Anal. Chem. 1086, 58, 2121-2126
dibenzo[bc,eflcoronene,190-31-8;dibenzo[eghi]indeno[1,2,3,4pqralperylene, 75449-96-6; acenaphtho[l,2-k]cyclopenta[cd]fluoranthene, 30909-04-7.
LITERATURE CITED Lee. M. L.; Novotny, M. V.; BarNe, K. D. Analytical Chemistry of Polycyclk Aromatlc Compounds; Academic Press: New York, 1981. Bjorseth, A. Hendbaok of Polycyclic Aromatlc Hydrocarbons; Marcel Dekker: New York, 1983. Lacassagne, A.; Buu-Hol, N. P.; Zajdela, F.; Saint-Ruf. G. C . R . Acad. Sei. Paris, Ser. D . 1968, 226, 301-304. McKay, J. F.; Latham. D. R. Anal. Chem. 1973, 45, 1050-1055. Romanowski,T.; Funke, W.; Grossman, I.; Konlg, J.; Balfanz, E. HRC C C , J . High Res. Chromatogr. Chromatogr. Commun. 1981, 4 , 209-214. Peaden. P. A.: Lee, M. L.: Hlrata. Y.; Novotny, M. Anal. Chem. 1980, 5 2 , 2268-2271. Hlrosa, A.; Wiesler, D.; Novotny, M. Chromatographie 1984, 78, 239-242. Hlrosa. A.; Wlesler. D.; Novotny, M. Anal. Chem. 1984. 5 6 , 1243-1248. Simonsick, W. J., Jr.; Hites, R. A. Anal. Chem. 1984, 56, 2749-2754. Peaden, P. A.; Wright, B. W.; Lee, M. L. Chromatographie 1982, 15, 335-340. Stenberg, V.; Alsberg, T.; Blomberg, L.; Wannman, T. I n Polynuclear Aromatic Hydrocar6ons; eds. Jones, P. W., Leber, P., E&.; Ann Arbor
2121
Sclence: Ann Arbor, MI, 1979; pp 313-326. (12) Dewar, M. J. S.;Thiel, W. J . Am. Chem. Soc. 1977, 99,4899-4907. (13) Dewar, M. J. S.; Thiel, W. J . Am. Chem. Soc. 1977, 9 9 , 4907-4917. (14) Ford, G. P.: Scribner, J. D. J . Am. Chem. SOC. 1981. 703. 4281-4291. (15) Mlrek, J.; Buda, A. 2.Netufforsch., A 198, 39a, 388-390. (16) Fetzer. J. C.; Blggs, W. R. J . Chromatogr. 1984. 295, 181-169. (17) Thiel, W. In Quantum Chemktty Program Exchange No. 438; Indlana Unlverslty: Bloomlngton, I N 1983. (18) Fletcher, R.; Powell, M. J. D. Comput. J . 1963, 6 , 183-168. (19) Jensen, T.; Kamlnsky, R.; McVeety, B.; Woznlak, T.; Hltes, R. A. Anal. Chem. 1982, 5 4 , 2388-2390. (20) Levln, R. D.; Lias, S. G. Natl. Stand. Ref. Data Ser. ( U . S . , Net/. Bur. Stand.) No. 77. 1982. (21) Moet-Ner, M. J . phvs. Chem. 1880, 8 4 , 2716-2723. (22) Clar, E.; Schmidt, W. Tetrahedron 1979, 35, 2673-2680. (23) Lee, M. L.; Hites. R. A. Anal. Chem. 1978, 48, 1890-1893. (24) Dias. J. R. Now. J . Chim. 1985, 9 , 125-134. (25) Dlas, J. R. J . Chem. I n f . Comput. Sci. 1984, 2 4 , 124-135. (26) Dias, J. R. J . Chem. Inf. Comput. Sci. 1982, 2 2 , 15-22. (27) Dlas, J. R. J . Ctwm. Inf. Comput. Sci. 1982, 2 2 , 139-152.
RECEIVED for review February 4, 1986. Accepted April 16, 1986. Supported by the US.Department of Energy (Grant NO. 80EV-10449).
