558
Anal. Chem. 1988, 58,558-561
pressure. CONCLUSIONS Despite the fact that accurate temperature (Finnigan) and pressure measurements were impossible on both instruments, relatively similar CHI NICI mass spectra for the polychlorinated test compounds were obtained after individual optimization of all parameters. Only the higher mass range was considered. The response factors and the signal-tobackground ratios are comparable for both instruments. However, the mass spectra recorded on the Finnigan system are dominated by the nonsignificant ions C1- and HC12- (in average responsible for about more than 80% of the total ion current). The filament of the Finnigan ion source is more easily corroded especially by OH-. The high background of rhenium oxides covers partly the signals of nanogram amounts of aza arenes in the total ion current chromatogram. The signalto-background ratio obtained on the Hewlett-Packard system is still on the order of 5:l for similar conditions. The OHNICI mass spectra of isomeric aza arenes recorded on both instruments show a high degree of similarity, which allows the identification of isomers using the mass spectra obtained on the other instrument. N2 is an interesting alternative to CHI. No rhenium ions and no carbon-containing ions of higher masses are formed, which greatly reduces the ion source contamination. The response factors are lower than for CH4but are still sufficient for most test compounds. C2H4was less suitable as reagent gas. Compared to CH4 a considerably higher background and reduced response factors were observed. Furthermore the formation of ion/ molecule adducts is possible. Based on these experiments the use of C 0 2 as reagent gas seems to be an interesting alternative. However, the tested
standard qualities contained a high level of impurities, which caused both an excessive background and undesired ion/ molecule reactions. More experiments are planned as soon as a better quality grade is found. Registry No. CzH4, 74-85-1;CHI, 74-82-8;N20, 10024-97-2; Nz, 7727-37-9. LITERATURE CITED Oehme, M. "Mass Spectrometry of Large Molecules"; Facchetti, S., Ed.; Elsevier: Amsterdam, 1985; pp 233-248. Budzikiewicz, H. Angew Chem., Znf. Ed. Engl. 1981, 20, 624-632. Brandenberger, H. "Recent Developments in Mass Spectrometry in Biochemistry and Medicine"; Frigero, A., Ed.; Plenum Press: New York, 1979; Vol. 2, pp 227-255. Hunt, D. F.; Sethi, S. K. "High Performance Mass Spectrometry: Chemical Applications"; Gross, M. L., Ed.; American Chemical Society: Washington, DC, 1978; ACS Symp. series 70, pp 150-156. Dougherty, R. C. Anal. Chem. 1981, 5 3 , 625A-636A. Stockl, D.; Budzlkiewicz, H. Org. Mass Spectrom. 1982, 17, 470-474. Stockl, D.; Budzikiewicz, H. Org. Mass Spectrom. 1982, 77, 376-381, Gregor, I. K.; Guilhaus, M. Znt. J . Mass Spectrom. Ion Proc. 1984, 56, 167-176. Hunt, D. F.; Stafford, G. C.; Crow, F. W.; Russell, J. W. Anal. Chem. 1978, 4 8 , 2098-2105. Oehme, M. Anal. Chem. 1983, 55, 2290-2295. Oehme, M.; Man@,S.;Stray, H. HRC C C , J . High Resolut. Chromatogr. Chromatogr. Commun. W82, 5 , 417-423. Daishima, S.;Iida, Y.; Kajlki, T. Nippon Kagaku Kaishi 1984, 5 , 439-744. Weast, R. C., Ed. "Handbook of Chemistry and Physics"; CRC Press: Cleveland, OH, 1975, pp F56-59. Hunt, D. F.; McEwen, C. N.; Harvey, T. M. Anal. Chem. 1975, 4 7 , 1730- 1736. Evans, S. Finnlgan, Ltd., Hemel Hempstead, England, personal communication.
RECEIVED for review June 17, 1985. Accepted October 18, 1985. This work was partly supported by a senior scientist fellowship of the Royal Norwegian Council for Industrial Research.
