Anal. Chem. 1888, 58, 2114-2121
2114
Characterization of High Molecular Weight Polycyclic Aromatic Hydrocarbons by 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 exchangr ch.mlcal kniutkn mam spdromolry (CE/CI)MShas k.n usod 19 analyze hlgh mokculrrr wdghl
Char*
Pdycyck aromatk hydmarbom (PAH) W a t d from a carbon black. Thermally dable capllMv cokmm were prepared to sqmrat. theso brg. PAH. Th. (WC1)MS workhg range waa expanded by Incncrring the Ion source pressure. Reliable k n l t a k n pdenUab (IP) were obtdrml by semiempklcal molecular or#lal c a l c u l wlng ~ the MNDO method. On the bad8 of the ratb of the (M H ) + h to the M+ kn andonthedadatod IP,
[email protected] were p d k t i ~ d ydbtin@hd. T ~ (CEIC1)MS o tWlwrlqW, CWpbdwm-ac, polwlmdulem~ ofseverd lsomerlc PAH as posslbk components In the extract. The method ako revealed that many komen presenl in a carbon black have not been documented In the Uterature.
+
Polycyclic aromatic hydrocarbons (PAH) currently represent the largest class of chemical carcinogens known to man. Because of their carcinogenic behavior PAH have been the subject of numerous research investigations (1,2). The majority of these studies have concentrated on PAH containing from two to six rings although the existence of higher molecular weight (>300)carcinogenic PAH have been documented (3). Limited solubilities, low vapor pressures, the absence of standard references compounds, and the lack of analytical techniques that allow isomer elucidation are the reasons for the scarcity of information on large PAH. McKay used gel permeation chromatography coupled with fluorescence spectroscopy detection to identify large PAH in petroleum distillates (4). PAH of molecular weights 300-402 were detected in airborne particulate matter by GC/MS (5). These high molecular weight PAH were characterized by electron impact mass spectrometry, which does not distinguish among isomeric PAH. High-performance liquid chromatography (HPLC) (6) and more recently microcolumn (capillary) LC (7,8)have been the most succe88ful methods for large PAH analysis. Unfortunately, unless one has standard compounds or reference spectra available, only molecular weights (5)or representative structures for an isomeric group typically are measurable (7, 8). This is unfortunate because the exact structure is important since only certain structural isomers are carcinogenic. Charge exchange chemical ionization mass spectrometry (CE/CI)MS was sucessfullyemployed to differentiate isomeric PAH in the absence of standard compounds (9). This is of particular interest for high molecular weight PAH of which there are few commercially available standards. Recent advancea in the preparation of thermally stable capillary column stationary phases have extended the upper temperature limit for gas chromatography (10, 11). Cross-linked phases are stable to operating temperatures of 400 O C , which facilitate 'Current address: E. I. du Pont de Nemours & Co., Marshall R&D Laboratory, Philadelphia, P A 19146. 0003-2700/86/0358-2114$01.50/0
the analysis of PAH possessing molecular weights in excess of 400 (11). We have prepared these columns in our laboratory and coupled them to a mass spectrometer operated in the CE/CI mode to be able to analyze high molecular weight PAH with isomeric specificity. Under (CE/CI)MS conditions, (M H)+ ions of PAH are formed by proton transfer with C2H5+,and M+ ions are generated by charge exchange with Ar+; these reagent ions are generated by using a mixture of argon and methane in an ion source operating at about 1torr. On the basis of the ratio of the MH+ to M+ ions [(M + H)+/M+ ratio] and on the measured first ionization potential (IP), isomeric PAH are predictively differentiated (9). Unfortunately, experimental IPS are not available for many four to six ring PAH and for most greater than seven ring PAH. Therefore, we have selected a semiempiricalmethod, modified neglect of diatomic overlap (MNDO) (12),for the calculation of IP's. The MNDO method has been previously demonstrated to yield calculated molecular parameters in accord with experimental results for PAH (13-15).
+
EXPERIMENTAL SECTION Chemicals and Reagents. Most of the PAH used in this study were obtained in pure form from Analabs (New Haven, CT). Several of the PAH that are not commercially available were furnished by M. L. Lee and G. R. West (Brigham Young University, Provo, UT) and Chevron research (16).Methylene chloride (MCB, Cincinnati, OH) solutions containing approximately 20 ng/rL of each compound were prepared. The high molecular weight PAH required slow heating of the solution to solubilize these chemicals. If we were unable to dissolve these compounds in warm methylene chloride, the PAH were dissolved in hot anisole (MCB, Cincinnati, OH). A 10-g aliquot of 260 nm particle size carbon black (Cabot Corp., Boston, MA) was Soxhlet extracted for 24 h with 400 mL of methylene chloride. The extract was reduced to a volume of approximately 5 mL and analyzed directly by (CE/CI)MS. Molecular Orbital Calculations. All MO computations were carried out by use of an enlarged version of Dewar and Thiel's MNDO prcgram (12) operating on Indiana University's CDC Cyber 170/855. The program was modified in order to accommodate PAH with molecular weights in excess of 400. Several arrays were enlarged to facilitate these calculationsaccording to the method described by Thiel (17). At the start of the calculation, the PAH molecules were assumed to be planar with uniform bond lengths and anglea. The aromatic C-C bonds were input as 1.4 8, and the C-H bonds were 1.1 A. All planar bond anglea were 120" except the five-membered rings where a value of 105" was used. The initial geometry was optimized by minimizing the total energy with respect to the planar bond lengths and angles using the Davidson-Fletcher-Powell (DFP) algorithm, which is incorporated into the MNDO program (18).
