Desorption electrospray ionization principles

A new ionization method called desorption electrospray ionization (DESI) was described by Cooks and his co-workers in 2004 [86]. This direct probe exposure method based on ESI can be used on samples under ambient conditions with no preparation. The principle is illustrated in Figure 1.36. An ionized stream of solvent that is produced by an ESI source is sprayed on the surface of the analysed sample. The exact mechanism is not yet established, but it seems that the charged droplets and ions of solvent desorb and extract some sample material and bounce to the inlet capillary of an atmospheric pressure interface of a mass spectrometer. The fact is that samples of peptides or proteins produce multiply charged ions, strongly suggesting dissolution of the analyte in the charged droplet. Furthermore, the solution that is sprayed can be selected to optimize the signal or selectively to ionize particular compounds. 

Techniques for the Ionization of Molecules The measurability of molecules by MSI is enabled through the local desorption and ionization of the molecules from a surface. In theory, all types of molecules that can undergo these two chemical processes can be measured. Many techniques have been developed or adapted to achieve desorption and ionization of molecules from surfaces, but three different desorption/ioniza-tion techniques made their way to commercially available products. The acronyms of these technologies are desorption electrospray ionization (DESI), MALDI, and secondary ion mass spectrometry (SIMS). The principles of these three methods and a comparison of their possibilities and limitations are outlined throughout this section and summarized in Figure 1 and Table 2, respectively. 

Animation on the principle of desorption electrospray ionization (DESl) ion source ... 

In Chapter 4, titled Principles of Mass Spectrometry Imaging Applicable to Thin-Layer Chromatography, the authors first introduce a budding analytical approach known as imaging mass spectrometry (IMS) strategy and then present some successful examples of its practical applications. Then, they introduce in detail three mass spectrometric techniques as those routinely used within the framework of IMS. These are secondary mass spectrometry (SIMS), matrix-assisted laser desorption/ ionization (MALDI-IMS), and desorption electrospray ionization (DESI). Finally, the authors discuss the advances and bottlenecks of these techniques when applied to TLC. 

Several modifications of the ESI principle were described, such as the desorption electrospray ionization (DESI), the desorption atmospheric pressure photoionization (DAPPI), the electrospray-assisted pyrolysis ionization (ESPI), the ambient sonic spray ionization (SPI), " the electrosonic spray ionization (ESSI), but also combined MALDI/ESI techniques, such as the matrix-assisted laser desorption electrospray ionization (MALDESI). ... 

Figure 14.1 Schematic view of a mass spectrometer. Its basic parts are ion source, mass analyzer, and detector. Selected principles realized in modern mass spectrometers are assigned El—electron impact. Cl—chemical ionization, FAB—fast atom bombardment, ESI—electrospray ionization, MALDI—matrix-assisted laser desorption/ionization. Different combinations of ion formation with mass separation can be realized.

Three popular ionization techniques are electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) and matrix-assisted laser desorption (MALDI). Electrospray is the most widely used ionization technique when performing LC-MS, and has proved to be a most versatile tool for soft ionization [72] of a large variety of analytes such as them described in paper I. Figure 6 shows the principle of the ESI. 

The introduction of Matrix-Assisted Laser Desorption/Ionization (MALDI) and Electrospray Ionization (ESI) (Chapter 1) has dramatically increased the mass range for molar mass analyses by mass spectrometry. In principle, both techniques are able to produce intact quasi-molecular ions of polymers with high molar mass (>100,000 Da). 

Suppression of ionization efficiency is important when the total ionizing capability of the ionization technique is limited, so that there is a competition for ionization among compounds that are present in the ion source simultaneously. In principle such a saturation effect must be operative for all ionization techniques, but in practice it is most important for electrospray ionization (Section 5.3.6), slightly less important for atmospheric pressure chemical ionization (Section 5.3.4), atmospheric pressure photoionization (Section 5.3.5) and matrix assisted laser desorption ionization (Section 5.2.2) it does not appear to be problematic under commonly used conditions for electron ionization and chemical ionization (Section 5.2.1) or thermospray (Section 5.3.2). Enhancement of ionization efficiency for an analyte by a co-eluting compound is less commonly observed and is, in general, not well understood. 

Fast atom bombardment, liquid-SIMS (secondary ion mass spectrometry), electrospray (ESI), and matrix assisted laser desorption (MALDI) ionization modes have been applied successfully for the investigations of biomolecules.However, ESI and MALDI are the two most frequently adopted techniques for investigations of biopolymersDetails involving the principles and application of all of these techniques can be found elsewhere. The samples may be introduced either directly or after liquid chromatographic separation. All of the above techniques, with the exception of MALDI, have been adopted for the LC/MS experiments. " Although most of the reported LC/MS investigations involved the electrospray ionization of the molecules, continuous flow-FAB ionization techniques have also been found useful. 

---