Anal. Chem. 1999, 71, 2482-2487
New Concepts in Sample Controlled Thermal Analysis: Resolution in the Time and Temperature Domains G. M. B. Parkes,† P. A. Barnes,*,† and E. L. Charsley‡
Centre for Applied Catalysis and Centre for Thermal Studies, University of Huddersfield, Queensgate, Huddersfield HD1 3DH, U.K.
In conventional thermal analysis the sample is subjected to a predetermined heating program (typically a linear rising ramp), while one or more of its properties (physical or chemical) is monitored. In contrast, a number of alternative approaches have been developed in which the heating rate is controlled, using a feedback loop, as some function of the rate of the reaction or process under study. It has been proposed that the generic name for all these techniques is sample controlled thermal analysis (SCTA).1 The first of these, termed constant rate thermal analysis (CRTA), is a method pioneered independently by Rouquerol2,3 and the Pauliks,4,5 in which the sample temperature is altered in such a way as to keep the rate of reaction constant. This reduces the temperature and pressure gradients which are found in materials subjected to conventional linear heating regimes and gives pseudoequilibrium conditions throughout the sample. For this reason, CRTA provides a more reliable measurement of the temperature of the material under analysis and also provides improved kinetic and mechanistic data.6-9 More recently developed techniques include Sorensen’s stepwise isothermal analysis
(SIA)10,11 and the dynamic rate method of TA Instruments.12,13 A comprehensive review of the field has been given by Reading.14 Previously we have described15,16 versatile equipment for carrying out both CRTA and SIA, using evolved gas analysis (EGA) and a new variant, “gas concentration” thermal analysis, in which the gas-phase composition is programmed rather than the temperature.17 More recently we have shown our techniques can be advantageously applied to temperature programmed reduction studies to obtain the mechanism and energetics of the process.18 Resolution in the Time and Temperature Domains. In thermal analysis, resolution concerns the extent to which adjacent, or partially overlapping, thermally induced processes are separated. In a conventional thermal analysis (TA) experiment (using linear heating) the measure of resolution in either the time or temperature domain is essentially the same. In other words, plotting the TA signal against time or temperature gives similar profiles. However, this is not so for SCTA experiments where the relationship between time and temperature is always nonlinear when a reaction is occurring. Consequently, SCTA curves displayed as a function of time (i.e., in the time domain) look markedly different from those displayed as a function of temperature (i.e., in the temperature domain). By using different forms of SCTA, as described below, it is possible to achieve remarkable increases in resolution in either the temperature or the time domains.19 Presentation of SCTA results in either domain is equally valid and useful, provided that the context is clearly stated and explained. In the work described below using EGA20 detectors, it should be noted that the magnitude of the signal is directly proportional
* Corresponding author: (fax) 44-1484-472-182, (e-mail)
[email protected]. † Centre for Applied Catalysis. ‡ Centre for Thermal Studies. (1) The proposal for the term ‘Sample Controlled Thermal Analysis’ first arose in discussions between the authors, Dr. J. Rouquerol and Dr. M. Reading at the ESTAC 6 (Grado, 1994) and TMG (Leeds, 1996) conferences and was endorsed at the workshop on SCTA methods held at the 11th ICTAC, Philadelphia 1996. (2) Rouquerol, J. Bull. Soc. Chim. Fr. 1964, 31, 67. (3) Rouquerol, J. Thermochim. Acta 1989, 144, 209. (4) Erden, L.; Paulik, F.; Paulik, J. Hungarian Patent 152197, 1962. (5) Paulik, F.; Paulik, J. Thermochim. Acta 1986, 100, 23. (6) Maelek, J.; Sestak, J.; Rouquerol, F.; Rouquerol, J.; Criado, J. M.; Ortega, A. J. Therm. Anal. 1992, 38, 71. (7) Criado, J. M.; Gotor, F. J.; Ortega, A.; Real, C. Thermochim. Acta 1992, 199, 235. (8) Reading, M.; Dollimore, D.; Whitehead, R. J. Therm. Anal. 1991, 37, 2165. (9) Reading, M. Thermochim. Acta 1988, 135, 37.
