TEMPERATURE ANALYTICAL PYROLYSIS

Modern analytical pyrolysis has conventionally been canied out only by thermal energy to break some covalent bonds in the sample molecules at elevated temperatures to produce smaller and/or volatile fragments (pyrolyzates). On the other hand, the reactive pyrolysis in the presence of organic alkaline, such as tetramethylammonium hydroxide [(CH / NOH] (TMAH) has recently received much attention especially in the field of chai acterizing condensation polymers. 

In addition to GC/MS, high performance liquid chromatography (HPLC/MS) has been used to analyse natural resins in ancient samples, particularly for paint varnishes containing mastic and dammar resins [34]. A partial limitation of chromatographic techniques is that they do not permit the analysis of the polymeric fraction or insoluble fraction that may be present in the native resins or formed in the course of ageing. Techniques based on the direct introduction of the sample in the mass spectrometer such as direct temperature resolved mass spectrometry (DTMS), direct exposure mass spectrometry (DE-MS) and direct inlet mass spectrometry (DI-MS), and on analytical pyrolysis (Py-GC/MS), have been employed as complementary techniques to obtain preliminary information on the... 

J.J. Boon, Analytical pyrolysis mass spectrometry new vistas opened by temperature resolved in source PYMS, Int. J. Mass Spectrom., 118/119, 755 787 (1992). 

H. Sato, K. Kondo, S. Tsuge, H. Ohtani, and N. Sato, Mechanisms of thermal degradation of a polyester flame retarded with antimony oxide/brominated polycarbonate studied by temperature programmed analytical pyrolysis. Poly. Degr. Stab., 62, 41-48 (1998). 

The concept of chemical modification (CM) is extremely popular in modern elec-trothermal-assisted atomic techniques. As per lUPAC s recommendations [47], in order to influence processes taking place in the atomizer in the desired way, reagents called chemical modifiers may be added. These can help to retain the analyte to higher temperatures during pyrolysis, remove unwanted concomitants or improve atomization in other ways . However, there is a tendency towards broadening the scope of this term, starting from the classical and still used term matrix modifier , matrix/analyte modifier or instrumental matrix modification to indicate the useful effects of the type, pressure and flow-rate of protective gas or gas mixtures internal matrix modifier for matrix constituent s) with favourable effects on processes in the atomizer, either by themselves (e.g. refractory components) or upon addition of suitable promoters permanent modifiers for... 

Analytical pyrolysis is considered somehow apart from the other thermoanalytical techniques such as thermometry, calorimetry, thermogravimetry, differential thermal analysis, etc. In contrast to analytical pyrolysis, thermoanalytical techniques are not usually concerned with the chemical nature of the reaction products during heating. Certainly, some overlap exists between analytical pyrolysis and other thermoanalytical techniques. The study of the kinetics of the pyrolysis process, for example, was found to provide useful information about the samples and it is part of a series of pyrolytic studies (e.g. [6-8]). Also, during thermoanalytical measurements, analysis of the decomposition products can be done. This does not transform that particular thermoanalysis into analytical pyrolysis (e.g. [9]). A typical example is the analysis of the gases evolved during a chemical reaction as a function of temperature, known as EGA (evolved gas analysis). 

Commonly, analytical pyrolysis is performed as flash pyrolysis. This is defined as a pyrolysis that is carried out with a fast rate of temperature increase, of the order of 10,000° K/s. After the final pyrolysis temperature is attained, the temperature is maintained essentially constant (isothermal pyrolysis). Special types of analytical pyrolysis are also known. One example is fractionated pyrolysis in which the same sample is pyrolysed at different temperatures for different times in order to study special fractions of the sample. Another special type is stepwise pyrolysis in which the sample temperature is raised stepwise and the pyrolysis products are analyzed between each step. Temperature-programmed pyrolysis in which the sample is heated at a controlled rate within a temperature range is another special type. 

The presence of water as a reaction product from the pyrolytic processes or as adsorbed water on the material to be pyrolysed is not unusual. However, in analytical pyrolysis, water is not commonly added to the sample. During some pyrolytic processes with industrial applications such as wood pyrolysis, water is sometimes added intentionally. The main effect of water during pyrolysis is hydrolysis. This takes place as the temperature elevates. For polymers like cellulose or starch, the chain scission by hydrolysis (instead of transglycosidation) is the main effect of water addition. This can be seen in the modification of the yields of different final pyrolysis products. Therefore, the reproducibility in analytical pyrolysis may be influenced by the variability of water content of the initial sample [9]. 

