6lB
Anal. Chem. 1981, 53, 619-625
Determination of Polyhalogenated Hydrocarbons by Glass Capillary Gas Chromatography-Negative Ion Chemical Ionization Mass Spectrometry Frank W. Crow"' and Alf Bjorseth2 Battelle Columbus Laboratories, 505 King Avenue, Columbus, Ohio 4320 1
Kenneth T. Knapp and Roy Bennett Environmental Sciences Research Laboratories, U.S.Environmental Protection Agency, Research Triangle Park, North Carollna 277 1 1
Negative ion chemical ionization m a s spectrometry is evaiuated as a method for qualitative and quantitative determlnation of polyhalogenated aromatic hydrocarbons. The major compounds of interest In this study are chlorlnated and brominated benzenes. Aroclor 1242, Aroclor 1268, Chloralkylene 12, and 1,2,3,4-tetrachiorodlbenzo-p4ioxin (TCDD) are also studied. Each compound or mixture of compounds is analyzed by glass capillary gas chromatography-mass spectrometry by using different modes of chemical ionization in the mass spectrometer. Instrumental parameters are kept constant so that reiatlve sensitivity comparisons between varlous modes of ionization are possible. The modes of ionization employed are methane postlve ion CIMS, methane negative ion CIMS, methane/CH,CI, chloride attachment negative ion CIMS, and methaneloxygen negative ion CIMS.
Halogenated aromatic hydrocarbons, such as chlorinated benzenes, chlorinated pesticides, or polychlorinated biphenyls (PCB) are ubiquitous pollutants in the environment. Due to their lipophilic and persistent characteristics, they exhibit bioaccumulating and biomagnifying properties. Consequently there is a need to develop sensitive and specific methods to detect and quantify low levels of these compounds in environmental samples. Currently the technique most commonly used to analyze for these compounds is gas chromatography with electroncapture detection. This method is very sensitive but does not provide evidence for compound identification other than retention time. With the complex mixtures usually encountered in environmental samples, this may give inconclusive results. An analytical method involving negative ion chemical ionization mass spectrometry (negative ion CIMS) is capable of similar sensitivities to that achieved by the above electroncapture detector method, but in addition it has the possibility of verifying the compound's identity through its mass spectrum. Negative ion CIMS is a new technique and ita current applications to environmental chemical analysis are scattered and diverse. Dougherty and co-workers have applied this technique to screening for environmental contamination by toxic residues, including chlorinated aromatic pesticides (I-3), pentachlorophenol, and 2,4,5-T ( 4 ) . Hunt and co-workers have shown that oxygen negative ion CIMS can provide a sensitive and specific probe for polychlorinated dibenzo-p-dioxins (5). Negative ion CIMS has been employed by Hass et al. to measure these compounds in biological samples (6). In this work the highest sensitivity was achieved by using methane Present adddress: Midwest Center for Mass Spectrometry, Department of Chemistry, University of Nebraska, Lincoln, NE 68588. Present address: Central Institute for Industrial Research, Blindern, Oslo 3, Norway.
as a reagent gas. However, a mixture of methane and oxygen produced sensitivities almost as high as with methane alone and gave better structural information. The use of pure oxygen as the chemical ionization reagent gas had lower sensitivity, Under optimal conditions, using selected ion monitoring, a sensitivity of 100 pg was achieved for 2,3,7,8-TCDD and 1 pg for 1,2,3,4,6,7,8-HpCDD. Hunt and co-workers have applied a simultaneous recording of positive and negative ion CIMS using a Townsend discharge ion source (7,B). Their results on aromatic compounds indicated that compared to positive ion CIMS, negative ion CIMS can provide a 100- to a 1000-fold increase in sample ion current, as well as unique structural information and confirmation of sample molecular weight (8). Atmospheric pressure ionization mass spectrometry has been used to show that certain chlorinated aromatic compounds are ionized in the presence of nitrogen which contains approximately 0.5 ppm of oxygen (9). Chlorobenzene and o-dichlorobenzeneyielded only chloride ions, while more highly chlorinated benzenes formed phenoxide ions. Sub-picogramlevel detection of 2,3,4,5,6-pentachlorobiphenyl was demonstrated by selective monitoring of the corresponding phenoxide ion (9). A review of negative ion chemical ionization mass spectrometry has recently been published (IO). This review includes a consideration of negative ion-molecule reactions with special attention to their possible applications. The studies cited above indicate that negative ion CIMS is a technique very well suited for providing additional information on organic pollutants in environmental samples. Of particular interest is the capability of detecting and identifying picogram quantities of chlorinated aromatic hydrocarbons. The purpose of this study is to investigate the application of negative ion CIMS to the qualitative and quantitative analysis of these coqpounds.
