GAS Recent technological advances improve prospects for the widespread acceptance of GC/IR/MS instrumentation Charles L. Wilkins Department of Chemistry University of California Riverside, Calif. 92521 For almost 20 years the idea of linking a gas chromatograph with both an infrared and a mass spectrometer to form an integrated analysis system (GCIIRI MS) has intrigued analytical chemists. For example, in one of the earliest GC/ IR papers, which reported GCIIR spectra of 10-18 wavenumher resolution with microgram-level sensitivity, Low and Freeman suggested the potential value of a GC/IR/MS system ( I ) . Of course, it was obvious then (in 1968) that a number of significant improvements in both IR spectroscopy and computer technology would he required for the practical realization of this visionary suggestion. Although powerful and inexpensive computers easily capable of the experiment control and data reduction needs are readily available today, that was most emphatically not the case in 1968. In fact, at that time computer systems with 8K words (16K bytes) of 16-bit memory equipped with paper tape input-output, a teletype, and modest data acquisition and control capabilities were available for $35,000-$40,000! (2, 3 ) . Programming support was equally limited (e.g., multipass paper tape-oriented FORTRAN compilers, memory resident BASIC compilers that occupied most of memory, or assemhly language). Consequently, development of requisite real-time control, data acquisition, data storage, and interpretation software would have been a formidable and extremely expensive undertaking. Most of these early difficulties have 0_003-2700,87 0359-571AiS01 50 0 s 1987 Amer can Cnem ca Sociel)
been overcome, and substantial progress has been made. It is therefore timely to review the current status and historical development of GCIIRIMS. The present article should complement the excellent related reviews by Griffiths and co-workers on chromatography-Fourier transform IR (FT-IR) spectrometry (4) and Heuiou and coworkers on liquid chromatographymass spectrometry (LCIMS) (5). All
Prior to the first demonstration of GCIIRIMS (12), there were numerous reports of various instrument systems exploiting complementary information for organic analysis. In one of the earliest papers, Uden and co-workers described a pyrolysis GC system in which samples were trapped and subjected to subsequent MS, elemental analysis, and GC/IR measurement, although the instruments were not linked (13).In a
these techniques are possible because of major technical advances made during the past 10 years or so.
later paper from the same laboratory, oil shale samples were characterized by joint use of GCIIR, MS data, and GC retention time (although, again, a directly linked system was not used) (14). Among publications emphasizing the potential value of complementary IR and MS information for computerized structural analysis is one of the very first papers by Isenhour and eo-workers on the application of pattern recognition to chemical analysis (15). In a 1968 paper Sasaki suggested linking spectrometer control computers to combine the information from IR, MS, and nuclear magnetic resonance (NMR) (to he obtained from parallel analysis of several samples) ( 1 6 ) . Hirschfeld's 1980 paper on hyphenated methods (17) describes an ambitious analytical chemistry system planned for Lawrence Livermore Laboratory that was to incorporate a linked GCI IRIMS system as a central element. By 1980, it was quite clear that GCI IRIMS was an inevitable development, under active investigation from a variety of perspectives in a number of lahoratories. The first actual implementation of a direct-linked GCIIRIMS sys-
Background I t appears that the first formal plans for an integrated GCIFT-IR/MS system were presented late in 1976 at a conference in Washington, D.C., and subsequently were published (6). The use of a high-performance sector mass spectrometer (Kratos MS-5076) and a commercial dual-heam GCIFT-IR spectrometer (Spectrotherm ST-10, subsequently withdrawn from the market) was proposed. This proposal included the idea of using an integrated on-line interpretive software package, following many of the principles discussed earlier by Naegeli and Clerc ( 7 ) . Unfortunately, although Griffiths (8, 9)and other co-workers (10, 1 1 ) have established the feasibility of dualbeam FT-IR for gas analysis, the ST10 instruments were announced but they were never delivered to customers because of production difficulties. Consequently, dual-heam GC/FT-IR instruments to date have not been used in GCIIRIMS systems.