Substituent Effects in Charge Exchange Chemical Ionization Mass Spectrometry William J. Simonsick,Jr.,l and Ronald A. Hites* School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405
We examined substituent effects In charge exchange chemical lonlzatlon mass spectrometry. A hear Hammett relatlonshlp was observed for a wrles of monorubstttuted benzenes and naphthalenes. Substituted aromatlc compounds not dhtlngulrhed by electron Impact mass spectrometry can be predkthrdy resolved by charge exchange ctwnkal bnlzatktn mass spectrometry.
to a parent PAH can dramatically affect its mutagenic activity. For example, aminopyrenes have been identified in the mutagenic fraction of a coal-derived heavy distillate (3), and nitropyrenes have been reported to be the most mutagenic chemicals known, although p p e n e itself shows no biological activity (4). Given the significant biological effects of certain substituted aromatic compounds, we thought it would be important to investigate the effect of substituents on the (M H)+/M+behavior for a series of substituted aromatic compounds. The ion/molecule reactions that occur in chemical ionization mass spectrometry are in many cases analogous to those that occur in solution chemistry (5,6). Therefore, the rules governing solution chemistry should be applicable in (CE/ C1)MS. For example, the application of the Hammett equation (7)is a well-known, useful way of correlating the effects of substituents upon reaction rates or equilibrium constants (8). The list is too long to cite all the applications of the Hammett equation for explaining substituent effects in mass spectrometry, but a few of the most important studies will be presented. Bursey used the Hammett equation to explain substituent effects observed under electron ionization (EI) conditions (9). Field studied the effect of substituents in chemical ionization mass spectrometry and has observed correlations between the rates of reactions of several substituted benzyl acetates and the substituent constants )'a( (10). Linear correlations exist between the ionization potential and u+ values for substituted benzenes (11). Recently substituent effects related to the Hammett constant ( u ) were used to explain trends observed in the newly developed technique of resonant two photon ionization spectroscopy (12). Based
+
Charge exchange chemical ionization mass spectrometry (CE/CI)MS has been shown to be a valuable tool for the analysis of unsubstituted polycyclic aromatic hydrocarbons (PAH) (1). Using a 15% mixture of methane in argon, (CE/CI)MS yields abundant M+, (M HI+, and (M C2H5)+ ions from PAH with minimal fragmentation. Molecular ions (M+) are formed by charge exchange reactions with the ionized argon, and protonated molecular ions [(M HI+] result from proton transfer between the analyte and C2H5+.Based on the ratio of the (M H)+ to the M+ ion currents and on the ionization potential (IP), this technique differentiates, in a predictable manner, isomeric structures of unsubstituted PAH (see preceding paper in this journal). Slight modifications in molecular structure, such as the addition of an alkyl group on a ring, can either enhance or diminish the carcinogenic properties of a compound. In the methylchrysene series, for example, only the &methyl isomer is a potent carcinogen while the others only show marginal activity (2). In the same fashion, the addition of a substituent
+
+
+
+
'Current address: E. I. du Pont de Nemours & Co., Marshall
R&D Laboratory, Philadelphia, PA 19146.
0003-2700/86/0358-2121$01.50/0
0 1986 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986
on this history of use, was have used Hammett constants to correlate our (CE/CI)MS data. We will also demonstrate that the (CE/CI)MS behavior of substituted PAH is predictable based upon ionization potentials obtained from molecular orbital calculations.
A)
E1 (70 av) M-
EXPERIMENTAL SECTION Mass spectra were acquired on a Hewlett-Packard 5985B GC/MS system equipped with a dual ionization source (E1 and CI). The mixed reagent gas (15% or 10% methane in argon) (Linde Co., Indianapolis, IN) was introduced into the ion source through a modified transfer line (13). The electron energy for the (CE/CI)MS experiments was set at 230 eV. The fluorinated benzenes were studied by use of a 10% mixture of methane in argon at an ion source pressure of 0.4 torr (capacitance manometer). All other investigationsutilized a 15% mixture of methane in argon. The flow of the mixed reagent gas was increased until an (M + H)+/M+ratio of approximately 1.0 was achieved for the internal standard, 1,2,3,5-tetrafluorobenzene(TFB) (Aldrich, Milwaukee, WI). This corresponded to an ion source pressure of about 0.6 torr. For this study, an ion source temperature of 250 "C was used. The ion source temperature was measured by a thermocouple located in the ion source body. One-microliter methylene chloride (MCB, Cincinnati, OH) solutions, containing approximately 20 ng/pL of each standard (Aldrich, Milwaukee, WI), were loaded in the splitless mode (injector temperature) 280 "C) for 0.7 min onto a 30-m (0.25-pm film thickness) DB-5 capillary column (J and W Scientific Inc., Ranchero, CA) employing helium as the carrier gas at a head pressure of 11 psi. The temperature program was adjusted according to the compounds being analyzed. The mass spectrometer was tuned prior to each set of experiments using a binary mixture of perfluorotributylamine (PETBA) (PCB Research Chemicals, Inc., Gainesville, FL) and TFB. A complete mass spectrum (50-300 daltons) was then obtained for each compound. All subsequent mass spectral data were collected by using selected ion monitoring. The complete M+ and (M + H)+ mass spectral peak profiles of each compound were examined by use of selected ion monitoring software. This was accomplished by stepping across the M+ and (M + H)+ peaks at 0.1-dalton intervals followed by integration h0.2 daltons from the peak maxima. The (M + H)+/M+ratios were corrected for the natural abundance of carbon-13. All molecular orbital calculations reported here were carried out as follows. Input geometric8 of PAH were entered into the next editor of Indiana University's CDC Cyber 170/855. The calculations were performed with an enlarged version of Dewar and Thiel's MNMI program (14).At the start of the calculation, the carbon skeleton and adjacent hydrogens were assumed to be completely planar (except methyl hydrogens) with uniform bond lengths and angles. The aromatic C-C bonds were input as 1.4 A, the aromatic aliphatic C-C as 1.5 A, C-0 as 1.4 A, C-N as 1.4 A, C-C1 as 1.7 A, C-F as 1.3 A, C = O as 1.2 A, C z N as 1.2 A, N-0 as 1.2 A, 0-H as 1.0 A, and C-H as 1.1A. All planar bond angles were 120O. The initial carbon skeleton geometry was then optimized by minimizing the total energy with respect to the planar bond lengths and angles using the Davidson-FletcherPowell (DFP) algorithm, which is incorporated into the MNDO program (15).In addition to the optimization listed above, the subetituent groups were also optimized with respect to the dihedral angle.
