Determination of isomeric polycyclic aromatic hydrocarbons in air

Chem. , 1983, 55 (14), pp 2290–2295. DOI: 10.1021/ ... Cite this:Anal. Chem. 55, 14, 2290-2295 .... P. G. Sim , W. D. Jamieson , R. K. Boyd. Organic...
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Anal. Chem. 1083, 55, 2290-2295

Determination of Isomeric Polycyclic Aromatic Hydrocarbons in Air Particulate Matter by High-Resolution Gas Chromatography/Negative Ion Chemical Ionization Mass Spectrometry M.Oehme Norwegian Institute for Air Research, P.O. Box 130, N-2001 Lillestrerm, Norway

High-resolution gas chromatography combined wRh electron capture negatlve ion chemlcal loniratlon (NICI) mass spectrometry looks to be a promising method for the detectlon of mutagenic and/or cafclnogenlc polycycilc aromatlc hydrocarbons (PAH). When methane/nitrous oxide mlxtures are used as reagent gas, (M H)-, M-, (M OH)-, (M H NO)-, and (M H N,O)- are formed In the ion source. Many lsomerk PAH can be identtfled by significant differences In the abundance of these ions. N I C I mass spectra and response factors oblalned by both technlques are reported for 22 reference PAH. Some practlcal and Instrumental aspects of the NICI mode are discussed. The method has been used for differentiation of isomeric bentofluoranthenes and benzofluorene$ from methylfluoranthenes as well as for the determination of PAH In air particulate extracts.

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The determination of polycyclic aromatic hydrocarbons has become an important field in environmental analysis. Sample cleanup as well as separation and identification of isomeric compounds are important aspects for qualitative and quantitative analysis. Often several hundred compounds can be separated. The recent improvements of the separation efficiency of capillary columns and the elaboration of a PAH retention index have considerably facilitated the identification of isomeric compounds (1-4). Mass spectrometry with electron impact ionization has found wide application for the identification of PAH (4-6). However, the analytical power of this method has been limited because electron impact spectra of isomeric PAH are almost identical. Positive ion chemical ionization has been used sucessfully for the differentiation of isomeric compounds in various substance classes such as olefins ( 7 , 8 ) ,aromatic hydrocarbons (8, 9 ) ,and alcohols (10). Some investigations have also been carried out to find methods that create differences in the mass spectra of isomeric PAH. The most promising approach has been described by Lee, Hites, and co-workers, which employed a charge exchange chemical ionization method (11-13). When an Ar/CH4 reagent gas mixture is used, M+ ions are formed by charge transfer and (M + I)+ ions by proton transfer. The rate of ionization by Ar+ is dependent on the ionization potential of the compound while the rate of protonation is not significantly affected. The ratio between the protonated molecular ion and the molecular ion, (M + l)+/M+, was correlated with the differences in the ionization potential of isomeric PAH. Some methylbiphenyls and tetracyclic PAH could be distinguished in this way. The availability of commercial mass spectrometers that can register negative ions has led to a more wide-spread application of negative ion chemical ionization (NICI) techniques (14-16). Normally methane is used as reaction gas to form thermal electrons in the ion source. Compounds with sufficient