(10) Sorensen, O. T. J. Therm. Anal. 1978, 13, 429. (11) Sorensen, O. T. J. Therm. Anal. 1992, 38, 213. (12) Gill, P. S.; Sauerbrunn, S. R.; Crowe, B. S. J. Therm. Anal. 1992, 38, 255. (13) Lever, T. J.; Sutkowski, A. J. Therm. Anal. 1993, 40, 257. (14) Reading, M. Thermal AnalysissTechniques and Applications; Charsley, E. L.; Warrington, S. B., Eds.; The Royal Society of Chemistry: London, 1992. (15) Barnes, P. A.; Parkes, G. M. B.; Charsley, E. L. Anal. Chem. 1994, 66, 2226. (16) Parkes, G. M. B.; Barnes, P. A.; Brown, D. R.; Charsley, E. L. Thermochim. Acta 1995, 269-270, 665. (17) Barnes, P. A.; Parkes, G. M. B.; Charsley, E. L. Thermochim. Acta 1998, 320, 297. (18) Tiernan, M. J.; Parkes, G. M. B.; Barnes, P. A. J. Phys. Chem. B 1999, 103 (2), 3, 338. (19) The concept of resolution in the time and temperature domains was first presented by the authors at the U.K. National Symposium on Thermal Analysis and Calorimetry, Leeds, U.K., 1996. (20) Barnes, P. A. Anal. Proc. 1990, 27, 150.
This paper describes the concepts of resolution in the time and temperature domains and illustrates the principles using two new sample controlled thermal analysis (SCTA) techniques. The use of SCTA, where the sample temperature is determined by the rate of reaction, offers a number of advantages over conventional (linear heating) thermal analysis methods. The operation, advantages, and disadvantages of the two new techniques are discussed with examples.
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to the rate of the reaction under study, dR/dt, where R is the extent of reaction which is expressed on a scale of 0 to 1. The slope of the EGA curve can be described, therefore, by the second differential d2R/dt2, and the rate of change of slope by d3R/dt3. The use of EGA-based SCTA methods provides an advantage over thermogravimetry (TG) in that an EGA signal is already in the form of dR/dt, whereas in TG this function has to be calculated from R and t. This often introduces the possibility of unwanted noise which makes the accurate control necessary for SCTA more difficult. INSTRUMENTATION The basic apparatus used has been described previously.15 In the present work, we employ a small water-cooled furnace (Stanton Redcroft) which has a low thermal mass, a small swept volume (∼5 cm3), and a rapid temperature response. The sample was contained in a ceramic crucible placed directly on a type R (platinum-rhodium) plate thermocouple for accurate temperature measurement. Evolved gases were passed to the EGA detectors by a steady flow of helium at 20 cm3 min-1 using an electronic mass flow controller (Brookes). The EGA detectors used in this work included a hygrometer21,22 (GEC), a hot-wire katharometer (GowMac), and a magnetic sector mass spectrometer (VG). The output signals from these detectors were fed, via a 16-channel, 16-bit analogue-to-digital converter (Comark), to a PC. The data were processed on-line using software developed by the authors which instructs a temperature programmer (Eurotherm 818P) to alter the furnace temperature in whatever manner is required by the experiment in hand. PART A: TEMPERATURE-RESOLVED SCTA Principles of Temperature-Resolved SCTA. Conventionally, high resolution in TA is achieved by using low (linear) heating rates and small sample masses so that the sample reacts under near-equilibrium conditions. However, this inevitably gives rise to long experiment times and, in the case of EGA or differential scanning calorimetry (DSC), reduced sensitivity. CRTA and SIA are broadly similar as, in essence, they heat slowly through thermal events and heat quickly between them. This provides all the benefits of a very low linear heating rate experiment but with a reduced time penalty (compared with an experiment using the same underlying heating rate), as the heating rate is faster during periods when there are no thermal events. The use of these heating strategies causes the processes to occur over a very narrow temperature range (sometimes zero in the case of SIA) in both cases. This gives an increase in resolution in the temperature domain, i.e., when R is plotted against temperature rather than against time. Details of a New Temperature-Resolved SCTA Technique: Proportional Heating. One limitation of both CRTA and SIA is the large increase in time taken to complete an experiment compared with that required to complete the equivalent conventional linear heating experiment which uses a typical heating rate of say 10 °C min-1. To obtain maximum temperature resolution in the minimum amount of time, we have developed an approach (21) Keidel, F. A. Anal. Chem. 1959, 31, 2043. (22) Barnes, P. A.; Stephenson, G.; Warrington, S. B. J. Therm. Anal. 1982, 25, 299.