RESULTS AND DISCUSSION The mass spectra of most substituted PAH obtained under electron impact (EI) and chemical ionization (CI) conditions are simple consisting of either a molecular ion or a protonated molecular ion with little fragmentation. For example, Figure 1shows the mass spectra of 1-cyanonaphthalene analyzed by four different types of ionization. The E1 mass spectrum (Figure la) is dominated by the molecular ion m/z 153 and a loss of HCN is observed ( m / z 126). The argon CI mass spectrum (Figure lb) is similar to the E1 mass spectrum; both spectra display intense molecular ions with corresponding losses of HCN. This finding is not surprising because Hunt and Gale found that nitrogen, another charge exchange gas,
[M-HCN].
It
, I
B) Argon CI
100
120
M'
160
140
1eo
200
H/Z
Mass spectra of l-cyanonaphthalene analyzed by different ionizing methods. All spectra were obtained at an ion source tern pertaure of 250 O C ; the CIMS spectra were acquired at an ion source pressure of 0.6 torr. Figure 1.
gave mass spectra similar to electron impact (16). The methane CI mass spectrum (Figure IC)is dominated by the protonated molecular ion and has adduct ions a t masses 168, 182, and 194 corresponding to [M + CH3]+, [M + C2Ha]+,and [M + C3H6]+. No fragmentation is observed. The (CE/CI) mass spectrum (Figure Id) has a characteristic of both the argon and methane CI spectra. Molecular, protonated molecular, and adduct ions are all observed. Therefore, in a single experiment one obtains both E1 and CI information. The mixed reagent gas mass spectra of the remaining substituted naphthalenes had characteristics similar to that of 1-cyanonaphthalene. The majority of the ion current (>go%) was concentrated in the M+, (M H)+, and (M C,H,)+ ions. The monobrominated and mononitrated compounds behaved differently. The monobrominated naphthalenes gave a base peak at m / z 156 corresponding to [M - Br + CzHa]+. The base peak of the mononitronaphthalenes was [M - 30]+,which we attribute to a loss of NO. The bond strengths of the bromo and nitro groups bonded to an aromatic ring are lower by at least 15 kcal/mol when compared to the other substituents (17). This may be the reason for the observed anomalies. Single Substituents. The u+ constants of Brown and Okamoto (IS)yield higher correlations with experimental IP's than do the normal Hammett constants (9). Since the (CE/CI)MS technique is very sensitive to the IP of a particular compound (I),we selected to use these modified constants. If a Hammett type relationship exists under (CE/CI)MS
+
+
ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986
Table I. IP and (M Benzenes 0. 2
I
1
-0
-
-0.1
-
*OH
1
I
-0.2
-0.6
/
/
*Ph
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-1.2
-0.8
-0.4
-0
Havtt content
0.4
0.8
( 0 9
Flgwe 2. Hammett plot for 1-substituted naphthalenes. All u+ values were taken from ref 8 except for CHO, which was taken from ref 20. ZHand Z,refer to the (M H)+/M+ ratios of the unsubstituted and substituted naphthalenes, respectively.