Capillary Column Preparation. Thermally stable capillary columns were prepared in the following manner: Approximately 50 m of new 0.25 mm i.d. fused silica capillary tubing (Spectran Corp., Surbridge, MA) was purged overnight with helium at 100 "C. The tubing was then rinsed with 10 mL of filtered (0.47wm) methylene chloride. The stationary phase employed was a methylphenylpolysiloxane containing 70% phenyl and 4% vinyl 0 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986
groups; it was obtained from M. L. Lee and G. W. West (Brigham Young University, Provo, UT). A small amount (0.2 mg) of the stationary phase was dissolved in equal portions of methylene chloride and n-pentane (10 mL each) and subsequently filtered (0.2 pm). The capillary tubing was then filled with the solution and a vacuum pulled to remove the solvent. The stationary phase (0.12-pm film thickenss) was cross-linked by purging the column with azo-t-butane(Alfa Products, Danvers, MA). Once the column was saturated with azo-t-butane vapors both ends were sealed. The column was then placed in a GC oven and heated from 40 O C to 225 O C at 4 deg/min and held at 225 O C for 1h. The column was then washed with 10 mL of methylene chloride and purged with helium. Conditioning of the column proceeded in the following manner: The column was installed in a GC oven that had been modified to accommodate a nitrogen purge line. A low flow of helium (about 1mL/min) was passed through the column as it was heated from 100 O C up to 400 "C at 4 OC/min. The final temperature was held for 12 h under a low purge of nitrogen. The column was evaluated by use of n-alkane standards. All GC retention data for PAH were acquired on a Carlo Erba 4160 gas chromatograph (Carlo Erba Strumentazione, Milan, Italy) equipped with an on-column injector and an FID detector. Retention index measurements of standards were accomplished by on-column injection of approximately 0.1-0.2 pL solutions of each PAH. Hydrogen was employed as the carrier gas at a head pressure of 2 kg/cm2. The GC was temperature programmed from 80 "C to 400 OC at 4 OC/min. The FID signal was output to a Hewlett-Packard 3392A integrator (Palo Alto, CA) from which retention indexes were calculated. Mass Spectrometry. The (CE/CI)MS epxeriments were all performed on a Hewlett Packard 5985B GC/MS system. The reagent gas, a 15% mixture of methane in argon (Linde Co., New York), was introduced to the ion source through a transfer line (19)that had been further modified to facilitate capillary column interchange. The ion source pressure was measured by a Baratron capacitance manometer (MKS Instruments, Burlington, MA) mounted outside the manifold housing. A stainless steel tube extending from the ion source block through a vacuum lock into the manometer provided the path for an accurate and rapid ion source pressure measurement. The ion source pressure was set with an automatic pressure controller (Granville Phillips Co., Boulder, CO) at approximately 0.8 torr. The CI gas flow was adjusted to achieve a (M + H)+/M+ratio of 1.5 for 1,2,3,5tetrafluorobenzene (TFB) (Aldrich, Milwaukee, WI). The ion source temperature was set at 250 O C and measured by a thermocouple located in the ion source body. The mass spectrometer was tuned prior to each set of experiments using binary mixtures of perfluorotributylamine (PFTBA) (PCR Research Chemicals, Inc., Gainesville, FL) and TFB. A complete mass spectrum (100-500 amu) was then obtained for each compound. All subsequent mass spectral data were collected by use of selected ion monitoring. A dwell time of 10 ms was used. The software examined the complete M+ and (M + H)+ mass spectral peak profiles of each compound analyzed. This was accomplished by stepping across the M+ and (M + H)+ peaks at 0.1 dalton intervals followed by integration h0.2 amu from the peak maxima. All (M + H)+/M+ratios were corrected for the natural abundance of carbon-13. RESULTS AND DISCUSSION Validity of the MNDO Method for IP Calculations of PAH. Semiempirical molecular orbital calculations can provide accurate and reliable information where experimental data are not available. The MNDO method (12)with optimization of geometry has been used in this laboratory to calculate the first ionization potential (IP) of over 200 PAH. The IP calculations with optimization of planar geometry take at least an order of magnitude more computer time than a single point calculation (12).Nevertheless, we chose to optimize the planar geometry because we found that the calculated IP's obtained by geometric optimization were significantly better (p < 0.01) when compared to experimental values (r2 = 0.98, N = 8) than those obtained from a singlepoint calculation (r2 = 0.90, N = 8). The calculated IP's, in
2115