electron affinity and cross section can be ionized by different electron capture mechanisms (15). For such substances as polychlorinated hydrocarbons, nitrated polycyclic aromatics, diketones, etc. a considerable gain in both sensitivity and selectivity can be obtained. In addition, most of the compounds with biological activities based on electron donor/ acceptor properties can be detected by electron capture ionization ( 1 7-19). Electron capture NICI (EC-NICI) is also a useful technique for the identification of biologically toxic cornpounds in different kinds of samples (20,21). Other reaction gases or gas mixtures such as CH4/N20 or CH4/CH3N02, which yield stable negative ions (as OH- (22) or CHSO-, respectively), can also be used for NICI. In this case, both ionization by electron capture and by ion/molecule reactions occur. The most common reactions are deprotonation and formation of ion/molecule adducts. The number of compound groups that form stable negative ions is considerably extended. In addition, isomeric compounds that have different acidities or form different ion/molecule adducts can be distinguished (23). More information about the use of different reaction gases is given elsewhere (14, 15, 24). Negative ion mass spectra of PAH have been reported first by von Ardenne (25), who used thermal electrons generated by argon discharge at low pressure torr). Hunt and co-workers employed oxygen as reagent gas for both negative and positive chemical ionization (14,24). 02+, 0+,02-,and 0- were formed by electron bombardment. The highly reactive 0- was removed by adding the scavanger gases Hz or CO. Isomeric PAH such as benzo[ghi]perylene and indeno[1,2,3-cd]pyrene could be distinguished through differences in the abundances of the formed M- and (M 15)- ions. However, the lifetime of the filament was not more than a few hours in an oxygen atmosphere of 1 torr. Therefore the use of a Townsend discharge ion source has been suggested (26). However, this type of source is not available for all commercially available mass spectrometers, which limits the application of this method for differentiation of isomeric PAH. Homing et al. investigated the possibility of detecting PAH by negative ion atmospheric pressure chemical ionization. However, for test compounds such as benz[a]anthracene neither M- nor (M - H + 0)- ions could be detected (27). In this work EC-NICI and NICI, using CH4/N20reaction gas mixtures, have been applied to detect mutagenic as well as carcinogenic PAH and to differentiate isomeric PAH. The hydroxyl ion, which is generated by CH4/Nz0mixtures, is a rather universal reaction gas ion (23). When applied to PAH, ionization occurs by either proton abstraction or electron capture of a thermal electron. In addition most PAH undergo ion/molecule reactions with OH- as well as NO. and NzO, which are also present in the mixture. Differences in the abundance of the formed ion/molecule adducts (M + OH)-, (M - H + NO)-, and (M - H + N20)-can be used to distinguish isomeric PAH. Results for isomers with molecular

0003-2700/S3/0355-2290$01.50/00 1983 American Chemical Society

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weight 202 to 276 are reported. When high-resolution gas chromatography (HRGC) is combined with NICI, a few nanograms of compound is sufficient for a proper identification. The mass spectra are compared with those obtained by ECNICI using methane as reaction gas. The sensitivity of NICI is compared with electron impact ionization (EI). Furthermore, EC-NICI is very useful for the selective detection of PAH with biological activity (such as mutagenicity or carcinogenicity, see also ref 28 and 29). HRGC/NICI has been applied to air particulate extracts containing PAH, and the results are compared with those obtained by EI. Furthermore some instrumental aspects such as filament lifetime and interferences by traces of oxygen are discussed.

EXPERIMENTAL SECTION Chemicals and Reagent Gases. All solvents were of analytical grade or higher. PAH reference compounds of 95-99% purity were used (Ultra Scientific, Hope, RI; Nanogens, Watsonville, CA). Methane (99.995%) and nitrous oxide (99.99%) were purchased from L'Air Liquide, 75321 Paris, France. The reference compounds were dissolved in toluene and diluted further with cyclohexane. The concentrations were between 4 and 10 ng/pL. Instrumentation. Chemical ionization and electron impact ionization mass spectra were recorded on a Hewlett-Packard 5985B mass spectrometer equipped with a Hewlett-Packard 5840 gas chromatograph The jet-separator and all shut-off valves were removed from the original GC/MS interface system. The effluent from the capillary column was introduced into the ion source through a homemade open-split interface equipped with a 0.1 mm i.d. fused silica transfer line. Reaction gases were introduced coaxially to the transfer line by using a makeup gas adapter from a Hewlett-Packard 5880 gas chromatograph. Construction details are available on request. A restriction made from a stainless steel capillary of 50 cm length and 0.25 mm i.d. was placed between the adapter and the shut-off valve (type SS-2H, Nupro Comp., Willoughby, OH) to avoid a too large pressure drop over the needle valves (type SS-BMG, Nupro Comp.). This measure helped considerably to maintain a stable ion source pressure over a period of at least 8 h. The ion source pressure was first monitored by a Batrachon capacitance manometer with feed-back system as described in ref 30, which was mounted on the calibration gas inlet system. Later it was replaced by a single thermocouple gauge and the stainless steel capillary restridor, which gave a comparable pressure stability in the ion source. However, only relative pressure measurements were then possible. Ionization of the reagent gas was accomplished with a 90-150 eV beam of primary electrons from a heated rhenium f i e n t with an emission current of 200 FA. A 70-eV electron beam was used for EL The source temperature was 175-200 "C and the source pressure was on the order of 0.4 torr (corrected) for methane and 0.5 torr (corrected) for methane/nitrous oxide mixtures (approximately 0.2 + 0.3 torr). The pressure and mixing ratio were optimized for both maximum sensitivityand yield of OH- ions. Optimum ion source conditions in the NICI mode were established using perfluorobenzonitrile ( m l e 193) and perfluorotributylamine ( m / e 452 and 633), respectively. Gas Chromatographic Separation. The following columns were used glass capillary, 25 m length, 0.32 mm i.d., coated with OV 1, 0.1 pm film thickness (G. and H. Jaggi, 9043 Trogen, Switzerland); fused silica capillary, 30 m length, 0.3 mm i.d. (Hewlett-Packard,Palo Alto,CA) coated with SE 54,0.17 pm film thickness. Separation conditions were as follows: injector temperature, 270 'C; interface temperature, 270 "C; carrier gas, He at a flow velocity of 35 cm/s; injection of 1FL sample splitless at 40 "C, 0.7 min at 40 "C, then 40 to 100 "C at 30 "C/min and 100 to 280 "C at 5 "C/min. Sampling and Cleanup of Air ParticulateMatter. Aerosols from 400 to 500 m3of air were collected on precleaned glass fiber filters of 142 mm diameter (Gelman Type 61635, Ann Arbor, MI) by using the same equipment as described earlier (31). Samples were Soxhlet extracted with 100 mL of cyclohexane for 8 h. Most of the interfering sample matrix was removed by liquid/liquid extraction according to Grimmer and co-workers (32).The crude PAH extract was purified further on a highly active silica column

