Anal. Chem. 1904, 56,2749-2754
2749
Analysis of Isomeric Polycyclic Aromatic Hydrocarbons by Charge-Exchange Chemical Ionization Mass Spectrometry William J. Simonsick, Jr., and Ronald A. Hites* School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405
Charge-exchange chemlcal lonlzatlon mass spectrometry (uslng a 15% mixture of methane In argon) yields abundant M+, M H+, and M C,H,+ Ions of polycycllc aromatlc hydrocarbons (PAH). On the bask of the relatlve lntensltles of the M H+ Ion to the M+ Ion, thls technique can dlfferentlate, In a predlctable manner, lsomerlc structures of PAH. The operatlng condltlons were evaluated and optlmlzed; the most significant contrlbutlons to the varlatlons In the (M H)+/M+ ratlos are the temperature, pressure, and condltlon of the Ion source. The optlmum Ion source temperature (250 "C) and pressure (0.8 torr) are easily controlled and regulated. The variable contamination of the Ion source Is compensated by an Internal standard 1,2,3,5-tetrafluorobenrene. The analyses of Standard Reference Materlals 1648 and 1649 and a carbon black extract are reported. Trlphenylene and chrysene, although not chromatographically resolved, can be quantltated by uslng thls technlque. The ldentlflcatlon of acephenanthrylene based on the measurement of ratlos and calculated lonlzatlon potentials Is also presented.
different isomers (8,9). Thus, in only one experiment, components of complex mixtures can be characterized by a PAH retention index, a combined E1 and methane CI mass spectrum, and the (M l ) + / M + ratio. (CE/CI)MS adds to the information content of a mass spectrum and allows the positive confirmation of individual PAH isomers that may or may not be chromatographically resolvable. Although simple to employ, requiring only an argonmethane reagent gas mixture and a mass spectrometer equipped to do chemical ionization, (CE/CI)MS has had limited use in the past because the lack of standard operating conditions resulted in nonreproducible data. The purpose of this study was to investigate the effect of operating parameters on the reactant ions in the ion source and hence on the CE/CI mass spectra of selected PAH. These operating parameters included ion source cleanliness, temperature, and pressure. The effect of using different mixed reagent gases (5%,lo%, E%,and 25% methane in argon) was also investigated. Once the effect of each variable was known, the system was optimized for maximum sensitivity and precision.
Some polycyclic aromatic hydrocarbons (PAH) are carcinogenic. Since this activity is dependent on the exact shape of the molecule, isomeric PAH can differ dramatically in biological effect. For example, benzo[a]pyrene is carcinogenic, but its isomer, benzo[e]pyrene, is not ( I ) . Thus, when analyzing mixtures of PAH, it is important to do so with complete isomer specificity. Gas chromatographic mass spectrometry (GC/MS) is now the most widely used tool for the analysis of complex mixtures of PAH. Unfortunately, since electron-impact (EI) mass spectra of isomeric PAH are almost always indistinguishable, the analyst must rely on the gas chromatographic column to separate isomeric PAH. The advent of fused silica capillary columns has made this task easier than i t once was (in fact, retention indexes of PAH have been compiled ( 2 , 3 ) )but , the fundamental limitation of E1 mass spectra remains. Because this limitation is a result of the E1 ionization process, which primarily forms molecular ions for PAH, other ionization strategies have been suggested to circumvent this limitation (4-7). These methods have had limited success because the resulting spectra could not be interpreted without the use of standard compounds, many of which are not available. In searching for a method which would be useful for analyzing PAH in the absence of standard compounds, we found that charge-exchange chemical ionization mass spectrometry [ (CE/CI)MS], with argon-methane mixtures as the reagent gas, gave different mass spectra of isomeric PAH and that the differences were predictable based on the ionization potential (IP) of the molecule (8). The ratio of the M H+ ion to the M+ ion, as measured by the (M + l)+/M+ ratio, was shown to correlate with the first ionization potential for the PAH studied (8). Since the ionization potentials of PAH isomers are dependent on the specific structure of each molecule, this (CE/CI)MS technique can produce different spectra for