EXPERIMENTAL SECTION Mass Spectrometry. All mass spectra were recorded on a Finnigan Model 3300 quadrupole mass spectrometer (Finnigan Corp., Sunnyvale, CA) equipped with a standard chemical ionization source. The CI reagent gas was ionized by using a 100-eV beam of electrons generated by a heated rhenium filament. Reagent gases were maintained at a pressure of 0.6-0.8 torr (gauge)., Positive ion CI mass spectra were generated by using methane as a reagent gas. Electron capture negative ion CI mass spectra were also obtained by using methane as a reagent (11). For the generation of chloride ions (C1-) as a reagent, approximately 0.05 torr of dichloromethane was bled into the CI plasma (12). Oxygen-enhanced methane negative ion CI mass spectra were generated by adding approximately 0.1 tom of oxygen to the CI plasma (6). Source temperatures were maintained in the range of 100-150 "C. The modifications done to the circuitry of the mass spectrometer to control the source and lens voltages to facilitate
0003-2700/81/0353-0619$01.25/0 0 1981 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 4, APRIL 1981
Table I. Retention Times of the Halogenated Benzenes on a 15-m SE-52 Capillary Columna compd retention compound no, time, min chlorobenzene bromo benzene 1,4-dichlorobenzene 1,3-dichlorobenzene 1,2-dichlorobenzene 1,3,5-trichlorobenzene 1,2,4-trichlorobenzene 1,4-dibromobenzene 1,3-dibromobenzene 1,2-dibromobenzene 1,2,3-trichlorobenzene 1,2,3,5-tetrachlorobenzene 1,2,4,5-tetrachlorobenzene 1,2,3,44etrachlorobenzene
1,3,5-tribromobenzene pentachlorobenzene hexachlorobenzene tetrabromobenzene hexabromobenzene Column temperature programmed at 4 "C/min from injection time,
8 1 11 10
9 14 13
4 3 2
12 16 17
2:38 2:53 3:38 3:43
4:02 5:42 6:41 6:51 6:52 7:27 7:30 10: 23
10:23
15
11:43
5
6
12:50 15:30 20:31 21:06
7
36:31
18 19
from 60 to 280 "C
collection of either positively or negatively charged ions are the same as described by Hunt et al. (8). Positive and negative ions were detected by using a Galileo Model No. 4770 continuous dynode electron multiplier (Galileo Electrooptics Corp., Sturbridge, MA) equipped with a conversion dynode (13). The use of this multiplier to obtain high negative ion signals with low noise is documented elsewhere (11). Both positive and negative ion data were acquired by using an INCOS data system (Finnigan Corp., Sunnyvale, CA). Gas Chromatography. Samples were introduced via a Finnigan Model 9500 gas chromatograph (Finnigan Corp., Sunnyvale, CA) modified with a Grob-type capillary column injector constructed in-house. The GC was interfaced to the mass spectrometer via a heated all-glass capillary transfer line which went from the end of the column directly into the CI ion source (14). This transfer line minimized tailing due to dead volume and active sites, as well as reduced sample loss. It also provided a coaxial inlet for the CI reagent gas. A 15-m SE-52 capillary column was employed throughout the study. Typical GC conditions were as follows: injector temperature, 280 "C; transfer line temperature, 280 "C; GC carrier gas, methane; split ratio, lO/l; column temperature, 60-280 "C at 4 "C/min; flow rate, 25 cm/s. The initial injection temperature was raised from 60 to 150 "C for the analysis of the larger chlorinated aromatics (PCBs and TCDD). Methane was employed as a carrier gas because it was found that helium or hydrogen tended to disturb the CI reagent ion plasma. The use of methane caused no noticeable change in the chromatography. Reagents and Chemicals. Methane (Matheson Grade UHP) was purchased from Matheson Gas Products, Joliet, IL,and passed through a hydropurge gas filter (Applied Science Laboratories, State College, PA) before use as GC carrier gas or CI reagent gas. The oxygen was purchased from Linde, Union Carbide Corp., New York, NY. Solvents were distilled-in-glass grade (Burdick & Jackson, Muskegon, MI). The Aroclor 1242, Aroclor 1268, Chloralkylene 12, 1,2,3,4-tetrachlorodibenzo-p-dioxin, and the halogenated benzenes were purchased from RFR Corp., Hope, RI.
