Anal. Chem. 1982, 5 4 , 817-820
(6) Van Duuren, B. L. J. Nsfl. Cancer Insf. (US.)1972, 48, 1539-1540. (7) Van Duuren, B. L. Inf. J. Envlron. Anal. Chern. 1972, 7 , 233-241. (8) Van Duuren, B. L. “Interaction of Some Mutagens and Carcinogenic Agents with Nucleic Aclids”; Proceedings of the International Symposium; Publisher: Location, 1988; p 149. (9) Pellizzarl, E. D. ”Development of Analytical Techniques for Measuring Ambient Atmospheric Carclnogenic Vapors”; EPA-600/2-75-076 U.S. Environmental Protection Agency; Nov 1975. ( I O ) Peiiizzari, E. D. “Analysis of Organic Air Pollutants by Gas Chromatography and Mass Spectrciscopy”; EPA-600/2-77-100; US. Envlronmental Protection Agency: June 1977. (11) Pellizzari, E. D.; Bunch, J.; Carpenter, B.; Sawlcki, E. Environ. Scl. Technol. 1975, 9 , 556. (12) Peliizzari, E. D.; Carpenter, B.; Bunch, J.; Sawicki E. Environ. Sci. Technol. 1975, 9 , 552. (13) Pelllzzari, E. D. “The measurement of Carcinogenic Vapors In Ambient Atmospheres”; EPA-600/7-77-055; US. Environmental Protection Agency; June 1977. (14) ”Eight peak Index of Mass Spectra”; Mass Spectrometry Data Centre; AWRE, Aldermaston: Reading, RF74FR, UK, 1970 I (Tables 1 and 2), I1 (Table 3). (15) Stenhagen, E., Ed “Registry of Mass Spectra Data”; Wiiey: New York, 1974: Vol. 4. (16) Smith, D. J.; Pelllzzari, E. D.; Bursey, J. T. “Quantltatlon of Volatile
(17) (18) (19) (20) (21) (22) (23)
817
Organics from Environmental Matrices Using Relative Molar Response Factors”, ASMS: St. Louis, MO, May 1978. Pelllzzarl, E. D. “Development of Method for Carcinogenic Vapor Analysis In Ambient Atmospheres”; EPA-650/2-74-121; U.S. Envlronmental Protection Agency; July 1974. Pelllzzari, E. D.; Bunch, J. E.; Bursey, J. T.; Berkley, R. E.; Sawlcki, E,.; Krost. K. And. Left. 1978, 9 , 579. Daniels, F.; Aiberty, R. A. “Physical Chemistry”; Wliey: New York, 1972; pp 607-611. Pelllzzari, E. D.; Bunch, J. E.; Berkiey, R. E.: McRae, J. Anal. Lett. 1976. - - - - ,9 . 45. -Bunch, J. E.; Castllio, N. P.; Smith, D.; Bursey, J, T.; Pellizzarl, E. D. “Evaluation of the Basic GC/MS Computer Analysis Technque for Pollutant Analysis”; EPA Contract No. 88-02-2998; 1980. Pelllzzari, E. D.; Bunch, J. E. “Ambient Air Carcinogenic Vapors: Irnproved Sampling and Analyticai Techniques and Field Studies”; EPA60012-79-081; U.S. Environmental Protection Agency; May 1979. Tore, J. C.; Kalins, G. J. Anal. Chem. 1974, 46, 1866.
-.
RECEIVED for review March 19,1981. Accepted Janauary 13, 1982. This work was supported by EPA Contract No. 6802-2262, 68-02-2998, and 68-02-1228.
Organic Anailysis with a Combined Capillary Gas Chromatograph/Mass Spectrometer/Fourier Transform Infrared Spectrometer Rlchard W. Crawford,” Tomas Hlrschfeld, Russell H. Sanborn, and Carla M. Wong Lawrence Livermore National Laboratory, Livermore, California 94550
We have demonstrated, for the first tlme, a linked capillary gas chromatograph/mass spectrometer/Fourier transform infrared spectrometer (GC/MS/FT-IR). Although the ilnklng of a packed column GC/MS/FT-IR has been reported, we feel the use of a high-resolution column (SCOT) significantly Increases the analytical usefulness of this technlque. The linking of the three provldes complementary information and eliminates the possiblllty of comparing spectra from different components due to shifts In retention time. Examples are glven by uslng a known mlx of alkyl benzenes and a completely unknown siloxane.