most cases, were higher than experimental values (see Table I), but an excellent linear correlation was observed (1.2 = 0.959, n = 60).We concluded that the IP's calculated by the MNDO method are reliable for our work, and we elected to use these values even when experimental values were available. Ion Source Pressure. As the PAH increase in size, the difference in ionization potentials between isomers decreases because larger r systems permit increased delocalization of the charge on radical ions (21). Therefore when larger PAH are analyzed, it is important to have a large change in (M + H)+/M+ for small changes in IP. From our earlier work (91, it was found that lowering the ion source temperature, increasing the percent methane in argon, or raising the ion source pressure caused larger (M H)+/M+differences for small IP changes. Because the ion source pressure has the most pronounced effect on the (M + H)+/M+ratios of PAH (9) and because it is the easiest parameter to vary, we chose to increase the ion source pressure in order to expand the (CE/CI)MS working range. The working range of (CE/CI)MS was increased by adjusting the flow of reagent gas to achieve an (M + H)+/M+ of 1.5 for our internal standard, TFB. With conditions adjusted to TFB approximately 1.0 (9), naphthalene and anthanthrene differed by 1.8 ratio units, but at TFB = 1.5 this difference is expanded to 3.1 ratio units. The precision for the (M H)+/M+ ratios obtained at the higher source pressure is quite good, averaging about 3% relative standard deviation (see Table 11). Correlations with Calculated Molecular Parameters. The experimental (M + H)+/M+ ratios of PAH seem to correlate well with the experimental ionization potentials (IP) (9). Therefore, a high correlation is expected between the MNDO IP's and the experimental (M + H)+/M+ ratios because of the agreement between the experimental and calculated IP's. However, before use of the MNDO IP as the definitive predictor of the (M + H)+/M+ratio, other possible predictors were investigated. In addition to the first IP, the MNDO program gave us the energies of the frontier orbitals, the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital), which are important properties of PAH. Although a statistically significant (p < 0.01) correlation exists (r2 = 0.75, N = 53) between the LUMO energy and the (M + H)+/M+ratios obtained at TFB = 1.5, the LUMO energy is not a better predictor of the (M + H)+/M+ behavior than the IP. Combining the IP and LUMO energy data for PAH and regressing on the (M H)+/M+ ratios did not yield a better correlation than that obtained by using only the IP. Clar and Schmidt found that the second IP (IP,) was a better indicator of chemical reactivity than only the first IP (22).Thus, we investigated the possibility that either IP2or the difference between the first and second IP's could be used to predict the (CE/CI)MS response of PAH; unfortunately, no correlation was observed. Additionally, we observed no higher correlation when we classified PAH according to structural type. We found the best predictor of (M + H)+/M+ behavior is simply the MNDO calculated IP. Therefore, (CE/CI)MS allows one to predictively assign specific structures to isomeric PAH based on a calculated IP and an experimentally determined (M + H)+/M+ ratio. Table I1 lists the calculated MNDO IP's, the (M + H)+/M+ ratios acquired at TFB = 1.5 and the experimental error for 53 PAH studied in this laboratory. It is obvious that an increase in the IP will be reflected by a corresponding increase in the (M + H)+/M+ratio. The equation that best relates the (M + H)+/M+ ratio and the calculated IP is
+
+
+
log [(M + H)+/M+] = 15.64 log [IP(MNDO)] - 14.16
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986 ~~
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Table I. Comparison between MNDO Calculated IP and Experimental Values mol compound
wt
benzene acenaphthylene indene naphthalene fluoranthene fluorene phenanthrene triphenylene acenaphthene benzo[c]phenanthrene chrysene picene dibenzo[c,g]phenanthrene benz[a]anthracene pyrene azulene anthracene benzo[b]triphenylene
78 152 116 128 202 166 178 228 154 228 228 278 278 228 202 128 178 278 302 302 278 328 278 300 328 278 328 278 328 328
dibenzo[de,qr]naphthacene dibenzoVg,op]naphthacene dibenz[a jlanthracene naphtho[l,2-b]triphenylene dibenzo[a,h]anthracene coronene benzo[h]pentaphene pentaphene benzo[c]picene benzo[b]chrysene dibenzok,p]chrysene naphtho[ 1,2-b] chrysene
ionization potential, eV exptln calcdb 9.24 8.22 8.15 8.15 7.95 7.91 7.90 7.87 7.79 7.61 7.60 7.52 7.50 7.46 7.43 7.43 7.42 7.41 7.41 7.40 7.40 7.38 7.38 7.36 7.33 7.29 7.28 7.23 7.19 7.19
9.386 8.778 8.759 8.575 8.466 8.609 8.479 8.505 8.525 8.327 8.261 8.239 8.243 8.112 8.029 8.072 8.049 8.172 8.163 8.198 8.175 8.222 8.148 8.077 8.184 8.116 8.149 7.971 8.161 8.044
ionization potential, eV exptl" calcdb
mol compound
wt
benzo[b]picene benzo[c]pentaphene benzo[ghi]perylene naphtho[ 2,3-g]chrysene dibenzo[b,pqr]perylene naphtho[ 1,2,3,4-rst]pentaphene benzo[a]coronene benzo[rst]pentaphene benzo[a]naphthacene tetracene dibenzo[a jlnaphthacene benzo[p]naphtho[1,8,7-ghi]chrysene dibenzo[bghilperylene dibenzo[b,k]chrysene dibenzo[a,c]naphthacene hexaphene naphtho[ 1,2,3,4-ghi]perylene benzo[pqr] naphtho[ 8,1,2-bcd]perylene anthanthrene benzo[b]perylene naphtho[8,1,2-bcd]perylene dibenzo[cd,lm]perylene benzo[a]perylene pentacene dibenzo[a,l]pentacene benzo[a]pentacene dibenzo[bc,eflcoronene dibenzo[bc,kl]coronene hexacene phenanthro[ 1,10,9,8-opqra]perylene
328 328 276 328 326 352 350 302 278 228 328 352 326 328 328 328 326 350 276 302 326 326 302 278 378 328 374 374 328 350
7.18 7.17 7.16 7.15 7.12 7.09 7.08 7.07 7.06 7.01 7.00 7.00 6.99 6.98 6.97 6.97 6.96 6.92 6.92 6.89 6.82 6.74 6.71 6.66 6.66 6.66 6.50 6.42 6.40 6.30
8.009 8.066 7.944 8.051 7.959 7.869 7.936 7.817 7.797 7.722 7.861 7.940 7.904 7.831 7.846 7.784 7.803 7.699 7.621 7.826 7.621 7.571 7.567 7.496 7.634 7.565 7.417 7.230 7.201 7.251
"All values taken from ref 20. bCalculated by MNDO method with optimization of planar geometry (12).