ANALYTICAL CHEMISTRY, VOL. 59, NO. 8, APRIL 15. 1987
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tem was in 1980, as described in a paper published early in 1981 (12).
Interface of GC/FT-IR/MS
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 8, APRIL 15, 1987
inshuments When a linked GC/FT-IR/MS system is designed, the first problem that must be addressed is the difference in sample requirements between the three instruments. Whereas gas chromatographs operate at pressures well in excess of 760 torr, which are readily accommodated by common GCPT-IR lightpipe interfaces, mass spectrometers require sample pressures at least eight orders of magnitude lower (between 10-5 and 10-8 torr). In the first report of GC/IR/MS, a packed GC column was used with a jet separator that eliminated much of the carrier gas prior to introduction into the mass spectrometer. Because of the relatively high required carrier flow rates and the inefficiency of packedcolumn GC, this approach was abandoned following the initial feasibility demonstration. Most subsequent studies have employed surface-coated open-tubular (SCOT) columns in conjunction with splitters to reduce the carrier gas flow to the mass spectrometer (18).Thus, in the second published report of GC/IR/MS (It?), Hirschfeld and co-workers used a SCOT column linked via a splitter to a quadrupole mass spectrometer and an FT-IR spectrometer in parallel. Because FT-IR is nondestructive, it is obvious that either parallel or serial linkage of this instrument with a gas chromatograph (input) and mass spectrometer (output) is possible. Of course, if the serial arrangement is used, the lightpipe dead volume is interposed between the GC and the mass spectrometer, with the concurrent potential for degrading chromatographic resolution. In fact, this is what is found. In an early report of linkage of GC/FTIR with a Fourier transform mass spectrometer (FT-MS), using a SCOT column for separation and a jet separator (employing helium make-up gas) as an interface, chromatographic resolution was degraded by almost 50% for the longest retained substances in two complex mixture separations (19).Furthermore, in these studies the disparity between the sensitivity of the infrared and mass spectrometers was such that, even in the absence of deleterious chromatographic effects, there was no need to use a serial configuration. Thus for GC/FT-IR/MS systems employing lightpipes, a parallel split interface between the GC and the two spectrometers, routing most of the effluent (95% or more) to the IR spectrometer and the balance to the mass spectrometer, is the method of choice. The rationale is that the full value of GC/IR/MS is realized only when both
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 8, APRIL 15, 1987
Instrumentation for GC/IR/MS One question that legitimately can be asked about complex analysis systems such as GC/IR/MS is whether their cost and complexity can be justified by the increased analytical capabilities they provide. If expensive high-performance IR and mass spectrometers are required for success, it is not likely that the method will be widely adopted for routine mixture analysis. For example, a review that appeared late in 1982 (20) discussed GC/IR/MS analysis studies using very expensive high-performance double focusing (12)and Fourier transform mass spectrometers (19) in combination with top-of-the-line FT-IR spectrometers. Such systems involved use of close to half a million dollars’ worth of equipment. Clearly they would not be practical for widespread use unless they were highly reliable and provided unique analytical power. There had been the Hirschfeld report of a system using a quadrupole mass spectrometer (18), but the only examples of relatively complex mixture analyses (peppermint oil and lacquer thinner) had relied on high-performance instruments. Fortunately, it was at precisely that time (1983)that low-cost mass spectrometers, such as the Hewlett-Packard mass selective detector (MSD) and the Finnigan ion trap instrument (ITD), became available, and unprecedented competition in the FT-IR field resulted in the introduction of numerous moderately high-performance instruments a t relatively low prices. These developments, occurring just as feasibility of GC/IR/MS had been convincingly demonstrated, laid the groundwork for the next stage of GC/IR/MS research, which would concentrate on the dual questions of data interpretation algorithms and evaluation of lower cost alternatives to the expensive prototype research systems of the preceding three years. Complementary information from GC/IR/MS Following the demonstrations of functional GC/IR/MS systems discussed above, there was renewed interest in evaluation of the use of complementary IR and mass spectral information for organic analysis. In our laboratory (12, 19), searches of computer-readable mass spectral and gas-phase IR spectral libraries were used for identification of mixture components. To accomplish specific identification, the same component was required to appear
