Anal. Chem. 1009, 65, 2819-2823
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Thermogravimetry/Gas Chromatography/Mass Spectrometry and Thermogravimetry/Gas ChromatographyIFourier Transform Infrared Spectroscopy: Novel Hyphenated Methods in Thermal Analysis William H. McClennenl. Richard M. Buchanan, Neil S. Arnold, Jacek P. Dworzanski, and Henk L. C. Meuzelaar Center for Microanalysis & Reaction Chemistry, University of Utah, Salt Lake City, Utah 84112
Two doubly hyphenated,thermogravimetry-based analytical techniques, viz. TG/GC/MS and TG/ GC/IR, are described. A valveless, quartz, heated sample path between TG furnace and GC column minimizes losses of products. Furthermore, combination of a pulsed automated vapor sampling inlet and a transfer line type GC column permits high-speed GC identification of individual TG products while maintaining sufficiently high temporal resolution with the 1-min sampling interval to provide kinetic information about the underlying reaction mechanisms. Example analyses on poly (a-methylstyrene),a styreneisoprene block copolymer, and wood demonstrate the techniques’ capability for monitoring specific minor products and isomers.
niques22 combining the direct measurement of weight loss as a function of temperature with the use of sensitive spectroscopic detectors. Such detectors permit qualitative and quantitative determination of the evolved volatile products to provide kinetic information about the specific reaction mechanisms. The TG/MS interfaces2have variously involved placing the TG in the vacuum of the MS16 or operating the TG in a higher pressure region connected to the MS vacuum either through orifices, often with differential pumping,”” or through a capillary restrictor2J2Js to limit the conduction of products into the MS. The TG/MS technique has found broad application in structure/reactivity studies including synthetic polymers,l fossil fuels,9-6J2and natural products,1OJ3 among others. Similarly, TG/IR has become so popular as to have led to the development of dedicated combined instruments21as well as systems readily configured from commercially available analytical instruments and components.1213J* The technique is especially useful for smaller molecules where the high specificity of strong IR absorption bands makes up for the INTRODUCTION generally lower sensitivity of IR detection compared to MS.193 Combined thermogravimetry/mass spectrometry (TG/ In addition, certain TG/IR configurations do have some MS)1-16 and thermogravimetry/Fourier transform infrared advantage over TG/MS in the handling of heavy tar products, spectroscopy (TG/IR)13J7-21 are powerful ‘hyphenated” techwhich can be analyzed as fine aerosols in a gas stream.18121 Despite the utility of these techniques, a distinct disad(1) Holdinesa, M. R. Thermochim. Acta 1984, 75, 361-399. vantage is that the presence of components at very low (2) Dollimore,D.;Gamlen,G.A.; Taylor,T. J. Thermochim.Acta 1984, concentrations may be masked by higher concentration 75. 59-69. (3) Ohrbach, K. H.; Klusmeier, W.; Kettrup, A. Thermochim. Acta interferants. Thus, some researchers have incorporated the 1984, 72, 165-169. separation power of gas chromatography (GC) by collecting (4) Behrens, R. Rev. Sci. Instrum. 1987,58,451-461. products in a trap or on the head of a capillary column for (5) Yun, Y.; Meuzelaar, H. L. C. Energy Fuels 1991,5, 22-29. (6) Emmerich, W. D.; Kaisersberger,E. J. Therm. Anal. 1979,17,197all or part of the TG run.14-16 However, these methods YLI. necessarily result in the loss of the time/temperature evolution (7) Earnest, C. M. Anal. Chem. 1984,56, 1471A-1486A. data for the products analyzed. (8) Kettrup, A. A.; Ohrbach, K. H.; Radhoff, G.; Klusmeier, W. Thermochim. Acta 1984, 7 4 , 8 7 4 3 . Recent development of an automated vapor sampling (9) Kaieersberger,E.; Emmerich, W. D. Thermochim. Acta 1985,85, (AVS) technique in our laboratory23.24 has illustrated the 275-278. feasibility of using high-speed “transfer line” gas chroma(10) Dyszel, S. M. Thermochim. Acta 1986,104, 85-92. (11) Charsley, E. L.; Manning, N. J.; Warringon, S. B. Thermochim. tography techniques for near real-time process monitoring Acta 1987, 114, 47-52. applications using MS.26.26 The so-called transfer line GC,27 (12) Buchanan,R. M.;Holbrook,K.M.;Meuzelaar,H.L.C.;Leibrand,
