Target preparation Electrospray

DESI Desorption electrospray ionization (DESI) is a recently developed technique that permits formation of gas-phase ions at atmospheric pressure without requiring prior sample extraction or preparation. A solvent is electrosprayed at the surface of a condensed-phase target substance. Volatilized ions containing the electrosprayed droplets and the surface composition of the target are formed from the surface and subjected to mass analysis (Takats et al., 2005 Wiseman et al., 2005 Kauppila et al., 2006). 

Electrospray sample deposition (ESDEP) is a sample preparation method where matrix and analyte solutions are sprayed on the target surface under the influence of a high-voltage electric field [44,45]. ESDEP is reported to yield much better shot-to-shot and spot-to-spot reproducibility than the dried-droplet method. The improved results are ascribed to the small and evenly sized crystals that are formed, and as a consequence, improved homogeneity of the MALDI sample surface. Hanton et al. [45] analyzed PEG1450, a narrowly distributed poly(ethylene oxide) sample, using an uncommon... 

Ambient MS is another advance in the field. It allows the analysis of samples with little or no sample preparation. Following the introduction of desorption electrospray ionization (DESI) [108,109], direct analysis in real time (DART) [110], and desorption atmospheric pressure chemical ionization (DAPCI) [111, 112], a number of ambient ionization methods have been introduced. They include electrospray-assisted laser desorption/ionization (ELDI) [113], matrix-assisted laser desorption electrospray ionization (MALDESI) [114], atmospheric solids analysis probe (ASAP) [115], jet desorption ionization (JeDI) [116], desorption sonic spray ionization (DeSSI) [117], field-induced droplet ionization (FIDI) [118], desorption atmospheric pressure photoionization (DAPPI) [119], plasma-assisted desorption ionization (PADI) [120], dielectric barrier discharge ionization (DBDI) [121], and the liquid microjunction surface sampling probe method (LMJ-SSP) [122], etc. All these techniques have shown that ambient MS can be used as a rapid tool to provide efficient desorption and ionization and hence to allow mass spectrometric characterization of target compounds. 

Finally, (nano)electrospray deposition can be used to deposit the analytes onto different kinds of predeposited matrix layers. MALDI sample preparations where the analyte solution is deposited on top of a previously prepared matrix layer are generally termed sandwich methods. The base layer of matrix may be prepared either by the standard dried droplet technique or by thin layer preparation. For (nano)electrospray deposition of peptides, for example, a 10 M solution is sprayed from a (nano)electrospray capillary onto the solid matrix layer. The advantage of nanoelectrospray over conventional electrospray is that very small droplets are formed, which arrive at the target as dry particles, and thus, do not wet and redissolve the matrix surface [41]. 

In Part IV of the book, analytical and practical issues of electrospray and MALDI are confronted. Chapter 13 presents an analytical comparison between electrospray and MALDI, with atmospheric-pressure chemical ionization thrown in for good measure. An investigation into the factors that influence charge state distributions in electrospray and MALDI appears in Chapter 14. Next comes a detailed overview of the utility of electrospray and MALDI for investigating noncovalent interactions that are preserved as gas-phase ions (Chapter 15). Attention then turns to discussion of the various ion activation techniques that are used to provoke dissociation of the typically stable intact precursors generated by electrospray and MALDI in preparation for tandem mass spectrometry experiments (Chapter 16). Chapter 17 presents a primer on interpretation of mass spectra specifically targeting decompositions of the even-electron ions produced by soft ionization techniques. 

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