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(see ref 32). The extracts were normally concentrated to 0.5 mL.

RESULTS AND DISCUSSION The control of PAH concentrations in both air particulate matter and the vapor phase has become an important routine analysis in many countries. Normally PAH with known carcinogenic properties and major PAH compounds, present in the sample, are quantified. One problem of the quantitative determination is that minor compounds with high carcinogenic potential are frequently disturbed in the gas chromatogram by other PAH with lower activity. Therefore a detection method, which is selective for carcinogenic and for mutagenic PAH, is highly desirable. Since there are big differences in the carcinogenic properties of isomeric PAH, this method also should be able to differentiate isomers. The majority of compounds with biological activities (mutagenic, carcinogenic, toxic, electron transmitter in cells, etc.) are also able to form stable negative ions (17-20). This property may be utilized by ECD or EI-NICI for analyzing such compounds. The promising results of von Ardenne (25) and especially Hunt et al. (14,24)in differentiating isomeric PAH by NICI have not stimulated further activities. One of the main reasons was the lack of commercially available instruments for negative ion mass spectrometry a t that time. Therefore in 1980 Lee et al, still concluded that "negative-ion mass spectrometry of PAH has not been generally accepted as a very useful tool because of the low yield of negative ions usually obtained" ( 4 ) . Since most instrument suppliers now offer the possibility to detect negative ions, the use of NICI mass spectrometry has become more attractive. However, as outlined in the Experimental Section, there is still a need to modify commercial instruments to improve both sensitivity and reproducibility of the results. NICI with Methane. The efficiency of the electron capture mechanism depends on the temperature and the pressure in the ion source as well as on the energy of the primary electrons. The pressure/temperature optimum was in the range 0.3-0.5 torr (corrected) and 150-200 "C (see also ref 33). An optimum value of 90-120 eV was found for the primary electron beam. NICI mass spectra of unsubstituted PAH with a minimum of three condensed rings give only molecular weight information (see Table I). PAH containing a =CH- or -CHzgroup in the ring system have the (M - H)- ion as base peak. Deprotonation induced by 0, and OH- ions, which are formed from traces of O2and HzO in the carrier gas of the chromatographic column, is one possibility and loss of a proton after electron capture ionization is another one. Deprotonation leads to an extended delocalization of the r-electron system and an adequate gain in resonance energy. The M- abundance for all such ring systems is very close to the isotopic 13C contribution, which indicates that ionization by electron capture is either not of importance or immediately followed by deprotonation. Some five- and six-ring PAH show (M + 1)- abundances which are considerably higher than the 13C isotope contribution. H-, which is also formed in the ion source, normally reacts immediately with traces of water in the system to OH(7). However, a direct reaction of H- with PAH cannot completely be excluded and may be the reason for the increased abundance of (M + 1)-. Ionization by reaction with H-has also been observed for hexachlorodibenzo-p-dioxins(34). (M - HI-is also the most abundant ion for methyl-substituted PAH with small ring systems. According to the Huckel molecular orbital (HMO) theory the energy gain caused by the extension of the delocalized r-electron system to C atoms outside the ring system decreases with the growing size of the delocalized system (35). This is in agreement with the mass spectrometric results, which show that deprotonation

