Temperature Rise Time and True Pyrolysis Temperature in Pulse Mode Pyrolysis Gas Chromatography Ram L . Levy, Dale L. Fanter, and Clarence J. Wolf Me Donnell Douglas Research Laboratories, Mc Donnrll Douglas Corp., St. Louis, Mo. 63166 Factors affecting the interlaboratory reproducibility of filament and Curie point pyrolyzers used in pyrolysis gas chromatography (PGC) are discussed. The temperature rise time (TRT) of the heat source is of particular concern. A direct method of measuring the true pyrolysis temperature based on monitoring the temperature-time profile of the heat source is described. The true pyrolysis temperature is below the final equilibrium temperature of the heat source and is related to the power supplied to the pyrolyzer. In addition, inherent characteristics of the ferromagnetic wires used in the Curie point method are discussed. New methods to obtain TRT’s as fast as 7 msec with filament units are presented.
PYROLYSIS GAS CHROMATOGRAPHY (PGC) has evolved into a potentially powerful analytical technique for the analysis of complex organic materials (1). The further development of P G C depends on the achievement of interlaboratory reproducibility and on standardization of the experimental conditions ( 2 ) . In fact, the Pyrolysis Gas Chromatography SubGroup of the Gas Chromatography Discussion Group (3-5) has undertaken a detailed study into the problems concerned with PGC reproducibility. Their survey indicates that approximately 80% of all the pyrolyzers used in PGC are of the so-called pulse mode variety. In these pyrolyzers, most of which are based on either the resistive heating of filaments or on high frequency induction heating of ferromagnetic wires, energy, consequently temperature, is applied as a pulse. One of the experimental parameters of prime interest in pulse mode pyrolyzers is the temperature rise time (TRT) of the heat source. Farre-Rius and Guiochon ( 6 ) showed that in most PGC studies, pyrolysis is complete at temperatures well below the equilibrium temperature (TeJ of the heat source. For example, even with a thermally stable material such as polytetrafluoroethylene, the half-time for decomposition was estimated to be 26 msec a t 600 “C. Since most (1) R. L. Levy, Chromotogr. Rec.. 8,48 (1966). (2) R. L. Levy, Panel Discussion in “Gas Chromatography 1968,” C. L. A. Harbourn, Ed., Institute of Petroleum, London, 1969. p 410. (3) S . G. Perry, J . Chroniarogr. Sci., 7, 193 (1969). (4) N. B. Coupe, C. E. R. Jones, and S. G. Perry, J . Chromutogr., 47, 291 (1970). ( 5 ) A. G. Douglas, J . Chronmtogr. Sci..9, 321 (1971). (6) F. Farre-Rius and G. Guiochon, A ~ A LCHEM., . 40. 998 (1968). 38
organic decompositions proceed by a complex mechanism involving several separate steps, the temperature of the system during decomposition is critical in determining the overall product distribution. In most cases encountered in PGC, the actual pyrolysis temperature of the sample remains unknown and depends on the sample heating rate (6). Thus, the pyrolysis temperature cannot be accurately reproduced in different laboratories. The achievement of interlaboratory reproducibility in PGC, similar to that of I R , MS, and N M R , adds a new dimension t o structure analysis. Normalized pyrograms obtained under standard conditions could be compiled and used as reference spectra. The accumulation of compiled reference data will permit the deduction of modes of thermal fragmentation which, in turn, could be used for interpretation of new pyrograms not listed in the compiled data. Limited studies on vapor phase, PGC, a continuous mode pyrolysis process, indicate that it is both reproducible on an interlaboratory basis and useful for structural elucidation (7, 8). The development ( 9 ) and refinement (10) of the induction heated or so-called Curie point devices and the introduction of novel methods for fast T R T (11-13) and control of the final temperature (13, 14) of filament units have elevated pulse mode PGC to a new level of reliability. The new pulse mode devices can now meet the basic requirement to produce a well defined square-wave temperature-time profile (15). The direct measurement of pyrolysis temperature (or range of temperatures) is highly desirable for the achievement of interlaboratory reproducibility. Janak (16) reported unsuccessful attempts to measure this important parameter. (7) D. L. Fanter, J. Q. Walker; and C. J. Wolf, ANALC H E W ,40, 2168 (1968). (8) R. A. Brown, ihid., 43, 900(1971). (9) W . Simon and H. Giacobbo: Cheni.