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among the top “hits” from both library searches. For compounds missing from either or both of the spectral libraries, this procedure obviously fails. In those cases, molecular weight information inferred from chemical ionization (CI) studies (when the FT mass spectrometer was used), combined with library search results, permitted inference of structural type. These analytical protocols proved highly reliable in the initial studies. Subsequent studies by Shafer et. al ( 2 0 , Gurka (22).and Biemann (23) of the utility of complementary MS and IR data for organic analyses effectively demonstrated the value of such an approach for environmental and other samples. Although these workers did not use directly linked analytical systems, they nevertheless clearly demonstrated the potential of GC/IR/MS for practical application. Other recent work in our own laboratory has focused on the utility of other mass spectral information in addition to conventional electron ionization spectra (24, 25). In particular, we took advantage of the ease of collection of CI spectra and accurate mass measurements (AMM) on GC eluents when a Fourier transform mass spectrometer is used. Figure 1 schematically depicts the information available when such data are collected with the instrument system diagrammed in Figure 2. When these data were analyzed for 45 unknown compounds, the results summarized in Table I were obtained (26).Table I also shows that good results are obtained for mixtures. As expected, identifications are highly reliable and, most important, false positives are eliminated. More recent work has included the use of negative CI data (27). One important aspect of automation of data interpretation for%C/IR/MS is development of software that accomplishes cross-correlation of spectral library search results automatically. This has been done in our laboratory through the use of unique Chemical Abstracts registry numbers (CA REGN) as a cross-correlation tool. Because each member of the spectral libraries contains the CA REGN for the compound it represents, it is possible to compare library search hits by their registry numbers. Thus the search for coincidences is reduced to a very rapid registry number comparison. A recent publication describes the procedure and demonstrates its application to 14and 23-component mixtures analyzed using an MSD w the mass spectrometer in the GC/IR/MS system (28).
Lower cost GWIWMS Returning to the questions of cost and complexity of GC/IR/MS that were raised in the above discussion of instrumentation, it is worthwhile to review 576A
...-. J 1. Information available from linkedaccurate mess measurement elecl ionization-chemical IonizationGC/IR/MS analysis. (Reprinted with permission from Reference 25.)
Figure 2. Block diagram of a GC/FT-IR/FT-MS system. (Reprinted with permission from Rderence 25.)
I
Table 1. Cornparisond various GC/IR/MS algorithmsa Srnpu
Peppermlm oil ( I 8 components)
llpalurn
IR
MS IRlMS
ca.*
InEmM
9 9 7
9 9 0
17-component mixture
IR/MS CI-RIMS
12 16
0
45 compounds
IR IR/AMM MS MSlAMM IRlMS AMM IRlMS
32 33 26 30 32 35
13 8 19 12 0 0
0
E-l
0 0 11
5 1 0 4
0 3 13 10
a IR or MS abne. beat library match is uaed. IRIMS, coincidence aigalthm is d. AMM is accurate mass msuremem filter of search date (see References 25-27). Ci indicates use of chemical imiz~lion date fm reduction of search list (see Refer25). if there is 110 coincidence of library search hits (IRNS). u if me best match is incmsism wlth AMM or Ci 68ta.M) identification Io m e .
Adapted wim psrmisim horn Reference 25.
ANALYTICAL CHEMISTRY, VOL. 59, NO. 8. APRIL 15, 1987
BringsThe Best Of BothWorlds Together
This Cryolect 4800 displag screen compares the mass spechum d o - c m o l with the IR fingerprint region d the same compound in the liquid, vapor and ma&isolated states. Note the wealth of clearly defined information within the matrix-isolated spechum, shown in green. Footnotes I . As speikd in H - P Publiroiim No. U-59%~0665. 2. Summoiy sto1men1, c ~ y o l e c l
U m i Meeting, Madison, Wl, iVwembec 1986.