RESULTS AND DISCUSSION Halogenated Benzenes. Gas Chromatography. Table I presents the retention times for the 19 halogenated benzenes analyzed. In only three cases does a problem in separation exist. Compounds 11 and 10, 1,6dichlorobenzene and 1,3dichlorobenzene, have very similar retention times. The same is true with the pairs l,4-dibromobenzene (4),1,3-dibromobenzene (3) and 1,2,3,5-tetrachlorobenzene(16), 1,2,4,5tetrachlorobenzene (17). In every other case either there is
GC separation of each compound or, where overlap takes place, the two compounds are separated by their mass spectra. Mass Spectrometry. Methane Positive Zon CIMS. The methane positive ion CI mass spectra of the halogenated benzenes were recorded to serve as a reference for sensitivity and identity information. Each compound was present in solution a t a concentration of 50 ng/pL and split injections (1:lO) were used. Each compound was therefore present at the level of 5 ng injected on column. Table I1 presents the methane positive ion CI mass spectra of these compounds. The sensitivity numbers are in arbitrary area units which can be compared not only within the table but also to the Sensitivity values in Tables 111-V. Each compound produces prominent M H+ ions and ions at M + C2H6+and M + C3H5+,which are typical of methane CIMS. The brominated benzenes undergo rearrangements or decompositions to produce the ions presented in the "other" column. The identities indicated are assigned by analyzing the isotopic patterns of the ion clusters in the mass spectra. The mechanisms producing these ions are unknown. There is a slight drop in sensitivity as the chlorine content of the compound increases. A similar but more noticeable decrease in response is observed with the brominated benzenes. There are two reasons for this decrease in sensitivity. Since there is a constant weight (5 ng) of each compound present, as the molecular weight of the compound increases the molar concentration decreases causing a smaller signal. This does not account entirely for the decrease in signal. As the number of halogen atoms on the molecule increases, the compound's ability to attach a proton and to stabilize the resulting positive charge decreases (Le., the proton affinity decreases). Methane Negative Ion CIMS. When methane is used in the negative ion mode, no negative reagent ions are produced. However, a large population of thermal electrons is produced which can be captured by molecules with a high electron affinity, mimicking the performance of an electron-capture detector. The only special consideration is to keep the mass spectrometer ion source cool. As the source temperature is increased the probability for dissociative electron capture increases and the probability of nondissociative electron capture decreases (15). This results in the production of halogen anions (C1- and Br-) which give little information as to the identity of the parent compound. Table I11 reports the negative ion CI mass spectra of the halogenated benzenes by use of the electron-capture mode. Several trends can be observed. The low halogenated compounds are not detected. Bromobenzene (l),chlorobenzene (8), and the dichlorobenzenes (9-11) produce no detectable signal. This correlates with the low signal intensity observed when using an electron-capture detector for these compounds. Electron affinity increases rapidly with increased bromine content, as can be seen by the strong signals produced by dibromo- to hexabromobenzene (2-7). However, even though the ion source is kept cool, dissociative electron capture dominates, producing primarily Br-. Only pentabromobenzene (6) and hexabromobenzene (7), show nondissociative electron capture, as indicated by the presence of the molecular anion. The lower brominated compounds do produce ions indicative of the molecular weight. They have a weak M-H- ion, also a product of dissociative electron capture. None of these M-H- ions have an intensity greater than 0.5%. The sensitivities for the detected brominated benzenes are higher than the sensitivities observed under positive ion CIMS conditions (except for 1,4-dibromobenzene). The chlorinated benzenes (8-19) do not typically show enhanced sensitivity using methane electron capture negative ion CIMS. The mono- and dichlorobenzenes are not detected and the tri- and
+
ANALYTICAL CHEMISTRY, VOL. 53, NO. 4, APRIL 1981
Table 11. Methane Positive Ion Chemical Ionization Mass Spectra of the Halogenated Benzenes" intensities other M t H' M t C,H,+ M t C,H,+ compd
a
1 2
90.1 75.6
8.6 8.7
1.3 0.9
3
79.0
8.5
1.2
4
77.2
9.7
1.0
5
54.5
5.3
1.7
6
43.4
2.7
7
46.6
8 9 10 11 12 13 14 15 16 17 18 19
87.9 86.1 86.2 84.0 86.9 87.5 89.3 90.1 91.0 89.0 93.5 92.1
10.4 12.5 12.4 14.8
1.7 1.4 1.4 1.2
12.0 12.0
0.5
(M t (M t (M t (M + (M t (M t (M t (M t (M t (M t (M t (M t (M t (M t (M t (M t (M + (M t (M t
H - Br)' H, - Br)' C,H, - Br)' H - Br)+ H, - Br)' C,H, - Br)' H - Br)' H, - Br)' C,H, - Br)' H - Br)' H, - Br)+ C,H, - Br)' H- Br)' H, - Br)+ C,H, - Br)' C,H, t H - Br)' H - Br)' H, - Br)' C,H, - Br)'
sensitivity 5.7 4.5 4.7 4.4 3.5 3.4 4.9 2.9 4.3 20.0 9.6 8.9 18.9 10.3 19.8 4.8 10.2 20.1 23.1
1.1
10.1
0.6 0.6
9.3 8.0
11.0 6.5 7.9 Intensities in terms of percent total sample ion current.