-O?g.-T~.ch.,37, 709 (1965). (10) Ch. Buhler and W. Simon, J . Chroniutogr. Sci., 8, 323 (1970). ( I 1) R . L. Levy and C. J. Wolf, 158th American Chemical Society Meeting, New York, N . Y . . September 1969. (12) R. L. Levy and D. L. Fanter, ANAL.C H E W 41, , 1465 (1969). (13) R. L. Levy and C. J. Wolf, 161st American Chemical Society Meeting, Los Angeles, Calif., April 1971. (14) M. Krejci and M. Deml. Collect. Czech. Chent. Comrv~ol.,30, 3071 (1965). (15) R . L. Levy. J. Gus C/1ron7rrtogr.,5, 107 (1967). (16) J. Janak. in “Gas Chromatography 1960.” R. P. W. Scott, Ed.. Butterworths, London, 1960, p 387.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 1, JANUARY 1972
b
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Figure 2. Schematic of the capacitor discharge circuit for rapid excitation of filament pyrolyzers
A new method to directly measure the “true pyrolysis temperature’’ is reported here. This method which is based on the measurement of the temperature-time profile is applicable t o both Curie point and filament pyrolyzers. In addition, limiting inherent characteristics of the Curie point method and novel methods t o attain rapid TRT in filament pyrolyzers are discussed.
Movable heater Glass enclosure
EXPERIMENTAL
P-
GC column (open tubular)
Figure l a . Diagram of the filament pyrolyzer
(double helix)
Figure l b . Detailed view of the filament holder
Filament Pyrolyzer. A diagram of the filament pyrolyzer unit is shown in Figure l a . The filament was fabricated from 3.5 cm of 0.03-cm diameter Pt wire wound into a double helix; a n expanded view is shown in Figure l b . The pyrolysis filament was welded t o 0.13 cm diam Pt leads which were positioned in a ceramic insulator [0.6 cm in.) o.d.1 and sealed (gas tight) with epoxy sealant. The insulator containing the Pt leads and pyrolysis filament was placed inside a 1-cm (”8 in.) 0.d. glass sleeve. A Swagelok union was used t o connect the insulator t o the sleeve and to admit the carrier gas. Capcitor Discharge Circuit. A capacitor discharge circuit used t o rapidly raise the filament to its equilibrium temperature (Teq)is schematically illustrated in Figure 2. The discharge of the 10,000-pF capacitor through the filament gives the rapid temperature boost while a constant current maintains the filament a t the desired TeQ. The capacitor charging voltage is set so as t o minimize TRT while boosting the filament temperature only t o Teq. Curie Point Pyrolyzers. Two Curie point pyrolyzers, a Fischer-Varian unit operating at 1200 kHz and 1500 W and a Phillips unit operating at 550 kHz and 30 W were used for comparative determination of TRT’s of various ferromagnetic wires. A schematic of a Curie point pyrolyzer is shown in Figure 3. The points a, 6, and c indicate the position of the thermocouple attachment relative to the rf coil. Sampling Procedures. Polystyrene samples were deposited on the filament or the ferromagnetic wire from a benzene solution (1-3 pl). The solution (50 grams of polystyrene per liter) was placed at the point of thermocouple contact. The samples were air dried for 10 min, then evacuated a t 1 Torr for 20 min to remove residual solvent. An acrylic acidacrylonitrile copolymer sample was applied by gently heating the filament t o approximately 100 OC and sprinkling the polymer powder onto the filament. Measurement of “True Pyrolysis Temperature.” Two different techniques were used to monitor the temperaturetime profiles of the filaments; a silicon photodiode and a thermocouple. The photodiode together with the filament were housed in a light-tight box, The output signal which corresponded to 5 V at 650 “C was displayed and photographed with a storage oscilloscope (Hughes “Memoxope”). The photodiode response is proportional to the temperature ANALYTICAL CHEMISTRY, VOL. 44, NO. 1, JANUARY 1972
39
v
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Input $#goalIVI
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Filament
