Desorption ESI

Desorption ESI (DESI) was introduced by Takatz et al. [102]. The phenomenon actually was observed earlier but was discarded as a nuisance (e.g., an analyte or calibration mixture that coated the entrance of the transfer capillary and contributed to undesired peaks in the spectra). The idea of using the electrospray for desorption is as clever as it is simple. The method is sensitive and large species such as proteins can be detected. The ions observed are more or less the same as with regular ESI. 

Because the high voltages required for ESI and plasma-APCI are independently applied and controlled, the dual ion source can be operated in ESl-only, APCI-only, or ESI + APCI modes to characterize polar, nonpolar, or both polar and nonpolar compounds, respectively. Laser systems, inclnding a CW laser and a pulsed laser, have been combined with the dual-ion sonrce to characterize chemical compounds on surfaces, where the desorbed analytes are introduced into the ESI and/or APCI plumes for postionization. In addition, analytes on sample snrfaces can also be directly desorbed and ionized by directing the ESI -H APCI plume toward the sampling area, an approach also known as desorption ESI -t APCI [47]. Chemical compounds including PAHs, peptides, drugs, diesel oils, and essential oils on stainless-steel and paper surfaces have been characterized by the dual ionization source. 

Common soft ionization techniques include ElectroSpray Ionization (ESI, outlined in Section 2.8.3), Desorption ESI (DESl, for solids), the closely related Matrix-Assisted Laser Desorption Ionization (MALDI), and Direct Analysis in Real Time (DART). Note that ESI-MS even allows structural information relevant to solution phases to be obtained [5]. 

Two new ionization methods to appear recently are desorption ESI (DESI) and direct analysis in real time (DART). They are the first in a new area of ionization mechanisms that are collectively referred to as open-air ionization. Both DART and DESI form ions in an open atmosphere and sample the resulting plume through an entrance cone into the mass spectrometer. 

Although this chapter covers certain aspects of the ESI and MALDI sources, earlier chapters of this book are recommended for details about these ion sources and especially for coverage of the new development of desorption ESI (DESI) by Cooks and co-workers in 2004. 

Electron impact (El), or Efectrospray ionization (ESI), or Matrix-assisted laser desorption ionization (MALDI)... 

Most biochemical analyses by MS use either electrospray ionization (ESI) or matrix-assisted laser desorption ionization (MALD1), typically linked to a time-of-flight (TOF) mass analyzer. Both ESI and MALDl are "soft" ionization methods that produce charged molecules with little fragmentation, even with biological samples of very high molecular weight. 

Matrix-assisted laser desorption mass spectrometry (MALDI-MS) is, after electrospray ionization (ESI), the second most commonly used method for ionization of biomolecules in mass spectrometry. Samples are mixed with a UV-absorbing matrix substance and are air-dried on a metal target. Ionization and desorption of intact molecular ions are performed using a UV laser pulse. 

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 chemical compositions of the isolated Au SR clusters were investigated by mass spectrometry [15,16,18, 22,32-35]. TEM was used to confirm that the species detected by the mass spectrometer represents the clusters in the sample. Figure 3a is a schematic representation of the top view of the mass spectrometer, which consists of five stages of differentially pumped vacuum chambers. The apparatus accommodates two t5 pes of ion sources, electrospray ionization (ESI) and laser-desorption ionization (EDI), and a time-of-flight (TOE) mass spectrometer with a reflectron. Details of the apparatus and the measurement protocols are described below. 

Fast atom bombardment (FAB) Plasma desorption (PD) Liquid secondary-ion mass spectrometry (LSIMS) Thermospray (TSP)/plasmaspray (PSP) Electrohydrodynamic ionisation (EHI) Multiphoton ionisation (MPI) Atmospheric pressure chemical ionisation (APCI) Electrospray ionisation (ESI) Ion spray (ISP) Matrix-assisted laser desorption/ionisation (MALDI) Atmospheric pressure photoionisation (APPI) Triple quadrupole (QQQ) Four sector (EBEB) Hybrid (EBQQ) Hybrid (EB-ToF, Q-ToF) Tandem ToF-ToF Photomultiplier... 

In direct insertion techniques, reproducibility is the main obstacle in developing a reliable analytical technique. One of the many variables to take into account is sample shape. A compact sample with minimal surface area is ideal [64]. Direct mass-spectrometric characterisation in the direct insertion probe is not very quantitative, and, even under optimised conditions, mass discrimination in the analysis of polydisperse polymers and specific oligomer discrimination may occur. For nonvolatile additives that do not evaporate up to 350 °C, direct quantitative analysis by thermal desorption is not possible (e.g. Hostanox 03, MW 794). Good quantitation is also prevented by contamination of the ion source by pyrolysis products of the polymeric matrix. For polymer-based calibration standards, the homogeneity of the samples is of great importance. Hyphenated techniques such as LC-ESI-ToFMS and LC-MALDI-ToFMS have been developed for polymer analyses in which the reliable quantitative features of LC are combined with the identification power and structure analysis of MS. 

The introduction and eventual commercialization of matrix-assisted laser desorption/ionization (MALDI) and electrospray (ESI) allowed biomarker status to be extended to proteins in 1996.15"17 With a few exceptions, ESI has been used in conjunction with extractions and high-pressure liquid chromatography (UPLC) interfaced with mass spectrometry. MALDI, on the other hand, has been widely adapted for rapid analysis of intact organisms, supported by bioinformatics.1819... 

It is therefore not surprising that the interest in PyMS as a typing tool diminished at the turn of the twenty-first century and hence why taxonomists have turned to MS-based methods that use soft ionization methods such as electrospray ionization (ESI-MS) and matrix-assisted laser desorption ionization (MALDI MS). These methods generate information-rich spectra of metabolites and proteins, and because the molecular ion is seen, the potential for biomarker discovery is being realized. The analyses of ESI-MS and MALDI-MS data will still need chemometric methods, and it is hoped that researchers in these areas can look back and learn from the many PyMS studies where machine learning was absolutely necessary to turn the complex pyrolysis MS data into knowledge of bacterial identities. 

The growing interest for the identification and characterization of polar and large compounds caused the development and the introduction of new ionization techniques, such as electrospray ionization (ESI)[4], and matrix assisted laser desorption ionization (MALDI),[5] and their more recent improvements, thus establishing new MS based approaches for studying large molecules, polymers and biopolymers, such as proteins, carbohydrates, nucleic acids. 

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...

The product of the desorption process is the formation of singly charged (protonated or deprotonated) molecules of the analyte, with dimensions ranging from a few hundred daltons to several hundred thousand daltons. It follows that, contrary to ESI, in the study of... 

Desorption Electrospray Ionization (DESI). DESI is a novel gentle ionization method for surface analysis (Figure 2.6).[19,20] Like classical ESI, it operates at atmospheric pressure. No sample preparation is required. A solvent passes through the capillary of the electrospray source charged droplets are produced (primary ions) and they are directed to a solid sample. Their impact with the surface causes sample molecules to be ionized and... 

---