+
-0
+ H)+/M+ Data for Fluorinated
1
0. 1
2123
ionization potential, eV (M + std d e 9 Nd exptl" MNDO H)+/Mtb
compound
mol wt
fluorobenzene 1,4-difluorobenzene 1,2-difluorobenzene 1,3-difluorobenzene 1,2,4-trifluorobenzene 1,2,3-trifluorobenzene 1,3,5-trifluorobenzene 1,2,4,5-tetrafluorobenzene 1,2,3,5-tetrafluorobenzene 1,2,3,4-tetrafluorobenzene pentafluorobenzene hexafluorobenzene
96 114
9.23 9.26
9.467 9.557
2.308 1.357
0.008 0.002
3 3
114
9.40
9.688
1.413
0.002
3
114
9.42
9.728
1.690
0.035
3
132
9.34
9.836
0.689
0.002
3
0.912
0.006
3
0.474
0.005
3
10.067
132 132
9.51
10.138
150
9.36
10.030
150
9.56
10.177
150
9.58
10.175
168
9.77
10.406
0.155
0.002
3
186
9.99
10.766
0.031
0.001
3
OValues taken from ref 16, except the IP of 1,2,4-trifluorobenzene was taken from ref 21. bConditions were an ion source temperature of 250 O C and a source pressure of 0.4 torr using a 10% mixture of methane in argon. All abundances have been corrected for the natural abundance of carbon-13. Standard deviation of an individual measurement. Number of measurements.
t
which is highly significant (p < 0.01). Figure 3 shows a similar trend for a series of substituted benzenes ( r = 0.83). We conclude that the Hammett equation is valid for (CE/CI)MS conditions. The slopes of the lines seen in Figures 2 and 3 are positive, signifying that electron-withdrawing groups increase the (M H)+/M+ ratio. These observations can be explained in the following manner. The Hammett constant characterizes the ability of various substituents to stabilize or destabilize an ionic state based on the substituent's electron-withdrawing or releasing properties. For example, an amino substituent (u+ = -1.3) strongly releases electron character into the electron cloud of PAH and will stabilize a radical cation. This results in a lowering of the IP; for example, 1-aminonaphthalene has a lower IP relative to naphthalene (7.38 eV vs. 8.15 eV) (19). A decrease in the IP increases the rate of charge exchange with argon, which results in a decrease in the (M + H)+/M+ratio. On the other hand an electron-withdrawing substituent such as the cyano moiety (u+ = 0.66) destablizes the ionic state; hence, an increase in the 1P of 1-cyanonaphthalene relative to naphthalene is seen (8.61 eV vs. 8.15 eV) (19). Raising the IP slows the rate of charge exchange although the rate of protonation is not affected (1). This results in a relative increase in the rate of protonation; hence, larger (M + H)+/M+ ratios are observed for electron-withdrawingsubstituents. No attempt was made to correlate the observed (M + H)+/M+ behavior with the numerous other modified u constants (8). Multiple Substituents. We would predict that the addition of more electron-releasing substituents (negative e+ values) would further decrease the (M + H)+/M+ratio. For example, we observed that as one increases the number of fluorine substituents, the (M H)+/M+ ratio decreases (see Table I). However, the addition of more than one fluororine
+
-0.4
-0. 5 -0.6
I
*"2
n - 8
I -1. 2
-0.B
-0. 4
-0
0. 4
0. 8
Hanrtt cmatont W+)
Flgwe 3. Hammett plot for monosubstituted benzenes. All u+ values were taken from ref 8 except for CHO, which was taken from ref 20. Z, and 2, refer to the (M H)+/M+ ratios of the unsubstituted and substituted benzenes, respectively.
+
conditions, then a plot of the following equation should be linear: where ZHand 2, are the experimentally determined (M + H)+/M+ ratios of the unsubstituted and substituted compounds, respectively, u+ is the Hammett u+ value of the substituent, and p is the reaction constant, which indicates the magnitude of an effect that a substituent exerts on the particular reaction. Figure 2 shows a plot of this equation for a series of ten 1-substituted naphthalenes. Neither 1-bromonaphthalene nor 1-nitronaphthalene were included in this plot for the reasons discussed above. The linear correlation coefficient (r) is 0.93,
+
2124
ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986
M*l/M Rotio
!
Table 11. Comparison between Experimental Ionization Potentials and Those Calculated by the MNDO Method for Substituted Aromatic Compounds
* \ compound name
i
\*
r
1 1.5 1
i 1
0.5
i 1
L
11 I
I
0
1
2
3
4
5
6
7
Nunbor o f f l w r i m c
a
+
H)+/M+ ratio vs. the number of fluorines substituted benzene ring. Conditions are listed in Table I.