All experiments were performed on a Hewlett-Packard 5985B GC/MS system equipped with a dual ionization source (E1 and CI). The mixed reagent gases (Linde Co., Indianapolis,IN) were introduced through a modified transfer line (IO)prior to the ion source. The ion source temperature was measured by a thermocouple located in the ion source body. The relative ion source pressure was measured by a thermogauge and by an ion gauge tube located on the side of the ion source manifold and was controlled by an automatic pressure controller (Granville Phillips Co., Boulder, CO). The PAH selected for this study were four sets of structural isomers: phenanthrene and anthracene, pyrene and fluoranthene, chrysene and triphenylene, and benzo[a]pyrene,benzo[e]pyrene, and perylene. One to two microliters of methylene chloride (MCB, Cincinnati, OH) solutions, containing approximately 20 ng/pL of each PAH standard (Analabs, New Haven, CT),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., Rancho Cordova, CA) employing helium as the carrier gas at a head pressure of 11psi. All mass spectral data were collected by using selected ion monitoring. The software examined the complete M+ and (M 1)+mass spectral peak profiles of each PAH isomer. This was accomplished by stepping across the M+ and (M + 1)' peaks at 0.1 amu intervals followed by integration 10.2 m u from the peak maxima. All (M + l)+/M+ ratios were corrected for the natural abundance of 13C. The two air particulate samples analyzed in this study (SRM's 1648 and 1649) were obtained from the Office of Standard Reference Materials, National Bureau of Standards, Washington, DC. One gram of each sample was Soxhlet extracted for 48 h with 450 mL of methylene chloride. Both samples were spiked prior to extraction with an internal standard solution containing the perdeuterated PAH fluorene-d,,, phenanthrene-dlo,chrysene-d,,, perylene-d12,and benzo[ghi]perylene-d12(MSD Isotopes, Merck and Co., Rahway, NJ). The methylene chloride extracts were reduced in volume to approximately1 mL with a rotary evaporator. The extracts were preadsorbed onto 3 g of neutral aluminum oxide (80-200 mesh,
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EXPERIMENTAL SECTION
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0003-2700/84/0356-2749$01.50/0
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0 1984 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
220
180
140
260
303
m/e
M+1' 180
200
220
240
280
260
Measured i o n s o u r c e temperature (OC)
M'
I
+
Flgure 2. Effect of ion source temperature on the (M l)+/M+ ratio of selected PAH at constant ion source pressure (0.8 torr) using a 10% methane in argon reagent gas mixture. "Average" refers to the av-
M+29
I r n M+I/M
=
1.606
ill
100
140
180
220
~
260
erage (M Section.
300
m/e
Flgure 1. Charge-exchange chemical ionization mass spectra of tetracene and triphenylene, two isomeric PAH of elemental composition CI8HlP. Conditions: 15% methane in argon reagent gas, 250 "C ion source temperature, and 0.8 torr ion source pressure.
Fisher No. -4-540, Fisher Scientific Co., Fairlawn, NJ) which had been dried in an oven at 180 "C for 2 days prior to sample workup. The adsorbed extracts were subsequently transferred to the top of a 6-g alumina (conditionedas before) column (10 cm X 11mm i.d.) and eluted with 100 mL of chloroform containing 0.75% of added ethanol (MCB, Cincinnati, OH). The eluate was reduced to approximately 1 mL and placed on the top of a 22 mm i.d. column packed with 60 g of Bio-Beads S-X12 (200-400 mesh, Bio-Rad Laboratories, Richmond, CA) and eluted with methylene chloride. The volume eluting between 55 and 180 mL was collected, and it contained the PAH components (11). The eluate was then reduced to a volume of approximately 1 mL. Fractionation into chemical classes was done as previously described (12);however, only the neutral PAH fraction was analyzed by GC/MS operating in the CE/CI mode using 15% methane in argon as the reagent gas. A 10-g aliquot of 260 nm particle size furnace black (Cabot Corp., Boston, MA) was Soxhlet extracted for 24 h with 400 mL of methylene chloride. The furnace black was spiked prior to extraction with the predeuterated PAH listed previously. The extract was reduced to a volume of approximately 5 mL and analyzed directly by (CE/CI)/MS.