This equation gives a very high correlation between t h e MNDO IP and the ( M + H ) + / M + ratio obtained at TFB = 1.5 (r2= 0.959, N = 53). Therefore, we conclude that m a n y authentic standards are not necessary t o characterize t h e ( C E / C I ) M S behavior of isomeric PAH, and we can reliably predict the (M + H ) + / M + response of isomeric PAH using the MNDO IP. Application to a Carbon Black Extract. The successful preparation of thermally stable capillary GC columns will facilitate the separation of large PAH, and (CE/CI)MS detection will permit the differentiation of high molecular weight PAH. T h e following paragraphs present an example of this approach. The demonstration sample we selected was an experimental, 260-nm carbon black from Cabot Corp. W e selected this particular sample for our investigations because this sample is well-known for its high concentration of high molecular weight, adsorbed PAH and because i t has been the subject of several other studies (6, 7,23). Recently, microcolumn LC was used to separate the large PAH in this sample; unfortunately, no positive identification of a n y of the constituents was made (7). In an earlier investigation of t h e same mixture, HPLC was used t o separate m a n y large PAH and detection was accomplished b y electron impact MS and by spectrofluorometry (6). T w o C28H14PAH, benzo[cd]naphtho[3,2,1,8-pqr,a]perylene [350(4)] and benzo[pqr]naphtho[8,1,2-bcd]perylene [350(6)], were positively identified as components in the extract by comparison t o known UV spectra (6). However, only tentative identifications of t h e remaining Cg8H14isomers and for two C30H14 isomers were suggested based on molecular weight information (6). Figure 1 shows a portion of the total ion chromatogram of the carbon black extract run under CE/CI conditions adjusted such that TFB = 1.5, and T a b l e I11 gives t h e data for each H ) + / M + ratios for P A H of peak. The experimental (M
+
m/z
-
350 n
r-•
I
m/r
-
374
iiJ
A
A
c 62
63
65
64
66
6)
*-. 6'0
n
/LA--. 6'9
70
7'1
Time (min)
320
340 Tomporoture
360 (e
380
C)
Figure 1. Partial (CE/CI)MS total Ion chromatogram of hlgh molecular weight PAH isolated from a carbon black. MS condfons were adjusted to TFB = 1.5 (seetext). GC conditions were as follows: 20 m X 0.25 mm i.d., 0.12 mm film thickness (methylphenylpolysiloxane phase), helium carrier gas, 11 psi head pressure.
molecular weights 350 and 374 found in this carbon black extract span a range of 0.637 ratio units as compared t o only 0.383 ratio units when working such that TFB = 1.0. Most of the isomeric C28H14 and C30H14 PAH are not commercially available; although some large PAH compounds have been synthesized. Our laboratory has recently acquired four of these large PAH, and Table IV summarizes t h e analytical information of importance for these compounds. Figure 2 lists t h e structures of all the isomeric C%H14( m / z 350) PAH for which the IP's were calculated in this laboratory. T h e eight PAH containing only fused six-membered rings (PAHG) [350(1-8)] represent all t h e possible PAHG structures
ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986
Table 11. Selected PAH with Their Calculated Ionization Potentials and Their Measured (M
(M + H)+/M' mol
IP
compound
wt
(MNDO)"
ratiob TFB = 1.5
dibenzo[bc,eflcoronene tribenzo[de,kl,rst]pentaphene pyranthene dibenzo[cd,lm] perylene anthanthrene naphtho [8,1,2-bcdlperylene dibenzo[a,l]pentacene dibenzo[ b,deflchrysene naphtho[2,1,8-qra]naphthacene benzo[pqr]naphtho[8,1,2-b~d]perylene naphth0[8,1,2-~bc]coronene tetracene benzo[a]naphthacene dibenzo[h,rs tlpentaphene naphtho[ 1,2,3,4-ghi]perylene benzo[rst] pentaphene benzo[blperylene benzo[a]pyrene per y1ene diindeno[ 1,2,3-c&1',2',3'- Im]per ylene benzo[a]coronene naphtho[ 1,2,3,4-deflchrysene
374 376
7.417 7.452
0.316 0.250
376 326 276 326
7.544 7.571 7.621 7.621
0.410 0.327 0.416 0.476
378 302 302
7.634 7.644 7.689
0.434 0.440 0.472
350
7.699
0.497
374
7.709
0.511
228 278 352
7.722 7.797 7.799
0.537 0.622 0.719
326
7.803
0.631
302 302 252 252 400
7.817 7.826 7.830 7.846 7.874
0.570 0.515 0.683 0.497 0.632
350 302
7.936 7.943
0.698 0.820
mol
Std
deV N
compound
0.020 2 benzo[ghi]perylene 0.013 2 dibenzo[bg]phenanthrene indeno[ 1,2,3-cd]pyrene 0.018 4 dibenzo[a,e]fluoranthene 0.005 4 pw=e 0.018 6 anthracene 0.022 4 coronene benz[a]anthracene 0.035 2 pentaphene 0.023 7 benzo[e]pyrene 0.010 3 cyclopenta[cd] pyrene dibenz[a,h]anthracene 0.032 4 benzo[k]fluoranthene benzo[b]triphenylene benzo v] fluoranthene 0.014 7 picene chrysene 0.024 8 benzo[a] fluorene 0.042 4 benzo[c]phenanthrene 0.022 3 benzo[blfluorene benzo[b]fluoranthene 0.039 5 4H-cyclopenta[deflphenanthrene 0.026 6 fluoranthene 0.001 3 phenanthrene 0.043 6 triphenylene 0.005 7 benzo[ghi]fluoranthene 0.042 3 naphthalene fluorene 0.020 5 indene 0.034 7 acenaphthylene benzene
wt
2117
+ H)+/M+Ratios
(M + H)+/Mt ratiob std (MNDO)" TFB = 1.5 deV N
IP
276 278 276 302 202 178 300 228 278 252 226 278 252 278 252 278 228 216 228 216 252 190
7.944 8.001 8.024 8.028 8.029 8.049 8.077 8.112 8.116 8.137 8.145 8.148 8.167 8.172 8.207 8.239 8.261 8.309 8.327 8.390 8.410 8.436
0.683 0.832 0.932 1.104 1.019 1.042 0.832 1.227 1.300 1.225 1.417 1.288 1.435 1.587 1.274 2.062 2.035 1.571 1.855 1.984 2.396 2.243
0.027 0.018 0.017 0.018 0.048 0.031 0.016 0.047 0.049 0.034 0.048 0.052 0.039 0.037 0.018 0.051 0.072 0.049 0.048 0.081 0.057 0.066
8 5 8 4 8 8 8 8 4 7 6 7 8 7 6 6 8 7 7 6 6 7
202 178 228 226 128 166 116 152 78
8.466 8.479 8.505 8.507 8.575 8.609 8.759 8.778 9.386
2.654 2.671 2.777 2.617 3.467 2.447 3.403 3.023 7.596
0.082 0.053 0.079 0.032 0.155 0.057 0.042 0.063 0.154
7 8 8 5 5 5 4 7 3
"Obtained using MNDO with planar optimization of geometry. *All ratios are corrected for natural abundance of carbon-13. The ratios were measured such that the internal standard, 1,2,3,5-tetrafluorobenzene(TFB), gave an (M + H)+/M+ ratio of 1.5 (TFB = 1.5). cStandard deviation of an individual measurement.