Determination of Selected Chlorodibenzofurans and Chlorodibenzodioxins Using T$vo-Dimensional Gas ChromatographyIMass Spectrometry Woodfin V. Ligon, Jr.,* a n d Ralph J. May General Electric Company, Corporate Research and Development, Schenectady, New York 12301
Polychlorinated biphenyl fluids, fly ash extract, and hexachlorophene have been successfully analyzed for 2,3,7,8tetrachlorodibenrofuran and 2,3,7,8-tetrachlorodlbenzodloxln at part per bllllon levels by two-dlmenslonal gas chromatography/mass spectrometry wlthout resort to prellmlnary Isolation procedures. Detailed experimental procedures are provided. Systematic errors, fundamental Ilmltatlons, and lnterferences are documented.
Chlorodibenzofurans and chlorodibenzodioxins are known to occur in the environment and in certain industrial materials at low levels. Techniques for the determination of these materials have generally involved multistep isolation procedures followed by gas chromatography/mass spectrometry (GC/MS). The work of Harless et al. is typical (I). A review
of various methods have been published (2). We wish to report that for certain classes of sample, important congeners of each of these materials can be determined at part per billion (ppb) levels by two-dimensional GCMS (GC/GC/MS) without the use of any prior isolation procedure whatsoever. A review of the historical development of GC/GC/MS can be found in ref 3. EXPERIMENTAL SECTION The GC/GC/MS apparatus utilized for these experiments has been described in detail previously ( 3 ) . 2,3,7,8-Tetrachlorodibenzofuran(2,3,7,8-TCDF)analyses were performed with a packed 10 f t Silar 1OC column in the primary chromatograph and a combined DB-17/SP-2330capillary in the secondary chromatograph exactly as has already been described ( 4 ) . This combination provides full "base line" resolution of 2,3,7,8-TCDF. The conditions for the primary and secondary separation have been reported ( 4 ) . 2,3,7,8-Tetrachlorodibenzo-
0003-2700/86/0358-0558$01.50/00 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 3, MARCH 1986
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Analysis of a 1.0-mg sample of Aroclor 1242 for 2,3,7,8TCDF by GCIGCIMS using electron ionization and selected ion monitoring. The analyte was monitored at mlz 304, 306, 308 and the internal standard was monitored at 316, 318, 320. The spectrometer resolution was 3000 (m/Am, 10% valley). The level determined was 40 ppb. Figure 1.
dioxin (2,3,7,8-TCDD)analyses were performed using the same primary column and a 30-m SP-2330 capillary column from Supelco, Inc., Bellefont, PA, as the secondary column. For this separation, the primary column was operated in an isothermal fashion at 230 “C and the secondary column was programmed as follows: hold 100 “C for 2 min then programmed from 100 to 180 “C at 20 “C min-’, and then programmed from 180 to 230 “C at 8 “C min-’. The mass spectrometer was a Vg ZAB operated in electron ionization mode at about 3000 resolution (m/Am,10% valley) unless otherwise specified. Selected ion monitoring (SIM) experiments on 2,3,7,8-TCDF involved the ions 304, 306, and 308 and 316,318, and 320. For 2,3,7,8-TCDD,the ions monitored were 320,322, 332, and 334. In both cases, a lock mass signal at mass 328 was also monitored. The lock mass was generated by admission of octafluoro-4,4’-diaminobiphenylthrough a heated reference inlet. All masses were selected at their precise value to six significant figures using the “DIGMID” in combination with the associated peak