Laser desorption, comparison

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. 

Ktihn, G. Weidner, S. Just, U. Hohner, S. Characterization of Technical Waxes. Comparison of Chromatographic Techniques and Matrix-Assisted Laser-Desorption/Ionization-MS. J. Chromatogr. A 1996,732, 111-117. 

The book is organized into three sections. The first contains the most recent views on fundamental aspects of particle bombardment. Discussions of 252Cf plasma desorption and laser desorption mass spectrometry have been included for comparison. The second section addresses the issues involved in instrument design, covering work on liquid metal and FAB ion guns. The last part presents representative applications of these bombardment methods. 

D. J. Harvey, T. J. P. Naven, B. Kiister, R. H. Bateman, M. R. Green, and G. Critchley, Comparison of fragmentation modes for the structural determination of complex oligosaccharides ionized by matrix-assisted laser desorption/ionization mass spectrometry, Rapid Commun. Mass Spectrom., 9 (1995) 1556-1561. 

S. G. Penn, M. T. Cancilla, M. K. Green, and C. B. Lebrilla, Direct comparison of matrix-assisted laser desorption/ionisation and electrospray ionisation in the analysis of gangliosides by Fourier transform mass spectrometry, Euro. J. Mass Spectrom., 3 (1997) 67-79. 

Strege summarized the technique of high-performance liquid chromatography-electrospray ionization mass spectrometry (HPLC-ESI-MS) in dereplication of natural products. In contrast to earlier electron impact ionization (El), ESI technique is applicable to virtually any ion in solution with a soft ionization process. A comparison of ESI with fast atom bombardment (FAB), matrix assisted laser desorption ionization (MALDI), atmospheric pressure chemical ionization (APCI) and other techniques demonstrates its superior sensitivity, compatibility and reliability when coupled with HPLC [51]. 

Rouse, J.C., Yu, W. and Martin, S.A. (1996) A comparison of the peptide fragmentation obtained from a reflector matrix-assisted laser desorption-ionisation time-of-flight mass and a tandem 4-sector mass spectrometer. J. Am. Mass Spectrom., 6 (9), 822-35. 

We report on the further physical and chemical characterization of the new forms of molecular carbon, C ) and C70. Our results demonstrate a high yield of production (14%) under optimized conditions and reveal only and C70 in measurable quantity, in an 8S IS ratio. These two new molecular forms of carbon can be completely separated in analytical amounts by column chromatography on alumina. Comparison among mass spectra obtained by the electron impact, laser desorption, and fast atom bombardment (FAB) methods allows a clear assessment of the composition of the mixed and pure samples, and of the fragmentation and double ionization patterns of the molecules. In addition, spectroscopic analyses are reported for the crude mixture by C NMR and by IR spectroscopy in KBr pellet, and for pure C o and C70 in solution by UV-vis spectroscopy. 

More success was obtained using on-line solid phase extraction before offline analysis by capillary electrophoresis with matrix-assisted laser-desorption ionization (MALDI) MS [18]. Because microdialysis is a method used for analysis of small molecules and peptides, MALDI is not used frequently with microdia lysis sampling. Another disadvantage is that MALDI cannot be used on-line with the separation because it is a vacuum ionization technique. However, if the pep-tide is large enough (0 1000 Da) MALDI can be useful. For the analysis of peptides in dialysate an appropriate separation is important before mass spectrometric detection. In a comparison with direct sampling of dialysate in MALDI, capillary electrophoresis provides the high efficiency separations necessary to resolve all... 

FIGURE 4.4 Schematic diagram of the SELDI Ciphergen mass spectrometer. After sample preparation, the ProteinChip arrays are analyzed by a laser-desorption/ionization (LDI) time-of-flight mass spectrometer (TOF MS). The TOF MS measures the molecular weights of the various proteins that are retained on the array. For comparison purposes, the software associated with the SELDI Ciphergen instrument is capable of displaying the resultant data as either a spectral, map, or gel view. 

This method was developed by MacFarlane [93], who, in order to induce desorption, used high kinetic energy fragments ( Tc, 100 MeV) produced by nuclear fission of californium. Different ionization modes may occur electron transfer (M and M ) [94], proton transfer ([M + H] and [M-H] ) and, finally, cationization ([M + Na] ). Figure 13 shows the comparison of spectra obtained under PDMS-conditioned laser desorption. 

Bullock J, ChowdhuryS, Johnston D (1996). Characterization of poly (ethylene glycol)-modified superoxide dismutase Comparison of capillary electrophoresis and matrix-assisted laser desorption/ionization mass spectrometry. Analyt. Chem. 68 3258-3264. 

Macht, M., Asperger, A., and Deininger, S. O. (2004) Comparison of laser-induced dissociation and high-energy collision-induced dissociation using matrix-assisted laser desorption/ionization tandem time-of-flight (MALDI-TOE/TOF) for peptide and protein identification. Rapid Commun. Mass Spectrom. 18, 2093-2105. 

A variety of volatilization/ionization methods have been applied to polymers a recent review or key paper is cited here for each. Extensive reviews that include mass spectrometry of pol5mrers can be found in Analytical Chemistry Other to )ical reviews are field desorption, laser desorption, plasma desorption, fast-atom bombardment, pyrolysis, and electrospray ionization. The present review will focus on polymer characterization using secondary-ion mass spectrometry (SIMS) in the high mass range comparison with other methods will be presented where appropriate. 

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