Table 111. Summary of Results for Isomer CzsHlland C,H,, PAH Isolated from 260-nmCarbon Black Extract" w
. 1
I
mol wt 350 350 350 350 350 350 350 350 350
retention indexc 616.70 f 0.71 (3) 624.57 f 0.44 (9) 627.60 0.49 (9) 629.26 f 0.56 (4) 631.10 f 0.62 (3) 635.22 f 0.53 (7) 637.46 f 0.63 (7) 639.29 f 0.70 (7) 641.63 f 0.64 (8)
(M + H)+/M+d 1.025 f 0.097 (3) 0.832 0.037 (5) 0.753 f 0.028 (5) 0.708 f 0.029 (5) 0.470 f 0.007 (5) 0.720 f 0.034 (5) 0.388 f 0.022 (5) 0.593 f 0.012 (5) 0.431 f 0.018 (5)
J K
374 374
663.56 f 0.59 (9) 678.08 f 0.78 (9)
0.917 0.019 (5) 0.521 f 0.027 (5)
peakb A B
C C
E F
G H
*
*
*
" Numbers in parentheses refer to number of determinations. bSee Figure 1. CRetentionindex based on extrapolation of chrysene = 400.00 and picene = 500.00 interval. dConditions adjusted to (M + H)+/M+ of TFB = 1.5; all ratios corrected for natural abundance of carbon-13. possessing the molecular formula CZ8Hl4as defined by Dias (24). We used the MNDO method and computed the IP's of
two additional PAH [350(%10)] which were listed in Chemical Abstracts. One of these compounds, diindeno[ 1,2,3cd:1',2',3'-jk]pyrene [350(9)] was reported, although not confirmed, to be present in this carbon black (6). The following discussion illustrates how the (CE/CI)MS method is used for the analysis of high molecular weight PAH in complex mixtures. Figure 3 displays the (M + H)+/M+ ratios (from Table 111)obtained from the (CE/CI)MS analysis of the CZ8Hl4( m / t 350) isomers found in the carbon black. The experimental (M + H)+/M+ratios are designated by the lettered vertical lines (corresponding to peaks A to I) which overlay the range bars that represent the 90% confidence limits of the (M + H)+/M+ratios estimated using the above equation. The two standards that we possess, benzobqrlnaphtho [ 8,1,2-bcd]perylene [350(6)] and benzo [a]coronene [350(8)], are designated by asterisks. Inspection of this figure reveals that neither phenanthro[1,10,9,8~opqralperylene [350(1)1, perinaphthacenonaphthacene [350(2)],benzo[cd]naphtho[3,2,1,8-pqra]perylene [350(3)], or acenaphtho[ 1,2-k]cyclopenta[cd] fluranthene [350(10)]are constituents of this carbon black because no (M + H)+/M+ (vertical bar) is within the 90% confidence limits
Table IV. GC Retention Data and (CE/CI)MS Behavior of Four High Molecular Weight PAH Standards" compound
compd no
GC retention indexb
(M + H)+/M+,
benzo[a]coronene benzo[pqr]naphtho[8,1,2-bcd]perylene naphth0[8,1,2-a bclcoronene dibenzo [bc,eflcoronene
350(8) 350(6) 374(4) 374(3)
635.16 f 0.27 (4) 638.91 f 0.49 (3) 678.60 f 0.44 (4) 683.62 f 0.17 (3)
0.689 f 0.020 (5) 0.497 f 0.032 (4) 0.511 f 0.014 (5) 0.316 0.020 (4)
*
"Numbers in parentheses refer to number of determinations. *Calculated by extrapolation of chrysene (RI = 400.00) to picene (RI = 500.00) interval. 'Conditions adiusted such that TFB = 1.5: all ratios corrected for natural abundance of carbon-13.
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986
@Q /
\
I
/
\
/
PhnnthmCI.lO,9.8-oalwta~hm C a p o u n d designation I P (MNOO) * 7.374 eV
M+l/M (calcl
-
- I50(3l
0.257
~ro[cd]ngktko[3.2.1 .8-pgralprylrw Canpound deslpnation I P (MOO) ' 7.610 cV M + l / M i c d l c i * 0.421
~
350(4)
Figwe 3. Range chart illustrating the resolving power of the (CE/CI)MS technique for the cornpounds given in Figure 2 wtth the experimental (M H)+/M+ ratios of PAH from the carbon black superimposed. The letters refer to the peaks in Figure 1.
