Thermal volatilization analysis systems

With respect to apparatus, the design and operation of a sub-ambient thermal volatilization analysis (TVA) system has been described. A technique, which is essentially TVA, has also been used for the qualitative and quantitative analysis of trace amounts of volatiles produced during polymer degradation. A pyrolytic mass spectrometric method has also been developed which gives information on the yield of volatile degradation products, on their nature and on the kinetics of their formation. 

Early applications of what would later become analytical pyrolysis of silicone systems include work by Wacholtz et al. [61] in which the pyrolysis products of the thermal degradation of a silicone system were sampled and analyzed in-line using a combination of FTIR and GC/MS. Such early in-line pyrolysis studies of silicone degradation would later form the basis of modern microanalytical pyrolysis methodologies for the analysis of silicone degradation. However, until comparatively recent times the field of analytical deg-radative analysis of sihcones was almost entirely dominated by the vacuum pyrolysis technique—thermal volatilization analysis (TVA). 

Volatiles Content (a) Thermal Gravimetric Analysis (TGA) using a DuPont 990 system and 951 TGA unit sample size 10 mg temperature 115°C atmosphere nitrogen or vacuum (b) Oven drying ... 

In this technique, in a continuously evacuated system the volatile products are passed from a heated sample to the cold surface of a trap some distance away. A small pressure develops which varies with the rate of volatilisation of the sample. If this pressure is recorded as the sample temperature is increased in a linear manner, a thermal volatilisation analysis thermogram showing one or more peaks is produced. The trace obtained is somewhat dependent on heating rate, which therefore should be standardised. 

Thermal evolution analysis is an excellent tool for polymer studies complementary to other thermal techniques such as DTA, TG and pyrolysis. Its applications include thermal degradation studies, determination of additives and contaminants, polymer composition and structure identifications. With small variations, the apparatus can also be used for vapour pressure measurements, and for determination of odorous materials in polymer systems. Coupling of TEA to GC for the identification of effluents is practicable and useful. TEA-CT-GC was used for the analysis of volatiles from ABS 10 ppb of styrene but negligible acrylonitrile was detected in the headspace of a typical ABS resin [42]. 

Headspace analysis involves examination of the vapours derived from a sample by warming in a pressurized partially filled and sealed container. After equilibration under controlled conditions, the proportions of volatile sample components in the vapours of the headspace are representative of those in the bulk sample. The system, which is usually automated to ensure satisfactory reproducibility, consists of a thermostatically heated compartment in which batches of samples can be equilibrated, and a means of introducing small volumes of the headspace vapours under positive pressure into the carrier-gas stream for injection into the chromatograph (Figure 4.25). The technique is particularly useful for samples that are mixtures of volatile and non-volatile components such as residual monomers in polymers, flavours and perfumes, and solvents or alcohol in blood samples. Sensitivity can be improved by combining headspace analysis with thermal desorption whereby the sample vapours are first passed through an adsorption tube to pre-concentrate them prior to analysis. 

Because little has been said concerning difficulties arising from derivati-zation of samples to render them suitable for GC analysis, replacement of GC by HPLC for non-volatile or thermally labile compounds is a possibility. However, the demands of reproducible solvent removal for a reliable LC-C-IRMS approach are formidable. Caimi and Brenna [685,686] have developed an instrument based on a moving wire transport system. The analytes are deposited on the wire as they elute from the HPLC column and, after solvent drying at 200 °C, are transported into an 800 °C combustion furnace loaded with CuO, where the resulting C02 is picked up by an He carrier stream and swept via a drying trap into the IRMS. 

These liner exchange systems make feasible yet another analysis mode direct thermal desorption (DTD). Here the liner or an insert is packed with the solid sample. The liner exchange system can then be used in place of a conventional autosampler. The liner is automatically inserted into the PTV and the volatiles thermally desorbed onto the column. Some analysts may feel uneasy about such desorption from the solid phase how does one know that all of the volatile analytes have been released from the sample crystal lattice However, where applicable, this approach may not be as difficult to validate as one might imagine. For instance, the PTV can be cooled after the analyte transfer, and then, at the end of the chromatographic temperature programme, reheated to repeat the process. Ideally all of the analyte should transfer in the first cycle and none in the second, demonstrating that complete desorption occurs in the method. 

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