RESULTS AND DISCUSSION The methane-argon reagent gas mixture gives different mass spectra for isomeric PAH. Figure 1 shows the (CE/ C1)MS spectra of two tetracyclic PAH of molecular weight 228. Protonated molecular ions (M I)+are formed by proton transfer from C2H5+while M+ ions are generated primarily by charge-exchange reactions between the analyte and Ar+. The optimized conditions for (CE/CI)MS were an ion source temperature of 250 "C, a 15% mixture of methane in argon, and an ion source pressure of 0.8 torr. In addition, it was necessary to control for ion source contamination with an internal standard. The next several paragraphs discuss the effect of these four variables and their optimization. Ion Source Temperature. The ion source temperature was observed to be a significant factor affecting the (M + l)+/M+ ratio of individual PAH. Figure 2 illustrates this behavior; a decrease in the (M 1)+/M+ ratio is seen with increasing ion source temperature at constant pressure. As the ion source temperature was raised, the relative intensity of Ar+ increased while that of the CzH5' complex decreased.
+
+
+ l)+/M+ ratio of the nine PAH listed in the Experimental
The ion source residence time of the reactive species decreases with increasing source temperature (13). This results in the extraction of Ar+ and primary methane species (CH3+,CH4+) before extensive formation of the protonating CzHb+species. The relative increase in Ar+, coupled with the decrease in C2Hb+,results in the negative temperature coefficient seen in Figure 2. Although we observe an increase in sensitivity with increasing temperature, there is a practical limit. A temperature of 250 "C is a good compromise. The source temperature is easily controlled, and its precision is estimated to be f2 "C. Reagent Gas Composition. The methane-argon reagent gas mass spectrum for all mixtures consisted of significant ions at m / e 15, 17,29, 40, 41, and 80 corresponding to CH3+,CHS+, CzH5+,Ar+, C3H5+,and Arz+; Ar+ and C2H5+comprised over 70% of the reactive ion current. The relative abundance of these ions varied with the ion source pressure. As the source pressure was raised, the relative abundance of Ar+ decreased while a corresponding increase of C2H6+was observed. This trend ceased at approximately 0.6 torr for all mixtures; at this pressure, the system seemed to be close to equilibrium since minor pressure changes did not cause large changes in the relative amount of reactive ions present. The 15% and 25% methane in argon mixtures both approached this steady-state condition at a lower pressure than either the 5% or the 10% CH4/Ar mixture, indicating a preference for a higher methane composition for greater precision. As one increased the methane percentage, a corresponding increase in (M + l)'/M+ ratio of PAH was also seen. For these reasons and because 15% methane in argon gave the best sensitivity (see below), this composition was selected for routine work. Ion Source Pressure. The reactive ion current was monitored as a function of the ion source pressure for the methane-argon gas mixtures. The 15% CH4/Ar gave the highest ion current followed by the 25%, l o % , and 5% mixtures, respectively. Figure 3 shows this effect for the 15% and 25% mixtures. One observes that at low source pressures (low concentration), the sensitivity drops significantly. This is a direct result of having a relatively low concentration of reactive species present. The drop in sensitivity at higher pressures is a result of the inefficiency of pulling ions out of the ion source. It should also be noted that the optimum ion source pressure (largest reactive ion current) for all gas mixtures occurs in the range of 0.6-0.9 torr. A value of 0.8 torr was selected for routine work. Figure 4 shows the effect of increasing the ion source pressure on the (M + 1)+/M+ratio of PAH; as the ion source
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
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+
Table I. Reproducibility of (M l)+/M+ Ratio of Selected PAH Run oyer a Period of 6 Weeks compound
MW
(M + 1)/M ratio
re1 std dev," %
phenanthrene anthracene fluoranthene pyrene chrysene triphenylene benzo[a]pyrene benzo[e]pyrene
178 178 202 202 228 228 252 252 252
1.474 0.640 1.542 0.653 1.193 1.606 0.474 0.757 0.373
2.52 2.77 3.66 3.14 2.85 2.24 3.21 2.50 3.02
perylene 0.4
0.8
0,8
0.7
0.8
0.0
1.0
1.1
Indicated i o n s o u r c e pressure ( T o r r )
Flgure 3. Effect of ion source pressure on the measured reactive ion current (C,H5+ pius Ar') at an ion source temperature of 250 OC.
".
7
/I
n N 2 15.