+
Colnpound deslgnation
IP ( W W ) * 7.658 ev W*llN (calc) = 0.465
- 35015)
@ 0 \
\
I
Capound designation I P (MOO) = 7.936 eV
Dilndw[l.2,3-cd:l'.2'.3'-j~l~~ CnpDund designition IP (MOO) * 8.019 CY U + l l M (talc) * 0.958
- 350191
. 150(8)
~cMphtko[l.2-k]Cyclopnt.[cd]flua~Ulm. Canpound deslpnation IP ()(WOO] = 8.188 ev (calc) * 1.326
- 350(10)
M+11M
Flgure 2. Structures of isomeric C2&I14PAH (mol wi 350) studied in this laboratory.
for these PAH. These compounds were not found in this mixture in an earlier investigation either (6). Peak A gave an (M + H)+/M+ of 1.025. This value falls within the 90% confidence limits (Figure 3) of diindeno[1,2,3-~&1',2',3/-jk]pyrene[350(9)],which has been suggested, but not confirmed, to be in this mixture (6). From the compounds available in our data set, we concluded that peak A was diindeno[I,2,3-~d:I',2',3'-jk]pyrene [350(9)1. Peak B gave an (M H)+/M+ ratio of 0.832. This falls within the 90% confidence limits (Figure 3) of both phenanthro[5,4,3,2-abcde]perylene[350(7)] and benzo[a]coronene
+
[350(8)]. Fortunately, we possess a standard of benzo[a]coronene, and we can eliminate this as a possiblity because its GC retention index 635.16 f 0.27 is significantly different from the index for peak B, 624.57 f 0.44. Therefore, in conjunction with a standard, the CE/CImodel predicts that peak B (Figure 3) is phenanthro[5,4,3,2-~bcde]peryIene [350(7)]. This PAH was previously suggested, although not confirmed, to be a component in this carbon black (6). Peak C gave an (M + H)+/M+ratio of 0.753, which also falls within the 90% confidence interval of both phenanthro[5,4,3,2-abcde]perylene [350(7)] and benzo[a]coronene [350(8)]. We rejected benzo[a]coronene [350(8)] as a possibility because its retention index was different (635.16 f 0.27) than the index for peak C (627.60 f 0.49). Hence, the CE/CI model predicts that peak C (Figure 3) is also phenanthro[5,4,3,2-abcde]perylene [350(7)]. This inconsistency will be explained later. Peak D yields an (M + H)+/M+of 0.708 which also suggests that this peak is benzo[a]coronene [350(8)] or phenanthro[5,4,3,2-abcde]perylene [350(7)]. Once again benzo[a]coronene may be eliminated as a candidate using retention index measurements, 629.26 f 0.56 (Table 111) vs. 635.16 f 0.27 (Table IV). Thus, the model predicts peak D as being phenanthr0[5,4,3,2-~bcde]perylene[350(7)]. The reason for this impossible prediction is as follows: We have limited out data set of CBHI4PAH to only those listed in Chemical Abstracts, and we have concentrated primarily on PAHG compounds because their occurrence in the environment is more prevalent than PAH containing five-membered rings. The number of PAHG isomers for a given molecular formula has been fully enumerated (25-27) whereas PAH containing five-membered rings have not. This is no trivial task. Dias predicts that the maximum number of five-membered rings that a PAH of molecular formula CBH14 can possess is six (27). When one considers all the possible PAH structures of molecular formula CsHl, containing from one to six pentagonal rings, the number of possible structures becomes overwhelming. Even when considering PAH containing only one or two pentagonal rings, the situation is still a complicated one. Therefore, we have performed molecular orbital calculations only on compounds that were found in Chemical Abstracts. Since we have not calculated (M +
ANALYTICAL CHEMISTRY. VOL. 58. NO. 11. SEPTEMBER 1986
H)+/M+ ratios for all isomers, some GC peak may not have a match with our model. Thus, of peaks B, C, and D, which all seem to be compound 350(7), at least two are compounds for which we have not calculated (M H)+/M+ ratios. Peak E gave an (M H)+/M+ ratio of 0.470. This falls within the confidence limits of three CBH14 PAH, namely, benzo[cd]naphtho[3,2,1,8-pqr,a]perylene[350(4)], benzo[lmn]naphtho[2,1,8-qr,a]perylene [350(5)],and benzo[pqr]naphtho[8,1,2-bcd]perylene[350(6)]. We can eliminate benzo[pqr]naphtho[8,1,2-bcd]perylene[350(6)] as a possiblity because we have a standard of this compound, and ita retention index is 638.91 f 0.49 as compared to 631.10 f 0.62 for peak E. Therefore, peak E is predicted to be either benzo[cd]naphtho[3,2,1,8-pqra]perylene[350(4)] or benzo[lmn]naphtho[ 2,1,8-gra]perylene [350(5)]. Peak F was confirmed to be benzo[a]coronene [350(8)]. The retention index for the standard was 635.16 f 0.27, and for peak F, a value of 635.22 f 0.53 was measured. The experimental (M H)+/M+ratio for the authentic compound was 0.689 f 0.020 while that of peak F was 0.720 f 0.034. This isomer has been suggested to be in this mixture by Peaden et al. (6). Peak G gave an (M H)+/M+of 0.388. Consulting Figure 3, we predict that this peak consists of benzo[cd]naphtho[3,2,1,8-pqra]perylene [350(4)]. This compound has been previously confirmed to be a constituent of this mixture (6). As discussed previously, we proposed peak E to be either benzo[cd]naphtho[3,2,1,8-pqra]perylene[350(4)] or benzo[Imn]naphtho[ 2,1,8-qra]perylene [350(5)]. We now have assigned benzo[cd]naphtho [3,2,1,8-pqra]perylene [350(4)] to peak G. Therefore, we predict that peak E is benzo[lmn]naphtho[2,1,8-qra]perylene[350(5)]. However, the