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R. Hewlett Packard Application Brief, 1991; IRD 91-4. (13) Dworzanski, J. P.; Buchanan, R. M.; Chapman, J. N.;Meuzelaar, H. L. C. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1991, 36 (2), 725-732. (14) Pavlath, A. E.; Gregorski, K. S.; Young, R. Thermochim. Acta 1985,92, 383-384. (15) C h u g , H. L.; Aldridge, J. C.; Ogden, M. W. R o c . 39th ASMS Conf.Mass Spectrom. Allied Top. 1991, 730. (16) Yuen, H. K.; Mappes, G. W. Thermochim. Acta 1983, 70, 269281. (17) McEwen, D. J.; Lee, W. R.; Swarin, S. J. Thermochim. Acta 1985, 86,251-256. (18) Carangelo, R. M.; Solomon, P. R.; Gerson, D. J. Fuel 1987, 66, 960-967. (19) Yong, W. J.; Quan, X. X.; Zheng, J. T. Thermochim. Acta 1987, I l l , 325-333. (20) DeGroot, W. F.; Pan, W. P.; Rahman, M. D.; Richards, G. N. J . A w l . Appl. Pyroly~is1988,13,221-231. 0003-2700/93/0365-2819$04.00/0
(21) Solomon, P. R.; Serio, M. A.; Carangelo,R. M.; Baesilakie, R.; Yu, Z. Z.; Charpenay, S.;Whelm, J. J. Anal. Appl. Pyrolysis 1991,19,1-14. (22) Hirschfeld, T. Anal. Chem. 1980,52,297A-312A. (23) Arnold,N. S.:McClennen. W. H.: Meuzelaar,H. L. C. Anal. Chem. 1991,63, 299-304. (24) McClennen, W. H.: Arnold, N. S.:Meuzelaar.H. L. C. U.S.Patent 4,970,905, 1990. (25) McClennen, W. H.; Arnold, N. S.;Roberts, K. A.; Meuzelaar, H. L. C.; Lighty, J. S.; Lindgren, E. R. Combust. Sci. Technol. 1990, 74, 297-309. (26) McClennen, W. H.; Sheya, S. A. N.; Arnold, N. S.; Meuzelaar, H. L. C.;Deng,X. X.;Lamon,F. S.;Silcox, G.Roeeedings,Znd International Congress Toxic Combustion &-Products: Formation Control; March 26-29, 1991, in presa. (27) Arnold, N. S.;Kim, M.-G.; McClennen, W. H.; Dworzanski, J. P.; Meuzelaar, H. L. C. R o c . 1992 Workshop Ion Mobility Spectrom. June 21-25, 1992, Mescalero, NM, 1992; pp 11-16. 0 1993 American Chemical Soclety
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 20, OCTOBER 15, 1993 I
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and TG/GC/MS instrument configuration.
as opposed to short-column GC,28 is optimized for fixed pressure drop situations where the ordinary inlet pressurebased optimization of capillary GC is not possible. Such techniques have been shown capable of GC preseparation to eliminate interferences, while providing repetitive sampling at 1-10 samples/min26 consistent with process monitoring applications. Although developed originally to utilize the vacuum outlet conditions28 of the mass spectrometer, recent work has shown that most of the analytical utility of this short-column approach, as measured in resolution and speed of analysis, can be maintained at relatively high outlet pressure^.^' The coupling of an AVS short-column GC interface to ion mobility spectrometry has illustrated that effective separations may be obtained with pressure drops as low as 1-2 psi below atmosphere29and have clearly demonstrated the feasibility of coupling the same device to gasphase IR detectors. Thus, the approach of this work is to transport products from the TG to the GC where they are rapidly separated on a short capillarycolumn. The regular injectionport is replaced by an automated vapor sampling inlet for repetitive, periodic injection of the products. The resulting TG/GC/MS and TG/ GC/IR configurations are capable of separating thermal decomposition products, while still allowing characterization of the evolution profile for each component. Although not shown here, further extension to a "triple-hyphenated" TG/ GC/IR/MS configuration is straightforward and may be appropriate for some especially tough analytical problems.