Advances in instrumentation and the incorporation of powerful computers have created some remarkable analytical tools. One of the most powerful and popular instruments in the realm of organic qualitative and quantitative analysis is the computerized gas chromatograph/mass spectrometer (GC/MS). This instrument combines the separatory properties of the chromatographic column with the fast fingerprinting capability of the mass spectrometer. The advent of the fast scanning Fourier transform infrared spectrometer coupled with light pipes and cryogenic detectors made possible the analogous technique of GCIFT-IR, offering some of the same advantages. Hirschfeld (1)has discussed the field of “hyphenated” instruments, that is, linking multiple instruments to give complementary data about the same sample. The linking of a GC/MS to a GC/FT-IR is a natural choice as they work with the sample in the gas phalse and have similar sensitivities and speeds and the information they provide is complementary. The development of tlhis instrumentation syst,em has depended upon technological advances in the field of high-
resolution gas chromatography and interferometric infrared spectrometry (FT-IR). Mass spectrometry has had, for many years, the speed and sensitivity needed to match these other components. Erickson (2)in his excellent review of GC/FT-IR shows an almost linear plot of sensitivity vs. year, starting with the work of Low (3) in 1968 who achieved a 10 pg sensitivity and ending in 1977 with 10 ng sensitivity reported by Wall and Mantz ( 4 ) , using packed columns. Wilkins has built on this technology (5-7) for his GC/MS/FT-IR using a packed column. Golay (8) introduced the capillary or wall-coated open tubular (WCOT) column in 1957. Halasz et al. (9) developed the support-coated open tubular (SCOT) column in 1961. Scientists were excited over the vastly superior resolution of these columns. In addition, a single column type was found to be applicable to a wide variety of sample types. Unfortunately, technical and patent problems limited their use until recent years. The first use of GC/FT-IR using a SCOT column was by Azarraga et al. (10)in 1974. This is considerably more difficult than packed column work as noted by Griffiths (11) due to the low flow rates and narrow peak widths. We have designed our instrument system (12-15) to successfully overcome these problems and make use of this technology. EXPERIMENTAL SECTION Instrumentation. Initial development work on the GC/ MS/FT-IR used a packed column and simple test mixes. We found the quality of separation to be so poor that it did not justify tying together two such expensive instruments. Therefore, we decided to try a capillary column for impraving the resolution. In switching then to a SCOT column, we found serious problems not encountered in the packed column GC/MS/FT-IR linkup. These major problems were (1) splitting effluent reliably, (2) eliminating cold spots in all lines carrying sample, and (3) es-
0003-2700/82/0354-0817$01.25/00 1982 American Chemical Society
818
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ANALYTICAL CHEMISTRY, VOL. 54,
Digilab FTS-ZOC
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Figure 1. Schematic of GCIMSIFT-IR.