matching unit. Dwell time at each mass was 100 ms. The data were acquired with an Incos data system. Quantifications were obtained by the isotope dilution method from the ratio of the ion currents obtained (area, not peak height) for the respective labeled and unlabeled molecular ions including their associated isotopomers. An Aroclor 1242 (Aroclor is a trademark of the Monsanto Corp.) sample was obtained from a small General Electric capacitor built in 1965. The sample was diluted 50% w/w with toluene and 2 pL was injected into the primary chromatograph. This means that the equivalent of about 1mg of pure PCB fluid was injected. The sample was spiked before injection at the 77 ppb level with 99 atom % 13C2,3,7,8-TCDF (4). The sample was coinjected with a retention index standard as previously described (4). The data presented in Figure 1were obtained by using a resolution of 3000 at the mass spectrometer. However, we have occasionally found it necessary to use a resolution of 10000 in order to completely remove interferences. This effect has been further evaluated by comparing data obtained for replicate samples using the direct method and using a Stalling (5) type cleanup procedure before analysis. See the Results and Discussion section for a more detailed description of this study. An extract of 22.8 g of municipal fly ash was provided by F. W. Karasek and H. Y. Tong of the University of Waterloo. The extract was prepared by these workers according to a published procedure (6). In our laboratory the crude extract was dissolved in 10 mL of toluene. One milliliter of this solution was separated and spiked at 30 ppb with 99 atom % 13C 2,3,7,8-TCDF. An aliquot of this solution (2 pL) was then coinjected onto the primary column with the retention index standard. Quantification was obtained as described above. Interferences in this sample were evaluated by injecting a more concentrated sample and obtaining full mass spectra. The remaining 9 mL of solution was concentrated to a final volume of 100 pL and 2 PL of that solution was coinjected onto the primary
559
column with an aliquot of retention index marker. Following primary and secondary separations, mass spectra were obtained over the range 150-450 amu with a total cycle time of 2 s and a mass resolution of 1100. A commercial sample of 2,2’-methylenebis(3,4,6-trichlorophenol) (“hexachlorophene”) was obtained from Aldrich Chemical Co., Lot 162655. This material (0.413 g) was dissolved in 500 pL of methanol and spiked with 1.5 ng of 99 atom % I3C 2,3,7,8-TCDD. This standard material was obtained from Cambridge Isotope Laboratories, Cambridge, MA, Lot AWN-1203-65. The spiking level was estimated by referring to the literature (7). 2,3,7,8-TCDD was found to coelute on Silar 1OC exactly with the tripropyl ester of benzenetricarboxylic acid. This material was used to locate the retention window for 2,3,7,8-TCDD on the primary column in a separate injection just before the analytical injection. The hexachlorophene solution (3 KL)was then injected and a 1min wide window for 2,3,7,8-TCDDwas collected in the cold trap. All other materials eluting from the primary column were diverted. The secondary stage of chromatography is described above. This particular analysis may more correctly be called “two-stage”rather than “two-dimensional”because the same liquid phase was used for both separations. Quantification was obtained by the isotope dilution method using the raw ratio of labeled to unlabeled ion currents without correction.