Ion Source Cleanliness. Another parameter which could account for a variation in the (M + l)+/M+ ratios is the condition of the ion source (cleanliness, amount of adsorbed water). Spectra were recorded when the ion source was clean under a given set of operating conditions. The (M + 1)+/M+ ratio of PAH were followed for a period of time during which time the instrument had extensive use (approximately 200 analyses). A steady increase in all the PAH (M + 1)+/M+ ratios with time was observed. This was a result of the ion source lenses accumulating a film which, in turn, results in an increase in the ion source residence time of the reactive ions. This causes a depletion in the Ar+ ion current and a corresponding increase in the C2H5+ ion current. The net effect was an increase in the (M + l),+/M+ ratio. By monitoring the ratio of the major reactive ions (C2H5' and Ar+),one might predict the (M l)+/M+ratio behavior. The amount of H30+was also monitored since it is an excellent proton donor and would raise the (M + l)+/M+ratio if a large excess was present. Initially, the (M l ) + / M +ratio of PAH tracked the C2H5+/Ar+ratio provided H30+was not too high. After approximately 150 analyses (30 days), this trend ceased and the C2H5+/Ar+ratio inflated without a corresponding increase in the PAH (M l ) + / M + ratio. Thus, the (M l ) + / M + ratio could not be reliably predicted by only monitoring the reactive ions. Since the (M + l ) + / M + ratio varied with ion source conditions, an internal standard was employed to monitor any changes occurring in the source. The ideal internal standard would have (a) a (M l)+/M+ratio close to unity for 15% methane in argon in the (CE/CI)MS mode, (b) a mass spectrum which would not interfere with the compounds which will be analyzed, (c) a high volatility so the compound may be continually bled into the source and pumped away to avoid interfering with other modes of GC/MS operation, such as negative chemical ionization, (d) a large ionization cross section, (e) commercial availability, and (f) a large (M + l)+/M+ ratio change for the variables examined. Many fluoro compounds fit the above criteria. The compound which seemed best suited for (CE/CI)MS operation was 1,2,3,5-tetrafluorobenzene(TFB). This standard is continually bled into the ion source through a modified transfer line. By adjlzsting the flow of the reagent gas, hence the ion source pressure, the same (M + l ) + / M + ratio of TFB can be set. This results in reproducible (M + l)+/M+ ratios of analytes. Table I lists the results obtained for a set of PAH standards run over a 6-week period. A long-term relative standard deviation of about 3% seems satisfactory. Thus, (CE/CI)MS has been shown to yield reproducible (M + l)+/M+ ratios for standard solutions of PAH. Applications. Complex mixtures of environmental significance may be analyzed both qualitatively and quantitatively with (CE/CI)MS. Recently, reverse-phase liquid chromatography with fluorescence detection was utilized to
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04
0,
06
OLI
07
09
10
1 1
1 2
I n d i c a t e d ion s o u r c e pressure (Torr)
+
Flgure 4. Effect of ion source pressure on the (M 1)+/M+ ratios of selected PAH employing a 1 0 % methane in argon reagent gas at an ion source temperature of 250 OC. "Average" refers to average (M l)+/M+ ratio of the nine PAH listed in the Experimental Section.
+
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pressure is raised, the (M l)+/M+ ratio significantly increases. This observation agrees with the results previously discussed concerning the effect of the ion source residence time on reactive ions. As the ion source pressure is increased, the ion source residence time of reactive species correspondingly increases (13). This results from an increase in [C2H5]+/[Ar]+. The (M l)+/M+ ratio increases accordingly, since the (M 1)+is directly related to the amount of CzH5+present. The most significant variable is clearly the ion source pressure which must be carefully controlled in order to obtain reproducible results. Initially, the H P 5985B GC/MS ion source was plugged during normal CI operation by inserting the calibrant probe into the ion source block. This reduced the vacuum pumping efficiency to the source. One may then work a t lower flow rates of reagent gases although still achieving the ion source pressure (0.1-1.0 torr) necessary for ion-molecule reactions in chemical ionization. The nonreproducible calibrant probe fit introduced a variability in the ion source pressure for a, given reagent gas flow. Depending on how the calibrant probe was inserted (tightness, alignment), one needed a higher or lower flow rate to achieve the desired ion source pressure. To eliminate this uncertainty, a stainless steel plug was fitted into the direct insertion probe hole of the ion source. The relative source pressure could then be manually set by adjusting the flow of reagent gas to achieve a thermogauge reading of approximately 0.8 torr and maintained by using the automatic pressure controller.