EXPERIMENTAL SECTION The TG/IR/MS, TG/GC/MS, and TG/GC/IR configurations described below consist of a standard Hewlett-Packard GC/IR/ MS system (includingthe Model 5890A GC, Model 5965A IRD, and Model 5971A MSD),coupled to a Perkin-Elmer Model 7 TG with high-temperature (maximum 1500 "C) furnace. As shown in Figure 1,a speciallyconstructed, heated transfer line assembly allows direct couplingof the TG system to the GC injection port. The GC oven acts as a convenient heated coupling and flow distribution module. The configuration of the transfer line interface to the TG is described in detail elsewhere.12 Basically, a quartz tube (1.0-mm i.d.) mounted in this interface extends upward to within 1 cm of the sample crucible. A fused-silica (28) Trehy, M. L.; Yost, R. A,; Dorsey, J. G. Anal. Chem. 1986, 58, 14-19. (29) Snyder, P. A.; Harden, C.S.; Brittain, A. H.; Kim, M. G.; Arnold, N. S.; Meuzelaar, H. L. C. Anal. Chem. 1993,65,299-306.
capillary transfer line protrudes several centimeters into the quartz tubing from its lower end. Both the quartz tube and the outlet of the ceramic furnace tube are vented by means of needle valves. This effectively removes all dead volume in both the furnace and the quartz tube, while efficiently directing TG effluent into the transfer line. Additionally, the quartz tube serves as a controlled condensation zone, preventing contamination and eventual plugging of the transfer lines by heavy tar components. The quartz tube can be conveniently cleaned or replaced after each run, if necessary. Finally,a means is provided to back-flush the quartz tube. This prevents air from entering the MS vacuum system when the TG furnace is open or not yet purged. Temperature control along the valveless, all-quartz sample path is accomplished by a combinationof techniques. A machined aluminum block surrounding the entire TG interface assembly is heated using the control system of the HP 5890A GC. From there, the transfer lines are led through an insulated, resistively heated stainless steel tube, which terminates inside the heated GC injection port. Once inside the GC oven, the transfer lines are easily directed to their respective destination instruments using the existing heated GC/IR and GC/MS interfaces. Severalsampleswere run by TG/IR/MSwithout GC separation to demonstratethe advantagesconferredby the latter technique. In this case,two separate capillarieswere led through the transfer line from the TG to the detectors (0.32-mm i.d. to the IR and 0.10-mm i.d. to the MS). The total helium flow through the TG was set at 60 mL/min with -30 mL/min entering the quartz tube and 10 mL/min traversing the transfer lines. All heated zones were maintained at 300 "C. The TG/GC/IR and TG/GC/MS analyses used a single 0.53mm deactivated fused-silica transfer line to carry TG effluent to the GC oven. Inside the GC oven this capillaryterminateswithin a specially designed repetitive vapor sampling inlet from which a second capillary transfer line, the GC column, leads to either the IR or MS modules. The AVS inlet is described in detail e l s e ~ h e r and e ~ ~involves ~~ the use of a protective flow of He carrier gas around the inlet end of the capillary column which is periodically interrupted to permit sample vapors to enter the column. The TG furnace was operated at -0.14 bar above ambientpressure to maintain sufficientflow through the transfer line,AVS inlet, GC column,and ambient-pressureIR. Operation of the Hewlett-Packard IRD at subambientpressure would have facilitated many aspects of interfacingto the TG and our short GC column. However,this nonstandardapplicationdid not work due to a leak in the technologically complex IR cell assembly which went undetected to our in-house maintenance. A 2.1 m X 0.32 mm i.d. HP-5 column was used for GC/IR separation,and a 1.7 m X 0.10 mm i.d. HP-5 column for GC/MS analyses. Apart from the TG furnace and GC oven, all heated zones were
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ANALYTICAL CHEMISTRY,VOL. 65, NO. 20, OCTOBER 15, 1993 io0
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RESULTS AND DISCUSSION The first example demonstrates the utility of the shortcolumn GC for allowing detection of minor components in the midst of similarly structured major decomposition products. Samples of poly(a-methylstyrene)were run by both TGIIRIMS and TG/GC/MS to demonstrate the enhanced capability for detection of minor products. In the TG data (not shown) the polymer was seen to degrade completely between ca. 250 and 350 OC. Panels a and b in Figure 2 show the TG/MS continuous time evolution profiles for ions at m/z 103 and 104, presumably both from the monomer (see Figure 2c). The 10% intensity of m/z 104 relative to m/z 103 isnearly entirelyaccountad for by the expected 9 % l9C content of m/z 103(C&+) derived by a methyl loss from the monomer molecular ion at m/z 118. The nearly identical shapes of the two curves offer no contrary evidence. Panels a and b also show the corresponding TG/GC/MS poly(cr-methylstyrene) data for m/z 103 and 104. Note that
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Flgure 3. TGIGCIIR data for a 14% styrene and 86% Isoprene ABA block copolymer. Time evolutkm profiles are shown by (a)the weight loss (TO) and rate of weight loss (DTG) data along with (b) the total Infrared response chromatogram. (c-1) Selected Infrared absorbance spectra corresponding to GC peaks in (b).
