Ionisation, work

Electrical Conductivity. In order to conduct electricity, the working fluid must contain charged particles, ie, it must be partially ionised. 

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. 

Applications Early MS work on the analysis of polymer additives has focused on the use of El, Cl, and GC-MS. The major drawback to these methods is that they are limited to thermally stable and relatively volatile compounds and therefore are not suitable for many high-MW polymer additives. This problem has largely been overcome by the development of soft ionisation techniques, such as FAB, FD, LD, etc. and secondary-ion mass spectrometry. These techniques all have shown their potential in the analysis of additives from solvent extract and/or from bulk polymeric material. Although FAB has a reputation of being the most often used soft ionisation method, Johlman el al. [83] have shown that LD is superior to FAB in the analysis of polymer additives, mainly because polymer additives fragment extensively under FAB conditions. 

Hyperthermal surface ionisation (HSI) is an ultrasensitive tuneable selective ion source [222,223] which is based on the very effective ionisation of various hyperthermal molecules upon their scattering from a surface with a high work function, such as rhenium oxide. Molecule-surface electron transfer constitutes the major and most important HSI mechanism for GC-MS. 

Soft ionisation modes, such as API, which leave the (pseudo)molecular ion intact without much fragmentation, offer more sensitivity, and are ideal for quantitative work at low levels (e.g. breakdown products). With the use of soft ionisation techniques in LC-MS, tandem MS... 

Much LC-MS work is carried out in a qualitative or semi-quantitative mode. Development of quantitative LC-MS procedures for polymer/additive analysis is gaining attention. When accurate quantitation is necessary, it is important to understand in depth the experimental factors which influence the quantitative response of the entire LC-MS system. These factors, which include solvent composition, solvent flow-rate, and the presence of co-eluting species, exert a major influence on analyte mass transport and ionisation efficiency. Analyte responses in MS procedures can be significantly affected by the nature of the organic modifier used in the RPLC... 

The use of ionisation techniques such as El and Cl for TLC stationary phases has generally been limited to relatively nonpolar and thermally stable molecules. Polar involatile compounds, separated on silica gel, generally strongly adsorb on to the matrix, and decompose when heat is applied for volatilisation [817]. Use of less-adsorbent phases, such as polyamide, is particularly useful for TLC-EIMS work, because the analytes are not as strongly adsorbed to this phase and do not require high probe temperatures [818,819]. For compounds that are not suitable candidates for TLC-EIMS, FAB can be employed. Chemical ionisation, although suitable for TLC-MS, appears to have been little used. 

Detection limits in the lOOfg range can be obtained with a tuneable UV laser working at a wavelength of maximum absorption for the compounds of interest. Continuous supersonic beams require high gas loads and combination with a pulsed ionisation technique (e.g. REMPI) is unfavourable in terms of sensitivity. Pulsed valves are a better approach for a GC-UV-MS interface [1021]. 

In a series of calculations on ethylene, butadiene and hexatriene, Deleuze and co-workers [105] showed that the ADC(3) method can provide a very accurate picture of the electronic processes associated with ionisation in the valence region. Poly(acetylene) has a large feature above 21 eV, which was previously assigned to shake up. The theoretical work showed conclusively that in fact even the band at around 17eV, which had previously been assigned to a C 2s excitation could not be explained by a single particle picture but was due to satellite excitations. 

Besides the well-established chromatographic/mass spectrometric or spectroscopic methods there is always a need for complementary methods for the study of organic materials from art objects. The application of laser desorption/ionisation mass spectrometry (LDI-MS) methods to such materials has been reported only sporadically [12, 45 48] however, it is apparently increasing in importance. After GALDI-MS had been applied to triterpenoid resins, as described in Section 5.2, this relatively simple method was evaluated for a wider range of binders and other organic substances used for the production or conservation of artwork. Reference substances as well as original samples from works of art were analysed. 

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