tablishing low-volume, leak-free connections. The initial effluent splitter was a three-way T valve. This was mounted in the GC oven to eliminate long lines to the MS. It functioned well in isothermal mode, but when the oven temperature was ramped, the valve expanded and cut off most of the flow to the MS. The valve was replaced with a low-volume T with a gold foil pin-hole leak to limit flow to the MS. There were problems sealing the gold, so we went to the present system which usea a short piece of Pt/Ir tubing that is pinched to give the correct split ratio of 10% to MS and 90% to FT-IR. The GC/MS interface was built into the commercial instrument and heated to 250 "C, so we extended heated lines at 250 "C between the GC and the FT-IR. When we injected mixes with high boiling compounds (250-400 "C) they appeared in the MS, but none of them appeared in the IR light pipe. The two major cold spots were (1)bringing cold helium in as makeup gas and (2) the space between the inner and outer wall of the GC oven where the line to the IR came through. These problems were eliminated and the high boiling components appeared. Fittings were a major problem. There are no commercially available ferrules that would work with the variety of tubing sizes and types such as a 1.04 mm 0.d. glass SCOT column, 0.36 mm 0.d. Pt/Ir tube, or 1.70 mm 0.d. nickel tubing. We finally purchased a set of microdrill bits and Vespel-40% graphite "no hole" ferrules (Alltech) and made our own as needed. Commercial unions and tees (Parker, Swagelock, SGE) designed for capillaries were used for couplings. Figure 1 is a schematic of the system. A Hewlett-Packard 5985B GC/MS using a HP 21MX computer and HP software was linked via a nickel tranfer line to a Digilab FTS-20C GC/FT-IR system using a NOVA 3 computer system and Digilab software. The capillary column was an SGE support-coated open tubular (SCOT) using SE-30 coating and was 47 m long and 0.5 mm i.d. Temperature programming from 40 to 260 "C was used and a helium pressure of 6 psig gave a flow of 3 cm3/min at 40 "C. The GC effluent was split so that approximately 90% of the sample went to the FT-IR. A low-volume T was attached to the end of the SCOT column and the 0.5 mm i.d. nickel tube leading to the IR was attached to one leg. A short length of glasa capillary, "welded" to a piece of PbIr tubing leading to the MS, was attached to the other. The Pt-Ir tubing was carefully pinched with a needle nose pliers to give the proper split ratio. The gas went directly to the MS through l/g in. glass-lined stainless steel tubing. An additional low-volume T was inserted in the nickel line leading to the FT-IR and makeup helium carrier gas was added to bring the flow up to 20 cm3/min. This was necessary to maintain chromatographic resolution in the large (60 cm by 2.4 mm) light pipe. The transfer line was held at 250 "C with heating tapes, variacs, and thermocouples. The light pipe was maintained at 250 O C with block heaters and a heating tape using thermistor sensors and a controller (Digilab). The light pipe was the standard Digilab gold-plated Pyrex tube (16)and the FT-IR detector a liquid nitrogen cooled Hg/Cd-Te detector. Special Parameters. Electron impact mass spectra were obtained by using a source voltage of 70 eV and a scan time (3 readings per 1/8 amu point) of 1.4 s from mass 40 to 400. The quadrupole mass filter provided unit resolution to mass 1000. FT-IRspectra were obtained with the light pipe at 250 "C and a dry argon purge. Spectra were collected with a resolution of 8 cm-' over the range of 650-4000 cm-l and either four or eight
spectra coadded. Each coadded spectrum was acquired in 0.635 s and 1.09 s was needed for storing the data on disk, so four spectra required 3.63 s and eight spectra took 6.17 s (new hardware and software allow speeds at least 3 times as fast). Samples. The known mix was made by adding small volumes of reagent grade chemicals to hexane such that a 1-WLsample would contain approximately 400 ng of each. This was injected into the GC at 40 O C using splitless mode, waiting 1 min before purge. The unknown mix was injected neat using 1001 splitting in the injector. A 1-pL sample was used. Spectral Searches. The mass spectral search used the compressed library of 32 000 compounds provided by Wiley. The search algorithm was the Hewlett-Packard version of McLafferty's ( 1 7) probability based search (PBS) program. The infrared library was the 2300 vapor-phase spectra collection of EPAs Leo Azarraga. The search algorithm was Digilab's version of the GIFTS software written by Hanna et al. (18). System Operation. To convert the GC/MS into GC/MS/ FT-IR operation, the effluent end of the column is disconnected from the direct inlet to the MS. The low-volume T and pinched Pt-Ir tubing are connected to the column, and the Pt-Ir tubing is "welded" to a short piece of capillary glass tubing leading to the direct inlet. The nickel tubing leading to the FT-IR is connected to the other leg of the T. This operation takes approximately 10 min. At the time of injection, the data acquisition system on the MS and the FT-IR are started simultaneously by manual action. The mass chromatogram is displayed in real time.