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
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Table 11. Summary of Quantitation of SRM's 1648 and 1649 and Comparison of (M
compound
M+
phenanthrene anthracene pyrene fluoranthene benz[a]anthracene benzo[e]pyrene benzo[a]pyrene perylene indeno[1,2,3-cd]pyrene benzo[ghi]perylene
178
178 202 202 228
252 252 252 276 276
SRM 1648 NBS," (CE/CI)MS, wg/g Kdg 4.8 0.31 6.1 7.4 2.7 6.8 3.3 0.52 4.6 6.8
+ l)'/M+
SRM 1649 (CE/CI)MS, NBS," @g/g M/P
4.6 0.36 7.0 8.4 3.0 3.0 0.67 4.7 5.5
4.8 0.50 5.8 7.0 3.3 3.6 2.8 0.57 3.4 4.4
Ratio with PAH Standards
(M + 111 M, std
(CE/CI)MS (M + I)/ M, 1648
(M + 1)/ M, 1649
1.474 0.640 0.653 1.542 0.788 0.757 0.474 0.373 0.644 0.418
1.572 0.591 0.672 1.584 0.744 0.678 0.469 0.406 0.680 0.454
1.560 0.599 0.634 1.538 0.779 0.746 0.466 0.380 0.613 0.427
4.6 6.4 7.0 2.6 3.6 2.6 0.76 3.3 3.9
"From ref 14. Table 111. Summary of Results for 260-nm Carbon Black Extract
peak"
compound
retention indexb
1 2
naphthalene acenaphthylene dibenzothiophene phenanthrene anthracene fluoranthene acephenanthrylene phenanthro[4,5-bcd]thiophene pyrene benzo[ghi]fluoranthene cyclopenta[cd]pyrene benz [a]anthracene chrysene benzofluoranthenes benzo[e]pyrene benzo[alpyrene perylene CzzHlzisomer indeno[1,2,3-cd]pyrene benzo[ghi]perylene anthanthrene coronene
200.00 233.52 293.57
3 4 5
6 7 8 9 10 11
12 13 14 15 16 17 18 19 20 21 22
300.00
301.50 342.00 345.53 347.06 349.73 389.85 397.15 398.75 400.00 452.29 454.12 457.25 490.08 493.51 501.26 504.77 540.84
(M + I)/ M std
(M + I ) / M obsd
concn (EI): rg/g
2.152 1.860 1.804 1.474 0.640 1.542
2.085 1.735 1.977 1.513 0.609 1.533 1.428 1.045 0.623 1.326 0.910 0.810 1.167d 1.209 0.768 0.491 0.366 1.082 0.619 0.458 0.306 0.571
4.8 3.5 17 39 1.9 84 (11) (27) 170 (30) (45) 4.8 7.2 (76) 56 78 30
0.653 0.788 1.193 0.757 0.474 0.373 0.644 0.418 0.320 0.567
280 490 160
210
concn (CE/CI), Pg/g 5.2 2.6 14
28 0.82 110
(6.9) (32) 150 (34) (27)
4.2 4.8 (71) 40 171
20 (26) 360 500 180 140
aSee Figure 5. *Retention index based on naphthalene = 200.00, phenanthrene = 300.00, chrysene = 400.00, and benzo[e]pyrene = 452.29 (2). No response factor available for calculation of concentrations given in parenthesis. Suggests that peak 13 is exclusively chrysene.
provide a sensitive and selective method for PAH analysis in urban air particulate samples (14). Identification and quantitation of 11 individual PAH in SRM 1648 and 13 in SRM 1649 were reported (14). A simple extraction and clean-up procedure (11,12)was used in conjunction with (CE/CI)MS analysis, and these SRM's wete analyzed. The results are summarized in Table 11. The measured (M l)+/M+ ratio of the PAH in the extracts compare quite well with the (M l)+/M+ ratio of standards; the average percent difference
+
+
is 4.4%. The quantitation of components, calculated by using response ratios of PAH to their predeuterated analogues, agree with literature values (average percent difference is 11%); this illustrates that (CE/CI)MS is also well suited for the quantitative analysis of complex mixtures. Triphenylene and chrysene, although not chromatically resolved, can also be quantitated by (CE/CI)MS. One can generate a linear calibration curve (r2= 0.992) for the (M + 1)+/M+ ratio vs. the ratio of triphenylene to chrysene. When the (M 1)+/M+ ratio of the eluting peak is determined, the composition is easily calculated. For example, the GC peak in SRM 1649 corresponding to chrysene and triphenylene gave a (M 1)+/M+ratio of 1.286. From the working curve, it was determined that this peak was composed of 27% triphenylene and 73% chrysene. The reported values for these compounds
+
+