Figure 1. Dehydration of 2.2 mg of copper sulphate pentahydrate using a linear heatig rate of 5 °C min-1 to 350 °C.
Figure 2. Dehydration of 2.2 mg of copper sulphate pentahydrate using PH with the heating rate varying between 5 °C min-1 and 0.1 °C min-1.
called proportional heating (PH)16 in which the heating rate is proportional to a function of the reaction rate. In essence, the technique smoothly alters the heating rate between preset maximum and minimum values in such a way as to provide the optimum balance between resolution (for which the maximum is sought) in the temperature domain and experiment time (for which the minimum is sought). The operation of the PH method depends on four main parameters. These are a maximum and minimum heating rate, a ‘target’ reaction rate, and a function relating heating rate to reaction rate. When the reaction rate is zero, the heating rate is set to its maximum value. When the reaction rate is at, or above, the preset target level, the heating rate is set to its minimum value. Between these limits, the heating rate is related to the reaction rate by one of a number of possible functions. The PH method is illustrated by the dehydration of copper sulfate pentahydrate (Aldrich). Figure 1 shows the curves obtained using conventional linear heating run with a 2.2-mg sample mass and a linear heating rate of 5 °C min-1. The typical 2:2:1 water loss,23,42 with the first two peaks being relatively poorly resolved, is clearly seen. Figure 2 shows an analogous experiment using PH, with maximum and minimum heating rates of 5 and 0.1 °C min-1, respectively, a target reaction rate approximately equivalent to a rate of water loss of 0.009 mg min-1 (a signal of 1 mV from the hygrometer), and a linear relationship between the heating and reaction rates. It should be noted that the target reaction rate chosen had no special significancesit was selected solely on the criterion that it would be suitable to demonstrate the operation of the technique in a reasonable amount of time. Other similar rates would produce comparable results. This figure shows how the heating rate is reduced through each peak and is increased between each peak. The heating rate was reduced less through (23) Zivkovic, Z. C. J. Therm. Anal. 1979, 16, 3. (24) Paulik, J.; Paulik, F.; Arnold, M. J. Therm. Anal. 1988, 34, 1455.
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Figure 4. Decomposition of 2.6 mg of ammonium hydrogen carbonate using a linear heating rate of 10 °C min-1 to 300 °C.
Figure 3. (a) Comparison of R vs temperature profiles for the dehydration of copper sulphate pentahydrate using linear heating (curve A) and PH (curve B). (b) Comparison of R vs time profiles for the dehydration of copper sulphate pentahydrate using linear heating (curve A) and PH (curve B). Table 1. Comparison of Peak Resolution for the Dehydration of Copper Sulphate Pentahydrate Using Linear Heating and Proportional Heating
peak A width/°C peak B width/°C peak separation/°C
linear heating (LH)
proportional heating (PH)
ratio (PH/LH)
25.8 50.0 3.2
6.5 29.0 17.7
0.25 0.58 5.53
the third peak as it generated water at an insufficient rate to attain the preset target, a common problem in temperature-resolved SCTA techniques. The improvement in resolution obtained by the use of the PH method is most clearly seen in Figure 3a where the results are plotted in the form of R against temperature. When the same profiles are plotted as a function of time (Figure 3b) it can be seen that, although the profile for the linear heating experiment appears identical to that in Figure 3a, the increase in resolution produced in the PH experiment is now less apparent. Table 1 compares the resolutions of the first two processes for the experiments performed under linear heating and PH conditions. The peak ‘width’ in this case is taken as the temperature range for 90% completion of each respective process, while the separation is the temperature between 95% completion of the first process and 5% completion of the second. It can be seen that PH has a marked effect not only on the separation (an increase of 5.5 compared with the linear heating experiment) but on the temperature range (peak width) over which each process occurred. In contrast, the time taken for each of the two events is considerably greater than that for the linear heating experiment, the PH experiment taking ∼180 min compared with 60 min for the linear heating experiment. However, if a linear heating experiment had been performed using a fixed heating rate of 0.1 °C min-1, to obtain resolution similar to that seen in the PH experiment it would have taken nearly 3,500 min. 2484 Analytical Chemistry, Vol. 71, No. 13, July 1, 1999
Figure 5. Decomposition of 2.6 mg of ammonium hydrogen carbonate using PH with the heating rate varying between 10 and -10 °C min-1 (CRTA emulation).