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Section of the total ion chromatograms of an air particulate extract containing P A H (A) electron impact ionization: (8)negative ion chemical ionization using methane: (C)use of CH,lN,O mixture as reaction gas. Due to overloading effects of the column, benz[a]anthracene was not separated from chryseneltryphenylene. The inactive: (+) carcinogenic activity of the compounds is marked: (0) medium actii: (++)highly active (for m e infwmathn. sea Appendix in ref 4 and 42). Flgure 1.

is suppressed with the increased number of C atoms in the ring system. Differences in the M- abundance8 of isomeric methyl-PAH were observed. The differences in the (M H)-/M- ratio are small (5-10%) and do not allow a proper identification. Exceptions are the differentiation between methylhenzophenanthrenes and -anthracenes as well as methylphenanthrenes and -anthracenes (see Table I). Since oxygen efficiently captures thermal electrons, even small leaks cause a considerable decrease in sensitivity. Removal of the jet separator and all shut-off valves from the GC/MS interface resulted in a pressure decrease from 6 X torr to 5 X torr for the closed system and a gain in sensitivity by a factor of 10. Monitoring of m / e 233 and 235 (rhenium trioxides formed by reaction of 0,with the hot filament (36))can be used to minimize the leakage of O2 into the mass spectrometer. Figure 1 shows a practical application of the EC-NICI technique. The PAH group with molecular weight 216 has a very complex composition in air particulate extracts. Members of this group are benzofluorenes, methylpyrenes,

and methylfluoranthenes, which cannot he separated completely by high-resolution gas chromatography. From all compounds of this group only methylfluoranthenes are mutagenic ( 4 ) and give a sufficiently high NICI response. NICI is in this case very useful to screen samples for these compounds. The concentration of this compound group in the vapor phase is even higher (37). Most of the compounds with carcinogenic and/or mutagenic effects shown in Figure 1can be detected by EC-NICI. Table III shows the response factors for both mutagenic and nonmutagenic PAH obtained by El-NICI. The 22 PAH can be divided roughly into two groups, one giving a response equal or less to E1 and one showing an increased response compared to EI. The latter contains most of the compounds with mutagenic activity. However, one has to be careful in equating a high EI-NICI response to mutagenic properties. Since the electron affinity of PAH increases with the number of ring systems, one should only compare the response factors within the same ring number group. While a perfect correlation can he obtained for the five-ring group with molecular weight 252, there are some discrepancies within the four-ring members. Fluoranthene, which is considered to be non- or relatively weakly mutagenic gives a stronger response than expected compared to benz(a1anthraceneknown to he a strong mutagen. Horning and co-workers (27) also found abnormal behavior in the latter compound under atmospheric pressure NICI conditions. No negative ions could be observed and no suitahle explanation could be given for this. Estimations of the EI-NICI response based on electron affinities predict a lower response for fluorantheue than for benz[a]anthracene which agrees with experimental results for the ECD given elsewhere (38). However, a resonable explanation of the response differences between fluoranthene and henz[a]anthracene using EC-NICI can be given by the HMO theory (35). Fluoranthene is a nonalternating PAH system. HMO calculations (see also ref 35) show that the lowest nonoccupied r orbital is lower than for the alternating benz[alanthracene system, which may explain the strong response of fluoranthene. On the other hand, the level of the lowest nonoccupied r orhital of the nonalternating henzofluorenes is higher than that of fluoranthene, which is in g d agreement with the response fadors obtained by EC-NICI. The following conclusions seem viable based on the results in Table I11 There is a strong indication that PAH compounds with EC-NICI responses higher than for E1 may have some mutagenic properties and should be investigated further. However, one has to be careful with the interpretation of response data obtained from nonalternating PAH, with low-lying nonoccupied r orbitals. For some compounds such as benzo[a]pyrene and perylene a better correlation between mutagenic properties and EINICI response is obtained than for the EDC (for comparison, see ref 29 and 40). Further work is in progress to characterize the EI-NICI response of other PAH compounds and to compare it to their mutagenic behavior. NICI with CH,/N,O Mixtures. Optimum sensitivity was obtained in the same temperature and pressure range as for EC-NICI. The composition of the CH4/N20 mixture was optimized to yield a maximum concentration of OH- ions in the ion source. For a clean ion source the mixing ratio was ahout 0.2 torr CH, and 0.3 torr N,O. Impurities in the N,O reagent gas such as 0, and H20 cause an increased background up to m / e 150 due to reagent gas polymerization products containing electrophilic groups. In addition, the formation of rhenium oxides is favored. PAH are ionized by different competitive mechanisms, which occu simultaneously in the ion sauce: electron capture