are 32% triphenylene and 68% chrysene (14). In the same way, it was determined that SRM 1648 contained 12% triphenylene and 88% chrysene; unfortdnately no values were reported for these compounds in this mixture (14). Thus, the (CE/CI)MS technique can be useful in the resolution of coeluting isomers. However, if three or more components are present in a single GC peak, the mixture cannot be quantitated even if all the components are identified. Even in this case, the (M l)+/M+ ratio would allow one to conciude that a given peak may contain more components than originally thought. Carbon black is a material of environmental concern due to the large number and high concentration of adsorbed PAH (15). A carbon black with an average particle size of approximately 260 nm was analyzed under (CE/CI)MS conditions. Figure 5 is the gas chromatogram of the PAH extracted from this carbon black. Table I11 lists the calculated retention indexes, the observed (M l)+/M+ratio, the (M + l)+/M+ ratio of PAH standards where available, and the quantitation of components obtained under E1 and (CE/CI)MS conditions. T h e measured (M l ) + / M + ratio of PAH found in the carbon black extract compare well with the ratios obtained from standards; the average percent difference is 4%. The quantitative analyses obtained by E1 and (CE/CI)MS of PAH
+
+
+
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
2753
Time Iminutml
0
10
20
Jo
40
e4
50
70
Temperaluri lDCl
m 250 Figure 5. Capillary gas chromatogram of a carbon black extract. Conditions: 30 M X 0.25 mm i.d., DB-5 column, hydrogen carrier gas, flame ionization detector. Peak numbers refer to compounds listed in Table 111. 50
150
1w
in the carbon black sample are also presented in Table 111. The response factors for PAH obtained under (CE/CI)MS conditions vary from 0.5 to 2.0, a much larger range than the response factors under E1 conditions which are in the vicinity of 1 when compared to their deuterated analogues. The experimental (M l)+/M+ ratios correlate well with the first ionization potential (IP) of the PAH studied (8). In addition, a correlation (r2= 0.902) exists between the (M l)+/M+ values and the experimental proton affinities (PA) (16) of several PAH. Combining the PAH data of these two fundamental properties and correlating these values with the l)+/M+ values did not yield a significantly higher (M correlation than the correlation obtained with only IP. It was, therefore, concluded that the IP was the best indicator of (M + 1)+/M+behavior. The implication of this observation is important: (CE/CI)MS allows one to predictively assign specific structures to isomeric PAH. Table IV lists the experimental first ionization potential (17) and the measured (M l)+/M+ ratio of PAH studied in this laboratory. Apparently, an increase in IP causes the rate constant for ionization by Ar+ (charge exchange) to decrease while the rate of protonation is unaffected. The overall effect is an increase in the relative rate of protonation resulting in an increase in the (M l)+/M+ ratio. The experimentally determined equation relating I P and (M l)+/M+ratio is
Table IV. Selected PAH with Their Respective Experimental Ionization Potential and Measured (M l)+/M+ Ratio
+
+
+
+
(M
+
+ l ) + / M + = 1.601(IP) - 11.049
(r2 = 0.947)
This correlation may be exploited where no GC retention characteristics, mass spectral behavior, or standards are available as in the case of an unknown compound (see peak 7, Figure 5 ) in the carbon black extract. This peak gave a molecular ion at m / e 202, eluted between fluoranthene (I)and pyrene (II), and yielded an (M l)+/M+ ratio of 1.428. If
+
compound
eV
(M + l)+/ M+b
anthanthrene perylene tetracene benzo[ghilperylene benzo[a]pyrene coronene dibenz[a,h]anthracene benzo[bltriphenylene anthracene pyrene benzo[e]pyrene benz[a]anthracene chrysene benzo[clphenanthrene acenaphthene triphenylene phenanthrene fluorene fluoranthene dibenzothiophene naphthalene acenaphthylene benzene
6.92 6.98 7.01 7.16 7.26 7.36 7.38 7.40 7.42 7.43 7.43 7.46 7.60 7.61 7.79 7.87 7.90 7.91 7.95 7.95 8.15 8.22 9.24
0.320 0.373 0.386 a.418 0.474 0.567 0.810 1.063 0.640 0.653 0.757 0.788 1.193 1.106 1.036 1.606 1.474 1.542 1.542 1.804 2.152 1.860 4.086
ionization potential,"
+
+
"All experimental ionization potentials taken from ref 16. *Conditionssuch that (M + l)+/M+ratio of TFB is approximately 1.
ratio = 1.6311 would suggest acephenanthrylene (IV) [IP(calcd) = 7.82 eV, Calculated (M l)+/M+ ratio = 1.4711 as the structure. Since the measured (M + 1)+/M+ratio value was 1.428, it was deduced that acephenanthrylene was the correct isomer.