Table 111. Methane Negative Ion Chemical Ionization Mass Spectra of the Halogenated Benzenes" intensities halogen other Mcompd (C1- or Br-) M - 11 2
3 4 5 6 7
60.6
0.2
0.5 0.5 0.4 4.3 30.4
(M - Br)(M - Br)(M - Br)m/e 282 (M - Br)m/e 390
0.2 0.1
0.2 2.6 8.3 0.7
200 260 120 71
38
120 330 300 230 420 3 20 210 130 170 170 130 170
sensitivity 460 250 150 770 680 260 ND ND ND ND
8 9
a
140 230
ND
99.8 99.5 99.3 99.5 92.9
10 11 12
89.6
13
88.8
18
90.9 90.4 90.6 90.4 5.0
19
0.8
14 15 16 17
621
10.4 11.2 9.1 6.4 5.6 9.6
10
92.3 97.0
Intensities in terms of percent total sample ion current.
tetrachlorobenzenes produce signals a t least 5 times and as much as 50 times less intense than those seen for the positive ion CIMS. The above performance abruptly changes with the pentaand hexachlorobenzenes (18, 19). Both of these compounds produce molecular anions with signal intensities 5-20 times
(M - C1)(M - C1)-
3.2 3.5
(M - C1)(M - 19)-
2.2 0.5 2.0 0.2
(M - (21)(M - 19)'
6 6 25 20 4 600 3200
more intense than the positive ion CIMS signals. Ion source temperature studies on the chlorinated benzenes indicated that the signal intensities for these compounds could be increased by raising the temperature of the ion source. However, this resulted in exclusive production of C1-. This route was not chosen, since molecular weight information is totally lost.
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 4, APRIL 1981
Table IV. Methane/CH,Cl, Chloride Attachment Negative Ion Chemical Ionization Mass Spectra of the Halogenated Benzenesa intensities other compd M(M - X)CH,Cl,Br' 1 2 3 4 5 6 7 8 9 10 11 12 13
7.0 60.6 9 2.4
5.0
100 100 100 93.0 39.4 2.6
ND ND
27.9
16
34.4
17.3
17
a
10 5.3 4 .O 19 29 130
ND
14 15
18 19
96.9 91.7
2.4 2.1
ND ND
(M- 1)(Mt 14)-
50.9 49.1
(M-1)(Mt 14)(M- 1 ) (M+ 14)(M-1)(M t 14)(M- 19)-
21.9 32.8 21.9 32.8 44.2 55.8 0.7 0.2
(M - 19)-
0.3
ND 4.2 4.0
1.7 560 2900
Intensities in terms of percent total sample ion current,
Table V. Methane/O, Negative Ion Chemical Ionization Mass Spectra ofthe Halogenated Benzenesa intensities other compd MM - 1M - 19 (or 63) 1 2 3 4 5 6
7
0.2 0.4 7.1
0.1 0.7 0.9
Br Br Br Br Br Br Br(M - Br)-
8 9 10 11 12 13 14 15 16 17 18 19 a
sensitivity ND
51.7 59.7 50.5 70.1 11.1 19.1 14.4 6.9 6.0 6.4 1.6 4.7
0.5 0.7 1.3 12.6 22.1 13.5 78.8 88.5
c1c1c1c1c1-
c1cl-
c1c1-
c1c1c1-
100 100 100 100 99.7 98.9 91.5 0.5 48.3 40.3 49.5 29.9 88.4 80.4 84.3 80.5 71.3 80.1 19.6 6.8
sensitivity 15 940 1300 530 2500 2700 1900 8.3 50 50 44 130 150 130 340 340 270 2900 3600
Intensities in terms of percent total sample ion current.
Methane/CH2 Chloride Attachment Negative Ion CIMS. Since the electron-captureprocess proved less than satisfactory for the analysis of the halogenated benzenes from the standpoint of both sensitivity and spectral quality, ion molecule reactions were explored. If one admits approximately 0.05 torr of CHzClzto the methane negative ion CI plasma, the CHzClzcaptures the thermal electrons and produces intense C1- ions by dissociative electron capture. These ions can be used as reagents to attach a chlorine, abstract a proton, or transfer a negative charge (12). Table IV presents the methane/CH,Cla negative ion CI mass spectra of the halogenated benzenes. This mode of ionization was also unsatisfactory for this analysis. Bromobenzene (1) was not detected. The dibromobenzenes (2-4) produced Br- by dissociative electron capture which subsequently reacted with the CHzCl2to produce CHzCl2Br-. The
remaining brominated benzenes show increased production of the molecular anion with increased bromine content in the molecule, but the signals are less intense than those seen by using electron capture (Table 111). The low chlorinated benzenes were not detected. The higher chlorinated benzenes produced molecular anions (12, 16,18, 19); however, the sensitivities were low. Pentachloro(18) and hexachlorobenzene (19) show intense molecular anions comparable with those seen by using electron capture. Methane/Oz Negative Ion CIMS. The introduction of approximately 0.1 torr of oxygen into the methane reagent gas has shown promise for chlorinated aromatic compounds. The exchange of an oxygen for a halogen results in the formation of a phenolic anion (9). Table V shows the oxygenenhanced methane negative ion CI mass spectra of the halogenated benzenes. This is the only negative ion mode studied
ANALYTICAL CHEMISTRY, VOL. 53, NO. 4, APRIL 1981
623
~
Table VI. Positive and Negative Chemical Ionization GC-MS of Aroclor 1242 by Using Various Modes of Ionizationa CH,- CH,- C1'- O,/CH4retention PCB POS peak neg neg neg time, min no. CIMS CIMS CIMS CIMS no. 1
2 3
4 5 6 7
8 9 10 11
12 13 14 15 16 17 18
19 20 21 22 23 24 25 26 27 28 29 30
1
2 2 2 2 3 3
2 3
3 3 3 3 3 4 4 4 4 4 3 4 4 4 4 4 5 5 5 5 5 5
1
8
33 6 14 73 4 40 4 3 26 4 56 26 11
Table VII. Positive and Negative Chemical Ionization GC-MS of Aroclor-1268 by Using various Modes of Ionization" CH,- CH,Cl-- 0,/CH4neg retention peak PCB pos neg neg no, CIMS CIMS CIMS CIMS time, min no.
