh (Me)a-Bu, Me-Et-Pr, or (E% (Me)&? (Me)z-(Eth (Me),-Et
23, 24, 26 30, 31, 32 27, 28, 29, 35, 36 37, 38, 40, 41, 42, 43 33, 34, 39, 44, 45, 46 ND~ 58 51, 52, 54, 56 47, 48, 49, 50, 55, 57,
59, 60, 61, 63, 64 ND 53,62
70 HP Me-Hx, Et-Pe, 73, 74, 75, 76, 77 or Pr-Bu 71, 72 3 (Me)z-Pe, Me-Et-Bu, Me(Pr)2,or (Et)z-Pr 2 65, 67, 68, 69, 78 (Me)d3u, (MeIz-Et-Pr, or Me-(Et), 66 1 (Me)4-Pror (Me)dEt)2 a Key: Me, methyl; Et, ethyl; Pr, propyl; Bu, butyl; Pe, pentyl; Hx, hexyl; and Hp, heptyl. None of these designations implies whether the alkyl substituent is normal or branched. bND, not detected. 178
5 4
-E5 I m
I
134 331
m/z
0 150 c
-
17
10
20
19
I
53
21
22
p"
m/z 164
1s
20
21
22
23
24
mlr 170
Retention Time (minutes) Flgure 3. Selected ion chromatograms of alkylbenzene isomers in diesel exhaust analyzed by chemical ionizatlon-proton exchange mass spectrometry. Each selected ion chromatogram was reconstructed from the total ion chromatogram generdted with a scan range of mlz 104 to 406. The mass displayed in each chromatogram is the (M 2) (M D') ion of the alkylbenzene species. Peak numbers refer to Table 111.
+
+
The CIPE spectra of the C3and C4 alkylbenzenes were also used to distinguish between meta- and ortho- or para-disubstituted isomers (Table III). Since few higher molecular weight disubstituted alkylbenzene standards were available, the determination of meta w. ortho or para isomers is not reported
for alkylbenzenes containing more than four alkyl carbons. The analysis of the complex hydrocarbon mixture found in diesel exhaust demonstrateB the ability of CIPE mass spectrometrty to provide structural information about alkylbenzene isomers that is generally unavailable from E1 spectra. The number of unsubstituted aromatic carbons can easily be determined without standard spectra or the study of fragmentation patterns, making CIPE ideal for the analysis of highly complex samples. A study of the relative intensities of a particular specie's CIPE spectra also allows identification of some positional isomers and is particularly useful for distinguishing ortho- and para- from meta-disubstituted alkylbenzenes. A greater understanding of the mechanism of proton exchange is needed before the potential of CIPE mass spectra to determine positional isomers can be fully exploited. Registry No. CHzOD, 1455-13-6;H2,1333-74-0;D2,7782-39-0; a-xylene, 95-47-6; m-xylene, 108-38-3;p-xylene, 106-42-3;ethylbenzene, 100-41-4;propylbenzene, 103-65-1;isopropylbenzene, 98-82-8; o-ethyltoluene, 611-14-3; m-ethyltoluene, 620-14-4; p ethyltoluene, 622-96-8; 1,2,3-trimethylbenzene, 526-73-8; 1,2,4trimethylbenzene, 95-63-6; 1,3,5-trimethylbenzene, 108-67-8;odiethylbenzene, 135-01-3; m-diethylbenzene, 141-93-5; p-diethylbenzene, 105-05-5; 1,2,3,4-tetramethylbenzene,488-23-3;
Anal. Chem. 1985, 57,698-704
698
1,2,3,5-tetramethylbenzene, 527-53-7;1,2,4,5-tetramethylbenzene, 95-93-2; l-methyl-4-isopropylbenzene,99-87-6; pentamethylbenzene, 700-12-9;n-pentylbenzene, 538-68-1;n-hexylbenzene, 1077-16-3; n-diisopropylbenzene, 99-62-7;p-diisopropylbenzene, 100-18-5; 1,3,5-triethylbenzene, 102-25-0; hexamethylbenzene, 87-85-4;butylbenzene, 104-51-8;dimethylethylbenzene,29224-55-3; butylmethylbenzene, 27458-20-4;ethylpropylbenzene,82162-13-8; diethylmethylbenzene, 25550-13-4; dimethylpropylbenzene, 82161-99-7; ethyltrimethylbenzene, 41903-41-7; methylpentylbenzene, 1320-01-0; butylethylbenzene, 82169-27-5; dipropylbenzene, 31621-49-5; ethylmethylpropylbenzene, 94278-81-6; ethyltetramethylbenzene, 94278-82-7;heptylbenzene, 1078-71-3; hexylmethylbenzene,27133-94-4;ethylpentylbenzene, 94278-83-8; butylpropylbenzene, 94278-84-9; dimethylpentylbenzene, 94278-85-0; butylethylmethylbenzene, 94278-86-1; dipropylmethylbenzene, 42300-93-6; diethylpropylbenzene, 94278-87-2; butyltrimethylbenzene,94278-88-3; dimethylethylpropylbenzene, 94278-89-4; methyltriethylbenzene, 41903-42-8; propyltetramethylbenzene, 94278-90-7;diethyltrimethylbenzene, 67143-86-6 n-octane, 111-65-9;n-decane, 124-18-5;n-undecane, 1120-21-4; n-dodecane, 112-40-3;n-tridecane, 629-50-5; n-tetradecane, 62959-4; methylpropylbenzene, 28729-54-6; butyldimethylbenzene, 82161-98-6;nonane, 111-84-2.