experimental (M H)+/M+ ratio for peak I is 0.431, which the (CE/CI)MS model predicts to be either benzo[lmn]naphtho[3,2,1,8-pqra]perylene [350(4)] or benzo[lmn]naphtho[2,1,8-qru]perylene[350(5)]. We can eliminate benzo[cd]naphth0[3,2,1,&pqra]perylene [350(4)] as a possibility because we have assigned this isomer to peak G, but we encounter a similar situation as before. We conclude that either peak E or peak I is benzo[lmn]naphtho[2,1,8-qra]perylene [350(5)];the other peak is an isomer that has not appeared in the literature, and thus we have not calculated its IP. (CE/CI)MS allows one to conclude that a given peak may contain more components than thought. This is the case of peak H. The retention index of this peak,639.29 f 0.70, agrees with the retention index, 638.91 f 0.49, of benzo[pqr]naphtho[8,1,2-bcd]perylene[see Figure 3; 350(6)];however, the observed (M + H)+/M+of peak H is 0.593 f 0.012 (Table 111) in contrast to 0.497 f 0.032 measured for the pure standard. It is reasonable to postulate that peak H contains compound 350(6) and at least one other compound. For this specific case where a standard was available, (CE/CI)MS provided the analytical information that led to the conclusion that peak H was multicomponent. The eluting GC peaks were assumed to contain only one component unless it was obvious that the peak was multicomponent (broadened or shoulders). Therefore, we assumed that the measured (M H)+/M+ratio originated from only one species. This was not always the case (for example, peak H). We did not have a method for determining how many isomers were present under an eluting GC peak because retention indexes have not been published and pure standards are not available for large PAH. The following example illustrates this potential problem. If an eluting peak was composed for equal amounts of perinaphthacenonaphthacene, [350(2)1, and phenanthr0[5,4,3,1-~bcde]perylene, [350(7)],an (M + H)+/M+ratio of about 0.5 would be predicted. Because we might not have any indication that the eluting peak was
+
2119
+
+
+
+
+
Figure 4. Structures of isomeric C3,,H,,
PAH (mol wt
= 374) studled
in this laboratory.
a binary mixture, erroneous conclusions could be reached. Both perinaphthacenonaphthacene [350(2)] and phenanthro[5,4,3,2-~bcde]perylene [350(7)] would be rejected as possible structures since the 90% conficence limits on an observed ratio of approximately 0.5 (see Figure 3) fall outside the 90% confidence limits for either of the two compounds. This limitation can, of course, be diminished by improvements in the chromatographic separation of PAH. The largest molecular weight isomer set we investigated was the C9OHl4group, which has a molecular weight of 374. Again, we only present those PAH that were listed in Chemical Abstracts. The three PAH6 listed in Figure 4 [374(1,3,4)]are all the possible PAH in thii isomer class containing only fused six-membered rings (24). However, the maximum number of pentagonal rings this molecular formula can have is eight; we did not consider any of these possibilities. Rather, we restricted our MNDO calculations to those compounds listed in Chemical Abstracts. Of the compounds listed in Figure 4, structures 374(4) and 374(5) have been suggested to be in this carbon black (6). Consulting Figure 5 shows that these two compounds are fully resolved by the (CE/CI)MS method. Figure 5 shows the (M + H)+/M+ratios obtained from the (CE/CI)MS analysis of the C&II4(m/z)374) isomers present in the carbon black; these ratios are shown by the lettered vertical bars as described before. The two standards we have, dibenzo[bc,eflcoronene [374(3)] and naphtho[8,1,2-abc]coronene [374(4)],are represented by asterisks. Examination of Figure 5 suggests that onIy naphtho[8,1,2-abc]coronene [374(4)]and dibenzo[rnn,qr]flureno[2,1,9,8,7-defgii]naphthacene [374(6)] are present in the mixture. Peak J crosses only the range bar corresponding to dibenzo[mn,qr]fluoreno[2,1,9,8,7-defghi]naphthacene[374(6)],and peak K intersects only the range bar for naphtho[8,1,2-abc]coronene [374(4)1. Peak K was confirmed to be naphtho[8,1,2-abc]coronene [374(4)]by a retention index comparison: 678.60 f 0.44 for the standard compared to 678.08 f 0.78 for peak K. Additionally, the experimental (M + H)+/M+ratio of naphtho-
2120
ANALYTICAL CHEMISTRY, VOL. 58, NO. 11,
l
374(3) ’
I
”
”
”
”
”
’
~
”
“
”
’
SEPTEMBER 1986 Table V. Summary of Structure Assignments of CzsHlland C3,,H14PAH Isolated from a Carbon Black
~
(M +
peak”
mol wt
H)+/Mtb
predicted compd
calcdC(M+ H)+/M+
A
350 350
350(9) 350(7)
C D E
350 350 350 350 350 350 350
1.025 0.832
0.958
B
0.750
350(7)
0.708
350(7)
0.792
0.470
350(5)
0.720 0.388 0.593 0.431
350(8) 350(4) 350(6) 350(5)
0.917 0.521
374(6) 374(4)
F
Mc
G
H
1
I
J K
374 374
0.792 0.792
0.465 0.814 0.421
0.506 0.465 0.879 0.516
OSee Figure 1. *Corrected for natural abundance of carbon-13. ‘Refer to Figures 2 and 4.
Figwe 5. Range chart IIlusWathg the resolving power of the (C€/CI)MS technique for the compounds ghren in Figure 4 with the experimental (M H)+/M+ ratios of C3,,Hl4 PAH from the carbon black superimposed. The letters refer to the peaks in Figure 1.
