Laser-Assisted Field Desorption Mass Spectrometry

A different, but no less severe problem, is encountered with the analysis of unknown biological substances by field desorption, laser desorption, and/or laser assisted field desorption mass spectrometry. A priori, it is not known at what point in time a component of interest is desorbed. This makes it difficult, if not impossible to time a scan or several scans in such a way that a representative spectrum is obtained. Again the answer to the problem is found by integrating the ion output over the entire sample or repeatedly over portions of the sample profile as it emerges from the ion source. 

Schulten (110, 111) has used laser-assisted field desorption mass spectrometry to study some inorganic and organometallic systems. This method is intermediate between LAMMA and simple FD. Metal cations predominate from inorganic salts. The technique also showed clusters of the type reported from both FAB and SIMS studies. By carefully controlling the laser, a chlorophyll molecular ion could be obtained as well as fragments relating to its structure. 

LAFDMS laser-assisted field desorption mass spectrometry... 

S Laser-Assisted Field Desorption Mass Spectrometry... 

Mass S >ectrometric Methods for Trace Analysis of Metab TaMe 9. Laser-assisted field desorption mass spectrometry of metals and allo) ... 

Frauenkron, M. Berkessel, A. Gross, J.H. Analysis of Ruthenium Carbonyl-Porphyrin Complexes a Comparison of Matrix-Assisted Laser Desorp-tion/Ionization Time-of-Flight, Fast-Atom Bombardment and Field Desorption Mass Spectrometry. Eur. Mass Spectrom. 1997, 5,427-438. 

The development of matrix-assisted laser desorption ionization (MAEDI) has advanced the entire field of mass spectrometry. To use this ionization method, the sample is mixed into a matrix that absorbs the laser wavelength extremely well (approximately 10,000 1 matrix analyte) and the mixture is placed on a solid substrate. Absorption of the laser causes the matrix to explode, ejecting the intact, nonvolatile molecules of interest into the gas phase. Proton exchange or alkali metal attachment occurs in the gas plume and the ionized species can be detected. 

An impetus in the field of mass spectrometry (MS) analysis occurred in the early 1990s with the invention of two novel and soft ionization methods, electrospray ionization (ESI) by John Fenn and matrix-assisted laser desorption ionization (MALDI) by Koichi Tanaka, who both shared the Nobel Prize in Chemistry in 2002. A second impetus, which is more diffuse, is currently occurring and consists of miniaturization. Whereas the intrinsic sensitivity of mass spectrometers has roughly remained the same for a couple of decades, the amount of material required for recording one spectrum has... 

In the last decade there has been a fast progress in the field of mass spectrometry employing soft ionization techniques such as electrospray ionization mass spectrometry (ESI-MS), Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), and matrix-assisted laser desorption ionization mass spectrometry (MALDI)-MS. These tools provide molecular level information on humic substances. 

The recent development of electrospray ionization (ESI) and matrix-assisted laser desorption-ionization (MALDI) sources for creation of macromolecular ions has brought about a renaissance in the field of mass spectrometry (MS). 

Lee, H. Williams, S. K. R. Wahl, K. L. Valentine, N. B. Analysis of whole bacterial cells by flow field-flow fractionation and matrix-assisted laser desorption/ ionization time-of-flight mass spectrometry. Anal. Chem. 2003, 75,2746-2752. 

Reschiglian, P. Zattoni, A. Cinque, L. Roda, B. Dal Piaz, F. Roda, A. Moon, M. H. Min, B. R. Hollow-fiber flow field-flow fractionation for whole bacteria analysis by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Anal. Chem. 2004, 76,2103-2111. 

Over the years, a lot of desorption ionization techniques have been introduced to MS, such as plasma desorption, field desorption, laser desorption, secondary ion mass spectrometry, fast atom bombardment, matrix assisted laser desorption and desorption electrospray ionization. Most of them are actually no longer used. In the following paragraphs, both matrix assisted laser desorption (MALDI) and desorption electrospray ionization (DESI) will be discussed. 

DGE a AC AMS APCI API AP-MALDI APPI ASAP BIRD c CAD CE CF CF-FAB Cl CID cw CZE Da DAPCI DART DC DE DESI DIOS DTIMS EC ECD El ELDI EM ESI ETD eV f FAB FAIMS FD FI FT FTICR two-dimensional gel electrophoresis atto, 10 18 alternating current accelerator mass spectrometry atmospheric pressure chemical ionization atmospheric pressure ionization atmospheric pressure matrix-assisted laser desorption/ionization atmospheric pressure photoionization atmospheric-pressure solids analysis probe blackbody infrared radiative dissociation centi, 10-2 collision-activated dissociation capillary electrophoresis continuous flow continuous flow fast atom bombardment chemical ionization collision-induced dissociation continuous wave capillary zone electrophoresis dalton desorption atmospheric pressure chemical ionization direct analysis in real time direct current delayed extraction desorption electrospray ionization desorption/ionization on silicon drift tube ion mobility spectrometry electrochromatography electron capture dissociation electron ionization electrospray-assisted laser desorption/ionization electron multiplier electrospray ionization electron transfer dissociation electron volt femto, 1CT15 fast atom bombardment field asymmetric waveform ion mobility spectrometry field desorption field ionization Fourier transform Fourier transform ion cyclotron resonance... 

Field M., Papac D., and Jones A. (1996), The use of high-performance anion-exchange chromatography and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry to monitor and identify oligosaccharide degradation, Anal. Biochem. 239, 92-98. 

The only mass spectrometric methods available during the era of the first cascade synthesis in 1978 [30] were electron impact (El) and field desorption (FD) mass spectrometry [31]. Fast atom bombardment (FAB) mass spectrometry is limited to fairly low mass ranges and not very suitable for compounds of low polarity. It was not until the development of new and gentle ionisation methods such as MALDI (matrix-assisted laser desorption ionization) [32] and ESI (electrospray ionization) [33] that the conditions were fulfilled for the start of intense research in the field of dendrimer chemistry. The following section will present the special features of these mass-spectrometric methods and their importance in dendrimer analysis. 

Other techniques that have been used to determine polycyclic aromatic hydrocarbons in soil extracts include ELISA field screening [86], micellar elec-tr okinetic capillary chromatography [ 87], supersonic jet laser-induced fluorescence [88,89], fluorescence quenching [90], thermal desorption gas chromatography-mass spectrometry [81,90,100], microwave-assisted extraction [91], thermal desorption [92], immunochemical methods [93,94], electrophoresis [96], thin layer chromatography [95], and pyrolysis gas chromatography [35]. 

Most of the mass spectrometry applications for combinatorial chemistry will be described in the following sections of this chapter. Here we will give a short overview of MS techniques utilized for the characterization of resin-bound molecules. The majority of publications in this field describe applications of matrix-assisted laser desorption ionization (MALDI), combined with time-of-flight (TOF) detection. The major difference of MS application for analysis of resin-bound molecules from the above-described NMR and IR applications is that analyte should not be covalently bound to solid support prior to mass measurement. Detachment of compound molecules from resin can be done chemically (for example, by bead exposure to TFA vapors) [30,31] or photochemically, such that cleavage, desorption, and ionization of molecules occur simultaneously upon stimulation by laser radiation [32], Since the... 

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