LITERATURE CITED (1) Daishima, S.;lida, Y. Shitsuryo Bunseki 1983, 37, 73. (2) Munson, M. S. B.; Field, F. H. J. Am. Chem. SOC.1987, 89, 1047. (3) Harrlson, A. G.; Lln, P. H.; Leung, H. W. Adv. Mass Spectrom. 1978, 7 8 , 1394. (4) Daishima, S.;Iida, Y. Shitsuryo Bunsekl 1982, 30,249. (5) Daishima, S.; Iida, Y. Shitsuryo Bunsekl 1982, 30,61. (6) Iseda, K. Nagoya Kogyo Guutsu Shikensho Hokoku 1982, 37, 306. (7) Buchanan, M. V. Anal. Chem. 1982, 54, 570. (8) Lin, Y. Y.; Smith, L. L. Biomed. Mass Spectrom. 1979, 6 , 15. (9) Hunt, D. F.; McEwen, C. N.; Upham, R. A. Anal. Chem. 1972, 4 4 , 1292. (10) Buchanan, M. V. Anal. Chem. 1984, 56, 546. (11) Hunt, D. F.; Sethi, S. K. J. Am. Chem. SOC.1980, 102, 6953. (12) Freiser, E. S.; Woodin, R. L.; Beauchamp, J. L. J. Am. Chem. SOC. 1975, 97,6893. (13) Martinsen, D. P.; Buttrill, S. E., Jr. Org. Mass. Spectrum. 1976, 7 7 , 762. (14) Hawthorne, S. E.; Sievers, R. E. Envlron. Sci. Techno/. 1984, 78, 483.
RECEIVED for review October 9, 1984. Accepted November 16, 1984. This work was performed under Cooperative Agreement No. DE-FC21-83FE60181for the US.Department of Energy, Office of Fossil Energy.
Laser Mass Spectra of Simple Aliphatic and Aromatic Amino Acids Cass D. Parker and David M. Hercules* Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
Mass spectra of amlno aclds have been studied using the LAMMA laser microprobe. Quasi-molecular Ions, (M H)’ and (M H)-, are produced In hlgh yields. The malor neutral fragment loss from (M H)’ corresponds to HCOOH. Loss of other acids is related to the amlno group postllon. For alkyl amino acids, add loss plus formation of imino Ions ( m / z 30) are the only slgnlficant fragments at threshold power densltles. Fragmentatlon Is llmlted In the negatlve Ion spectra; (M H)- and CN- are the only Ions observed at low power densltles. Fragmentatlon In both the posltlve and negative Ion spectra Is Influenced by substltuents on the alkyl or aryl chalns; the electron denslty of an aryl group has a signlflcant effect. I n a-@flsslon, the phenylalanine quasl-molecular Ion gives the a-fragment, but no a-fragment Is observed for tryptophan. Slgnlflcant thermal decomposltlon of amlno acids does not occur In laser mass spectrometry; spectra are comparable to chemical ionlzatlon using reagent gases havlng hlgh proton afflnltles.
-
+
+
-
The use of mass spectrometry to study nonvolatile organic compounds has been expanded in recent years, particularly for biomolecules which require derivatization for conventional chemical ionization (CI) or electron impact (EI) methods. Introduction of fast-heating direct insertion probes (1) and new ionization sources such as field desorption (FD) ( 2 ) , secondary ion mass spectrometry (SIMS) ( 3 ) , fast atom bombardment mass spectrometry (FAB) ( 4 ) ,plasma desorption mass spectrometry (PDMS) ( 5 ) , and laser mass spec-
trometry (LMS) (6)make derivatization no longer necessity for many compounds. Mass spectra of the amino acids have been reported with several ionization sources: electron impact (EI) (7), FD (8), CI (9),SIMS (3),FAB ( l o ) ,and LMS (11). Mass spectra of the amino acids obtained by these techniques show similarities; for example, all show quasi-molecular (M + H)’ ions (except E1 which shows M+3 and peaks corresponding to loss of formic acid from the quasi-molecular ion. Negative ion spectra have been reported by SIMS and low energy electron impact (LEEI) (12). SIMS reported detection of (M - H)- quasimolecular ions, while LEEI reported (M - H)-, with fragmentation arising from M-e; losses included NH2, HzO, C02H, and the alkyl chain. We report here results obtained for amino acids using laser mass spectrometry (LMS). The positive-ion spectra resemble those obtained by using other techniques. Of particular interest is the similarity to CI mass spectra obtained by using reagent gases having relatively high proton affinities. This attests to the “softness” of LMS as a technique for obtaining spectra of organic compounds. Fragmentation is not extensive at threshold power densities, particularly in negative-ion LMS.
EXPERIMENTAL SECTION Laser mass spectra were obtained with commercially available instrumentation: the Leybold-Heraeus LAMMA-500 and LAMMA-1000 laser microprobes. Mass spectra of DOPA, phenylalanine methyl ester hydrochloride, and tryptophan were obtained only on the LAMMA-1000; other samples were run on both instruments; fragmentation patterns did not differ significantly. The LAMMAdOO is described in detail elsewhere (13). The major difference between the two instruments is that the laser beam
0003-2700/85/0357-0698$01.50/00 1985 American Chemlcal Society