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Table I. Electron Capture Negative Ion Chemical Ionization Mass Spectra of Some Polyaromatic Hydrocarbons'" compound

mol wt

(M - H)-

142 142 166 178 178 192 192 202 202 21 6 216 21 6 216 24 2 24 2 242 252 252 252 252 252 252 266 276 276 276

2-methylnaphthalene 1-methylnaphthalene fluorene phenanthrene anthracene 2-methylanthracene 1-methylphenanthrene fluoranthene pyrene benzo [a]fluorene benzo [b] fluorene 2-methylpyrene 2-methylfluoran thene 3-meth ylbenzolc]phenanthrene 8-methylbenz [a]anthracene 10-methylbenz[alanthracene benzo[ blfluoranthene benzo [j]fluoranthene benzo[ hlfluoranthene benzo [elpyrene benzo[a]pyrene perylene 7-methylbenzo[o]pyrene indeno[1,2,3d]pyrene benzo[ghi]perylene anthanthrene

M-

(M t 1)-

12.6 13.2 14.8

100 100 100

b

100 58.5 23.2 100

20.1 100 100

13.7 17.3

b

19 lgc 29 31.4 25.7 100 100 100 100 100

100 100 100 100 100 20.0 13.4

b 100 100

100 100 100 100

24.8 21.9 25.4d 26.5d 38.0d 23.2 24 24.1 22.2 24.3 24.1

a All relative abundances are expressed as percentage of the base peak. Average of four parallels, standard deviation Not determinable, too low response. Uncertain due to between 5.5 and 8.5%. Ions below 2% have been omitted. contribution, see text. Higher than expected from low response (see Table 111).

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Table 11. Negative Ion Chemical Ionization Mass Spectra of Polyaromatic Hydrocarbons with Methane/Nitrous Oxide Methane/Nitrous Oxide Mixturesa compound

mol wt

(M - 1)-

M-

(M

+

1 ) - (M t OH)-

(M - 1)NO- (M - 1)N,O-

14.1 166 100 100 17.5 47.0 202 78.5 4.3 80.5 10.9 3.8 2.8 100 202 58.5 19.5 216 100 19.0 216 100 100 216 18.5 216 80.5 100 3.2 24.5 228 100 3.2 22.0 20.2 228 100 4.6 43.5 18.9 228 100 4.7 45.7 53.3 228 100 7.6 79.4 2.8 30.3 100 19.3 242 242 100 4.9 30.6 252 16.0 100 32.0 4.2 100 252 33.9 21.5 21.5 5.3 100 252 16.5 22.5 3.6 4.4 74.0 252 100 16.5 2.1 5.1 24.5 100 252 10.5 22.5 2.1 100 53.0 252 10.0 32.0 5.5 22.1 276 4.4 100 24.0 2.9 276 15.9 100 23.5 9.3 100 276 4.5 24.5 2.8 a All relative abundances expressed as percentage of the base peak. Average of four parallels, standard deviation between 6 and 9%. Ions below 2% have been omitted. fluorene fluoranthene pyrene benzo[a]fluorene benzo [ blfluorene Bmethylpyrene Pmethylfluoranthene benz [a]an thracene chrysene triphenylene benz[ blanthracene 3-methylbenzo[clphenanthrene 8me thylbenz [a]an thracene benzo[ b] fluoranthene benzo [jlfluoranthene benzo[h] fluoranthene benzo[e]pyrene benzo [alpyrene perylene indeno [ l ,2,3-cd]pyrene benzo [gh ilperylene anthanthrene

of thermal electrons creating the molecular ion M:, deprotonation yielding (M - H)-, and ion/molecule adduct formation with OH- giving (M + OH)- and (M - HI- by loss of HzO. Furthermore, there are two possibilities for the formation of the ion/molecule adducts (M - H + NO)- and (M - H N20)-: addition of NO- or N20 to (M - H)- or deprotonation of MNO. and MN20 adducts. As can be seen from Table I1 the abundance of the ions formed varies considerably within a group of isomeric PAH containing only condensed ring systems. Most isomers can be easily identified by their characteristic mass spectra. In