because the vapor samples were taken at 1-min intervals, the time profiles for a specific compound can be followed by identifying GC peaks with the same column retention time, i.e., the amount of time between the vapor sampling event and the appearance of a peak. For example, the largest GC peak in Figure 2a resulted from a vapor sampling event at 9.0 min, but occurred at a total elapsed time of 9.8 min, corresponding to a retention time of 0.8 min. In Figure 2b it is now clear that there are two sources contributing to the signal a t mlz 104with the larger series of peaks corresponding to those for m/z 103 in (a) while the second series elutes with a retention time of only 0.4 min. The complete mass spectra of the two peaks from the 10-min TG/GC/MS sample are illustrated inFigure 2c,d andare identriedas a-methylstyrene and styrene, respectively. At a concentration of only 1% relative to a-methylstyrene, the styrene could not have been detected without the GC separation. The second example further demonstrates the usefulness of the on-line separation step for isomer products in the hyphenated techniques. Figure 3 showsTG/GC/IRdata from an ABA block copolymer of 14%styrene (A) and 86 7% isoprene (B).Figure 3a shows the TG data with the weight loss curve (TG) plotted as a function of the run time along with the first derivative of the TG curve (DTG), which represents the rate of sample weight loss. As expected, the rate of product evolution, as indicated by the DTG curve, is mirrored closely by the connected peak tops in the IR detector total response profile in Figure 3b. Figure 4a shows the MS response profiie from a similar run with the same material. (The MS response is slightly delayed due to a shift in TG evolution during the
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 20, OCTOBER 15, 1993
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TG/GC/MS run.) Again, vapor samples were taken at 1-min intervals. In this case, the presence of both IR and MS data contributes to identification, as can be seen through selected IR and mass spectra shown in Figures 3c-f and 4b-e, respectively. The first eluting large series of GC peaks (short dashed line with t = 0.09 min in the GC/IR and t = 0.11 min in the GUMS data) shows clear alkyl (2930-2970 cm-l) absorptions in the IR spectra as well as a parent ion signal a t mlz 68 in the MS data, thus confirming the presence of the isoprene monomer (Figures 3c and 4b). Two additional early evolving sets of peaks are seen at t = 0.33 min (dotted line) and t = 0.50-0.53 min (dash/dot line) respectively, the latter being much stronger in intensity. Comparison of Figures 3d,e and 4c,d shows very few differences between either the mass or IR spectra of these two compounds; most noticeable are minor shifts in the MS peak ratios at m/z 107, 121, and 136. The presence of the m/z 136 peak indicates that both of these compounds are probably dimers of isoprene (mlz 68), consistent with the decrease in alkene IR absorption relative to the monomer in Figure 3c. The two very similar compounds are most likely 2,4-dimethyl-4-vinylcyclohexene ( t = 0.33 min) and 1-methyl-4-isopropenylcyclohexene (dipentene, t = 0.500.53 min) with their relative amounts due to the initial polymer structure and/or the degradation m e ~ h a n i s m . ~ The J ~ two distinct isomers would have been difficult or impossible to
detect without GC separation, especially considering the high degree of spectral resemblance between the two compounds. The TG/AVS-GC technique facilitates monitoring of the relative amounts of the two products as a function of evolution time and sample temperature. One other significant compound (t = 0.24 min, long dashed line) peaks at -3-min run time (80 OC) later than the others. Extensive aromatic absorptions (3030-3100 cm-l) and a parent ion at m/z 104 (Figures 3f and 4e) identify this species as styrene. In addition to these four compounds observed in both data sets, several other GC peaks can be identified in the TG effluent from this material using the MS data. This is due to the inherently greater sensitivity of the MS compared to the IR system. In most laboratory applications the selection of GC column is primarily a problem of selecting an appropriate stationary phase and film thickness while utilizing a commonly supplied length and inside diameter. Any remaining optimization is usually accomplished by head pressure adjustments. In the present application, the TG system is designed to operate at ambient pressure and, with some modifications, a few tenths of a bar above ambient. In this context, column length and radius become the primary means for optimizing a separation (along with stationaryphase). The columns used in this work were selected t,o have the same stationary phase, to yield appropriate volumetric flows to the detector (IR 3.7 mL/min and MS 0.2 mL/min at ambient temperature and pressure), and to provide short retention times (
