Desorption ionization techniques, analysis

Electrospray ionization [21] is one of the most widely utilized ionization techniques employed today for the analysis of thermally fragile molecules. As such, it has assumed an important role in the analysis of biologically important molecules. ESI is a desorption ionization technique. This means that ions are formed before or during the transition from the liquid phase and need not be volatilized in advance of the ionization event (as is the case for El, Cl, etc.). Like APCI and APPI, ESI occurs at atmospheric pressure outside the vacuum chamber of the mass spectrometer (Fig. 11.5). A solution of the analyte passes through... 

Why are desorption ionization techniques well suited to the analysis of biomolecules (they do not depend on thermal energy to volatilize analytes). 

The purpose of the MS techniques is to detect charged molecular ions and fragments separated according to their molecular masses. Most flavonoid glycosides are polar, nonvolatile, and often thermally labile. Conventional MS ionization methods like electron impact (El) and chemical ionization (Cl) have not been suitable for MS analyses of these compounds because they require the flavonoid to be in the gas phase for ionization. To increase volatility, derivatization of the flavonoids may be performed. However, derivatization often leads to difficulties with respect to interpretation of the fragmentation patterns. Analysis of flavonoid glycosides without derivatization became possible with the introduction of desorption ionization techniques. Field desorption, which was the first technique employed for the direct analysis of polar flavonoid glycosides, has provided molecular mass data and little structural information. The technique has, however, been described as notorious for the transient... 

Desorption Ionization Techniques for Quantitative Analysis of Drug Molecules... 

The GCTMS approach, although of great utility in composition analysis, is time consuming and labor intensive and requires large sample amounts. Attention has now shifted to several desorption-ionization techniques, such as fast atom bombardment (FAB), MALDI, and ESI. In some cases it may be... 

See also Atomic Mass Spectrometry Inductively Coupled Plasma Laser Microprobe. Gas Chromatography Mass Spectrometry. Liquid Chromatography Liquid Chromatography-Mass Spectrometry. Mass Spectrometry Ionization Methods Overview Atmospheric Pressure Ionization Techniques Electrospray Liquid Secondary Ion Mass Spectrometry Matrix-Assisted Laser Desorption/ionization. Surface Analysis Secondary Ion Mass Spectrometry of Polymers Laser Ionization. 

Traditionally, products and adsorbates had to be volatile enough so that they could be carried from the cell into the mass spectrometer, either by headspace sampling, or, more commonly for near-simultaneous analysis (referred to as differential electrochemical mass spectrometry), across a nanoporous, gas-permeable membrane (e.g., Teflon) supported at the tip of a microcapillary placed close to the electrode. Alternatively, a Pt-coated membrane electrode can be used. But the advent of the so-called soft atmospheric pressure desorption/ionization techniques associated with liquid chromatography-mass spectrometry has allowed the sampling of the solvent and involatile solutes directly. The spectra are more... 

A turning point in atmospheric pressure desorption-ionization techniques for the analysis of chemical compounds was reached very recently with the introduction of DESI (U.S. Patent Application 20050230635). In this approach, charged droplets from an electrosprayed solution are directed toward a solid sample by means of a high-velocity gas stream. The charged droplets ablate the exterior of the sample, removing and ionizing chemical compounds present on the surface. Accordingly, DESI permits the direct analysis of condensed-phase samples with minimal or no sample preparation. 

Bergman, N., Shevchenko, D., and Bergquist, J. 2014. Approaches for the analysis of low molecular weight compounds with laser desorption/ionization techniques and mass spectrometry, Ana/. Bioanal. Chem., 406 49-61. 

Some solid materials are very intractable to analysis by standard methods and cannot be easily vaporized or dissolved in common solvents. Glass, bone, dried paint, and archaeological samples are common examples. These materials would now be examined by laser ablation, a technique that produces an aerosol of particulate matter. The laser can be used in its defocused mode for surface profiling or in its focused mode for depth profiling. Interestingly, lasers can be used to vaporize even thermally labile materials through use of the matrix-assisted laser desorption ionization (MALDI) method variant. 

Two relatively new techniques, matrix assisted laser desorption ionization-lime of flight mass spectrometry (MALDI-TOF) and electrospray ionization (FS1), offer new possibilities for analysis of polymers with molecular weights in the tens of thousands. PS molecular weights as high as 1.5 million have been determined by MALDI-TOF. Recent reviews on the application of these techniques to synthetic polymers include those by Ilantoif54 and Nielen.555 The methods have been much used to provide evidence for initiation and termination mechanisms in various forms of living and controlled radical polymerization.550 Some examples of the application of MALDI-TOF and ESI in end group determination are provided in Table 3.12. The table is not intended to be a comprehensive survey. 

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... 

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