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the group with molecular weight 228, only chrysene and triphenylene are difficult to differentiate while five different mass spectra are obtained for the six compounds with molecular weight 252 (39). In general the differences in the mass spectra become smaller with the increasing number of rings. The mass spectra of all PAH containing =CHor -CHz- groups in the ring system have (M - H)- as base peak and do not show any adduct ions. The M- abundance corresponds to the 13Cisotope contribution of the (M - H)- ion. The gain in resonance energy makes deprotonation to the dominant reaction of the ion source. Differentiation of such

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ANALYTICAL CHEMISTRY, VOL. 55, NO. 14, DECEMBER 1983

FLUORANTHENE

IOj

20

,

,

,

,

,

,

,

,

,

,

rn/e

,I

60 20

,2[2J

200 220 240 260

I

PYRENE

"1

I BENZ0lj)FLUORANTHENE

269

20-

20

281 289 295

60

201

60

BENZOlbjFLUORANTHENE

245

217 231

1

rn/e

100

*le BENZldANTHRACENE

20

200 220 240 260

y7 257

m/e

271

220 240 2M) 280 100.

0~~

BENZOlghiIPERYLENE

6020

20

rn/e

220 240 260 280

I m/e

293

'i9

260 280 300 320

Flgure 2. NICI mass SDectra of some isomeric PAH obtained with CH,/N,O reaction gas mixture. Abundances are expressed as percentage of the base peak.

PAH isomers by CH4/N20mixtures is not possible. The use of weaker Brernsted bases (7) to suppress deprotonation and to favoring ion/molecule adduct formation will be studied to obtain differences in the mass spectra of this PAH class. Formation of (M - H)- by deprotonation is also the main reaction for methyl-substituted PAH. The gain in resonance energy obtained by extending the a-electron system to the C atom outside the ring system is dependent on the PAH structure. It is therefore possible to differentiate between, e.g., methylbenzanthracenes and methylbenzophenanthrenes or methylpyrenes and -fluoranthenes (see Table 11) but not between individual isomers. Nevertheless this helps to characterize groups of methylated PAH with a large number of isomers, which are not available as reference compounds. Figure 2 shows some examples of CH4/N20mass spectra of isomeric PAH. Response factors were compared by using the same optimization procedure (see Experimental Section) of the ion source for the E1 and the CH4/N20-NICImode. On the basis of the same scan range (60-350 m u ) and signal-to-background ratio, the response factors for NICI were on average a factor of 10 higher than for E1 (see Table 111). A similar increase in the sensitivity for PAH has been described by Grimsrud et al. using an electron capture detector with nitrogen makeup gas containing 0.2% oxygen (40). General advantages of the CH4/N20-NICI technique are as follows: all PAH give an increased response as compared to the E1 mode, there are smaller variations in the response factors as compared to the EC-NICI technique, and total ion current chromatograms are more comparable to those obtained by electron capture detection. Furthermore the differentiation of many isomeric PAH is possible with compound amounts of 1-2 ng. As can be seen from Figure 1the information from the E1 and EC-NICI total ion current gas chromatograms can be obtained simultaneously when the CH4/N20mixture is used. This makes the method useful for the search of PAH in a sample of unknown composition. Disadvantages are decreased selectivity in comparison to EC-NICI (unsaturated compounds, phenols, substituted benzenes, etc. are also ionized

Table 111. Comparison of Response Factors for Polyaromatic Hydrocarbons Obtained by NICI with Methane or Methane/Nitrous Oxide and Electrom Impact Ionizationai b

