Desorption-based techniques

Nondestructive radiation techniques can be used, whereby the sample is probed as it is being produced or delivered. However, the sample material is not always the appropriate shape or size, and therefore has to be cut, melted, pressed or milled. These handling procedures introduce similar problems to those mentioned before, including that of sample homogeneity. This problem arises from the fact that, in practice, only small portions of the material can be irradiated. Typical nondestructive analytical techniques are XRF, NAA and PIXE microdestructive methods are arc and spark source techniques, glow discharge and various laser ablation/desorption-based methods. On the other hand, direct solid sampling techniques are also not without problems. Most suffer from matrix effects. There are several methods in use to correct for or overcome matrix effects ... 

S. Hillenkamp, F De Vries, M.S. Comparative Mass Spectrometric Analyses of Photofrin Oligomers by Fast Atom Bombardment Mass Spectrometry, UV and IR Matrix-Assisted Laser Desorption/lonization Mass Spectrometry, Electrospray Ionization Mass Spectrometry and Laser Desorption/Jet-Cooling Photoionization Mass Spectrometry, J. Mass Spectrom. 34, 661-669 (1999). Powell, K.D. Fitzgerald, M.C. Accuracy and Precision of a New H/D Exchange- and Mass Spectrometry-Based Technique for Measuring the Thermodynamic Properties of Protein-Peptide Complexes, Biochemistry 42,4962-4970 (2003). 

Because of some of the problems with bioassays and immunoassays, liquid chromatography (LC)-based techniques are increasingly applied as an alternative. While modern LC-based assays have a comparable sensitivity to immunoassays, they oftentimes are characterized by a higher selectivity [18, 19]. Muller et ah, for example, used LC/mass spectrometry with matrix-assisted laser desorption ionization in ex vivo pharmacokinetic studies in combination with enzyme inhibition experiments to investigate the complex metabolism of dynorphin Al-13, a peptide with opioid activity, up to the fifth metabolite generation [20, 21]. 

Parallel to this progress, new ionization methods have been developed that are based on the direct desorpHon of ions from polymer surfaces. With the introduction of "desorption/ionization" techniques, it has become possible to eject large molecules into the gas phase directly from the sample surface, and thereby mass spectra of intact polymer molecules have been produced. Much progress to date has been made using matrix-assisted laser desorp-tion/ionization (MALDl-MS), which is capable of generating quasimolecular ions in the range of 10 Daltons (Da) and beyond. 

Similarly, enzymatic reactions can be performed directly on a MALDI plate, quenched, and the reaction products analyzed [43]. Various laser desorption/ionization (LDI)-based techniques facilitate such off-line measurements [44,45]. When the operations of initiating and quenching the chemical reactions are carried out manually, the temporal resolution of the LDI-MS-based methods is typically in the order of a few minutes. However, by implementing flow mixing and quenching methodology, one can perform observations of sub-second phenomena with this kind of off-line MS detection (see, e.g., [46-48]). 

There are several MS-based techniques that can provide chemical information for thin and thick layers [12]. For very thin layers (sub to 1-2 monolayers), nondestructive techniques such as static SIMS [13], ion scattering MS [14], or MS of recoiled ions [15] are suitable. These techniques are also the best adapted for examining surface contamination. They are all based on surface interactions of an ion beam with the solid surface. For depth profiling of thin and thick layers, MS is associated with a destructive source of neutrals or ions dynamic SIMS, secondary neutron mass spectroscopy (SNMS), glow discharge mass spectroscopy (GD-MS), matrix-enhanced SIMS, laser desorption/ionization MS, and desorption electrospray ionization (DESI) MS [16]. Ions are either desorbed from the solid surface or generated by postionization of neutrals sputtered off the surface. 

In general, as newer detection techniques and sensitivities are developed in the analytical laboratory, the demands are more stringent regarding the purity of the new products whether they are solvents or thin-layer plates. This is particularly true for the silica gels, bonded phases, or binders used in HPTLC plates to be used in TLC-MS work. If doing elution-based TLC-MS or desorption-based TLC-MS, the requirements are the same. 

Compared to densitometry and desorption-based approaches (atom and ion bombardment, laser light beam, spray beam, excited gas beam) where only a few micrometers of the plate surface is being sampled, elution-based TLC-MS is a destructive technique as it extracts all or most of the sample, respectively. This sets aside one of the main advantages of TLC, which is the possibility to reevaluate chromatograms at a later time (i.e., after weeks or months). 

Due to the open planar surface of UTLC layers, especially ambient desorption- or elution-based techniques were used to transfer analytes from the layer to the ionization region (Table 9.2). In addition, MALDI applications under vacuum were reported and the introduction of the whole ultrathin layer plate was possible without any special mounting devices, due to the small plate dimension. As UTLC-MS is a very new hyphenation, only a few applications of MS detection were reported after separation of different substances on UTLC layers. In addition, desorption- or elution-based approaches for detecting analytes without a separation directly from the UTLC layer are mentioned to show the capabilities of this hyphenation technique. However, as there is no chromatography (separation), it may not be termed UTLC-MS. 

The rapid rise in popularity of SPME had led to an increased interest in all types of sorbent-based extractions. Stirbar sorptive extraction (SBSE) is an outgrowth of SPME that employs a coated stirbar as the organic phase. Finally, there are a number of flowthrough extraction configurations that are combined with thermal desorption. These techniques offer interesting alternatives to classical solvent and headspace extractions. 

As an alternative to wet ehemical routes of analysis, this monograph deals mainly with the direct deformulation of solid polymer/additive compounds. In Chapter 1 in-polymer spectroscopic analysis of additives by means of UV/VIS, FTIR, near-IR, Raman, fluorescence spectroseopy, high-resolution solid-state NMR, ESR, Mossbauer and dielectrie resonance spectroscopy is considered with a wide coverage of experimental data. Chapter 2 deals mainly with thermal extraction (as opposed to solvent extraction) of additives and volatiles from polymerie material by means of (hyphenated) thermal analysis, pyrolysis and thermal desorption techniques. Use and applieations of various laser-based techniques (ablation, spectroscopy, desorption/ionisation and pyrolysis) to polymer/additive analysis are described in Chapter 3 and are critically evaluated. Chapter 4 gives particular emphasis to the determination of additives on polymeric surfaces. The classical methods of... 

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