1
1 1 8
2 1
2 10 6 4 8 6
1
7 3 6 4 6
1
40 47 65 290 35 250 70 290 100 660 360 300 60 180 170 150 200 200 290 220 400 420 390 140
3:18 3:45 3:58 4:06 4:33 5:05 5:07 5:19 5:33 6:02 6:18 6:35 6:49 6:57 7:21 7:29 7:35 8:OO 8:06 8: 23 9:lO
9:19 9:25 1o:oo 10:16 90 10:27 11:02 60 11:12 98 11:21 60 11:33 31 2 140 11:56 32 5 60 12:31 33 5 100 5 34 85 13:28 40 14:OO 35 6 a Values are in relative total ion current peak heights. 1 1
which produced mass spectra for every compound in this series (1-19) at the 5-ng level. Sensitivities vary from hexabromobenzene (7) whose CH4/O2 negative ion CIMS sensitivity is 50 times higher than that for the positive ion mode, to bromobenzene (I) and chlorobenzene (8), which have sensitivities more than 10 times less than that with positive ion CIMS. The CH4/02 mode is the most sensitive detection mode for all of the brominated benzenes except bromobenzene (1). This sensitivity is also evidence for all chlorinated benzenes containing more than three chlorines (15-19). The oxygen-enchanced methane negative ion CI mass spectra show primarily dissociative electron capture producing C1- and Br-. The production of the oxygen exchange ion (M+ 0,-OC1 or M- O,-Br) takes place only with the compounds with a large number of halogen atoms. As the number of halogen atoms increases, so does the amount of oxygen exchange. Pentachlorobenzene (18) and hexachlorobenzene (19), the only two chlorobenzenes which produce molecular anions, have greatly enhanced (M - 19)- ions and greatly enhanced sensitivities. Polychlorinated Biphenyls (PCBs). The various modes of ionization under investigation in this report were applied to two representative polychlorinated biphenyls (Aroclor 1242 and Aroclor 1268). Each PCB mixture was analyzed via capillary column GC-MS using the same operating parameters, except in order to shorten the elution times the initial injection was made with the column temperature at 150 "C. The temperature was then programmed to 280 "C at 4 "C/min. One microliter of a 500 ng/pL PCB solution was injected in
2 3 4 5
6 7 7
7 7 7 7
3
8
8
9
8 7
4 2 1
8
1
6
7
10 11
110
14 3
290 34 63 44
21 5 10 11
15 54 14 260 44 26 41 230 120 170 96 350 410 270 130 170 380 150
13:39 14:08 14:58 15:16 15:27 15:49 16:08 16:27 16:44 17:22 17:35 18:37 18:48 19:51 20:08 20:42 22:07 23:13
96 170 850 110 670 220 41 9 1 43 8 2 320 9 30 1000 230 52 18 10 4 340 a Values are in relative total ion current peak heights. 12 13 14 15 16 17
8
8 9
19 16 13
810
the split mode. With 50 ng injected on column and approximately 20 peaks in the chromatogram, the average amount of sample per peak was about 2.5 ng. Aroclor 1242 gave 35 GC peaks and Aroclor 1268 gave 18 peaks. A tabular presentation is used to summarize the data. Tables VI and VI1 report the data obtained on each mixture. The peak numbers are in order of retention time. The PCB number corresponds to the number of chlorines in the particular isomers contained in that peak. The columns labeled with the various ionization modes contain peak intensities. These intensities are consistent between the two tables. FroD Table VI it is evident that there are two choices for the analysis of the smaller PCBs (e.g., Aroclor 1242). Methane positive ion CIMS yields intense M + H+, M + CaH5+,and M C3HS+ions for the compounds which are detected. However, the sensitivity drops off rapidly as the chlorine content of the PCB increases. The pentachloro and hexachloro isomers in Aroclor 1242 are not detected. The negative ion mode using either electron capture (methane) or C1- as a reagent are not sensitive. Neither mode shows intense signals for any of the compounds. Electron capture negative ion CIMS shows weak signals and primarily chloride ions (Cl-) in the spectrum. Peak number 12, a trichlorobiphenyl, gives a spectrum which contains an M - H-ion (10%) and an M - Cl- ion (15%), unlike all of the other PCBs in Table VI which show only C1- ions. These chloride peaks yield no molecular weight information. Using C1- as a reagent produces a molecular anion for one peak in the Aroclor 1242 chromatogram (peak No. 3). This is the only peak detected and the sensitivity is low when compared with the other ionization modes under investigation. The use of oxygen-enhanced methane as a negative ion reagent gas mixture increases the negative ion sensitivity to the point where it is more sensitive that the positive ion mode by a factor of up to 100 and dows detection of the pentachloro and hexachloro isomers not detected in the positive ion mode. The types of ions produced in the Oz/CH4 negative ion CIMS mode change with the number of chlorines in the PCB. Trichlorobiphenyl is a representative low chlorinated PCB. The base peak is Cl- produced by dissociative electron capture. A weak (10%) (M - H)- ion is also produced. A very weak ion corresponding to the oxygen exchange reaction can be seen at M - 19-. Starting with the tetrachloro-PCB isomers the oxygen exchange reaction starts to become the dominant