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Figure 3. Cross-section diagram of a Curie point pyrolyzer showing points of attachment of low mass thermocouples averaged over the entire length of the filament. Since a real sample, however, coats only a small fraction of the filament surface, it is difficult to obtain representative temperature-time profiles of the sample. Thus, the “true pyrolysis temperature” cannot be determined readily with the silicon photodiode detector. Therefore, a direct measuring system based on the attachment of a low mass thermocouple was developed. It is important t o note that the thermocouple method measures the temperature-time profile at the actual point of contact between the filament and the sample. Chromel-alumel thermocouple leads (0.0075-cm diameter) were indivjdually arc-welded onto the bottom loop of the filament thus forming a n “unbeaded” thermocouple (see Figure lb). The rise times (Le., the T R T of the filament) measured with both methods are identical within experimental error ( + 5 %). This observation indicates that the response of the measuring system is not the limiting factor, but the measurements represent the true temperature-time characteristics of the pyrolyzer. The small (0-20 mV) thermocouple signal which is superimposed upon the larger capacitor discharge transient and the steady state voltage (3-4 V) is isolated by a high gain differential amplifier (Tektronix Type “D” preamp on a Tektronix 585 A oscilloscope, see Figure 4). The thermocouple leads are attached directly t o each preamp input. When operated in the “A minus B” dc mode, the differential amplifier isolates the net thermocouple signal. The “true pyrolysis temperatures” were directly determined from the difference in the temperature-time profiles of the filament obtained with and without the polymer sample. “Unbeaded” thermocouples were arc-welded onto the ferromagnetic wires for direct measurement of TRT. For these measurements, the positive thermocouple lead was fed directly into the preamp with a 0.68 pF capacitor shunt t o ground to filter out rf. RESULTS AND DISCUSSION
The reproducibility of the experimental conditions in PGC depends on the control of all parameters involved. One of the most important parameters is the time required for the heat source to reach Te,,--i.e., the TRT. By following the approach developed for thermogravirnetry (/7), Farre-Rius and (17) H. H. Horowitz and G. Metzger, ANAL. CHEM.,35, 1465 (1963). 40
ANALYTICAL CHEMISTRY, VOL. 44, NO. 1, JANUARY 1972
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io2
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Figure 5. Characteristic pyrolysis temperature of poly0 as a function of the temperature rise styrene T, and T, time (TRT)
+
A-T,
e-T,+8
Guiochon (6) calculated the characteristic pyrolysis temperature of polystyrene and polytetrafluoroethylene for different heating rates. The characteristic temperature, T,, is defined as that temperature at which a fraction l / e (37%) of the sample remains unpyrolyzed, assuming that the sample temperature increases linearly. The temperature at which pyrolysis is completed is defined as T, 0. The characteristic temperature, T,, and T , 0 for polystyrene as a function of TRT’s are shown in Figure 5 . This curve indicates that the characteristic pyrolysis temperature of polystyrene varies by 300 “C when the rise time varies from 5 X t o l o 3 sec. It is important to note that the TRT’s of filament and Curie point pyrolyzers encountered in normal PGC operation range between 15 and 5 X sec. According t o the data shown in Figure 5 , within this range of TRT’s pyrolysis temperatures of polystyrene would range between 570 and 350 “C. However, one should not be left with the impression that interlaboratory reproducibility is asymptotically approached as T R T is decreased. It is rather the relationship of the T R T to the half decomposition time that determines the effect of TRT on the reproducibility. When half decomposition times are long compared to TRT, the effect of the latter will be small. The practical conclusion is that pyrolysis temperatures should be sufficiently low to make half decomposition
+
+
Traw a
Trace b
Figure 6. Expanded scale tracings of temperature-time profiles of a filament with and without sample showing the true pyrolysis temperature Trace a, Filament. Trace b, Filament plus 150 of acrylic acid-acrylonitrile copolymer
pg