RESULTS AND DISCUSSION This paper is intended to demonstrate the analytical utility of GC/GC/MS for the determination of trace contaminants in the environment and in industrial materials. The analytes chosen have been widely studied by a variety of methods and therefore represent especially appropriate models since direct comparisons with many other techniques is possible. For example, see ref 1 and 2. The quantifications reported are provided purely to demonstrate the potential of this apparatus for providing quantitative data with high confidence. We do not intend to imply that these levels are typical for other samples of this general type nor do we draw any conclusions regarding the toxicological significance of the levels observed. Figure 1 provides the result of our analysis of a polychlorinated biphenyl fluid (PCB, Aroclor 1242) directly from a capacitor without cleanup. The mass spectrometer was operated in the selected ion monitoring mode. The entire analysis including quantification was accomplished in less than 45 min. A level of 40 ppb was determined by using the isotope dilution method relative to a previously established linear calibration curve. Chlorine isotope ratios were in good agreement with theory. This represents the injection of 1mg of PCB fluid and the quantification of 40 pg of a minor contaminant. We have not observed problems with analyte memory effects in the primary chromatograph although blanks are routinely run between samples as a precautionary measure. Matrix memory effects are observed, however, and a subsequent trace analysis for PCB fluids themselves cannot be performed without extensive baking of the apparatus and possible exchange of the primary column. The GC/GC/MS method has been found useful for PCB fluids with chlorination levels which do not exceed that of Aroclor 1242. This limitation occurs because the more highly chlorinated biphenyl congeners cannot be adequately resolved away from the 2,3,7,8-TCDF elution window. It may be seen from Figure 1 that isomers other than 2,3,7,8 have also been trapped from the primary column. It must be noted, however, that no conclusions may be drawn regarding the relative quantities of these isomers in the original sample. Since these other isomers do not coelute exactly with 2,3,7,8 ( 4 ) , the fraction of these isomers trapped from the primary column is unknown. The amounts observed represent a lower limit for the amounts actually present. Quantification is possible only for the exact species selected. PCB fluids represent an especially difficult matrix for a GC/GC/MS analysis. In addition to the PCBs themselves,
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 3, MARCH 1986
a wide variety of other chlorinated aromatics are also present at significant levels (3). For this reason particular care has been taken to evaluate the role of interferences in this experiment. Potential interferences at the analytical mass have been evaluated in three ways. First, we have required that the measured isotope ratio should not be altered by changing the mass spectrometer resolution from 3000 to 10000. This test ensures that materials of very similar mass are not generating an interference. For the example presented in Figure 1, these two results were consistent. However, some PCB samples have failed this test. Discrepancies in the measured value have ranged from 20 to 50%. A second test for interferences involves comparison of results from a direct GC/GC/MS analysis (3000 resolution) with a replicate analysis performed after the sample has undergone a Stalling (5) type cleanup. The agreement found for this particular sample (Figure 1) was acceptable, but good agreement has not been found in every case. Discrepancies were similar to those found in the first test. Increasing spectrometer resolution to 10000 in the direct method has, however, invariably produced good agreement. The results of tests one and two, taken together, suggest that an interfering component may be present in some PCB fluids that is apparently removed by the Stalling procedure and that may be selected against by increasing spectrometer resolution. It is suggested, therefore, that for the direct GC/GC/MS analysis of PCB fluids, the use of 10000 resolution should be considered necessary as a precautionary measure. In accordance with theory, such an increase in resolution causes a directly proportional loss in sensitivity. A detailed discussion of the factors controlling sensitivity in GC/GC/MS appears in reference 3. The possibility of residual matrix influencing quantification has been evaluated by the standard addition method. In this case known amounts of natural 2,3,7,8-TCDF were added to the sample before analysis in increments of 20 ppb to determine if the measured isotope ratio would be altered by the expected amount. Results of this test indicated a strictly linear response. It has also been shown that the measured ratio is independent of the weight of PCB injected for a sample size range of a t least a factor of 3 provided the analyte quantity does not fall below the quantification limit that has been conservatively set at about 10 pg. The normal injection size is 1 mg of PCB. A fourth and especially subtle type of interference can occur a t the lock mass during this analysis. In order to allow centroid determination, the full profile of the lock mass signal must be acquired. This signal is especially sensitive, therefore, to interference from materials having very similar masses. Such interference can result in loss of mass measurement accuracy for all of the analytical masses since it produces an apparent shift in lock mass centroid thereby inducing unnecessary and improper corrections. This type of interference can be evaluated by careful monitoring of the lock mass intensity. Any change in lock mass intensity indicates an interference and loss of mass selection accuracy at the analytical masses must be considered likely. Shouldering by minor components has a smaller effect if the lock mass intensity is kept relatively high. These considerations are applicable to most high-resolution selected ion monitoring experiments and are not unique to the GC/GC/MS experiment. Interferences of this type have been observed in this analysis but as of this date, none has been observed near the retention time of 2,3,7,8-TCDF. Analyses of these materials without internal standard have shown no interferences at the retention time of the internal standard. Figure 2 shows the result of an analysis of a crude extract of municipal fly ash for 2,3,7,8-TCDF using GC/GC/MS and
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by GC/GC/MS using electron ionization and selected ion monitoring. The analyte was monitored at m l z 304, 306, 308 and the internal standard was monitored at 316, 318, 320. The spectrometer resolution was 3000 (mlAm, 10% valley). The level determined relative to the weight of the original fly ash was 2.3 ppb.