In addition to providing high resolution in the minimum time, PH has the added bonus of being a versatile technique, as alterations in the control parameters can be used to change the nature of the experiment. Although not essential to the understanding of the concept of resolution in the time and temperature domains, it serves to illustrate the close relationship between temperature-resolved SCTA techniques. If the minimum heating rate is set to zero and a “step” function relating the reaction and heating rates is used, the experiment becomes, in effect, SIA. Using a very high target reaction rate causes the experiment to emulate conventional linear heating. If, on the other hand, a low target is used and a negative value is set for the minimum heating rate, the technique can emulate CRTA. For the experiment described below we adopt here the abbreviation ‘PH (CRTA)’ to signify this emulation. However, it should be noted that, in this case, PH is functionally indistinguishable from CRTA achieved by conventional means. Where the reaction(s) being studied are complex, e.g., those involving nucleation, PH (CRTA) can provide an insight into the mechanisms of the processes involved. This can be seen (Figures 4 and 5) by comparing the results of linear heating and PH (CRTA) decomposition experiments on ammonium hydrogen carbonate (BDH 99.9%) which decomposes with the co-evolution of ammonia, water, and carbon dioxide.25 In this work, the reaction was followed using a mass spectrometer set to continuously monitor carbon dioxide (m/z ) 44). Figure 4 shows the result of the linear heating experiment on a 2.66-mg sample using a 10 °C min-1 heating rate to 300 °C which produced a single peak centered on 125 °C. Figure 5 shows the corresponding experiment using PH (CRTA) with a target approximately corresponding to a rate of decomposition of 0.04 mg min-1 and maximum and minimum heating rates of +10 and -10 °C min-1, respectively. Again, the selected target rate had no special significance other than being suitable to demonstrate the operation of the technique. (25) Muehling, J. K.; Arnold, H. R.; House, J. E. Thermochim. Acta 1995, 255, 347.
Figure 6. (a) Comparison of R vs time profiles for the decomposition of 2.6 mg of ammonium hydrogen carbonate using linear heating (curve A) and PH (CRTA emulation) (curve B). (b) Comparison of R vs temperature profiles for the decomposition of 2.6 mg of ammonium hydrogen carbonate using linear heating (curve A) and PH (CRTA emulation) (curve B).
This experiment produced a more complex EGA curve than the conventional linear heating run, as it shows the presence of a nucleation stage in the form of the large initial peak. Nucleation frequently requires a higher temperature than propagation13,26 so once the former is complete the temperature is above that required for the preset target reaction rate and so it falls. The remainder of the decomposition then proceeds requiring a slowly rising temperature to maintain an approximately constant rate. Eventually, as the decomposition reaches completion, the heating rate returns to its maximum value. Figure 6a compares the data from the linear heating and PH (CRTA) experiments in the form of R vs time profiles. Two main features are observable. First, the greater duration of the PH (CRTA) experiment compared with that of the linear heating experiment is apparent. Second, the gradient of R vs time for the PH (CRTA) experiment is approximately linear over the majority of the decomposition, demonstrating that the process occurred at a nearly constant rate. Figure 6b compares the same R profiles but plotted as a function of temperature. Here, the shape of the profile for the PH (CRTA) experiment clearly reveals the existence of nucleation,27 as evidenced by the higher temperature required for the initiation stage. It can also be seen that, with the control parameters used, the decomposition occurred at a lower temperature in the PH (CRTA) experiment than in the corresponding linear heating experiment. Neither of these features is apparent in the time-based plots of Figure 6a. A full discussion of the ability of PH to emulate other temperature-resolved SCTA techniques is beyond the scope of this paper and is described in detail elsewhere.28 (26) Bamford, C. H.; Tipper, C. F. H. Comprehensive Chemical Kinetics: Vol. 22 Reactions in the Solid State: Elsevier: Amsterdam, 1980. (27) Ortega, A.; Perez-Maqueda, L. A.; Criado, J. M. Thermochim. Acta, 1994, 239, 171. (28) Parkes, G. M. B.; Barnes, P. A.; Charsley, E. L., in preparation.