compound

negative ion chemical ionization methane/ electron nitrous impact methane oxide

dibenzothiophene 0.84 0.26 13.3 phenanthrene 0.94 0.04 11.6 anthracene 0.89 0.54 12.3 Bmethylanthracene 0.35 10.4 0.78 1-methylphenanthrene 0.84 0.30 12.7 fluoranthene 8.90 17.9 0.99 pyrene 0.03 11.3 1.00 benzo [a]fluorene 0.12 11.2 0.99 benzo[ blfluorene 1.07 0.16 9.1 benz [alanthracene c 6.50 1.09 triphenylene 1.11 Of4 1.3 benzo[ blfluoranthene 1.07 38 21.2 benzo[j] fluoranthene 1.12 42d 23.8 benzo [k]fluoranthene 1.02 36d 22.7 benzo [elpyrene 0.01 5.5 0.80 benzo [alpyrene 120d 31.0 0.89 perylene 0.90 1.5 5.2 indeno[ 1,2,3-cd]pyrene 0.81 120d 45 benzo [ghilperylene 0.87 64d 20.1 anthanthrene 170d 35.5 0.75 a Response factors for electron impact ionization relative to pyrene. Response factors for negative ion chemical ionization relative to electron impact ionization. The results are the average of four parallels. Standard deviation between 4 and 9%. Not determined. Close to saturation of the ion source. to some extent), a shorter lifetime of the filament (in this case some weeks), contamination of the ion source by rhenium oxides, and an increased background in comparison to ECNICI below m/e 70. The cleaning intervals for the ion source

ANALYTICAL CHEMISTRY, VOL.

are in the order of 2 to 3 weeks within continuous use. The optimum pressure in the ion source shifts very rapidly to values above 0.8 torr when the reaction chamber in the ion source becomes excessively dirty. The CH4/N20-NICImethod has been used sucessfully to determine the concentration of benzoljl- and benzo[b]fluoranthene in particulate matter. These compounds cannot be separated by gas chromatography on stationary phases normally used for routine analysis such as SE 52, SE 54, and OV 1, but the differences in the mass spectra (see Table 11) allow the quantification of both compounds by an iterative process. As reported recently they can now be separated on liquid crystal phases (41). However, these stationary phases are not commercially available a t the moment. Another application is the detection of interferences in the quantitative analysis of benzo[a]fluorene. Some samples from industrial sites (PAH released from coal electrodes used in aluminum smelters) seemed to contain higher concentrations of benzo[a]fluorene relative to benzo[ blfluorene than urban air samples. With CH4/N20-NICIit could be shown that the benzo[a]fluorene signal was partly due to interference by a considerable amount of 1-methylfluoranthene. The mass spectruin of benzo[a]fluorene has m/e 215 as base peak (a 4 H 2 - group PAH) while 1-methylfluoranthene gives m/e 216 as the most abundant ion. This allows a qualitative check whether the signal area of benzo[a]fluorene is influenced by the coeluting 1-methylfluoranthene or not.

ACKNOWLEDGMENT The author thanks D. J. Dixon, Hewlett-Packard, Waldbronn, West Germany, for valuable suggestions during this work and J. Clench-Aas for reading the manuscript. Registry No. 2-Methylnaphthalene, 91-57-6; l-methylnaphthalene, 90-12-0; fluorene, 86-73-7; phenanthrene, 85-01-8; anthracene, 120-12-7; 2-methylanthracene, 613-12-7; l-methylphenanthrene, 832-69-9; fluoranthene, 206-44-0;pyrene, 129-00-0; benzo[a]fluorene, 238-84-6; benzo[b]fluorene, 243-17-4; 2methylpyrene, 3442-78-2;2-methylfluoranthene, 33543-31-6;3methylbenzo [c]phenanthrene, 2381-19-3; 8-methylbenz [ a ] anthracene, 2381-31-9; 10-methylbenz[a]anthracene,2381-15-9; benzo[b]fluoranthene, 205-99-2; benzo[i]fluoranthene,205-82-3; benzo[k]fluoranthene,207-08-9; benzo[e]pyrene, 192-97-2;benzo[a]pyrene,50-32-8;perylene, 198-55-0; 7-methylbenzo[a]pyrene, 63041-77-0;indeno[1,2,3-cd]pyrene,193-39-5;benzo[ghi]perylene, 191-24-2; anthanthrene, 191-26-4.

LITERATURE CITED Vassilaros, D. L.; Kong, R. C.; Later, D. W.; Lee, M. L. J. Chromatogr. 1982, 252, 1-20. Janssen, F. Int. J. Anal. Chem. 1982, 13, 37-54. Lee. M. L.; Vassilaros, D. L.; White, C. M.; Novotny, M. Anal. Chem. 1979, 57,768-773. Lee, M. L.; Novotny, M. V.; Bartle, K. D. "Analytical Chemistry Chemistry of Polycyclic Aromatic Compounds"; Academic Press: New York, 1981.

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RECEIVED for review April 14,1983. Accepted August 26,1983. This work was partly presented at the 9th International Mass Spectrometry Conference, Vienna, Austria, 1983.