Low-Temperature FAB

Cooled FAB probes have been designed to prolong the acquisition time for FAB measurements with more volatile matrices. [117] Research on sputtering processes from solid gases has contributed to FAB at cryogenic temperatures. [118,119] 

More recently, studies concerning cluster ion formation from solid or deeply cooled liquid alcohols [120-122] have gained new interest. [123,124] Low-temperature fast atom bombardment (LT-FAB) of frozen aqueous solutions of metal salts provides a source of abundant hydrated metal ions. [125-127] Organic molecules can also be detected from their frozen solutions. [128] Such LT-FAB applications are particularly interesting when enabling the detection of species that would otherwise not be accessible by mass spectrometry, because they are either extremely air- and/or water-sensitive [129,130] as the phosphaoxetane intermediate of the Wittig reaction [131] or insoluble in standard FAB matrices. [106,132] 

LT-FAB mass spectra are obtained during thawing of the frozen solution in the ion source of the mass spectrometer, thereby allowing to employ almost any solvent as matrix in LT-FAB-MS. Consequently, neither volatility nor unwanted chemical reactions with the matrix restrict the choice of a matrix. Instead, the solvent matrix may be tailored to the analyte s requirements. 

Example Selective activation of C-H bonds is rarely observed in saturated alkyl groups, but the iridium complex 1 does react by C-H insertion of the metal into a ligand bond upon treatment with LiBr in solution. The reaction can be tracked by LT-FAB-MS (Fig. 9.17). A decreasing intensity of the molecular ion of 1, m/z 812.4, and increasing of 2, m/z 856.4, indicate the progress of this reaction. Furthermore, the halogen exchange is indicated by the isotopic pattern. 

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Gross, J.H. Giesa, S. Kratschmer, W. Negative-Ion Low-Temperature FAB-MS of Monomeric and Dimeric [60]Fullerene Compounds. Rapid Commun. Mass Spectrom. 1999,73,815-820. 

Boryak, O.A. Stepanov, I.O. Kosevich, M.V. Shelkovsky, V.S. Orlov, V.V. Blagoy, Y.P. Origin of Qnsters. I. Correlation of Low Temperature FAB Mass Spectra With the Phase Diagram of NaCl-Water Solutions. Eur. Mass Spectrom. 1996,2,329-339. 

II. Distinction of Two Different Processes of Formation of Mixed MetalAVater Clusters Under Low-Temperature FAB. Eur. Mass Spectrom. 1997,5,11-17. 

Gross, J.H Use of Protic and Aprotic Solvents of High Volatility As Matrixes in Analytical Low-Temperature FAB-MS. Rapid Common. Mass Spectrom. 1998,12,1833-1838. 

Althought this decomposition mechanism is not yet well-defined (homolytic cleavage, reductive dimerization. hydride formation, or other mechanisms), it is practically meaningful. Styrene polymerization by CpTiMea/FAB carried out in the low-temperature regime (below the decomposition temperature) produces only atactic polystyrene, possibly via a car-bocationic polyaddition. However, at higher polymerization temperatures (above the decomposition temperature), highly syndiotactic polystyrene can be obtained by coordinative 2,1-polyinsertion. ... 

Conocurvone la is a deoxy-trimer of tereti-folion B (2a), a compound that has been known for longer and was first isolated from Conospermum teretifolium. The fast atom bombardment (FAB) mass spectrum of the tri-meric quinone revealed a molecular ion corresponding to the formula CeoHseOn, but the structure of la was in the end only fully elucidated by synthesis. The reason for this was that atropoisomeric equilibria were formed and led to more or less complex H-NMR spectra that varied with solvent and with temperature. The compound therefore appeared to be a complex mixture. However, the synthetic product was identical to the natural material, even in its chiroptical parameters, thus confirming the structure and also the low rotation barrier around the quinone-quinone axis, a property which has also been found for other quinonoid-quinonoid-coupled oligomers. 

There are several advantages to the FAB technique. The instrumentation is simple and the sensitivity is high. Analytes such as surfactants have been measured quantitatively at concentrations as low as 0.1 ppb. It is difhcult to get very large molecules into the gas phase because of their low volatility, and it is difficult to ionize large molecules and retain the molecular ion in many ionization sources. The FAB process works at room temperature volatilization is not required, so large molecules and thermally unstable molecules can be studied. The duration of the signal from the sample is continuous and very stable over a long period. 

The exposure potential from the use of NMP in paint stripping is completely different from its use in semiconductor processing as a photoresist stripper. In fab use, NMP is isolated from the fab worker in processing equipment that is a closed system. The low vapor pressure of NMP (0.342 mm Hg at 25°C) limits the saturated vapor concentration to about 450 ppm at room temperature. It has been reported that hydrolysis in air at relative humidities of 40-60% (a typical level found in a semiconductor fab area) can reduce maximum concentrations to approximately 130 ppm.1 1 The potential inhalation exposure hazard is low, but NMP is readily absorbed through the skin, and repeated or prolonged skin contact with the liquid can cause severe dermatitis. 

Kong J, Fab X, Xie Y, Qiao W (2004) Study on molecular chain heterogeneity of linear low-density polyethylene by cross-fractionation of temperature rising elution fractionation and successive self-nucleation/annealing thermal fractionation. J Appl Polym Sci 94 1710-1718... 

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