RESULTS AND DISCUSSION The analytical chemist in the real world is faced with two common, yet difficult, problems in organic analysisdetermining the composition of complex mixtures and doing structural elucidation on synthesis products (often also mixtures). Rarely is it possible to optimize GC conditions so thoroughly that similar compounds will show significantly different retention times, especially if these similar compounds are simultaneously present and better resolution than may be obtained using a SCOT column is called for. In practice, then, one is dependent upon the discriminating abilities of the GC/MS or GC/FT-IR spectrometers. Both MS and IR have weaknesses in their chemical discrimination, as an example, the MS in distinguishing chemical isomers and the IR in distinguishing long chain homologues. These differences allow a combined GC/MS/FT-IR to deal with both types of problems. Many authors (6,11,19-25) have pointed out the complementary nature of these techniques. The silicone polymer coated capillary column used in a temperature ramped mode is as close to a universal separation system as we have discovered. We have used this column type for almost all liquid samples submitted to us with the exception of very volatile liquids (bp < 80 "C). The SCOT column was selected instead of the WCOT (wall-coated open tubular) as its larger volume of liquid phase increased the capacity by a factor of 5 to 20 without an unacceptable degradation in resolution. The increased capacity was necessary to match the routine detection limit of approximately 100 ng for the FT-IR. One should remark here that this lower resolution of the SCOT column assumes an equal length column and a similar GC run time. However, in GC/MS or GC/FT-IR analysis of unknown samples, the total analysis time is actually dominated by data interpretation time, making longer GC run times inconsequential. Figures 2 and 3 are GC/MS total ion chromatographs of oil shale extract in CHzClz (1:lO) using splitless injection (I kL). They illustrate that by using a longer column and slower ramp rate, the SCOT column can achieve separation comparable to a WCOT column. Manual verification of computer selections of library spectra vs. unknown spectra is presently the single most time-consuming task in the GC/MS analysis of mixtures. The analyst must pass judgment on all but the most trivial or obvious selections by visually comparing the unknown spectra with
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
819
Table I. Comparison of Library Search Outputs for the MS and FTIR on the Known Mix MS
FTIR
Carnoaiind
l " h X
Index
Compound
Peak 1 0.4245 0.4420 0.4452 0.4680
Valeronitrile,4-methyl-2-phenyl-. Ben~ene.Z-methylpropenvI-,
Benzene,ethyl-. Sulfide.Phenyl 3-Phenylpropyl,
0.9808 0.9808 0.9806 0.9806
Ethylhenzene Ethylbenzene Ethylbenzene Ethvlbenzene
0.9807 0.9807 0.9807 0.9807
0-Xylene M-Xylene P-xylene O-XylenE
0.9807 0.9807 0.9807 0.9807
0-Xylene M-Xylene P-Xylene 0-Xylene
0.9810 0,9810 0,9810 0,9809
1.2.4-Trimethylbeniene 1.2.4-Tilmethylben2ene
Peak 2
Figure 2. Oil shale extract using a 25-m WCOT column ramped at 10 OC/min.
0.9279 0.9670 1.0430 1.0504
P-xylene M-xylene Naphthalene,l-mBthyl-.
0.4697 0 5854 0 7100 0,8140
0-Xylene
0.5380 0.7223 0.7280 0.7612
Benzene,1,3,5-trmsthyl-. 2.4-Lutidine 5-colirdine Cumens,3,5-d1methyl-,
0,4151 0.5236 0.5289 0.5457
Beiiiene,l-Iropropyl-4-methyl-,
Acetsnil1de,2,3*-dichloro-,
Peak 3
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1 11:
31
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Naphthalene,l,Z,3,4-Tetrahydro-.
Dibenmthiophene Phenyldisulfide Peak 4
1.3.5-Trimethylb0nzene 1.2.3-Trimethylbenzene
Peak 5
TIIlE,
PlIH.
