Anal. Chem. 1984, 56, 1755-1758
LITERATURE CITED (1) Glukhova, L. Yu.; Perov, P. A. Khlm. Tekhnol. Vody 1981, 3 , 236237; Chem. Abstr. 1981, 95, 138270(16). (2) Shaglaeva, N. S.;Brodskaya, E. I.; Rzhepka, A. V.; Lopyrev, V. A.; Voronkov, M. G. Vysokomol. Soedln., Ser. A 1979, 27, 950-952; Chem. Abstr. 1979, 0 1 , 5628(2). (3) Hawkins, 1.incoln W. “Polymer Stabllizatlon”, 1st ed.; Wiley-Interscl-
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ence: New York, 1972; Chapter 3. (4) Rosen, Stephen L. “Fundamental Principles of Polymeric Materials”, 2nd ed.; Wlley-Interscience: New York, 1982; p 142.
RECEIVED for review January 24,1984. Accepted March 26, 1984.
On-Column Sampling Device for Thermogravimetry/Capillary Gas Chromatography/Mass Spectrometry L. F. Whiting* and P. W. Langvardt Analytical Lctboratories, Michigan Division, Dow Chemical Company, Midland, Michigan 48640 Thermogravimetry coupled with mass spectrometry (TG/MS) has proven to be a useful combination in the investigation of thermal decomposition reactions ( I ) . Unfortunately, when the decomposition of a material generates multiple volatile products with similar or overlapping mass spectra, it may become very difficult, if not impossible, to identify the individual components. This situation is most often encountered with the decomposition of large complex molecules or complex mixtures of similar volatility. In order to overcome this problem, off-line trapping of the effluent gas from the thermogravimetric analyzer (TGA) has been carried out in several instances (2-4). The trap contents can then be devolatilized into a gas chromatograph (GC) for separation followed by mass spectrometric identification. Although this approach provides a great deal more analytical information to the experimenter, it can be somewhat cumbersome and time consuming. Great care must be taken in heating any transfer lines and valves between the TGA and trap and between the trap and the GC column to minimize condensation and cross contamination between samples. At the same time, one would prefer not to heat the gaseous effluent at all in order to prevent unwanted wall reactions or simple gas-phase pyrolysis of the TGA off-gases in the transfer lines or the trap (4). A simplified thermogravimetry/gas chromatography/mass spectrometry (TG/GC/MS) system has been developed which completely eliminates the off-line trapping and devolatilization step mentioned above and the associated sample transfer problems. There are no valves or transfer lines between the TGA furnace and the GC. The gaseous products generated in the TGA are taken from the immediate vicinity of the sample in the furnace and are cryogenically condensed on the front end of a wall-coated open tubular fused silica capillary chromatographic column. This minimizes undesired gas-phase pyrolysis and/or wall reactions in the furnace environment and results in a condensed off-gas sample which is already prepared for chromatographic separation. In addition to the above advantages in sampling the gaseous materials from the TGA furnace, the fused silica capillary column also provides other benefits. It is well known that glass capillary columns give better separations, shorter analysis times, and better sensitivity and are more inert than conventional packed GC columns (5). The use of these columns also enables one to change from the TG/MS to the TG/ GC/MS configuration in a matter of minutes so that both kinds of data may be acquired to better elucidate the nature of thermally induced reactions.
EXPERIMENTAL SECTION The coupled instruments used in this design were a modified Du Pont Model 951 thermogravimetric analyzer controlled by a Du Pont 990 programmer/recorder, an LKB 9000 gas chroma0003-2700/84/0356-1755$01.50/0
tograph/mass spectrometer with the stock chromatograph replaced with a Hewlett-Packard 5710A gas chromatograph, and a Digital Equipment Corp. PDP 8/e computer with OS/8 operating system for data reduction. Gas chromatographic separations were carried out on a J&W DB-1 bonded-phase fused silica capillary column, 0.32 mm i.d., 15 m, 0.1 km film thickness. The microneedle valves used to control the flow to the jet separator and the glass-lined stainless steel tubing and tee were obtained from Scientific Glass Engineering, Inc., Austin, TX. The coal used in this study was obtained from the U.S.Bureau of Mines and is a Pittsburgh Seam bituminous coal from Bruceton.