Using the integrated form of this equation (see rel. (7)), the values for WA/Vq for cellulose can be obtained for different temperatures and pyrolysis times. Assuming a pyrolysis time of 10 s in isotherm conditions at different temperatures [in rel. (15). T should be expressed in ° KJ, several values for W/Wq expressed in % are given in Table 3.2.1. These calculated values for WA/Vq used E and A extrapolated for a wider range of temperatures than those reported in literature [8] for cellulose. The only purpose of these calculations is to illustrate the effect of temperature on the rate of pyrolysis. From Table 3.2.1 it can be seen that around 200° C cellulose is not significantly decomposed. Around 400° C the decomposition starts, and around 600° C the decomposition is practically complete. In analytical pyrolysis, 600° C and 10 s pyrolysis time (total heating time THT) could, therefore, be recommended for the experimental conditions of cellulose pyrolysis. 

Analytical pyrolysis requires heating of the sample at a temperature significantly higher than ambient, commonly between 500° C and 800° C. For special purposes this temperature can be higher. The pyrolytic process is done in a pyrolysis unit (pyrolyser) which commonly interfaces with an analytical instrument (see Section 1.2). The analytical instrument is used for the measurement of the pyrolysis products. It is also possible to perform "off line pyrolysis (no direct interface to an analytical instrument), followed by the analytical measurement. The pyrolysers have a source of heat where the sample is pyrolysed. The pyrolysis products are usually swept by a flow of gas from the pyrolyser into the analytical instrument. 

There are several procedures to perform pyrolysis flash pyrolysis (pulse mode), slow gradient heating pyrolysis (continuous mode), step pyrolysis, etc. Commonly, the pyrolysis for analytical purposes is done in pulse mode. This consists of a very rapid heating of the sample from ambient temperature, targeting isothermal conditions at a temperature where the sample is completely pyrolysed. Controlled slow temperature gradients are also possible in pyrolysis, but their use in analytical pyrolysis is limited. Step pyrolysis heats the sample rapidly but in steps, each step following a plateau of constant temperature kept for a limited time period. 

The pyrolysis time (THT) in this example is 1.0 s. At temperatures up to 560° C, process (2) will dominate the pyrolysis, while at temperatures higher than 560° C, the pyrolysis will be dominated by process (1). If the pyrolysis products for process (1) are different from those for process (2), it can be seen that a small variation in the temperature profile may significantly modify the analytical results producing more of the products from process (1) or more from process (2). Therefore, the importance of TRT is more significant when rapid degradation reactions occur during pyrolysis. Equal TRT values are essential for the reproducibility of analytical pyrolysis mainly for fast processes. 

One way to produce a rapid heat transfer to the sample is to diminish the sample size [5]. This implies that the amount of heat required by the sample to reach a certain temperature is small and that the heat can be transferred rapidly. Typical sample sizes in analytical pyrolysis vary from a few ng to a few mg. A small sample size is, however, related to other effects, some advantageous and some not. Secondary reactions during pyrolysis are diminished for a small sample, but the contact with metal surfaces may... 

A very interesting subject is the application of analytical pyrolysis for the study of biomarkers in extraterrestrial samples [2], Several meteorites and lunar samples were studied using this technique. Also, Viking Lander used a Py-GC/MS system to explore the Martian atmosphere and surface [74], Commonly, a stepped pyrolysis technique has been used in these studies to determine organic components in an inorganic matrix [75], The procedure involves a set of four or five temperatures that allow the analysis of trapped gases, analysis of small volatile molecules, and the performance of true pyrolysis on macromolecules. 

Another special type is stepwise pyrolysis, in which the sample temperature is raised stepwise and the pyrolysis products are analyzed between each step. Temperature-programmed pyrolysis, in which the sample is heated at a controlled rate within a temperature range, is another special type. This type of pyrolysis can be used for analytical purposes but is not very common. 

Another parameter selected for analytical pyrolysis experiments is the temperature rise time (TRT). This parameter measures the time necessary for the heating element of the pyrolyzer to reach Teq. The goal in flash pyrolysis is to have a very short TRT, such that the decomposition of the sample takes place, virtually, in isothermal conditions. 

An excessively high temperature may diminish the chance of detecting some structural characteristics that can be obtained only from the study of the dimers, trimers, tetramers, etc. of the polymer. If only the monomer and small molecules are generated during pyrolysis, information on properties such as stereospecificity or copolymeric structure is lost. For this reason, Tgq values around 600° C may be more appropriate for certain studies using analytical pyrolysis. 

The variability in analytical pyrolysis results as a function of equilibrium temperature Tqq in the pyrolysis of a polymer is further exemplified for poly(diallyl isophthalate), which has the idealized structure shown below ... 

A novel, relatively simple method for analytical pyrolysis of polymers is pyrolysis-fractography (Py-F) [18]. In this method, the pyrolysate generated when a polymer is instantaneously pyrolyzed at 600° C is introduced together with the carrier gas into a short length deactivated stainless steel capillary tube. This tube is placed in the oven of a gas chromatograph. The oven temperature is linearly raised, so that the pyrolysate is fractionated based on the distillation temperature, and a fractogram is obtained using a mass spectrometer or a flame ionization detector. 

---