+
SUMMARY I
I1
& \
111 /
IV
the isomer were aceanthrylene (111) [IP(calcd) = 7.28 eV, calculated (M + l)+/(M)+= 0.6061, a ratio close to that of pyrene would be expected [IP(calcd) = 7.47 eV, calculated (M + l ) + / M + ratio = 0.9101; however, a ratio similar to fluoranthene [IP(calcd) = 7.92 eV, calculated (M + l)+/M+
(CE/CI)MS has a sensitivity comparable to electron-impact mass spectrometry and provides a combined E1 and methane CI mass spectrum. (CE/CI)MS is a reproddcible method for the analysis of PAH in complex mixtures and for the identification of PAH where no standards are available. This technique is also useful for resolving of coeluting isomers such as chrysene and triphenylene. Mixed reagent gas chemical ionization will not replace electron-impact mass spectrometry, but it will increase the information content of mass spectra for the identification and quantitation of components in complex mixtures.
ACKNOWLEDGMENT We thank M. L. Lee and W. R. West of Brigham Young University, Provo, Utah, for supplying some of the PAH used in this study and for providing the calculated ionization PO-
Anal. Chem. 1984, 56,2754-2759
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tentials of the ClsHIo isomers. The electron-impact mass spectral analysis of the carbon black extract was performed in this laboratory by T. D. Behymer. Registry No. I, 206-44-0; 11, 129-00-0;IV, 201-06-9; naphthalene, 91-20-3; acenaphthylene, 208-96-8; dibenzothiophene, 132-65-0;phenanthrene, 85-01-8;anthracene, 120-12-7;phenanthro[4,5-bcd]thiophene, 30796-92-0; benzo[ghi]fluoranthene, 203-12-3;cyclopenta[cd]pyrene, 27208-37-3; benz[a]anthracene, 56-55-3; chrysene, 218-01-9; benzo[e]pyrene, 192-97-2; benzo[alpyrene, 50-32-8; perylene, 198-55-0;indeno[l,2,3-cd]pyrene, 193-39-5;benzo[ghi]perylene, 191-24-2;anthanthrene, 191-26-4; coronene, 191-07-1;tetracene, 92-24-0; dibenz[a,h]anthracene, 53-70-3;benzo[b]triphenylene, 215-58-7; benzo[c]phenanthrene, 195-19-7;acenaphthene, 83-32-9; triphenylene, 217-59-4; fluorene, 86-73-7; benzene, 71-43-2; methane, 74-82-8.
LITERATURE CITED Lee, M. L.; Novotny, M. V.; Bartle, K. D. "Analytlcal Chemlstry of Polycyclic Aromatic Compounds"; Academic Press: New York, 1981. Lee, M. L.; Vassilaros, D. L.; White, C. M.; Novotny, M. Anal. Chem.
(5) Keough, T. Anal. Chem. 1982, 5 4 , 2540-2547. (6) Oehme, M. Anal. Chem. 1983, 55, 2290-2295. (7) Hiipert, L. R.; Byrd, G. D.; Vogt, C. R., paper presented at the Pitts-
burgh Conference on Analytical Chemistry and Applied Spectroscopy, March 5-9. 1984, Atlantic City, NJ; No 415. (8) Lee, M. L.; Hites, R. A. J . Am. Chem. Soc. 1977, 99, 2008-2009. (9) Lee, M. L.; Vassilaros, 0. L.; Pipkin, W. S.; Sorensen, W. L. "Trace Organic Analysis: A New Frontier in Analytical Chemistry"; National Bureau of Standards: Washington, DC, 1978; NBS Spec. Pubi. (US.) NO.519, pp 731-738. (10) Jensen, T.; Kaminsky, R.; McVeety, B.; Wozniak, T.; Hites, R . A Anal. Chem. 1982, 5 4 , 2388-2390. (11) Lee, M. L.; Vassilaros, P. L.; Later, D. W. Int. J . Environ. Anal. Chem 1982, 1 1 , 251-262. (12) Later, D. W.; Lee, M. L.; Bartle, K. D.; Kong, R C.; Vassilaros, D. L. Anal. Chem. 1981, 53, 1612-1620. (13) Sroka, G.; Chang, C.; Meisels, G G. J . Am. Chem. Soc. 1972, 9 4 , 1052-1056. (14) May, W. E.; Wise, S. A. Anal. Chem. 1984, 5 6 , 225-232. (15) Lee, M. L.; Hites, R. A. Anal. Chem. 1976, 4 8 , 1890-1893. (16) Meot-Ner, M. J . Phys. Chem. 1980, 8 4 , 2716-2723. (17) Levin, R. D.; Lias, S. G. "Ionization Potential and Appearance Potential Measurements, 1971-1981". Natl. Stand Ref. Data Ser. ( U S . , Natl. Bur. Stand.) 1982, No. 71.