Figure 4. (M on
to a benzene ring no longer stabilizes the radical cation as reflected by an increase in the experimental IP. We would predict from our CE/CI model that an increase in the IP would result in a corresponding increase in the (M + H)+/M+ ratio, although the opposite trend is observed (see Figure 4). The reason for this contradicting observation may be the following: It has been suggested for alkyl and halo substituents that ring protonation occurs in a position para to the site of substituent (20). Therefore, by the addition of another fluorine atom in the para position we prevent protonation a t that location. We still observe a large amount of (M + H)+formation, which suggests that protonation occurs a t several sites on the molecule. Our model does predict corredly the ordering of the (M H)+ratios for the difluoro compounds among themselves. Examination of the trifluorobenzenes reveals the same behavior. A higher I P is observed relative to the less substituted compounds, but a lower (M + H)+/M+is observed because less sites are available for protonation. The trifluoro compound with the larger IF' also has the higher (M + H)+/M+ ratio. The extreme case is hexafluorobenzene, which has the highest IP of the fluorobenzenes although it yields the lowest ratio. This is a consequence of the nearly complete blocking of all protonating sites. Analytical Utility. The resulting E1 mass spectra of positional isomers of substituted PAH are, in most cases, indistinguishable. For example, neither 1-or 2-naphthylamine nor 1- or 2-chloronaphthalene are distinguished by EIMS (22). Because the (CE/CI)MS technique is sensitive to small differences in molecular structure, similar compounds give different CE/CI mass spectra. As discussed above, the power of the (CE/CI)MS technique is its ability to predict the (CE/CI)MS behavior of isomers without possessing the compounds in pure form ( I ) . All that is necessary are IP's of the isomers in question and a mass spectrometer equipped to do chemical ionization. Because we did not want to limit ourselves to the analysis of compounds for which IP's are available from the literature, we decided to use calculated IP's obtained by the MNDO method of Dewar and Thiel with geometric optimization (14).The
+
1-aminonaphthalene 1-methoxynaphthalene 1-naphthol aniline 2-methoxynaphthalene 2-naphthol 2-methylnaphthalene 1-methylnaphthalene fluorenone phenalene-1-one biphenyl 1-naphthaldehyde p-xylene o-xylene 1-nitronaphthalene 1-cyanonaphthalene phenol m-xylene 2-nitronaphthalene benzo[clcinnoline toluene chlorobenzene fluorobenzene l,4-difluorobenzene 1,2,4-trifluorobenzene 1,2,4,5-tetrafluorobenzene
1,2-difluorobenzene 1,3-difluorobenzene 1,3,5-trifluorobenzene 1,2,3,5-tetrafluorobenzene 1,2,3,4-tetrafluorobenzene
cyanobenzene pentafluorobenzene nitrobenzene hexafluorobenzene
lit." 7.38 7.72 7.77 7.86 7.87 7.88 7.97 7.98 8.20 8.36 8.37 8.43 8.46 8.58 8.60 8.61 8.61 8.63 8.67 8.69 8.85 9.09 9.23 9.26 9.34 9.36 9.40 9.42 9.51 9.56 9.58 9.70 9.77 9.93 9.99
IP, eV MNDO*
8.348 8.292 8.345 8.209 8.459 8.486 8.569 8.549 8.757 8.953 8.650 8.853 9.178 9.235 9.302 8.937 8.889 9.233 9.299 9.130 9.281 9.625 9.468 9.557 9.836 10.030 9.688 9.728 10.137 10.177 10.175 9.817 10.406 10.311 10.766
"Values taken from ref 19; IP of 1,2,4-trifluorobenzene taken from ref 21. *Calculated using geometric optimization. MNDO method has been previously demonstrated to yield calculated molecular parameters in accord with experimental results for PAH (23-25). Table I1 compares the calculated IP's for 35 substituted aromatic compounds to literature values. A high linear correlation coefficient was found (r = 0.97, N = 35). Hence, we conclude that the IP's calculated by the MNDO method are reliable for our work and elected to use these values over experimental ones. Naphthalenes. Table I11 summarizes the data for the mono-, di-, and trisubstituted naphthalenes. There are only two isomers of each group of monosubstituted naphthalenes. We could not resolve the monomethylnaphthalenes employing only (CE/CI)MS; however, the resolution of these species is easily accomplished by GC. The remaining groups of isomeric monosubstituted naphthalenes are easily distingished using (CE/CI)MS. In all casea, the isomer with the larger calculated I P had the larger (M H)+/M+ ratio. For example, the calculated IP for 2-naphthol is greater than the I P for 1naphthol by 0.141 eV, which is reflected by an increase of 0.317 ratio units. Likewise, 2-methoxynaphthalene has a calculated I P that is 0.167 eV larger than 1-methoxynaphthalene and has a correspondly higher (M + H)+/M+ ratio (0.255 ratio units). Notice that as the degree of methyl substitution is increased from zero (naphthalene) to three (2,3,6-trimethylnaphthalene) a corresponding decrease is seen in the (M + H)+/M+ratio. We attribute this to a combination of the decrease of the available sites for protonation and the lowering of the IP relative to naphthalene.
