Thermal Volatilisation and Desorption Techniques

The rate of loss of additives from polymers is determined by migration and volatilisation. Physical loss of additives from polymers is increasingly important in food applications and medical uses of polymers, but also other situations lead to a rapid decrease in antioxidants, such as mechanical extraction of tyres (at temperatures up to 100°C at high speed conditions) or simple solvent extraction of seals and hoses, etc. 

Thermal desorption (TD), as the name implies, is a thermal extraction technique and is essentially a dynamic process. As with liquids, solid samples are heated under a continuous flow of inert gas such that (semi)-volatile organics are extracted from the sample matrix into the gas stream and transferred in the vapour phase to an analyser. The terminology is not always strictly adhered to and thermal desorption is 

TEA techniques thus allow continuous monitoring of materials thermally evolved from a sample on controlled heating. Stepwise detection of such volatiles as a function of temperature or time, and quantitative measurement and identification of these materials provide very useful information. 

TG Volume change Gas density Electrochemical Photometric Radioactivity MS Chemical 

The capability of a thermal technique for materials characterisation is greatly increased by hyphenation to identify further either the residue or the effluence (preferably both) during a certain thermal event. Detectors offering the best combination of sensitivity and versatility are MS and FTIR. They enable the simultaneous identification of the gaseous species emitted from a sample, according to their 

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Table 2.42. Characterisation of thermal volatilisation and desorption techniques ...

Alternative approaches consist in heat extraction by means of thermal analysis, thermal volatilisation and (laser) desorption techniques, or pyrolysis. In most cases mass spectrometric detection modes are used. Early MS work has focused on thermal desorption of the additives from the bulk polymer, followed by electron impact ionisation (El) [98,100], Cl [100,107] and field ionisation (FI) [100]. These methods are limited in that the polymer additives must be both stable and volatile at the higher temperatures, which is not always the case since many additives are thermally labile. More recently, soft ionisation methods have been applied to the analysis of additives from bulk polymeric material. These ionisation methods include FAB [100] and LD [97,108], which may provide qualitative information with minimal sample pretreatment. A comparison with FAB [97] has shown that LD Fourier transform ion cyclotron resonance (LD-FTTCR) is superior for polymer additive identification by giving less molecular ion fragmentation. While PyGC-MS is a much-used tool for the analysis of rubber compounds (both for the characterisation of the polymer and additives), as shown in Section 2.2, its usefulness for the in situ in-polymer additive analysis is equally acknowledged. 

For non-volatile sample molecules, other ionisation methods must be used, namely desorption/ionisation (DI) and nebulisation ionisation methods. In DI, the unifying aspect is the rapid addition of energy into a condensed-phase sample, with subsequent generation and release of ions into the mass analyser. In El and Cl, the processes of volatilisation and ionisation are distinct and separable in DI, they are intimately associated. In nebulisation ionisation, such as ESP or TSP, an aerosol spray is used at some stage to separate sample molecules and/or ions from the solvent liquid that carries them into the source of the mass spectrometer. Less volatile but thermally stable compounds can be thermally vaporised in the direct inlet probe (DIP) situated close to the ionising molecular beam. This DIP is standard equipment on most instruments an El spectrum results. Techniques that extend the utility of mass spectrometry to the least volatile and more labile organic molecules include FD, EHD, surface ionisation (SIMS, FAB) and matrix-assisted laser desorption (MALD) as the last... 

In general terms, absolute quantification by means of TD-GC-MS or thermal volatilisation techniques is a doubtful exercise because total desorption of the analyte(s) at a given temperature is not assured, internal standards are difficult to use and mass spectrometry is not exactly weU known for its quantitative excellence. No reports are available on the use of direct TD-CIS-GC-MS for quantification purposes. 

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