+
[8,1,2-abc]coronene [374(4)] is 0.511 as compared to 0.521 for peak J. This is good agreement. Dibenzo[bc,eflcoronene [374(3)] was probably absent or below the (CE/CI)MS detection limits because no GC peak had a molecular ion of 374 daltons and gave an (M H)+/M+ within the range of 0.244 t o 0.324 ratio units (see Figure 5). A retention index measurement confirmed our hypothesis. The retention index of compound 374(3) was 683.62 f 0.17 (Table 111-4), and there were no peaks observed within that GC interval. Additionally dibenzo[bc,kl]coronene [374(1)]and dicyclopenta[aj]coronene[374(2)] were rejected as possible components in the extract based on the predicted (M + H)+/M+ values. Dibenzo[ eghi]indeno[1,2,3,4-pqra]perylene [374(5)] has been reported to be in the carbon black (6),but our calculated (M + H)+/M+ ratio does not support this. The experimental (M H)+/M+ ratios of 0.521 and 0.917 fall outside the 90% confidence interval (Figure 5 ) for dibenzo[ e,ghi]indeno[ 1,2,3,4-pqra]perylene [374(5)]. A summary of our data obtained for this carbon black is presented in Table V. Using thermally stable capillary columns with (CE/CI)MS detection, we were able to reject many compounds as components in this mixture, tell if a given peak contained more than a single component (provided we had a standard), and predict the exact structure of certain high molecular weight isomeric PAH.
+
+
CONCLUSION We have shown that the IP’s of PAH calculated by the MNDO method compare well with those obtained by experimental methods. A high correlation was found between the (M H)+/M+ ratio of PAH and these calculated IP’s. Fused silica capillary columns with a thermally stable stationary phase were successfully prepared in this laboratory. These columns facilitated the characterization of PAH containing up to nine rings. The resolving power of (CE/CI)MS for large PAH was evaluated for the CBH,, and the C3,,HI4 isomeric series. The (CE/CI)MS method reliably predicts the mass spectral behavior of large isomeric PAH. The method was successfully employed for the analysis of high molecular weight PAH extracted from a carbon black. The method allowed us to reject several isomeric species as constituents of this mixture. We were also able to deduce that one GC peak had more
+
than one component, and we were able to assign specific structures to certain isomeric PAH without possessing standards. The major limitations of the model were revealed. The first drawback is a result of the insufficient number of isomers documented in the literature. Although a full enumeration of PAH6 has been presented, the PAH containing one or two pentagonal fused rings have not. This results in the assignment of more than one GC peak to the same compound. Additionally, the method is hampered by the GC separations. Erroneous conclusions can be drawn if an eluting GC peak is multicomponent, and this may go unrecognized. However, the (CE/CI)MS technique still provides the most information available for the analysis of isomeric PAH. ACKNOWLEDGMENT The authors are grateful to M. L. Lee of Brigham Young University and John Fetzer of Chevron Research for supplying many of the large PAH used in this study. We also thank Indiana University’s Bloomington Academic Computing Services for the use of their computing facilities. Registry No. Benzo[cd]naphtho[3,2,1,8-pqra]perylene, 6208-20-4; benzo[lmn]naphtho[2,1,8-qra]perylene,75449-94-4; benzo[pqr]naphtho[8,1,2-bcd]perylene, 190-71-6; phenanthro[5,4,3,2-abcde]perylene,75449-92-2;benzo[a]coronene, 190-70-5; diindeno[l,2,3-~d:1’,2’,3’-jk]pyrene, 191-23-1; naphtho[8,1,2abc]coronene, 6596-38-9; dibenzo[mn,qr]fluoreno[2,1,9,8,7defghilnaphthacene,76759-99-4; dibenzo[bc,eflcoronene,190-31-8; tribenzo[de,kl,rst]pentaphene,188-72-7;pyranthene, 191-13-9; dibenzo[cd,lm]perylene, 188-96-5; anthanthrene, 191-26-4; naphtho[8,1,2-bcd]perylene,188-89-6;dibenzo[a,l]pentacene, 22709-8; dibenzo[b,deflchrysene, 189-64-0; naphtho[2,1,8-qra]naphthacene, 196-42-9;tetracene, 92-24-0;benzo[a]naphthacene, 226-88-0; dibenzo[h,rst]pentaphene,192-47-2;naphtho[ 1,2,3,4ghilperylene, 190-84-1;benzo[rst]pentaphene, 189-55-9;benzo[blperylene, 197-70-6;benzo[a]pyrene, 50-32-8; perylene, 198-55-0; naphtho[1,2,3,4deflchrysene,192-65-4; benzo[ghi]perylene, 191-24-2;dibenzo[bglphenanthrene, 195-06-2;indeno[1,2,3-cd]pyrene, 193-39-5;dibenzo[a,e]fluoranthene,5385-75-1;pyrene, 129-00-0; anthracene, 120-12-7; coronene, 191-07-1; benz[a]anthracene, 56-55-3; pentaphene, 222-93-5; benzo[e]pyrene, 192-97-2; cyclopenta[cd]pyrene, 27208-37-3; dibenz[a,h]anthracene, 53-70-3;benzo[klfluoranthene, 207-08-9; benzo[b]triphenylene, 215-58-7; benzolilfluoranthene, 205-82-3;picene, 213-46-7;chrysene, 218-01-9;benzo[a]fluorene, 30777-18-5;benzo[c]phenanthrene, 195-19-7;benzo[b]fluorene, 30777-19-6;benzo[b]fluoranthene, 205-99-2; 4H-~yclopenta[deflphenanthrene, 203-64-5; fluoranthene, 206-44-0; phenanthrene, 85-01-8;triphenylene, 217-59-4; benzo[ghi]fluoranthene, 203-12-3; naphthalene, 91-20-3; fluorene, 86-73-7; indene, 95-13-6; acenaphthylene, 208-96-8; benzene, 71-43-2; phenanthro[1,10,9,8-opqra]perylene, 190-39-6; perinaphthacenonaphthacene, 188-50-1; 4552-79-8;dibenzo[bc,phenanthro[1,10,9,8-opqra]pentaphene, kl)coronene, 190-55-6; dicyclopenta[a,j]coronene,90207-46-8;
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 ppene 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
