Analytical Techniques Used with Pyrolysis

Chapter 5. Analytical Techniques Used with Pyrolysis 

1 The Selection of the Analytical Technique and the Transfer of the Pvroivsate to the Analytical Instrument. 

Selection of the analytical instrumentation for the analysis of the pyrolysate is a very important step for obtaining the appropriate results on a certain practical problem. However, not only technical factors are involved in this selection the availability of a certain instrumentation is most commonly the limiting factor. Gas chromatography (GC) and gas chromatography-mass spectrometry (GC/MS) are, however, the most common techniques utilized for the on-line or off-line analysis of pyrolysates. The clear advantages of these techniques such as sensitivity and capability to identify unknown compounds explain their use. However, the limitations of GC to process non-volatile samples and the fact that larger molecules in a pyrolysate commonly retain more structural information on a polymer would make HPLC or other techniques more appropriate for pyrolysate analysis. However, not many results on HPLC analysis of pyrolysates are reported (see section 5.6). This is probably explained by the limitations in the capability of compound identification of HPLC, even when it is coupled with a mass spectrometric system. Other techniques such as FTIR or NMR can also be utilized for the analysis of pyrolysates, but their lower sensitivity relative to mass spectrometry explains their limited usage. 

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An important aspect of the use of analytical pyrolysis is its capability to provide complementary information to other analytical techniques used for polymer characterization. One such technique is IR analysis of polymers. Although IR spectra can be used as fingerprints for polymer identification, the success of this technique can be questionable when the polymer is not pure or is in a mixture with other compounds. The IR spectra are particularly difficult to use when a polymer is present only at a low level in a particular material and cannot be easily separated. The use of Py-GC/MS allows identification of polymers even in low concentration in specific mixtures because it couples pyrolysis with a chromatographic technique. On the other hand, some polymers generate by pyrolysis a low proportion of easily identifiable molecules, producing mainly char and small uncharacteristic molecules such as HF, H2O, CO2, etc. In these cases, IR is the technique of choice. Since for an unknown sample each technique can be misleading, the use of both types of information is always beneficial. 

One of the advantages of GFAAS is the direct analysis of solid samples without prior decomposition, this technique has been reviewed by Bendicho and de Loos-Vollebregt (1991). The solid samples can be introduced directly or as a slurry. Calibration can be with aqueous standards, synthetic solids, standard additions or with SRMs. SRMs have been used for calibration for solid sampling of plant material (Schmidt and Falk, 1987). Examples of the use of solid samples with ETAAS are shown in Table 9-4. In many cases a matrix modifier is used with the sample, this allows the matrix to be volatilised at the pyrolysis stage without analyte loss, particularly important with volatile analytes such as Cd and Pb. (Ure, 1990). 

As indicated previously (see Section 1.2) pyrolysis must be associated with an analytical technique in order to provide information on a sample. Several common analytical techniques such as GC, GC/MS or GC/FTIR have been utilized either hyphenated with pyrolysis or off-line and were described previously. Less frequently, techniques such as HPLC, preparative LC, TLC, SFE/SFC, or NMR also have been used for the analysis of pyrolysates. These types of techniques are commonly applied off-line. They are used mainly for obtaining information on that part of the pyrolysate that is difficult to transfer directly to an analytical system such as a GC or for the analysis of materials associated with the char. However, the analysis of the non-volatile part of pyrolysates is frequently neglected, although this leads to an incomplete picture regarding the chemical composition of pyrolysates. 

The analysis of simple lipids can be done with good results using common analytical methods without any need for decreasing the molecular weight of the sample by techniques such as pyrolysis. HPLC, SFC or GC procedures were applied for simple lipid analysis, and even the mass spectra of some simple triglycerides are known. As an example, Figure 8.1.1 shows the El mass spectrum of tripalmitin (standard ionization condition). 

The widespread use of synthetic polymers has led to the development of a considerable number of analytical tools for polymer characterization and analysis. Analytical pyrolysis, consisting of pyrolysis coupled with an analytical technique, is one of these tools. The technique can be invaluable in solving many practical problems in polymer analysis. It can be used alone or can provide complementary information to other techniques such as thermal analysis, infrared spectroscopy, or even nuclear magnetic resonance. 