INSTRUMENT DESIGN AND OPERATION The instrument configuration used for both TG/MS and TG/GC/MS is illustrated in Figure 1. In order to minimize problems with the transfer of the TGA effluent from the furnace to the capillary column and the gas chromatograph, the Du Pont 951 TGA furnace tube was modified to operate in a vertical configuration rather than the normal horizontal configuration. As shown in Figure 1,the TGA furnace was rewired to allow vertical operation and placed directly on top of the inner wall of the GC oven. The balance housing was mounted on the end of the 951 TGA module so that the balance arm extended past the end of the module. A special furnace tube was made with ground glass joints for making connections to the balance housing and for access to the platinum hang-wire from which the sample pan is suspended. Samples were loaded by opening the top ground glass joint, grasping the top of the long platinum hang-wire and lifting it and the sample pan out of the furnace tube, placing sample in the platinum pan, and reversing the process. The sampling of the gases evolved from material being heated in the TGA is accomplished with on-column focused cyrogenic trapping (6). In order to implement on-column focused cryogenic trapping in thermogravimetry, a unique approach was taken. First, the inlet of the capillary column in the GC was setup to operate a t atmospheric pressure with the column outlet at reduced pressure to obtain flow through the column. In this way the front end of the capillary column can be inserted directly into the TGA furnace and positioned near the sample pan without disturbing the experiment (see inset A of Figure 2). Second, after trapping volatiles by means of a cryogenic trap positioned on the first loop of the capillary column, the front end of the capillary column is retracted into a glass-lined stainless steel tee which supplies carrier gas to the column for the chromatographic separation. The cryogenic trap is turned off and conventional GC/MS analysis is then carried out on the trapped compounds (see inset B of Figure 2). It should be noted that insertion of the fused silica capillary column into the TGA furnace poses certain limitations on the TGA operating conditions. The stationary phase of the GC 0 1984 Amerlcan Chemlcal Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUGUST 1984
Figure 3. Operation of TG/MS. The glass-lined tube is inserted into the TGA furnace. The other end enters a needle valve upstream of the jet separator. The cryogenic trap is off and the gas chromatograph is heated to 250 "C.
Design of TGlMS and TG/GC/MS instrumentation. Components are identified as follows: (A) platinum sample pan, (B) front end of fused silica capillary GC column, (C) fused silica capillary GC column, (D) '/,a in. 0.d. glass-lined stainless steel tee, (E) liquid nitrogen-cooled cryogenic trap using '/ in. 0.d. Teflon tubing, (F) flow controller adjusted to deliver 25 cmI/min helium makeup gas to the jet separator, (G) flow controller adjusted to deliver approximately 10 cm3/min helium carrier gas to the inlet of the capillary column, (H) TGA furnace, (J) sample thermocouple, (K) balance arm with platinum hang-down wire, (L) 35/25 ground glass joint used as sample loading port, (M) TGA purge gas Inlet, (N) needle valve used for TG/MS operation, (P) needle valve used for TG/GC/MS operation, (a) in. 0.d. glass-lined stainless steel tubing for TG/MS. Flgure 1.
Thermogravimetric analysis curve of a Pittsburgh bituminous coal sample: 17.79 mg, 10 "C/min, helium purge rate at 100 mL/min.