PART B: TIME-RESOLVED SCTA Principles of Time-Resolved SCTA. Current temperatureresolved SCTA techniques described in the literature operate by controlling the sample temperature as some function of the difference between the measured reaction rate and a preset ‘target’ reaction rate. This has the drawback that it is difficult to set a single level that is equally appropriate for complex systems which produce events of greatly different magnitudes. The basic principle of time-resolved SCTA can be seen by considering a simplified implementation applied to a model system of two well-resolved peaks. In this example, time-resolved SCTA is achieved by maintaining a linear heating rate throughout a thermal event and reducing it to a very low level between the two consecutive processes, i.e., once the first event has reached completion. The initial heating rate during the reaction imparts high sensitivity, as the process is forced to occur over a short time interval. Between events, the very low heating rate increases the time between the two reactions and so gives improved resolution in the time domain. This simple strategy can be viewed as being the opposite to that adopted in temperature-resolved SCTA, but will only work when the two peaks are completely resolved in the first place. In cases where the two peaks partially overlap, the simplistic approach described above is not possible as the second event commences before the first is completed, and therefore, a more complex strategy is required. We adopt an approach which dynamically analyzes the profile of the EGA curve as it is formed and switches the heating rate at an appropriate stage in the first reaction before it is complete and before the second reaction achieves a measurable rate. In the simplest case, this can be when the peak reaches its maximum (d2R/dt2 ) 0), but for systems where the degree of overlap between the two peaks is high, it is possible to switch the heating rate even earlier by identifying other points such as that of the greatest acceleration of the reaction rate (d2R/dt2 reaches its largest value), etc. Further details of the latter approach are given elsewhere.29 Once the first process has reached completion, as indicated (in an ideal case) by the return of the EGA signal to zero, two possibilities arise. First, the heating rate can be switched to a low value (e.g., 1 °C min-1) to increase the time taken before the temperature is sufficient for the second process to start. Once the second process is detected, the heating rate can be returned to its original value to provide high sensitivity, as for the first event. This approach maximizes the time resolution but increases the overall duration of the experiment, especially in cases where the second event is at a significantly higher temperature. To maintain the high peak separation and minimize the experiment time, the second possibility is to raise the heating rate back to its original level immediately when the end of the first process is detected. Clearly, once the two events have been resolved, the heating rate between them can be adjusted, as required, to increase the separation in the time domain. This discussion assumes Gaussian peaks and, for simplicity, ignores the possibility of interactions between the two adjacent processes (i.e., where the gas evolved from the first process inhibits the second, etc.). However, for many systems these (29) Reading, M.; Parkes, G. M. B.; Barnes, P. A.; Charsley, E. L., in preparation.
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Table 2. Comparison of Peak Resolution for the Decomposition of a Physical Mixture of Copper Hydroxycarbonate and Zinc Hydroxycarbonate Using Linear Heating and Peak-Slope Heating
Figure 7. Decomposition of 3 mg of a 1:2 mixture of copper and zinc hydroxycarbonates using a linear heating rate of 5 °C min-1 to 400 °C.
peak A width/s peak B width/s peak separation/s peak A/valley height
linear heating (LH)
peak-slope heating (PSH)
ratio (PSH/LH)
254 152 406 2.3
249 139 1019 8.86
0.98 0.91 2.51 3.85
Figure 8. Decomposition of 3 mg of a 1:2 mixture of copper and zinc hydroxycarbonates using PSH with the heating rate being switched from 5 to 0 °C min-1 at the maximum of each EGA peat (dR/dt ) 0).
assumptions are sufficiently valid for the time-resolved SCTA approach to provide significant benefits. Implementation of the New Time-Resolved SCTA Technique: Peak-Slope Heating. We describe here our first timeresolved SCTA technique, peak-slope heating (PSH), which derives its name from the fact that the heating rate is switched to zero when the rate of change of the reaction rate goes from positive to negative, i.e., immediately following the peak maximum. The basic operation of PSH is as described above, with the heating rate being increased as soon as the end of an event is detected. A slight modification has been made to the basic algorithm, in that there can be a progressive transition between heating rates to obtain the optimum balance between resolution and a smooth response. Figure 7 shows the decomposition of 3 mg of a 1:2 w/w mixture of copper hydroxycarbonate (Aldrich) and zinc hydroxycarbonate (Aldrich) using linear heating at 5 °C min-1 . The components of this system produce both carbon dioxide and water as they decompose,30,31 but for this work only the rate of water evolution was monitored, using a hygrometer. Figure 8 shows the corresponding experiment performed using PSH with maximum and minimum heating rates of 5 and 0 °C min-1, respectively. In this experiment the switch to zero heating rate on the falling edge of each peak is rapid, while the increase in heating rate, as the first approaches completion, is slower. It can be seen that the switching point between heating and isotherming is at the same relative point on each peak, despite their difference in size. Table 2 shows the peak widths at half-maximum peak height and peak separation for the linear heating and PSH experiments. In addition, the ratio between the height of the first peak to the (30) Stacey, M. H.; Shannon, M. D. Mater. Sci. Monogr. 1985, 28B, 713. (31) Klissurski, D.; Uzonov, I. Kumbilieva, K. Thermochim. Acta 1985, 93, 485.