Analytical pyrolysis can be coupled with different analytical techniques for providing information on polymers. Among analytical pyrolysis techniques, Py-GC and Py-GC/MS are probably the most common. The pyrolysis process typically generates a very complex mixture of molecules. For this reason, a chromatographic technique is very important for the separation of pyrolysate components. The fingerprint generated by Py-GC can be used for polymer identification. However, the detection associated with compound identification provided by GC/MS is invaluable in many applications. The exceptional sensitivity and identification capability of mass spectrometric analysis make Py-GC/MS technique the most important analytical pyrolysis technique. 

All the different techniques used in chemical analyses of organic material can and are being used in the analysis of deposits. Gas chromatography (GC) methods have been developed for the analysis of volatile extractive components and for analysis of non-volatile polymeric components pyrolysis GC can also be used. Size exclusion chromatography (SEC) is a convenient way of fractioning the sample, after which each fraction may be analysed with standard organic analytical techniques, e.g. nuclear magnetic resonance (NMR), C NMR and IR (of FTIR). 

The four Py-MS spectra — for pollen, bee feces, and two of the unknowns — presented as a figure in this report (but not shown in this chapter) clearly showed distinct differences, but the authors found that the complexity present in the 93 spectra made visual classification of the samples impossible. A discussion of the data analysis techniques used for the classification of these samples is beyond the scope of this chapter, but a brief summary of the results can be made. (Those readers with interest in statistical methods for analysis of data generated by analytical pyrolysis-mass spectrometry should find this paper interesting and may also want to read more recent work in this area. - )... 

Improvements in analytical capability for the analysis of complex pyrolysate mixtures have appeared during the last decade high-resolution capillary GC with more polar and selective stationary phases coated on inert fused-silica colmnns coupling of capillary GC with sensitive, selective, and lower-cost mass spectrometric detectors enhanced pyrolysis-MS techniques hyphenated analysis methods, including GC-Fourier-transform infrared spectroscopy (GC/FTIR) and tandem MS and better strategies for handling complex multidimensional pyrolysis data. The present chapter reviews the known chemotaxonomy of miCTOorganisms, summarizes practical considerations for the use of pyrolysis in microbial characterization, and critically discusses selected applications of analytical pyrolysis to microbial characterization. 

The analysis of material trapped on solid sorbents is done in a number of ways. Arguably the most common analytical technique is desorption in a suitable solvent with gas chromatographic analysis of the resulting solution. There has also been a considerable body of work done on thermal desorption of the trapped material, with or without preconcentration of the desorbed material prior to gas chromatographic analysis. Thermal desorption introduces the potential for pyrolysis of the material of interest, and thus its use is somewhat limited, in comparison to solvent desorption. Mass-selective (MS) detectors and the flame-ionization detectors (FIDs) are the most commonly used detectors for this purpose. Both detectors are considered to be universal detectors for organic compounds the FID is used mostly for... 

This area uses a variety of analytical methods. To identify and characterize lipids the main analytical technique employed is gas chromatography (GC), often is association with mass spectrometry (GC-MS). Newer advancements include the use of pyrolysis-GC-MS and the measurement of the stable isotope values of individual compounds in GC-C-IR-MS. To identify proteins a range of immunochemical identification techniques are used, including enzyme-linked immunosorbent assay and radioimmunoassay. 

Understanding the relationship between the molecular structure and the thermal stability (decomposition temperature and rate) of the organoclays and the subsequent influence on the stability of the polymer host is critical. Several analytical techniques have been used to determine the thermal stability of different organoclays and to indentify the decomposition products conventional and high-resolution thennogravimetric analysis (TGA) coupled with Fourier transform infrared spectroscopy (FITR) and mass spectrometry (MS), pyrolysis/gas chromatography (GC)-MS, and solid phase microextraction (SPME) [6-12]. 

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