+
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 4, APRIL 1981
Table VIII. Mass Spectra of 1,2,3,4-Tetrachlorodibenzo-p-dioxin
ionization mode C%CIMS C%cIMS
c1-
NICIMS O,/CH, NICIMS
(rn/e),% total ion current
(M + H)+, 80.2; (M t C,H,)+, 14.4; (M t C,H,)+, 5.4 M-, 65.3; (M - Cl)., 12.3; ci-, 22.4 M-, 100 (M - 1 9 ) - , 43.9; Cl-, 56.1
sensitivity 3 3 12 190
ionization process. The relative intensity of this ion continues to increase with the chlorine number until it is the major ion in the Oz/CH4 negative ion CI mass spectra of the higher chlorinated PCBs. For example nonachlorobiphenyl produces a 100% M - 19- ion and a 50% Cl- peak with no other peaks in the spectrum. Table VI1 presents similar GC-MS data on Aroclor 1268. Oxygen-enhanced methane was the mode which detected the greatest number of GC peaks with little loss in sensitivity. The positive ion mode was the least sensitive. Electron capture negative ion CIMS of the higher chlorinated biphenyls produced a molecular anion as the base peak. Other ions present are M - C1- and C1- ions (about 5% each), which result from dissociative electron capture, and a small M - 19- ion (about 2%) resulting from an ion-molecule reaction with residual oxygen in the reagent gas. Chloride attachment negative ion CIMS produces virtually the same ions BS did electron capture, implying that the signals result from electron capture, not chloride attachment. No M + Cl- ions are observed. A large (20-70%) ion is observed at M - 19-. This is probably a result of air admitted in trace quantities to the reagent plasma along with the CHzCl2. Since the PCBs and the CHzClzare both competing for the available electrons in the oxygen-enhanced methane mode, it is not surprising that the sensitivities of the PCBs decrease. Small (less than 3%) ions corresponding to (M - C1)- are also observed in this mode. The oxygen-enhanced methane negative ion CI mass spectra of all of the PCBs in Aroclor 1268 have a large M - 19- ion, the oxygen exchange ion. This ion is produced in complete deference to the production of a molecular anion, the primary ion produced by electron capture. A major advantage of Oz/CH4 as a negative ion reagent gas over pure methane can be seen by comparing Tables VI and VII. Electron capture produces intense signals for the highly chlorinated PCBs but produces little to no signal for the lower PCBs. This is a result of changing electron-capture crOss sections with chlorine content. The ion-molecule reactions when using Oz/CH4 seem less dependent on the chlorine content of the molecule and only fail to detect one PCB isomer, a monochlorobiphenyl. Chloralkylene 12. Chloralkylene 12, an alkyated polychlorinated biphenyl, was subjected to the same GC-MS analysis as were the PCBs. The major component is a dichlorobiphenyl isomer (two other isomers at 10% levels were also observed). Other components in the mixture are present at 5 1 5 % of the dichlorobiphenyl. These Components produce M H+, M C2H5+,and M + C3H5+ions which indicate that they are alkylated dichlorobiphenyls containing three additional carbons. Analysis of the chloralkylene 12 xpixture by negative ion CIMS produced no useable spectra. This is not surprising considering the low level of chlorination in this mixture. 1,2,3,4-Tetrachlorodibnzo-p-dioxin.Table VI11 presents a summary of the mass spectra and sensitivities obtained by employing the various CI modes described in this paper. It can be seen that oxygen-enhanced methane negative ion CIMS
+
+
provides a great increase in sensitivity with no loss in molecular weight information and hence no loss in selectivity. Although in the chloride attachment negative ion CIMS mode only the molecular anion is reported, there is undoubtedly C1produced. This is masked by the large excess of C1- already present as a reagent.
