III. ANALYTICAL PYROLYSIS — INSTRUMENTATION
Analytical pyrolysis should aim to avoid combustion and secondary thermal reactions such that the larger molecular weight pyrolysis products of organic matter can be detected. Pyrolysis products therefore need to be removed rapidly from the reaction zone and sample size is typically in the microgram range. Attention should be paid to ensure a low dead volume and that pyrolysis products are readily removed by the carrier gas stream, with a linear gas flow of at least 150 mm/sec. The total heating time and the final temperature are decisive in determining the nature of pyrolysis products evolved.
To address this, analytical pyrolysis units usually incorporate design features such as:
1. pyrolysis in a vacuum, or in a rapid stream of inert gas such as helium
2. provision for operating the pyrolysis unit at 200°C and preheating of the carrier gas
3. the importance of adequate carrier gas velocity, plus small dead volume in order to avoid secondary reactions
4. the presence of a replaceable liner (usually quartz) tube surrounding the pyrolysis wire, to avoid cross contamination between samples
5. small sample sizes of less than 100 /Ag
6. temperature rise time of less than 200 msec in total
A diagram of a commonly employed pyrolysis unit incorporating these design features (Pyrojector, Scientific Glass Instruments, USA) is shown in Figure 8.1.
The true value of analytical pyrolysis for the characterization of organic matter is realized when combined with analytical methods such as mass spectrometry (Pyrolysis-Mass Spectrometry, Py-MS).23 High sensitivity, specific and fast analysis are widely recognized characteristics of mass spectrometry (MS), which have earned this technique its reputation as one of the most powerful analytical tools for organic materials available today. With the total number of library mass spectra
figure 8.1 Pyrolysis unit, Scientific Glass Engineering (SGE) Pyrojector. 1. Sample loading area, 2. Quartz sample tube (reaction zone), 3. Sample loading cover interlock, 4. Valve body, 5. O-ring type seal, 6. Carrier gas inlet, 7. Check valve, 8. Carrier gas vent, 9. Valve spindle, 10. Heat deflectors, 11. Micro furnace, 12. Quartz replaceable liner, 13. GC interface nut, 14. Carrier gas inlet, 15. GC injection port septum, 16. Needle unit.
exceeding 100,000, it is tempting to credit MS with near universal applicability.23 However, unfortunately most organic matter samples consist of molecular assemblies of a complexity and size far beyond the capabilities of even direct MS techniques. Hence, the use of MS does not provide positive identification of products. It also does not reliably detect products which have high ionization potentials distinguish between compounds of the same molecular weight. This can only be achieved when the products are first separated based on chemical or physical properties. Gas Chromatography/MS (GC/MS) provides a convenient separation and identification procedure and it can be readily interfaced with most pyrolysis systems. Indeed, commercial pyrolysis units are designed to be directly interfaced with the GC injection port. The GC column effluent can flow through the appropriate GC/MS interface, now a standard installation on all mass spectrometers, and positive identification of pyrolysis products may be made using the mass spectral library and associated software. Optional chemical ionization (CI) can aid in the identification by giving a molecular ion for compounds which do not yield a molecular ion in the electron ionization (EI) mode.22
To avoid dead space and secondary pyrolysis reactions due to low flow rate, a split stream should also be used. Most commercial gas chromatographs have gas splitters, to permit sufficient flow rate (20 to 30 cm3/min) through the pyrolysis unit and the optimum flow rate through the column (1 to 5 cm3/min). An alternative procedure is to cold trap (cyrofocus) the pyrolysis products on the head of the column, then optimize the column flow rate, and release the products by rapid warming to the starting temperature of the GC program (usually below 100° C).
Generally, one of three heating techniques is employed:
1. Curie point pyrolysis uses a ferromagnetic probe that is inductively heated. The sample is pyrolysed by a high-frequency field that causes inductive heating of the ferromagnetic wire. Depending on the strength of the field, the wire may heat up to the Curie point temperature of the ferromagnetic alloy in a time ranging 0.1 to 5 sec. Under appropriately selected frequency as well as the dimension and alloys of the wires, the temperature will automatically stabilize within a few degrees of the Curie-point, (358°C [Ni], 770°C [Fe], 1128°C [Co] and intermediate values for various ferromagnetic alloys).
2. Microfurnaces provide a constantly heated, isothermal pyrolysis zone into which solid samples are introduced.
3. Flash pyrolysis involves the exposure of the sample to high temperatures for very short periods of time, e.g., 0.001 to 0.1 sec.
With these points in consideration, a wide variety of commercial analytical pyrolysis units are now available. The great advantage of Py-GC/MS over chemical degradation methods is the small sample size required and that no sample pretreatment is needed after the initial isolation of organic matter. However, Py-GC/MS is limited by the lack of final identification of the pyrolysis products, since their mass determinations can only give the molecular formula. Therefore Py-GC/MS data
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