Figure 3. Oil shale extract using a 50-m SCOT column ramped at 4
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up to 10 library spectra for each GC peak. The use of GC/ MS/FT-IR can reduce this task considerably by creating two lists of library searches for each peak that can be readily scanned for congruence. The use of the joined or "hyphenated'' instruments rather than separate GC/MS and GC/FT-IR analysis of the same sample guarantees that the spectra being tested for congruence are indeed from the same chromatographic peak. To test this in practice, we injected a mixture of ethyl benzene, 0 - , m-, and p-xylene, 1,3,5-trimethylbenzene, and 1-isopropyl-4-methylbenzene (p-cymene) into the GC/MS/ FT-IR. This mixture produced five GC peaks. Table I shows the top four choices from both the IR and MS library searches for these peaks. The cross correlation gives an obvious identification for peaks 1, 3, 4, and 5. Peak 2 seems to be either p- or m-xylene. Figure 4 shows the IR spectra of the leading and trailing edge of peak number 2. Visual matching of these spectra to reference spectra show that the leading edge is m-xylene, the trailing edge, p-xylene. This GC overlap cannot be observed in the MS data, which fails to differentiate these isomers.
The next obvious step is to link the output of the two library searches and allow a computer to check congruence and flag the correct choice. For this to work, all libraries have to contain a unique indentification for each compound such as the CAS index. In practice, the analyst often faces the task of determining the structure of unknowns ab initio rather than by comparison to a reference, since there are less than 20 OOO unique reference mass spectra and 7000 computerized IR vapor-phase spectra, as compared with 5 X lo6 identified organic compounds. This is especially true when. supporting a synthesis effort, as in the following example from our laboratory which illustrates the power of the GC/MS/FT-IR method. We received the end product of the reaction of dimethylamine and 1,7-dichlorooctamethyltetrasiloxane.The hoped for end product was 1,7-bis(dimethylamine)octamethyltetrasiloxane. The mass chromatogram from a WCOT column showed five major peaks. Peak number two had a peculiar mass spectrum as shown in Figure 5. The siloxanes typically have the largest peak at their molecular weight minus 15, a methyl group. The large peak at 294 indicates a molecular weight of 309 and the small peak at 309 confirms this. The isotope ratio of 295 and 296 to 294 indicates four silicons and eight carbons. The odd molecular weight indicates one or three nitrogen atoms. If there are four silicons, then there are probably three oxygens if the molecule is a siloxane. Taking all of the above together, a molecule with the elemental formula C9H2,N1Si403emerges. Additional structural information is provided by the mass spectrum. The lack of intense
820
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
compound shown in Figure 7, 1,2,2,4,4,6,6,8,8-nonamethyl-1azacyclotetrasiloxane.
ACKNOWLEDGMENT 4
-
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LITERATURE CITED
1
1000 Wavenumberr
We thank Leroy Schrawyer for mechanical design and construction and Robert Meisenheimer and Klaus Ernst for initial encouragement and support. Michael Riley provided the synthesis products.
(ern-')
Figure 6. IR spectra from leading and trailing edge of second peak in slloxane mix.
Figure 7. Probable structure of siloxane based on mass and infrared spectra.