1979, 5 1 , 773-788.
Vassllaros, D. L.; Kong, K C . ; Later, D. W.; Lee, M. L. J . Chromatogr. 1982, 252, 1-20. Shushan, B.; Safe, S. H.; Boyd, R. K. Anal. Chem. 1979, 51, 156-1 58.
RECEIVED for review May 29,1984. Accepted August 7, 1984. This work was supported by the U.S. Department of Energy (Grant DE-AC02-80EV10449).
Isomer-Selective Determination of Tetrachlorodibenzo-p -dioxins Using Hydroxyl Negative Ion Chemical Ionization Mass Spectrometry Combined with High-Resolution Gas Chromatography Michael Oehme* and Paul Kirschmer Norwegian Institute for Air Research, P.O. Box 130, N-2001 Lillestram, Norway
OH- negative Ion chemical lonizatlon (NICI) mass spectrometry can be used for the isomer-speclflc determination of tetrachlorodlbenro-p-dloxlns (TCDDs). When CH4/N,0 mlxtures are used as a reagent gas, (M - H)-, M-, (M OH)-, and (M CI)- are formed In the ion source. When thls iechnlque Is combined with high-resolution gas chromatography, 14 Isomers including 2,3,7,8-TCDD can be identified by slgnlflcant dlfferertces In the abundances of these Ions. Monitorlng of the (M OH)- adduct ion (formed by attack of the dioxin rlng) allows an Increase In the seiectlvlty for TCDDs against polychlorinatedblphenyis, pestlcldes, etc. A detecilon OH)- (signal-to-nolse 1 O : l ) limlt of 25 pg at m / z 337 (M was obtained under standard condltlons.
+
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In recent years the analysis of tetrachlorodibenzo-p-dioxins (TCDDs) has been the subject of considerable interest due to the extreme toxicity of some isomers (1-5). In addition TCDDs are highly stable compounds with mutagenic and carcinogenic properties (6,7). They are found as contaminants in herbicides, chlorinated phenols, and other polychlorinated compounds (1). Furthermore they are formed by different combustion processes and have been found in fly ash and flue gasses from municipal incinerators (1-5, 8, 9). Since TCDDs normally are present in parts-per-billion to parts-per-trillion levels in environmental and biological samples, a highly sensitive and selective method is required to
avoid interferences by other chlorinated compound classes. This is especially important for the determination of TCDDs in incinerator emissions which also contain large amounts of polychlorinated biphenyls, napthalenes, styrenes, terphenyls and diphenyl ethers, etc. (10). Different methods to separate polychlorinated dibenzo-p-dioxins (PCDDs) from other polychlorinated compounds have been described using singlestep or more complex liquid chromatographic methods (10-12). High-resolution gas chromatography (HRGC) and/or high-resolution mass spectrometry (HRMS) are employed to obtain the required specifity of the TCDD determination (13-16). In the past mainly quantitative results for the most toxic 2,3,7,8-TCDD were reported. However, interest has increased considerably in obtaining more information about the presence of other TCDD isomers enabling differentiation between sources. While HRMS does not supply any isomer specific information, both high-performance liquid chromatographic separation techniques (HPLC) and HRGC allow the separation of TCDD isomers (12,13). However, the only parameter for a positive isomer identification is the correct retention time. Therefore a detection method which gives additional isomer specific information would help to identify TCDDs with a higher degree of reliability. Negative ion chemical ionization (NICI) mass spectrometry employing O2 as reaction gas has been successfully used for the selective detection of TCDDs ( 17). The most dominant ions formed by attack of 0; are the 1,2-dibenzoquinoneradical
0003-2700/84/0356-2754$01.50/00 1984 American Chemical Society