+
ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986
Table 111. IP and (M Naphthalenes
compound naphthalene 1-naphthol 2-naphthol l-methylnaphthalene 2-methylnaphthalene 2,3-dimethylnaphthalene 2,6-dimethylnaphthalene 2,3,6-trimethylnaphthalene l-naphthaldehyde 2-naphthaldehyde l-methoxynaphthalene 2-methoxynaphthalene
+
+ H)+/M+ Data for Substituted
Table IV. IP and (M H)+/M+ Data for Substituted Phenanthrenes and Anthracenes
ionization potential (eV) (M + mol w i explta MNDO H)+/M+b std de9 Nd 128 144 144 142
8.15 1.77 7.88 7.98
8.515 8.345 8.486 8.549
2.080 1.497 1.814 1.655
0.097 0.024 0.021 0.030
4 11
142
7.91
8.569
1.677
0.023
10
156
7.89
8.556
1.364
0.060
4
156
8.558
1.186
0.041
4
170
8.548
0.912
0.010
4
8.853
3.223
0.023
4
8.867
3.464
0.081
4
156
8.43
156
2125
20 4
158
7.72
8.292
1.182
0.036
4
158
7.87
8.459
1.437
0.028
4
“Values taken from ref 19. *All ratios corrected for natural abundance of carbon-13. Standard deviation of individual measurement. d Number of measurements.
Phenanthrenes and Anthracenes. Table IV summarizes the data for the substituted phenanthrenes and anthracenes. The addition of a methyl group lowers the (M + H)+/M+ ratio relative to the parent PAH for the reasons previously discussed. Particularly interesting is the case of both 9,lO-dimethylphenanthrene and 4,5-dimethylphenanthrene.These compounds yield much lower (M + H)+/M+ratios (0.382 and 0.545, respectively) than would be expected based on the calculated P.For the former, we think that the methyl groups block protonation at the K region where protonation would be expected because of the high electron density of this area. The low ratio of the 4,5-isomer also suggests that significant protonation occurs at this region of the molecule. Although we are not able to predictively distinguish all positionally substituted isomers, we can easily tell if a methyl-substituted PAH with a molecular weight of 192 is a methylanthracene or methylphenanthrene. This cannot be done with EIMS. Note that the methylanthracenes cluster in one group (with an average (M + H)+/M+ ratio of 0.59) while the methylphenanthrenes cluster in another group (with an average ratio of 1.31). This observation could be used in the petroleum industry where it is generally accepted to report only the total amount of structural isomers present (26). (CE/CI)MS provides additional data in a predictive manner. Amino polycyclic aromatic hydrocarbons (amino-PAH) have been cited as the principal microbial mutagens in coal liquids (3). Amino-PAH comprise about 1%of the weight of these liquids while they account for approximately 98% of the mutagenic activity. We have employed (CE/CI)MS for the analysis of two amino-PAH standards: 1-aminoanthracene and 9-aminophenanthrene. These compounds have lower ratios than their unsubstituted homologues because of the amino group’s electron-donating character. The (CE/CI)MS
compound anthracene phenanthrene g-methylanthracene l-methylanthracene 2-methylanthracene 3-methylphenanthrene 9-methylphenanthrene l-methylphenanthrene 2-methylphenanthrene 4,5-dimethylphenanthrene 1,8-dimethylphenanthrene 3,g-dimethylphenanthrene 9,lO-dimethylphenanthrene 1-aminoanthracene 9-aminophenanthrene
IP(MNDO), (M + H)+/M+‘ std devb Ne eV mol w t 178 178 192
8.049 8.479 8.026
0.661 1.518 0.540
0.030 0.072 0.012
57 68
192
8.032
0.602
0.011
4
192
8.049
0.627
0.021
4
192
8.462
1.258
0.005
3
192
8.463
1.231
0.036
4
192
8.465
1.364
0.052
4
192
8.486
1.406
0.023
3
206
8.417
0.545
0.019
3
206
8.436
0.971
0.030
3
206
8.439
1.193
0.021
3
206
8.441
0.382
0.012
3
193 193
7.952 8.331
0.468 0.617
0.002 0.004
2 2
4
a All ratios corrected for natural abundance of carbon-13. Standard deviation of an individual measurement. Number of measurements.
technique correctly predicts the ordering of ratios between these compounds (see Table IV). This illustrates that (CE/CI)MS may be suited for difficult to resolve mutagenic amino-PAH.
CONCLUSION This study examined substituent effeds in (CE/CI)MS. We have shown that the Hammett equation was valid under (CE/CI)MS conditions and that the (CE/CI)MS behavior is a function of ionization potential and steric factors for a series of multisubstituted methylated PAH and fluorinated benzenes. We have demonstrated that calculated IP’s agree well with experimental I P S for substituted compounds. We were able to predict the correct isomer in the absence of standard compounds for substituted naphthalenes, and we think this could be extrapolated to larger ring systems, particularly the highly mutagenic amino-PAH. Unfortunately, the nitro-PAH fragmented extensively, and we were not able to predict their (CE/CI)MS behavior. Finally, the method was evaluated as a potential predictive method for methylated-PAH. We were able to distinguish groups of methylated isomeric PAH arising from different parent PAH.