You know the individual limitations of G U M S and GC/FT-IR. G U M S has picogram detection limits, but needs IR confirmation to identify many isomers and ring compounds. Traditional GC/FT-IR has always had molecular specificity, but not the sensitivity you demand. Even together, they can't provide proof-positive identification because light pipe techniques don't equal G U M S sensitivity. And that includes GC-dedicated light pipes such as the 1RD.l Now know this: only the Cryolect 4800 IR/MS combines the specificity you've always needed with the revolutionary sensitivity that only mat& isolation can bring to FT-IR. That's because only matrix isolation enhances IR bands with 100 times more intensity and extraordinarily sharper resolution than ever before. That means only the Cryolect 4800 - whether working with your G U M S or our optional mass selective detector - can fully meet your needs for thorough, fail-safe analysis of virtually any organic compound. And the Cryolect 4800 is supported by an extensive matrix-isolated spectral library backed by Mattson Instruments' exclusive relationship with Fluka Chemicals. The new Cryolect 4800 IR/MS. Users in laboratories everywhere are regulating the environment, identifying flavor chemicals, determining competitive formulations, in fact ". .. solving analytical problems daily which, in the past, were considered impossible."2 And when you discover how . much the Cryolect 4800 can enhance your lab's capability, you'll 1 see that you can't afford to be without it. Find out what the Cryolect 4800 can do for you. Send for a free, detailed brochure. Or ask us to uut the Crvolect 4800 to work on one of your tough identification problems. The Cryolect 4800 IR/MS.. it brings the best of both worlds together. 1
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some interesting recent developments. Late in 1986 reports of GC/IR/MS systems based on low-cost mass spectrometers such as those mentioned above began to appear (29,30).These meeting repor- described the use of both a Finnigan ion trap (29) and a Hewlett-Packard MSD (30) for GC/ IR/MS. Publications describing research using MSD mass spectrometers linked with Fr-IR spectrometers from two different manufacturers (28, 31) also appeared. In the first published report (31). Gurka analyzed six complex environmental samples containing a total of 106 analytes. lnterest in the use of lower cost quadrupole instrumenu has also recently involved somewhat more elaborate quadrupole mass spertrometers (3.2). Figure 3 shows typical reconstructed chromatograms that are obtainable with such a low-cost system, and Tahle 11 summarizes the analytiral resulta obtained from the combined IR/MS library search and comparison software discussed above (28). These data, and the recent introduction of an integrated GC/IR/MS system based on a new dedicated infrared CC detector (IRD) (33) coupled with an MSD. suggest widespread acceptance of this analytical methodology may develop in the near future.
Flgure 3. Reconstructed chromatograms. (a) FT-IR mconsbuc1Bd c h m t o g r a m for a 14-componemmlxhre. (b) MSD total Ion chromatogram for the same sample. (Reprimed lrom Refwewe 28).
Table 11. Search results for 1Ccomponent mixture -4
Peak m
I-Octene Ethylbenzene Anisole Isobutyl methacrylate Benzaldehyde c-Chlorotoiuene pChlorotoiuene &Pinene ODichlorobenzene Nitrobenzene Undecane 2.4-Dimethylphenol pChlorophenol Naphthalene
1 2 3 4 5 6 7 8 9 10 11 12 13 14
m-In
YLlD
MlCh
MICh
m.*
110.9
d
1 1 1
1
1 1 1
1
4 w 1 1 1
3 2 2
d
‘AB algorithm (Nicolet search rofhvare)-comparison of absolUte differenceOf reference and m p i s spectra. 3The lower the match number. the better lha match. PoBition when moieCUisr weight intmation is considered wpws in parentheses. “Not found in top 10 search output posiiim. Source: Reference 28.