selected ion monitoring. The organic fraction obtained from 22.8 g of fly ash was dissolved in 10 mL of toluene and 2 pL of that solution was coinjected onto the primary column with retention index marker. Quantification was obtained by the isotope dilution method with reference to a predetermined calibration curve. The average level found for three separate determinations was 2.3 ppb with a relative standard deviation of 14%. This value refers to the level in the original fly ash sample, not the level in the extract, and assumes quantitative extraction. Chlorine isotope ratios for the analytical masses agreed closely with theory. It should be noted that in spite of the exceptional complexity of this sample (6), the data obtained are exceedingly "clean" and apparently interference free. Because the level of 2,3,7,8-TCDF was high in the extract itself and as a further demonstration of the lack of interferences, a portion of this sample was concentrated by a factor of 90 and a second determination made with the mass spectrometer in "scanning" mode rather than in SIM mode. Figure 3 shows the mass spectrum and the associated reconstructed ion chromatogram obtained for 2,3,7,8-TCDF under these conditions. Ion signals due to instrument background and GC-column bleed have been subtracted from the mass spectrum. The relative intensities in the molecular ion region are slightly distorted because the ion current obtained exceeded the range of the analog to digital converter. It is clear from this spectrum that interferences at the analytical mass do not exist and that, in fact, no other chlorinated species are present including, for example, chlorinated diphenyl ethers. When present, coeluting chlorodiphenyl ethers can provide a direct interference in the analysis of chlorodibenzofurans (81. Figure 4 presents the result of an analysis of hexachlorophene for 2,3,7,8-TCDD. The injected mass of hexachlorophene was 2.5 mg containing 9 pg of 99 atom % 13C 2,3,7,8-TCDD. The observed chlorine isotope ratios were in good agreement with theory. Quantification was obtained by the isotope dilution method as described in the Experimental Section. A value of 4.7 ppb for 2,3,7,8-TCDD was obtained based on a single determination. This value falls in the same range as the very limited data on hexachlorophene available in the literature (7). Because of the notable absence of other chromatogxaphic peaks in the data, we regard the likelihood of positive interference at the retention time of 2,3,7,8-TCDD to be negligible. Accordingly, no rigorous attempt has been made to demonstrate the absence of such interferences. An analysis of this sample without internal standard showed no response at masses 332 and 334, thereby eliminating the possibility of positive interference at the mass of the internal
ANALYTICAL CHEMISTRY, VOL. 58, NO. 3, MARCH 1986
Figure 3. Analysis of an extract of municipal fly ash for 2,3,7,8-TCDF by GClGClMS using electron ionization and magnetic scanning to obtain full mass spectra. The top half of the figure shows the reconstructed ion chromatogram for the sum of the Ion currents for the ions 304, 306, 308. These Ions constitute the molecular ion cluster for 2,3,7,&TCDF. The signal for 2,3,7,&TCDF is indicated. The lower half of the figure shows the mass spectrum obtained for 2,3,7,&TCDF after removal of instrument background ions by subtraction. I
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Flgure 4. Analysis of a 2.5-mg sample of 2,2'-methylenebis(3,4,6trichlorophenol) for 2,3,7,8-TCDD by GClGClMS uslng electron ionization and selected ion monitoring. The analyte was monkored at m l z 320, 322 and the internal standard was monitored at 332, 334. The spectrometer resolution was 3000 ( m l A m , 10% valley). The level determined was 4.7 ppb.