ACKNOWLEDGMENT We are grateful to Indiana University’s Bloomington Academic Computing Services for use of their facilities. We also thank Beth Stemmler for helpful discussions and Milos Novotny for furnishing some of the methylated-PAH used in this study. Registry No. Naphthalene, 91-20-3; 1-naphthol, 90-15-3; 2-naphthol, 135-19-3; 1-methylnaphthalene, 90-12-0; 2-methylnaphthalene, 91-57-6; 2,3-dimethylnaphthalene,581-40-8; 2,6dimethylnaphthalene, 581-42-0; 2,3,6-trimethylnaphthalene, 829-26-5; 1-naphthaldehyde, 66-77-3; 2-naphthaldehyde, 66-99-9; 1-methoxynaphthalene,2216-69-5; 2-methoxynaphthalene,93-04-9; anthracene, 120-12-7;phenanthrene, 85-01-8;9-methylanthracene,
Anal. Chem. 1986, 58,2126-2129
2126
779-02-2; 1-methylanthracene, 610-48-0; 2-methylanthracene, 613-12-7;%methylphenanthrene,832-71-3;9-methylphenanthrene, 883-20-5; 1-methylphenanthrene,832-69-9; 2-methylphenanthrene, 2531-84-2; 4,5-dimethylphenanthrene,3674-69-9; 1,8-dimethylphenanthrene, 7372-87-4; 3,6-dimethylphenanthrene,1576-67-6; 9,10-dimethylphenanthrene,604-83-1; 1-aminoanthracene,61049-1; 9-aminophenanthrene, 947-73-9; fluorobenzene, 462-06-6; l&difluorobenzene, 540-36-3; 1,2-difluorobenzene, 367-11-3; 1,3-difluorobenzene,372-18-9; 1,2,4-trifluorobenzene,367-23-7; 1,2,3-tritluorobenzene,1489-53-8;1,3,5trifluorobenzene,372-38-3; 1,2,4,5-tetrafluorobenzene, 327-54-8 1,2,3,5-tetrafluorobenzene, 2367-82-0; 1,2,3,4-tetrafluorobenzene,551-62-2; pentafluorobenzene, 363-72-4; hexafluorobenzene, 392-56-3; l-cyanonaphthalene, 86-53-3; 1-aminonaphthalene, 134-32-7; aniline, 62-53-3;fluorenone, 486-25-9; phenalen-1-one, 548-39-0;biphenyl, 92-52-4;p-xylene, 106-42-3;o-xylene, 95-47-6; 1-nitronaphthalene, 86-57-7;phenol, 108-952; m-xylene, 108-38-3;2-nitronaphthalene, 581-89-5; benzo[c]cinnoline, 230-17-1; toluene, 108-88-3;chlorobenzene, 108-90-7;cyanobenzene, 100-47-0;nitrobenzene, 98-95-3.
LITERATURE CITED Slmonsick, W. J., Jr.; Hltes, R. A. Anal. Chem. 1984, 56, 2749-2754. Hecht, S. S.; Loy. M.; Hoffman, D. I n Carclnogenesls-A Comprehensive Survey; Jones, R. W., Freudenthal, R. I., Ed.; Raven Press: New York, 1976; Vol. 1, p 325. Wllson, E. W.; Pelroy, R.; Cresto, J. T. Mutagen Res. 1980, 79, 193-202. Mermelstein, R.; Kirlazides, D. K.; Butler, M.; McCoy, E. C.; Rosenkranz, H. S. Mutagen Res. 1981, 89, 187-196. Jennings, K. R. I n Gas Phase Ion Chemistry; Bowers, M. T.. Ed.; Academlc Press: New York, 1979; Vol. 2, p 123. Keough, T. Anal. Chem. 1982, 5 4 , 2540-2547.