peaks below the M - 15 peak is very indicative of a cyclic compound with no side chains larger than a methyl group. We ran the sample again using the SCOT column linked to both the MS and FT-IR. Only four major peaks appeared, but both the mass spectra and infrared spectra showed different spectra on the leading and trailing edge of peak number two. The mass spectra of the leading edge was identical with the spectra of our hypothetical ring compound. The infrared spectra of the leading and trailing edge of peak two are sho,m in Figure 6. The band at 1080 cm-l is the Si-0-Si stretch and 1260 cm-' is the Si-C stretch. It is further apparent that no C-C or C-0 bonds are present and that CH3 is the only carbon containing group present. The band at 900 cm-l in the' leading edge is the key-it is the Si-N-Si stretch. It almost guarantees a ring structure. Additionally, the lack of any C-C stretch confirms no groups larger than a methyl. Also, C-0 bonds are not in evidence. The GC/MS/FT-IR combination thus identified the synthesis product to be the
Hlrschfeld, T. Anal. Chem. 1980, 5 2 , 299 A. Erlckson, M. D. Appl. Spectrosc. Rev. 1979, 15,261. Low, M. J. D.; Coleman, I.Spectrochim. Acta 1966, 2 2 , 369. Wall, D. L.; Mantz, A. W. Appl. Spectrosc. 1977, 31, 552. Wllklns, C. L.; Giss, G. N.; Steiner, S.;Brissey, G. M.; White, R. L. Presented at the 22nd Rocky Mountain Conference on Analytical Chemlstry, Denver, CO, Aug 1980; Paper 28. Wllkins, C. L.; Giss, G. N.; Brissey, G. M.; Steiner, S. Anal. Chem. 1981, 5 3 , 113. Wllkins, C. L.; Bissey, G. M.;Giss, G. N. Presented at the Pittsburgh Conference on Analytical Chemlstry and Applled Spectroscopy, Atlantic City, NJ. March 1981; Paper 720. Golay, M. J. E. I n "Gas Chromatography, I.Symposium, East Lansing Mich., Aug. 1957"; Coates, V. J., Noebels, H. J., Fagerson, J. S.,Eds., Academic Press, New York, 1958; p 1. Halasz, I.; Wegner, E. E. Brennst.-Chem. 1961, 42, 261. Azarraga, L. V.; McCall, A. C. Infrared Fourier Transform Spectrometry of Gas Chromatographic Effluents; EPA 660/1-73-034, 1974. Grifflths, P. R. Automated Measurements of Infrared Spectra of Chromatographically Separated Fractlons; EPA 600/4-79-064, Oct 1979; p. 36. Crawford, R. W.; Hlrschfeld, T. B.; Wong, C. M.; Sanborn, R. H.; Meisenhelmer, R. G.; Schrawyer, L. R. General Chemlstry Dlvision Quarterly ReDOrt, Lawrence Llvermore Laboratory, Llvermore, CA, 1980; UCIDil 5644-79-4. Hirschfeld, T. Presented at the Pittsburgh Conference on Analytical Chemlstry and Appiled Spectroscopy, Atlantic City, NJ, March 1980; Paper 451. Hirschfeld, T.; Crawford, R.; Wong; C.; Sanborn, R. Presented at the Second Chemical Congress of the North Amerlcan Continent, Las Vegas, NV; Symposlum on Analytical Chemistry In the 1980s; Aug 1980, Paper 113. Crawford, R.; Sanborn, R.; Hlrschfeld, T.; Wong, C.; Brand, H. Presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, March 1981; Paper 890. Krishnan, K.; Curbelo, R.; Chiha, P.; Noonan, R. C. J. Chromatogr. Scl. 1979, 17, 413. McLafferty, F. W.; Hertel, R. H.; Villwock, R. D. Org. Mass Spectrom. 1974, 9, 690. Hanna, A.; Marshall, J. C.; Isenhour, T. L. J. Chromatogr. Sci. 1979, 17, 434. Low, M. J. D.; Freeman, S. K. J. Agric. Food Chem. 1968, 16, 525. Jurs, P. C.; Kowaiski, B. R.; Isenhour, T. L.; Reilley, C. N. Anal. Chem. 1969, 41, 1949. Naegell, P. R.; Clerc, J. T. Anal. Chem. 1974, 46, 736 A. Wilklns, C. L. J. Chem. I n f . Comput. Sci. 1977, 17, 242. Zupan, J. Anal. Chim. Acta 1978, 103, 273. Men, P. C.; Carpenter, A. P.; Hackett, H. M.; Henderson, D. E.; Siggia, S. Anal. Chem. 1979, 5 1 , 38. Shafer, K. H.; Lucas, S. V.; Jakobsen, R. J. J. Chromatogr. Scl. 1979, 77, 464.
RECEIVED for review October 22,1981. Accepted January 22, 1982. Work performed under the auspices of the U.S. Department of Energy at Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48. Reference herein to any specific commercial products by manufacturer does not imply its endorsement by the United States Government or the University of California. The opinions of authors expressed herein do not necessarily reflect those of the United States Government and shall not be used for advertising or product endorsement purposes.