Flgure 4.
t
Flgure 2. Operation of TG/GC/MS. (A) In the trapping mode the front end of the fused silica capillary column is extended Into the TGA furnace tube downstream of the sample. The cryogenic trap is operating. (6) In the analyze mode the front end of the capillary column is retracted into the glass-lined tee. The cryogenic trap is not operating.
column will thermally degrade if the temperature of the end of the column rises too high. We have found that in a helium atmosphere the end of J&W DB-1 can be heated to 350 "C for short periods of time, less than 20 min, without observing a large increase in chromatographic background signal. If the TGA furnace is programmed to temperatures higher than this, the end of the capillary column may be positioned closer to the exit of the TGA furnace so that the column temperature does not exceed 350 "C. This position is determined empirically with a thermocouple to measure the axial furnace temperature profile at the maximum program temperature. In oxidizing atmospheres such as air, the maximum operating temperatures for the capillary column inlet will of necessity be much lower than 350 "C but will depend on flow rate. Exact conditions for operating with air atmosphere have not yet been determined. Sample degradation products may react with the capillary column stationary phase. For example, poly(viny1chloride) thermally decomposes to form hydrogen chloride which reacts with DB-1 stationary phase even at room temperature. Samples which decompose to reactive products should be avoided.
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250
350
./z
Mass spectrum acquired at 260 "C during TG/MS experiment on Pittsburgh coal sample. Flgure 5.
This instrument design also provides for direct mass spectrometry of the TGA effluent using the TG/MS mode shown in Figure 3. In this mode, the GC column is inoperative, the GC oven is heated at 250 "C for 300 "C to prevent condensation in the transfer line, and the needle valve, N, to the jet separator is opened to give the desired flow to the mass spectrometer. Any necessary helium makeup gas can be provided to the jet separator by adjusting needle valve P and flow controller F.
RESULTS AND DISCUSSION Coal is a material which decomposes in a complex manner on heating. For that reason it was selected as a test compound to demonstrate the capabilities of the instrumentation described above. Figure 4 illustrates a TGA curve of the first 3% weight loss from a Pittsburgh Coal sample. During the heating of the sample in the TGA, a portion of the effluent
ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUGUST 1984
I
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li
10
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13
23
4
Figure 6. Reconstructed total ion current gas chromatogram from
TG/GC/MS experiment on coal sample, 0-1.6% weight loss.
Table I. Tentative Assignment of Gas Chromatographic Peaks from TG/GC/MS Experiment on Coal Sample, 0 to 1.6% Weight Loss 1 2
3 4 5 6
I 8
9 10 11
12 13 14 15 16 17 18
19 20 21 22
23 24 25 26 27
28 29 30
31 32 33 34 35 36 31 38 39 40 41 42 43
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carbon dioxide sulfur dioxide methylbutane methylene chloride 2-methylpentane n-hexane cyclohexane methylcyclohexane toluene 2-methylheptane xylene isomer xylene isomer methylethylbenzene trimethylbenzene methylethylbenzene 1,3,5-trimethylbenzene n-decane dimethylethylbenzene n-undecane tetramethylbenzene naphthalene n-dodecane methylnaphthalene methylnaphthalene n-tridecane or a methyldodecane 2,6- or 2,7-dimethylnaphthalene 1,5- or 1,6-dimethylnaphthalene n-tetradecane !2,3-dimethylnaphthalene
1,2-dimethylnaphthalene C,,-alkane C3 alkylated naphthalene pentadecane trimethylnaphthalene(s) trimethylnaphthalene(s) hexadecane phenylbenzaldehyde or methyldibenzofuran >CI6 branched alkane anthracene or phenanthrene alkane methylanthracene or methylphenanthrene methylanthracene or methylphenanthrene alkane
gases was analyzed by TG/MS. Mass spectra were acquired at 10-s intervals between m/z 14 and m / z 400 with a scan time of 8 s/scan. A mass spectrum taken at a sample temperature of 260 "C is shown in Figure 5. The complexity of the spectrum indicates that several different volatile species are being formed during the low-temperature pyrolysis of this coal sample. In order to identify some of these species, the TG/GC/MS instrumentation described above was used to analyze the TGA effluent during the first 0 to 1.6% weight loss. A reconstructed total-ion current chromatogram is il-
150
Mass spectrum of peak number 27 of chromatogram shown in Figure 6, TG/GC/MS experiment on coal sample, 0-1.6% weight loss. The peak is tentatively identlfled as either 1,5- or 1,6-dimethylnaphthalene. Flgure 7.
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