Hammett. L. P. J. Am. Chem. Soc. 1837, 59, 96-103. Lowry, T. H.; Richardson, K. S. In Medrenkrm and Theory In Organic Chemlstry; 2nd ed.;Harper and row: New York. 1981; pp 113-225. Bursey, M. M. Org. Mass Spectrom. 1988, 7 , 31-46. Field, F. H. J. Am. Chem. Soc. 1980, 9 7 , 6334-6341. Levsen, K. I n Fundamental Aspects of Organic Mass Spectrometry; Wehheim: New York. 1978 pp 19-21. Tembreull, R.; Sin, C. H.; Ping, L.; Pang, H. M.; Lubman, D. M. Anal. Chem. 1984, 5 7 , 1186-1192. Jensen, T.; Kaminsky, R.; McVeety, E. D.; Woznlak, T. J.; H b s , R. A. Anal. Chem. 1982, 5 4 , 2388-2390. Dewar, M. J. S.; Thlel, W. J. Am. Chem. Soc.1977, 99,4894-4907. Fletcher, R.; Powell, J. J. D. Comput. J. 1983, 6 , 163-166. Hunt, D. F.; Gale, P. J. Anal. Chem. 1984, 56, 1111-1114. Cox, J. D.; Piicher, G. I n Thermochemlstry of Organlc and Organometallic Compounds; Academic Press: London, 1970. Brown, H. C.; Okamoto, Y. J. Am. Chem. SOC. 1958, 80, 4979-4987. Levin, R. D.; Lkrs, S. 0.Natl. Stand. Ret. Data Ser. ( U S . Natl. Bur. Stand.) 1982, No. 7 1 . Lau, Y. K.; Kebarle, P. K. J. Am. them. SOC.1858, 98, 7452-7453. Rosenstock, H. M.; Herron, J. T.; Draxl, K.; Steiner, E. W. “Energetics of Gaseous Ions”, J. Phys. Chem. Ref. Data Suppl. 1877, 6 , 1365. Hltes, R. A. Handbook of Mass Spectra of Envlronmental Contamlnants; CRC Press: Boca, Raton, FL, 1985; pp 84, 85, 188, 189. Dewar, M. J. S.;Thlel, W. J. Am. Chem. Soc.1977, 99,4907-4917. Ford, G. P.; Scribner, J. D. J. Am. Chem. SOC. 1981, 703, 428 1-4291. Mirek, J.; Buda, A. 2.Naturforsch ., A 1984, 39a. 386-390. White, C. M. I n Handbook of Po&cycllc Aromatic Hydrocarbons, Ed., Bjorseth, A,, Ed.; Marcel Dekker: New York, 1963; pp 525-616.
RECEIVED for review February 4,
1986. Accepted April 16, 1986. Supported by the U.S. Department of Energy (Grant NO. 80EV-10449).
Application of Particle Desorption Mass Spectrometry to the Characterization of Minerals W. R. Summers and E. A. Schweikert* Center for Chemical Characterization & Analysis, Texas A&M University, College Station, Texas 77843-3144
Four m h r a l specknerw were analyzed by parUcle desorption mass spectrometry using callfomlum-252 as the prbnary ion source. Poilucite, amblygonlte, mkrocllne, and IepMdlte, chosen for thek high alkatl metal content, were characterized wkh no apparent decrease In the extractlon fleld strength were thick and M a t l n g . Further, even though the samglven the very low bombarding ion thence, no complkatlons due to charge accurnrlatkn on the sample were encountered. Ouatitatlve resuits 0Wahed by PDMS were valklated by XPS, EYP, and SIMS. PDMS provldes for a rapkl, nondestructive determination of alkall metals In complex natural matrices wlth minimal sample preparation.
Geological materials represent a challenging case for surface characterization methods that employ charged particles as the probe or signal beam, due to their heterogeneity and insulating nature. Techniques currently applied in the analysis of geological specimens include SIMS, LAMMA, and especially EMP (I). Each of these techniques has strengths and limitations. EMP can provide quantitative data for elements of 2 2 11and with detection limits 20.1 atom % (2,3).SIMS and LAMMA both can provide isotopic and molecular information with detection limits as low as lo4 atom % (4).For geological specimens, SIMS is semiquantitative because the 0003-2700/86/0358-2126$01.50/0
yield of the secondary ions emitted is strongly matrix dependent (5). LAMMA is less prone to charge buildup problems since the probe beam consists of photons and the signal beam is of extremely low current; however, the sample spot addressed is destroyed in the analysis process (6-8). The present study examines the feasibility of applying particle desorption mass spectrometry for examining geological specimens. The method is based on the desorption of atomic and molecular species from surfaces bombarded by fast heavy ions (21 20; E 1 0.5 MeV/amu). This phenomenon was first observed by Macfarlane (9). Particle-induced desorption coupled with time-of-flight mass spectrometry (usually referred to as PDMS) has been applied successfully for the characterization of large involatile biomolecules (10,I1 1. In the realm of materials characterization, the application of PDMS in a microscopic mode has recently been discussed (12). The present paper presents the first results of PDMS applied on geological materials. The strong suits of PDMS are (a) the sample amount consumed is negligible and (b) the primary ion current required for PDMS is low; thus charge buildup and sample damage are avoided. This study like most PDMS work to date used as the beam of heavy ions the fission fragments from a low-intensity 252Cf source. The discrete nature (in time) of the fission event lends itself conveniently to time-of-flight mass spectrometry. This 0 1986 American Chemical Society
