Electrospray small-molecule MALDI

One drawback for the use of MALDI with small molecules is that sample preparation is more intensive than that for electrospray because of the need for matrix addition. To some extent this sample preparation can, however, be automated. A more significant drawback is the presence of a low-mass-noise background arising from the matrix. This noise background can interfere with the direct assessment of the spectrum for the compound of interest. Considerable research efforts have been directed at approaches that eliminate the low-mass... 

Electrospray ionization (ESI) is one of the two newer ionization techniques (the other being MALDI) that has revolutionized the application of mass spectrometry to biochemistry and molecular biology, and may fairly be said to have revolutionized these disciphnes also. This is the result of the ability of ESI-MS to provide molecular mass information of hitherto unthinkable accuracy and precision for fragile biopolymers, particularly proteins, and to provide amino acid sequence information for specific peptides present at trace levels in complex mixtures characteristic of biological extracts. The discipline of proteomics would not exist without the development of ESI and MALDI. However, this book is concerned with quantitation of small molecules present at trace levels in complex matrices, rather than the essentially qualitative data typically acquired in proteomics experiments, although application of the approach to quantitation of proteins and peptides is discussed in Section 11.6. Nonetheless, development of methods to produce and characterize gaseous ions from macromolecules was very important in the development of ESI-MS, and this history will be briefly described here. Excellent reviews of this history as it pertains to macromolecule characterization (Penn 1990 Smith 1991, 1992 Fernandez de la Mora 1992) are available, while... 

Nowadays, interfacing separation methods such as high-performance hquid chromatography (HPLC) or CE [54] to MS is already a routine technique. MS methods based on so-called soft ionization techniques, which include fast atom bombardment, laser desorption and electrospray ionization (ESI), have allowed the analysis of biological macromolecules that in the past could have been analyzed only by extensive cleavage and derivatization. Of these methods, the two most preferred for biomolecules are ESI [55] and matrix-assisted laser desorption/ionization(MALDI) [56,57], for which time-of-flight (TOE) and ion trap mass analyzers are the most frequentiy used mass analysis methods. However, these methods are equally suitable for small molecules, such as metabolites or the products of organochemical reactions. 

IMS experiments may be performed using one of several MS ionization techniques secondary ion mass spectrometry (SIMS), laser desorption/ionization (LDI), desorption electrospray ionization (DESI), and matrix-assisted laser desorption/ionization (MALDI) (2). In general, these techniques offer complementary capabilities (3). SIMS imaging is favored for higher spatial resolution imaging over a low mass range (<1,000 Da), MALDI IMS covers a much wider mass range (up to and over 100,000 Da) but at somewhat lower spatial resolution, and DESI uses ambient pressure for analysis of small molecules but at lower spatial resolution. 

The classical area of application of mass spectrometry has been with small volatile compounds, although non-volatile samples could be analysed if they were suitably derivatised. The application of mass spectrometry to large complex molecules like proteins has been made possible by the development of novel ionisation techniques which enable large molecules (> 200 kDa) to be introduced into the mass spectrometer in an intact form suitable for analysis (Siuzdak 1996 Dass 2000). Of the various techniques that have been developed, electrospray ionisation (ESI) and matrix-assisted laser desorption ionisation (MALDI) are the ones best suited for use with macromolecules and macromolecular complexes. 

The analysis of proteins by mass spectrometry has emerged as the techifique of choice for obtaining high performance results from small amounts of analyte. Interest in this analytical technique has increased primarily as the result of the introduction of the electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) sources. These innovations permit nonvolatile molecules to be readily introduced into the gas phase. 

Recently, matrix-assisted laser desorption ionization-mass spectrometry (MALDI-MS) has been exploited to determine the complete profile of peptide messengers contained in a small sample of nervous tissues and even single neurons. This method is fast, sensitive, and provides highly specific and semiquantitative data of the peptides contained in complex mixtures of biological molecules. Other mass spectrometric methods, such as fast atom bombardment and electrospray mass spectrometry, are less sensitive and are also less suitable for the analysis of crude biological samples. 

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