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times long relative to the T R T of the pyrolyzer at hand. It is also frequently desired t o pyrolyze a t lower temperatures so that the nature and the distribution of the fragmentation products will carry more information about the structure of the parent material. However, in a normal P G C system such lowering of the pyrolysis temperature may cause loss of GC resolution due t o broadening of the sample input profile. As a general rule, it is desirable to have the T R T at least a n order of magnitude faster than the half decomposition time, provided that GC resolution can be preserved. I n their calculations of T,, Farre-Rius and Guiochon (6) tacitly assumed that the power consumed by the pyrolyzersample was not a limiting factor. I n a n actual system, however, pyrolysis of the sample consumes a considerable fraction of the power supplied to the heat source. When this occurs, the rate of temperature rise decreases resulting in a change in the temperature-time profile. I n cases where the power needed t o sustain the pyrolysis process equals the power applied, a plateau will be observed in the temperature-time curve. The oscilloscopic tracings of the temperature-time curves of a filament with and without sample are shown in Figure 6. The appearance of the plateau below T,, in Figure 6b indicates the temperature region where pyrolysis actually occurs. The length of this plateau is indicative of the sample lifetime o n the filament. This observation permits the direct measurement of the true pyrolysis temperature which we define as that temperature at which the rate of energy (power) consumed by the sample is equal t o the net power supplied to the system. The true pyrolysis temperature as defined here is related t o the characteristic temperature, T,, which was derived to estimate the actual pyrolysis temperature range as a function of heating rate (6). The true pyrolysis temperature represents a direct experimental measurement of the actual pyrolysis temperature as it occurs under experimental conditions. From oscilloscopic traces obtained during filament pyrolysis of 50 pg of polystyrene (rise time 10 msec), we measured a
I
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I 400
true pyrolysis temperature of 575 + 15 "C. This value should be compared with the calculated Ts value of 500 "C for a rise time o f 10 msec (see Figure 5 ) . These results are not in disagreement when one considers the approximations made in the calculations. A possible explanation for the higher experimental value is the almost inevitable lag in the sample temperature relative to the filament temperature. The Curie point pyrolyzer introduced by Simon and Giacobbo ( 9 ) led many workers to believe that it would resolve the problem of interlaboratory reproducibility. These expectations did not materialize because most commercial versions of the Curie point system did not meet the specifications of the original device. I n addition some inherent characteristics concerning the absorption of the rf energy by the ferromagnetic wires as a function of temperature may contribute to interlaboratory irreproducibility. Our measurements of the T R T of various ferromagnetic conductors carried out with two commercial Curie point pyrolyzers of different rf power output are summarized in Table I. The TRT's obtained with the 30 W Phillips unit are almost a n order of magnitude longer than the TRT's obtained with the 1500-W Fischer unit. Wires of different Curie points are obtained by varying the alloy composition. The composition of the ferromagnetic alloy, however, affects not only the Curie point but the T R T as well. For instance, the 600 and 610 "C wires have close Curie points, but the 610 "C wire exhibits a TRT almost twice that of the 600 "C wire. Correlation between the alloy composition and the corresponding TRT's indicates that alloys of higher nickel content show increased TRT's while higher cobalt content produces faster TRT's. Also, it should be noted that the 400, 600, and 700 "C wires, which were specially fabricated for Curie point PGC, exhibit relatively fast TRT's. A number of other factors affect the T R T of Curie point pyrolyzers. The measurement of the T R T at different points o n the ferromagnetic wire (see Figure 3) revealed that the TRT varies from point to point along the wire. For example, the
Table I. Temperature Rise Times (TRT) of Ferromagnetic Wires Obtained with rf Generators of Different Power Outputs. Composition of the Wires Is Also Indicated TRT(msec) Pyrolyzer Specified Curie Fischer-Varian Phillips Ferromagnetic alloy composition point of wire, "C 1500 W 30 W Fe Ni co 358 400 510 600 610 700 770
300 40 150 70 130 90 110
1300
500 700
500 1150 1350 2100
0 61.7 50.6 42 29.2 33 100
100 0 49.4 41 70.8 33 0
0 38.3 0 16 0 33 0
ANALYTICAL CHEMISTRY, VOL. 44, NO. 1, JANUARY 1972