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* ANALYTICAL CHEMISTRY, VOL.
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Futue developments in GC/IR/MS One of the most promising new developments is research aimed at improving the sensitivity of GCAR measurements (for a 23-component mixture analysis, quantities of components hetween 55 and 625 ng and averaging 200 ng were injected on column [28]) to better match the sensitivity obtainable with GCIMS. In a recent presentation, Bourne described a GCIIRMS system employing an open split interface that sends half of the GC effluent to a matrix isolation GC/FT-IR instrument and the other half to an MSD for mass spectral measurements (344). Although manual correlation of the spectral data was used, it is clear that this is a promising approach because of the potential for improvement of IR detertion sensitivity by two orders of magnitude or more &e., to the 100picogram level). In f a d , one manufacturer, Mattson Instruments. has announced a GCAR/MS system based on this approach. Research along similar lines is also underway in our laborato. ry. Costs are relatively low, ranging from $150,000 to $200,000 for the Mattson approach and from about 5100.000 to 51‘25,000 for the HewlettPackard GC/IRD/MSD mentioned above. With the availability of practical commercial GC/IR/MS systems. the next major challenge is u, develop adequate computer algorithms and soft-
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proved high-quality spectral databases of all kinds, particularly IR, MS, and NMR databases. These will be essential to further progress in the development of integrated organic analysis systems.
I
Figure 4. Block diagram of matr
= stion (i
ware to take full advantage of the extensive information that they produce. Figure 4 is a diagram of a new GC/IR/ MS system presently heing developed in ow laboratory. A key feature of this
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3WMbvte
:/FT-iR/MS system.
new system is a much more capable computer than was used in previous research. Equally important to continued development of all types of hyphenated systems is the need for im-
Acknowledgments Research on GC/IR/MS in our laboratow was suDwrted hv the National Science Foundation -under grants CHE-79-10263, CHE-81-13612. CHE82-08073, and CHE-85-19087. 'Partial support also was received under Cooperative Agreement CR-811370 between the University of California (UC), Riverside, and the Environmental Protection Agency (EPA) Environmental Monitoring Systems Laboratory, Las Vegas. Partial support from the UC Riverside Toxic Substances Research and Training Program is also achowledged. This paper has heen reviewed in accordance with EPA's peer and administrative review policy and has heen approved for publication. Mention of trade names does not constitute endorsement or recommendation for use. Ref(1) Low, M.J.D.; Freeman, J. K.J . A &. Food Chem. 1968,16,525-28. (2) Wilkins. C. L.:Kloofenstein. C. E. Pro-
ceedings of the Conf&enee oncomputers in the Undergraduate Curriculum; Uni-
How to close the critical gapbetween measurement
versity of Iowa: Iowa City, 1970; pp. 10.110.5. (3) Wilkins, C. L.; Klopfenstein, C. E. Proceedings of the Conference on Computers in the UndergraduateCurriculum;Dartmouth College: Hanover, 1971; pp. 269276. (4) Griffiths, P. R.; Pentoney, S. L., Jr.; Giorgetti, A.; Shafer, K. H. Anal. Chem. 1986,58,1349 A-1366 A. (5) Covey, T. R.; Lee, E. D.; Bruins, A. P.; Henion, J. D. Anal. Chem. 1986. 58, 1451 A-1461 A. (6) Wilkins, C. L. J. Chem. Inf. Comput. Sci. 1977.17, 24249. (7) Naegeli, P. R.; Clerc, J. T.Anal. Chem. 1974,46,739 A-744 A. (8) Kuehl, D.; Griffiths, P. R. Anal. Chem. i978,50,418-22. (9) Gomez-Taylor, M. M.; Griffiths, P. R. Anal. Chem. 1978,50,422-25. (10) Bar-Lev, H. Infrared Phys. 1967, 7, 93-98. (11) Low, M.J.D. Anal. Lett. 1968, I , 819')A _.
