Direct laser desorption/ionization

Matrix-free direct laser desorption ionization of analyte has been studied on different kinds of surfaces without real success because degradation of the sample is usually observed. However, good results were obtained with the method called surface-activated laser desorption ionization (SALDI) [43] which uses graphite as the surface. But the use of porous silicon as a new surface is more promising and has led to the development of a new method called desorption ionization on silicon (DIOS) [44], Unlike the other matrix-free laser desorption ionization methods, DIOS allows ion formation from analyte with little or no degradation. 

Gord JR, Bemish RJ, Freiser BS (1990) Collision-induced dissociation of positive and negative copper-oxide cluster ions generated by direct laser desorption ionization of copper-oxide. Int J Mass Spectrom Ion Processes 102 115... 

Matrix-assisted laser desorption ionization is, as its name implies, a desorption technique that uses a laser to impart energy to a substance for volatization and ionization. While direct laser desorption ionization is possible, it usually imparts too much energy to molecules and fragmentation occurs. Almost simultaneously, Karas and Hillenkamp (1988) and Tanaka et al. (1988) reported indirect laser desorption approaches in which laser energy is transmitted directly to a matrix in which the target analyte is mixed. Karas and Hil-... 

While MALDl-MS is widely used for polymer characterization, it does have certain limitations. The first limitation is that not all narrow-polydispersity polymers can be ionized by MALDl. For example, polyethylene, perfluoropolymers, and polycationic polymers are difficult to analyze by MALDl-MS. Direct laser desorption ionization with the assistance of metal powder can be used to ionize low-molecular mass polyethylene (<5000 Da) [112], but high-mass polyethylene is currently not amenable to MALDl analysis. 

Direct Laser Desorption/Ionization Mass Spectrometry (LDI-MS) An alternative technique used to eliminate matrix and cationization salt effects is LDI. 

The matrix-assisted effect refers to enhanced ionization efficiency of molecules that is observed when they are desorbed with another molecule in the mixture that acts as a primary chromophore as compared to their direct laser desorption ionization efficiency as a pure compound. Thus, in a matrix-assisted laser desorption/ionization (MALDI) experiment, one must always add the matrix to enhance the ionization efficiency of the analytes. Good matrices generally have high absorption coefficients for the laser wavelength of interest and are usually acidic to be able to donate a proton (positive ion mode) in the plume. It is also often desirable for a matrix to form large, flat crystals upon evaporation of solvent. This latter requirement is particularly true for time-of-flight mass analyzers because the resolving power is dependent on the flatness of the sample surface. 

The unseparated digest mixture was studied directly by mass spectrometry using matrix-assisted laser desorption ionization (MALDI) and this showed six of the polypeptides detected by LC-MS and three of the expected polypeptides that had not been detected by LC-MS. In contrast, MALDI did not show three polypeptides observed by LC-MS. 

Krishnamurthy, T. Ross, P. L. Rapid identification of bacteria by direct matrix-assisted laser desorption/ionization mass spectrometric analysis of whole cells. Rapid Commun. Mass Spectrom. 1996,10,1992-1996. 

The focus of this chapter is the development of a technique often called wholecell matrix-assisted laser desorption ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS) or whole-cell MALDI-TOF MS. Some groups prefer to use terms such as intact or unprocessed rather than whole, but the intended meaning is the same regardless of which word is used. As noted in the first chapter of this book, there are many different methods for the analysis of bacteria. However, for the analysis of intact or unprocessed bacteria, whole-cell MALDI-TOF MS is the most commonly used approach. This method is very rapid. MALDI-TOF MS analysis of whole cells takes only minutes because the samples can be analyzed directly after collection from a bacterial culture suspension. Direct MALDI MS analysis of fungi or viruses is similar in approach1,2 but is not covered in this chapter. MALDI-TOF MS of whole cells was developed with very rapid identification or differentiation of bacteria in mind. The name (whole cell) should not be taken to imply that the cells are literally intact or whole. Rather, it should be taken to mean that the cells that have not been treated or processed in any way specifically for the removal or isolation of any cellular components from any others. In whole-cell analysis the cells have been manipulated only as necessary to... 

Wang, Z. Russon, L. Li, L. Roser, D. C. Long, S. R. Investigation of spectral reproducibility in direct analysis of bacteria proteins by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Rapid Comm. Mass Spectrom. 1998,12,456-464. 

Proteomics ultimately hinges upon protein identification to reveal the meaning behind the masses, spots, or peaks detected by other means. Because fraction collection is a natural component of HPLC separations, intact proteins can be readily collected either for direct analysis or for proteolytic digestion and identification using peptide mass fingerprinting (PMF) in conjunction with matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). 

The ProteinChip System from Ciphergen Biosystems uses patented SELDI (Surface-Enhanced Laser Desorption/Ionization) ProteinChip technology to rapidly perform the separation, detection, and analysis of proteins at the femtomole level directly from biological samples. ProteinChip Systems use ProteinChip Arrays which contain chemically (cationic, anionic, hydrophobic, hydrophilic, etc.) or biochemically (antibody, receptor, DNA, etc.) treated surfaces for specific interaction with proteins of interest. Selected washes create on-chip, high-resolution protein maps. This protein mass profile, or reten-tate map of the proteins bound to each of the ProteinChip Array surfaces, is quantitatively detected in minutes by the ProteinChip Reader. 

The most discriminating technique for proving the identity and purity of analyte peak of a chromatogram, especially for analyzing biological samples and natural products, is by using online LC-UV/MS or GC-MS/FTIR methods [15]. Alternatively, one could use a combination of TLC and MS, where direct determination on the TLC plates is made by matrix-assisted laser desorption ionization mass spectrometry (MALDI MS) [16]. 

Sugiura Y, Shimma S, Moriyama Y, et al. Direct analysis of cultured cells with matrix-assisted laser desorption/ionization on conductive transparent film. J. Mass. Spectrom. Soc. Jpn. 2007 54 25-31. 

Schwartz S, Reyzer M, Caprioli R. Direct tissue analysis using matrix-assisted laser desorption/ionization mass spectrometry practical aspects of sample preparation. /. Mass. Spectrom. 2003 38 699-708. 

Figure 2.1 Mass spectrometric approach. Dl, direct inlet GC, gas chromatography HPLC, high performance liquid chromatography CZE, capillary zone electrophoresis El, electron ionization Cl, chemical ionization ESI, electrospray ionization DESI, desorption electrospray ionization APCI, atmospheric pressure chemical ionization MALDI, matrix assisted laser desorption ionization B, magnetic analyzer E, electrostatic analyzer...

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... 

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