CONCLUSION In comparing the various modes of chemcial ionization used for the analysis of polyhalogenated aromatic hydrocarbons, one finds that there is a trade-off between sensitivity and uniformity. While one is able to detect each of the halogenated benzenes by using positive ion CIMS, the electron capture negative ion CIMS mode yields enhanced sensitivities for many of the compounds. However, sensitivities vary from high to not detected. Chloride attachment produces a similar variation in sensitivity. Oxygen enhanced methane negative ion chemical ionization allows detection of each compound a t the 5-ng level. One drawback is that bromobenzene and the dibromobenzenes produce only Br- with no ions indicating molecular weight. High sensitivity for the polyhalogenated benzenes is observed in this mode. The enhanced sensitivity gained by use of Oz/CH4 negative ion CIMS is especially noticeable in the analysis of the PCBs and the dioxin isomer. It is important to be aware of the fact that the electroncapture process is competitive. If two components are in the source at the same time, the one with the largest electroncapture cross section will get the most electrons. Therefore, low concentration components or components with low electron-capture cross sections might not be observed in the presence of compounds more capable of capturing electrons. Although higher sensitivities can be obtained by using negative ion CIMS, it is also true that negative ion signals saturate at lower levels than do positive ion signals. Negative ion signals tend to saturate at the 10-ng level while positive ion signals saturate between the 10- and 100-ng levels. These last two considerations emphasize the importance of the use of capillary chromatography in this analysis. Capillary columns allow maximum separation of components, which helps avoid competition between overlapping compounds. These columns handle small volumes and produce sharp detectable GC peaks for small amounts of compound, thus avoiding use of too much sample. Oxygen is a difficult reagent gas to work with because it decreases the lifetime of the filament. A further complication is observed while running in the negative ion mode using oxygen. Large background ion clusters are seen at mle 233, 235 and m / e 249,251. The ion ratios are 233/235 = 0.6 and 249/251 = 0.6. This corresponds to the anions of rhenium trioxide and rhenium tetraoxide, which are produced from the reaction of oxygen with the hot rhenium filament. The short life of the filament and production of these background ions suggest that further work with oxygen should employ a Townsend discharge as an electron source (7). It has been shown that oxygen enhanced methane negative ion chemical ionization is a sensitive technique for the analysis of polyhalogenated aromatic compounds. Negative ion mass spectrometry is still in its infancy. There is not a large amount of data available to aid in interpretion of unknown spectra. This research increases the number of reference spectra available and provides a comparison of spectra under various negative ion ionization conditions.
LITERATURE CITED (1) Dougherty, R. C.; Roberts, J. D. Anal. Chem. 1975, 47, 54. (2) Dougherty, R. C.; Piotrowski, K. Symposium on Mass Spectrometry, 89th Annual Meeting of the AOAC; Washington, DC, Oct 13-16, 1975. (3) Dougherty, R. C.; Dalton, J. Org. Mass Spectrum. 1972, 6 , 1171. (4) Dougherty, R. C.; Piotrowski, K. Proc. Natl. Acad. Scl. U.S.A. 1976, 73, 1777. (5) Hunt, D. F.; Harvey, T. M.; Russell, J. W. J. Chem. Soc., Chef??. Commun. 1975, 151.
Anal. Chem. 1981, 53, 625-631
(6) Hass, J. R.; Friesen, M. D.; HaNan, D. J.; Parker, C. Anal. Chem.
825
(14) Crow, F. W.; F o l k R. L. presented to the 27th Annual Conference on Mass Spectrometry and Allled Toplcs; Seattle, WA; ASMS June 3-8, 1979% Paper TPMP14. (15) Crow, F. W.; Foltz, R. L. presented to the 26th Annual Conference on Mass Spectrometry and Allied Topics; St. Louis, MO; ASMS May 26June 2, 1978; Paper TP35.
1978, 50, 1474. (7) Hunt, D. F.; McEwen, C. N.; Harvey, T. M. Anal. Chem. 1975, 47, 1730. (8) Hunt, D. F.; Stafford, G. C.; Crow, F. W.; Russell, J. W. Anal. Chem. 1976, 48, 2098. (9) Dzidic, I.; Carroll, D. I.; Stillwell, R. N.; Horning, E. C. Anal. Chem. 1975, 47 1308. (10) Jennings, K. R. Mass Spectrom. 1977, 4 , 203. (11) Hunt, D. F.; Crow, F. W. Anat. Chem. 1975, 50 1781. (12) Tannenbaum, H. P.; Roberts, J. D.; Dougherty, R. C. Anal. Chem. 1975, 47, 49. (13) Stafford, G. C. Flnnigan Corp., Sunnyvale, CA, Patent Pending.