(12) Wilkins, C. L.; Giss, G. N.; Brissey, G. M.; Steiner, S. Anal. Chem. 1981,53,11%
1982,54,817-20. (19) Wilkins, C. L.; Giss, G. N.; White, R. L.; Brissey, G. M.; Onyiriuka, E. C. Anal. Chem. 1982,54,226&64. (20) Wilkins, C. L. Science 1983,222,29196. (21) Schafer, K. H.; Hayes, T. L.; Brasch, J. W.; Jakobsen, R. J. Anal. Chem. 1984.56, 23740. (22) Gurka, D. F.; Betowski, L. D. Anal. Chem. 1982,54,1819-24. (23) Chiu, K. S.; Biemann, K.; Krishnan, K.; Hill, S. L. Anal. Chem. 1984.56,161015. (24) Laude, D. A., Jr.; Brissey, G. M.; Ijames, C. F.; Brown, R.S.; Wilkins, C. L. Anal. Chem. 1984,56,1163-68. (25) Laude, D. A,, Jr.; Johlman, C. L.;Wilkins,C. L. 0 p t . E n g . 1985,24,1011-13. (26) Laude, D. A., Jr.; Johlman, C. L.; CooDer. J. R.: Wilkms. C. L. Anal. Chem. 1985,57,104449. (27) Laude, D. A,, Jr.; Johlman, C. L.; Brown. R. S.: Wilkins. C. L. Frensenius 2. Anal. Chem.'1986.324,839-45.
1-
I,.
(13) Uden, P. C.; Henderson, D. E.; Lloyd, R. J. J. Chromatogr. 1976,126,225-37. (14) Uden, P. C.; Carpenter, A. P., Jr.; Hackett, H. M.; Henderson, D. E.;Sigia, S. Anal. Chem. 1979,51,3843. (15) Jurs, P. C.; Kowalski, B. R.; Isenhour, T. L.; Reilly, C. N. Anal. Chem. 1969.41, 1949-53. (16) Sssaki, S.; Abe, H.; Ouki, T.; Sakamoto,M.;Ochiai,S.Anal. Chem. 1968,40, 2220-23. (17) Hirschfeld, T. Anal. Chem. 1980, 52, 291 A312 A. (18) Crawford, R. W.; Hirschfeld, T.; Sanborn, R. H.; Wong, C. M. Anal. Chem.
(30) Coop&, J. R.; Brown, R. S.; Johlman,
C. L.; Laude, D. A,, Jr.; Wilkins, C. L. Abstract 634. Presented at 13th Annual FACSS Meetine. St. Louis.. Ma... Sentem. ber-October 19%. (31) Gurka, D. F.; Titus, R. Anal, Chem. IJRK. 58.21R!L04. . .. , . . ,. . . . ... 2) Fuiiwara. H.: Solsten. R. T.: Wratten. (3% S. J. Ab&& 464. Presented k 13th An: nual FACSS Meeting, St. Louis, Ma., September-October 1986. (3%) ,.., Darland. E. J.: Mealer. L. D.; Michnowicz, J. A.;~Tang,C.; Hirschlreld, T. Abstract 494. Presented at 1986 Pitts~
~~~~~
~
burgh Conference on Analyrical Chemis. try and Applied Spectroscopy. Atlantic City. N. J.. March 1986. (34) Bourne. S. Abstract 632. Presented at 13th Annual FACSS Meeting, St. Louis. Ma.,Septemher-October 1986.
Charles L . Wilkins is professor of chemistry and chairman of the Department of Chemistry at the University of California, Riverside, where he has been a faculty member since 1981. I n addition to his interest in the development of combined organic analysis systems such as GCIIRIMS, he has long been interested in computer-assisted analysis. For the past several years he has also pursued extensive research in analytical applications of FT-MS. Other interests include new approaches to analytical nuclear magnetic resonance, such as flow NMR and HPLCINMR techniques.
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