standard. Interferences at the lock mass were not observed. This successful analysis of hexachlorophene is especially pertinent because it provides a particularly realistic model for the analysis of a wide variety of commercial chemicals that consist largely of a single component but contain trace or ultratrace quantities of undesirable side products. Systematic E r r o r a n d Fundamental Limitations. One possible route for systematic errors using this method has been identified. This involves the potential thermal conversion of furan and dioxin precursors into dioxins and furans in the injection port. Such processes can result in an overestimation of the analyte concentration and can potentially occur in any
561
GC experiment on a crude sample. This problem has been discussed by others (2). In the case of the PCB fluid reported here, such a possibility was eliminated by comparison with data from purified samples. For the case of fly ash, such precursors are exceedingly unlikely because of the thermal history of the sample. In the analysis reported for hexachlorophene, interference of this type has not been rigorously excluded at this time. The good agreement found with published data for hexachlorophene suggests, however, that if present, the effect is small. A fundamental limitation of the GC/GC/MS approach is the inherent loss of information for all impurities except the analyte specifically selected. In some cases (of which hexachlorophene is typical), it is possible to trap very large elution windows from the primary column, thereby trapping a variety of trace components. However, for complex matrices such as fly ash extract or PCB fluids, the window collected must be restricted to a very narrow region in order to eliminate as much matrix as possible. In these cases, data are limited to a single isomer and data for other isomers can be obtained only via a second analysis. In those cases where the amount of sample is limited, such repeated analyses may not be possible. This limitation could, of course, be overcome by construction of an apparatus with multiple cold traps. CONCLUSION Based on these data, we conclude that GC/GC/MS can be a very valuable tool for the analysis of chlorinated dibenzodioxins apd chlorinated dibenzofurans in a variety of matrices. Data can be acquired quickly and quantification of high quality is readily obtained by use of labeled internal standards. Unusually difficult chromatographic separations are facilitated because of the o ortunity for selection of complementary PP phases in the primary and secondary separations ( 4 ) . The method provides'qn additional confidence factor when compared witb other GC/MS methods since an analyte must have the correct refeeption time on two GC columns. As in other GC/MS experiments, the analyst must be aware of the possible synthesis of analyte in the injector port of the primary chromatpgraph. When the current data are considered with earlier results (3), it is clear that the GC/GC/MS approach can have general utility for trace analysis in complex matrices. ACKNOWLEDGMENT We thank James C. Carnahan of this laboratory for performing the ipternal standard spiking of these samples and for isolation of chlorodibenzofurans from the PCB fluid by the Stalling method. We thank F. W. Karasek and H. Y. Tong for the sample of fly ash extract. Registry No. 2,3,7,8-TCDF, 51207-31-9; 2,3,7,8-TCDD, 1746-01-6;Aroclor 1242, 53469-21-9; 2,2'-methylenebis(3,4,6-trichlbrophenol), 70-30-4. LITERATURE CITED (1) Harless, R. L.; Oswald, E. 0.;Wilkinson, M. K.; Dupuy, A. E., Jr.; McDaqiel, D. D.; Tai, H. Anal Chem. 1980, 52, 1239. (2) Baker, P. G.; Hoodless, R . A.; Tyler, J. F Pesfic. Sci. 1981, 12, 297. (3) Ligon, W. V.; May, R. J J . Chromafogr. 1984, 294, 77. (4) Ligon, W. Y.; May, R. J. J . Chromafogr. 1984, 294, 87 (5) Huckins, J. N.; Stalling, D. L.; Smith. W. A. J . Assoc. Off Anal. khem. 1978, 61, 31 (6) Karasek; F. W ; Tong, H. Y. J . Chromarogr 1984, 285, 423. (7) Firestone. D. Chem. Eng. News. 1984, 62 (Nov 26), 4. (8) Buser, l4. R. J Chromatogr. 1975, 107, 295.
RECEIVED for review July 29,1985. Accepted October 22,1985.