RECEIVEDfor review February 13, 1980. Resubmitted December 19,1980. Accepted December 19,1980. This research was supported by the Environmental Protection Agency (Contract No. 68-02-2457).
Liquid Chromatography of Azaarenes Henri Colin, Jean-Marie Schmitter, and Georges Guiochon' Ecole Polytechnique, Laboratoire de Chimie Analytique Physique, Route de Saclay, 9 1 128 Palaiseau, France
Reversed-phase liquid chromatography (RPLC) makes possible the separation of azaarenes according either to the carbon atom number, with little selectivity toward positional Isomers (long chain packlngs, methanol-water solvent mixtures), or to the position of the nitrogen atom with interferences occurring between compounds of different molecular weight (short chain packings, tetrahydrofuran and acetonitrile-water solvent mixtures). In normal-phase chromatography, the solute retention is malniy governed by the steric hindrance of the nitrogen atom.
The separation and quantitation of polynuclear azaheterocyclic hydrocarbons (azaarenes) is a difficult and stimulating problem. Many isomers and alkyl-substituted compounds of this series have been identified in sources such as tobacco smoke (1-3), automobile exhausts (4),high boiling petroleum distillates (51, crude oil (6), shale oil (71, and recent lake sediments (8). I t has also been well established that these compounds are carcinogens or cocarcinogens (9); certain azaarenes are even more carcinogenic than the most active neutral polyaromatic hydrocarbons (10). Various chromatographic techniques have been used for the separation of nitrogen bases: thin-layer chromatography (11, 12),gas chromatography (6,13,14),and conventional liquid chromatography (15,16). High-performance liquid chromatography (HPLC) has received only little attention so far even though this technique has proved to be most useful for the separation of polyaromatic hydrocarbons. Frei and co-workers have used Corasil bonded with nitrophenyl isocyanate (17) and Zipax coated with silver (18),and more recently Dong h d co-workers have used C18 bonded silica and straight silica (3, 19) for the separation of azaarenes. HPLC is a very attractive tool for studying azaarenes in complex mixtures because it can be used a t three levels: as an analytical technique, as a preparative (or micropreparative) technique, and as a cleanup method. We report here some general results on the use of HPLC for the separation of azaarenes. This work has been done with reference compounds ranging from bicyclic molecules (e.g., quinoline) up to tetracyclic ones (e.g., benzoacridine), demonstrating various alkyl substitutions and positions of the nitrogen atom. The analytical aspect is investigated by using 0003-2700/81/0353-0625$01,25/0
both normal-phase (NP)and reversed-phase (RP) techniques with various mobile phases. The retention patterns are discussed.
EXPERIMENTAL SECTION Liquid Chromatography. Various combinations of liquid chromatographicpumps and detectors were used. The pumping systems included a Waters Model 6000 A (Waters Associates, Milford, MA) and a Tracor Model 995 (Tracor, Austin, TX). Solutes were detected with Waters Model 440 and Tracor 960 absorbance detectors. Injectionswere made with a Rheodyne 7125 sampling valve (Rheodyne, Berkeley, CA). The columns (15 cm X 4 mm) were home packed with Lichrosorb RP 18,5 pm, Lichrosorb Si 100, Hypersil C8 5 pm, and Lichrosorb NH2, 5 pm (Merck, Darmstadt, GFR). The preparation of the buffered silica was made with a 0.1 M Na2HP04solution (pH 10.0) (20). All the solvents were analysis grade from Merck. The mobile phase compositions are given in volume percent. Reference Compounds. The list of standards used in this study is reported in Table I, with the molar weights (MW),number of carbon atoms and PKBvalues. The corresponding structures are shown in Figure 1. Some standard azaarenes (compounds 1-11) were commercially available from Merck, Fluka AG (Buchs, Switzerland), or Aldrich (Beerse, Belgium). Samples were dissolved in acetonitrile. The concentration was approximately 0.1 pg/pL. RESULTS AND DISCUSSION Reversed-Phase Liquid Chromatography, Several parameters can modify the retention pattern in RPLC: the nature of the stationary phase (mainly the length of the alkyl chain), the composition of the mobile phase (the water content, the nature of organic modifier(s), the pH and the concentration of various species for ion pairing, ligand exchange, etc.), and the temperature. We report here the results obtained with various solvents on C8 and C18 chemically bonded silica. The effect of the temperature has not been systematically investigated because preliminary results have suggested that in the present case this parameter has only a secondary role on the selectivity. The study of ion pairing and ligand exchange chromatography will be published later (21). Because of the aprotic character of the solutes studied, the shapes of the elution peaks are sometimes very poor with usual RPLC solvents (dramatic tailing). This results from the presence of unreacted silanol groups a t the surface of the bonded phase and/or from slow equilibrium between the 0 1981 American Chemical Society