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Figure 9. (a) Comparison of R vs time profiles for the decomposition of 3 mg of a 1:2 mixture of copper and zinc hydroxycarbonates using linear heating (curve A) and PSH (curve B). (b) Comparison of R vs temperature profiles for the decomposition of 3 mg of a 1:2 mixture of copper and zinc hydroxycarbonates using linear heating (curve A) and PSH (curve B).
‘valley’ height between the peaks is given. The peak widths and separation have units of time rather than temperature, in contrast to the results from the PH experiment shown in Table 1. It can be seen that, under the conditions used, PSH significantly increases the resolution of the two peaks without distorting their respective shapes. It is interesting to note that the individual peaks are very slightly more narrow (in terms of time) than those for the linear heating experiment, presumably because each peak has less contribution from the other in their profile. Figure 9a shows the R profiles as a function of time for the linear heating and PSH experiments, again emphasizing the increase in resolution of the latter at the expense of a slightly increased run duration. Figure 9b shows the same data plotted in the temperature domain, where the increase in resolution produced by PSH is less apparent. The PSH R profile appears particularly unusual as half of each of the two processes occurs under linear heating while the other half occurs under isothermal conditions and demonstrates why the profiles are normally plotted in the time domain. PSH has the great advantage over current temperatureresolved SCTA techniques in that the change in heating rate will take place at the same relative point on the peak (i.e., same value of R) irrespective of its absolute magnitude. This eliminates the need to select a target reaction rate prior to the start of each experiment.
CONCLUSIONS The results presented here illustrate the concept of resolution in the time and temperature domains using two new SCTA techniques: proportional heating (PH) and peak-slope heating (PSH). Techniques, such as PH, which heat slowly through thermal events but rapidly between them give enhanced resolution in the temperature domain at the expense of decreased sensitivity and increased experiment times. Conversely PSH, which heats quickly into thermal events and slowly between them gives enhanced resolution in the time domain while maintaining much of the sensitivity of conventional linear experiments but, again, at the expense of increased experiment times. In both cases the time penalty incurred for the increase in resolution is significantly less than that incurred with a conventional experiment using a linear heating rate sufficiently low to produce a similar gain in resolution. The high resolution obtained in the time or temperature domains finds uses in different areas of application. In addition to maximizing the resolution/experiment time ratio, a temperatureresolved SCTA technique (such as PH or CRTA) can reveal information on the mechanism of a reaction which is not apparent from a linear heating experiment. (32) Dollimore, D.; Gamlen, G. A.; Taylor, T. J. Thermochim. Acta 1984, 75, 59. (33) Barnes, P. A. Anal. Proc. 1993, 27, 150.
Identification of evolved gases, for example, by MS32 or TGMS33 under conventional linear heating conditions can be difficult in applications where two adjacent events overlap, as the mass spectra will contain a mixture of m/z fragments from both gases. Using a slower heating rate can increase the resolution, but the sensitivity decreases correspondingly, making the detection of minor m/z fragments more difficult. Time-resolved SCTA techniques (such as PSH) may lend themselves to the study of such systems as the high sensitivity found using a high linear heating rate is maintained, while the increased separation between events minimizes the overlap of the mass spectra, making identification of each species much simpler. Time-resolved SCTA techniques are also beneficial in situations where it is necessary to isolate and collect the gaseous products (or, indeed, the remaining solids) of a thermal analysis experiment for subsequent identification with other instrumentation.
Received for review November 17, 1998. Accepted March 16, 1999. AC9812781
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