Desorption techniques

Alternative ( soft ) ionization techniques are not usually required for aromatic isothiazoles because of the stability of the molecular ions under electron impact. This is not the case for the fully saturated ring systems, which fragment readily. The sultam (25) has no significant molecular ion under electron impact conditions, but using field desorption techniques the M + lY ion. is the base peak (73X3861) and enables the molecular weight to be confirmed. 

A physical method of desorption is, for example, the desorption stimulated by the impact of ions or electrons 3-8). Another method is desorption by a strong electrostatic field (9). Irradiation by a current of photons can lead to a photodesorption process (10-14). Even desorption by means of phonic energy has been described (15), Finally, the desorption by thermal energy falls into this group of desorption techniques. 

The flash desorption technique is applied usually in ultrahigh vacuum conditions. Then all the mentioned contributions to S and F should be accounted for in the evaluation of the experimental desorption curves. The effect of Sw on the results of desorption measurements is discussed in... 

A different approach consists of stepwise changing the adsorbent temperature and keeping it constant at each of the prefixed values Tx, Ts,. . ., Tn for a certain time interval (e.g. 10 sec), thereby yielding the so-called step desorption spectra s(81-85). The advantage of this method lies in a long interval (in terms of the flash desorption technique) for which the individual temperatures Ti are kept constant so that possible surface rearrangements can take place (81-83). Furthermore, an exact evaluation of the rate constant kd is amenable as well as a better resolution of superimposed peaks on a desorption curve (see Section VI). What is questionable is how closely an instantaneous change in the adsorbent temperature can be attained. This method has been rarely used as yet. 

With porous materials, a slow diffusion in the pores can sometimes control the rate of desorption. This may give rise to complications because diffusion in the pores may be complex and difficult to treat mathematically. Cvetanovi6 and Amenomiya (48) gave a model treatment for their modification of the thermal desorption technique. 

Application of a Temperature-Programmed Desorption Technique to Catalyst Studies... 

Examples of such compounds include anionic surfactants whose analysis had previously been limited to desorption techniques such as FAB and thermospray but which yielded interpretable El spectra when using a parUcle-beam interface... 

Coyne LB, Warren JS, Cerbus CS. 1987. An evaluation of a new desorption technique for air sampling sorbent tubes. Am Ind Hyg Assoc J 48 668-670. 

Comparison between xf a as determined on the basis of Eq. (3.1.15) from the microscopically determined crystallite radius and the intracrystalline diffusivity measured by PFG NMR for sufficiently short observation times t (top left of Figure 3.1.1), with the actual exchange time xintra resulting from the NMR tracer desorption technique, provides a simple means for quantifying possible surface barriers. In the case of coinciding values, any substantial influence of the surface barriers can be excluded. Any enhancement of xintra in comparison with x a, on the other side, may be considered as a quantitative measure of the surface barriers. 

Fig. 3.1.10 Molecular lifetimes xintra and. aii in H-ZSM-5 crystallites obtained using the NMR tracer desorption technique and calculated via Eq. (3.3.15), respectively. Tracing by probe molecules (methane, measurement at 296 K) after an H-ZSM-5 catalyst has been kept for different coking times in a stream of n-hexane (filled symbols) and mesitylene (open symbols) at elevated temperature. The inserts present the evidence provided by a comparison of xintra and r]1,]]], with respect to the distribu-...

It is worth noting that within a range of 20 %, five different methods of analyzing the crystallite size, viz., (a) microscopic inspection, (b) application of Eq. (3.1.7) for restricted diffusion in the limit of large observation times, (c) application of Eq. (3.1.15) to the results of the PFG NMR tracer desorption technique, and, finally, consideration of the limit of short observation times for (d) reflecting boundaries [Eq. (3.1.16)] and (e) absorbing boundaries [Eq. (3.1.17)], have led to results for the size of the crystallites under study that coincide. 

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. 

Liquid (solvent) extraction is not the only way of sample preparation, but stands along with various forms of heat extraction (headspace, thermal desorption, pyrolysis, etc.) and with laser desorption techniques. 

Principles and Characteristics Solid-phase microextraction (SPME) is a patented microscale adsorp-tion/desorption technique developed by Pawliszyn et al. [525-531], which represents a recent development in sample preparation and sample concentration. In SPME analytes partition from a sample into a polymeric stationary phase that is thin-coated on a fused-silica rod (typically 1 cm x 100 p,m). Several configurations of SPME have been proposed including fibre, tubing, stirrer/fan, etc. SPME was introduced as a solvent-free sample preparation technique for GC. 

Sterically hindered phenols and other additives containing thioesters, phosphites, phosphonites and hindered amine moieties were analysed by FAB-MS and LD-FTMS. The laser desorption technique was preferred for analysis of polymer additives because of undesirable fragmentation from FAB experiments [93]. 

Laser desorption methods (such as LD-ITMS) are indicated as cost-saving real-time techniques for the near future. In a single laser shot, the LDI technique coupled with Fourier-transform mass spectrometry (FTMS) can provide detailed chemical information on the polymeric molecular structure, and is a tool for direct determination of additives and contaminants in polymers. This offers new analytical capabilities to solve problems in research, development, engineering, production, technical support, competitor product analysis, and defect analysis. Laser desorption techniques are limited to surface analysis and do not allow quantitation, but exhibit superior analyte selectivity. 

In the thermal desorption technique excavated soil is heated to around 200 to 1000°F (93 to 538°C). Volatile and some semivolatile contaminants are vaporized and carried off by air, combustion gas, or inert gas. Off-gas is typically processed to remove particulates. Volatiles in the off-gas may be burned in an afterburner, collected on activated carbon, or recovered in condensation equipment. Thermal desorption systems are physical separation processes that are not designed to provide high levels of organic destruction, although some systems will result in localized oxidation or pyrolysis. 

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