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Figure 7. Expanded scale tracing showing the upper 100 "C portion of the temperature time profile of a 510 "C Curie point wire
500
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Temperature
top, center, and bottom thermocouples attached 5 mm apart t o a 400 "C wire exhibited TRT's of 70, 40, and 250 msec, respectively, with the 1500-W Fischer unit. A similar variation was noted with the 30-W Phillips unit. These measurements indicate that sample position with reference to the rf coil is extremely important. The sample must be deposited o n a narrow region of the wire and positioned in the center (both axially and longitudinally) of the rf coil. Coiled ferromagnetic wires sometimes are used for increased sample capacity and convenience in application. The TRT's of these wires were found to be approximately 10 times longer than those of the corresponding straight wires. The power consumption of the ferromagnetic wire decreases rapidly when the temperature of the wire approaches the Curie point. As a result, depending upon the particular wire used, the time for the temperature to rise the last 30-50 "C may greatly exceed the time for the temperature t o rise the first several hundred degrees. In Figure 7, the temperature-time profile of a 510 "C wire obtained with the 1500-W Fischer unit illustrates this effect. The rise time up to 450 "C is 25 msec while the time to reach the last 60 "C requires 125 msec. A similar, although much less pronounced, effect occurs with the filament. For example, the TRT t o 550 "C is 10 msec while the temperature increase from 550 t o 575 "C requires a n additional 10 msec (see Figure 6). The rf power consumption as a function of temperature for a typical ferromagnetic wire is shown in Figure 8. Note that in the region where the rate of pyrolysis is high, the power absorption by the wire is low. Thus, the true pyrolysis temperature may be considerably lower than the Curie point. This effect will not influence intralaboratory repeatability but can lead t o interlaboratory irreproducibility. Filaments pyrolyzers have been widely used since the inception of PGC (1). The TRT's of conventional filament pyrolyzers heated by a constant voltage usually range between 5-25 sec. The need for faster TRT's was realized quite early in the development of PGC ( I , 18). A simple technique of (18) A. Barlow, R. S.Lehrle, and J. C. Robb, SCZ(Soc. Cliem. Z r d , Loridon) Monogr., 17, 267 (1963).
42
ANALYTICAL CHEMISTRY, VOL. 44, NO. 1, JANUARY 1972
True pyrolyria temperaturerange
1
Tc
Figure 8. Temperature dependence of the rf power consumption of an iron conductor indicating possible true pyrolysis temperature range and Curie point temperature T, applying a "boosting current" for a predetermined interval of time (=l sec) (18) was used for decreasing the T R T to the 1-2 sec range (19). A more sophisticated system based on discharge of a large capacitor through the filament for achievement of fast T R T was introduced by Levy (15). A schematic of a recent version of the capacitor discharge circuit is shown in Figure 2. Using this circuit, we were able to achieve a TRT to 700 "C in 12 msec which is the fastest rise time reported so far in PGC. The development of a filament pyrolyzer offering fast T R T combined with control of T,, was considered an important objective in PGC. A circuit to accomplish this objective which is based on using the filament as one arm of a Wheatstone bridge was described by Krejci and Deml(14). A new circuit which combines the fast T R T features of the capacitor discharge method and the temperature control capability of the bridge circuits was developed by Levy and Bellina (20). This circuit is based on a Kelvin bridge rather than on a Wheatstone bridge to eliminate the effect of contact resistances occurring in the system. The TRT's achieved with this circuit are the same as those obtained with the capacitor discharge method-Le., 7-12 msec. The above mentioned devices represent the second generation of filament pyrolyzers which offer fast TRT and control of Teq,thus elevating the filament type units to a new level of reliability.
RECEIVED for review July 14, 1971. Accepted August 20, 1971. This research was conducted under the McDonnell Douglas Independent Research and Development Program.
(19) G. Bagby, R. S. Lehrle, and J. C. Robb, Makromol. Chem., 119,122 (1968). (20) R. L. Levy and J. J. Bellina, Jr., McDonnell Douglas Research Laboratories, unpublished data, 1970.
