Desorption electrospray ionization technique

Earlier methods of ionization applied to carotenoids, including electron impact (El), chemical ionization (Cl), a particle beam interface with El or Cl, and continuous-flow fast atom bombardment (CF-FAB), have been comprehensively reviewed elsewhere (van Breemen, 1996, 1997 Pajkovic and van Breemen, 2005). These techniques have generally been replaced by softer ionization techniques like electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI), and more recently atmospheric pressure photoionization (APPI). It should be noted that ESI, APCI, and APPI can be used as ionization methods with a direct infusion of an analyte in solution (i.e. not interfaced with an HPLC system), or as the interface between the HPEC and the MS. In contrast, matrix-assisted laser desorption ionization (MALDI) cannot be used directly with HPEC. 

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. 

Several innovative ion sources that are based on ESI were developed, including secondary electrospray ionization (SESI), desorption electrospray ionization (DESl) (Figure 4.4), and nanoelectrospray ionization (nESI). These were described in some detail in Chapter 3 in the context of sample introduction techniques and are only briefly discussed here. 

El and Cl methods can be used if the compound to be studied is sufficiently volatile and stable to be vaporized intact. However, only 20% of the organics found in surface water are volatile enough to be amenable to GC-EI-MS or GC-CI-MS. Today, there are a variety of other ionization techniques available electrospray ionization (ESI), atmospheric pressure chemical ionization, matrix-assisted laser desorption ionization, and fast atom bombardment. Each of these has its advantages and disadvantages. A simple guideline to the most likely optimum ionization technique for a given class of substance is given in Table 1. 

Matrix-assisted laser desorption/ionization (MALDI) is one of the two soft ionization techniques besides electrospray ionization (ESI) which allow for the sensitive detection of large, nonvolatile and labile molecules by mass spectrometry. Over the past 27 years, MALDI has developed into an indispensable tool in analytical chemistry, and in analytical biochemistry in particular. In this chapter, the reader will be introduced to the technology as it stands now, and some of the underlying physical and chemical mechanisms as far as they have been investigated and clarified to date will be discussed. 

The mass spectrometry (MS) of nudeic acids (NAs) has a history similar to that of proteins and other biomolecules. Although earlier work had been made possible by field desorption and fast atom bombardment (FAB) and other ionization techniques, the field did not really take off until the soft ionization techniques of electrospray ionization (ESI) [1] and MALDI [2] became available. Even with the benefits of these new ionization techniques, however, the analysis of NAs turned out to be substantially more difficult than that of proteins and, as a result, MS-based technologies are not nearly as prominent in genetics and genomics as they are in proteomics. The widespread utilization of MS for NA analysis has been further limited by the competition of powerful techniques that can rely on amplification, hybridization, and fluorescence detection. MALDI-MS has found its place in these fields only more recently, and its performance must consistently be measured against that of the competing techniques in any given analytical task. 

Chapter 6, titled Selection of Ionization Methods of Analytes in the TLC-MS Techniques provides an overview of mass spectrometric techniques that can be coupled with TLC and act as specific detectors in this hyphenated approach. The mass spectrometric techniques discussed in this chapter are secondary mass spectrometry (SIMS), liquid secondary ion mass spectrometry (LSIMS), fast atom bombardment (FAB), matrix-assisted laser desorption/ionization (MALDI), atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDI), electrospray ionization (ESI), desorption electrospray ionization (DESI), electrospry-assisted laser desorption/ionization (ELDI), easy ambient sonic spray ionization (EASI), direct analysis in real time (DART), laser-induced acoustic desorption/electrospray ionization (LIAD/ESI), plasma-assisted multiwavelength laser desorption/ionization (PAMLDI), atmospheric-pressure chemical ionization (APCI), and dielectric barrier discharge ionization (DBDI). For the sake of illustration, the authors introduce practical examples of implementing TLC separations with detection carried out by means of individual mass spectrometric techniques for the systematically arranged compounds belonging to different chemical classes. 

In recent years, TLC was successfully combined with different ionization techniques, matrix-assisted laser desorption/ionization (MALDI), ESI, atmospheric pressure chemical ionization (APCI), desorption electrospray ionization (DESI), electrospray-assisted laser desorption ionization (ELDI), and LDI for identification and quantification of organic and biomolecules. In this section, the interfacing of TLC techniques with MALDI-ESI/MS, DESI-MS, ELSI-MS, and LDI-MS will be described, performance will be discussed, and selected applications in the separation and identification of lipids, gangliosides, dyes, drugs, and medicinal compounds will be presented. 

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. 

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]. 

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. 

Over the years, a lot of desorption ionization techniques have been introduced to MS, such as plasma desorption, field desorption, laser desorption, secondary ion mass spectrometry, fast atom bombardment, matrix assisted laser desorption and desorption electrospray ionization. Most of them are actually no longer used. In the following paragraphs, both matrix assisted laser desorption (MALDI) and desorption electrospray ionization (DESI) will be discussed. 

A new family of ionization techniques allows ions to be created under ambient conditions and then collected and analyzed by MS. They can be divided into two major classes desorption electrospray ionization (DESI) and direct analysis in real time (DART). 

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... 

Mass spectrometry is also used for nanoparticles investigations. Two ionization techniques often used with liquid and solid biological samples include electrospray ionization (ESI) and matrix-assisted laser desorption/ioiuzation (MALDI). Inductively coupled plasma (ICP) sources are mainly used for metal analysis. In general,... 

Electrospray (ESI) is an atmospheric pressure ionization source in which the sample is ionized at an ambient pressure and then transferred into the MS. It was first developed by John Fenn in the late 1980s [1] and rapidly became one of the most widely used ionization techniques in mass spectrometry due to its high sensitivity and versatility. It is a soft ionization technique for analytes present in solution therefore, it can easily be coupled with separation methods such as LC and capillary electrophoresis (CE). The development of ESI has a wide field of applications, from small polar molecules to high molecular weight compounds such as protein and nucleotides. In 2002, the Nobel Prize was awarded to John Fenn following his studies on electrospray, for the development of soft desorption ionization methods for mass spectrometric analyses of biological macromolecules. ... 

A mass-spectrometric screening for molecular ions using electrospray ionization techniques or matrix-assisted laser desorption/ionization time-of-... 

There are many different ionization techniques available to produce charged molecules in the gas phase, ranging from simple electron (impact) ionization (El) and chemical ionization (Cl) to a variety of desorption ionization techniques with acronyms such as fast atom bombardment (FAB), plasma desorption (PD), electrospray (ES), and matrix-assisted laser desorption ionization (MALDI) (Mano and Goto 2003). 

In 1974, Comarisov and Marshall60 developed Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). This technique allows mass spectrometric measurements at ultrahigh mass resolution (R = 100000-1000000), which is higher than that of any other type of mass spectrometer and has the highest mass accuracy at attomole detection limits. FTICR-MS is applied today together with soft ionization techniques, such as nano ESI (electrospray ionization) or MALDI (matrix assisted laser/desorption ionization) sources. 

Two recently developed mass spectrometric techniques have had a major impact on the analysis of large biomolecules matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) and electrospray ionization mass spectrometry (ESI-MS). MALDI-MS was first introduced by Karas and Hillenkamp66 and Tanaka et al.61 in 1988 and has experienced an exponential development. It has become a widespread soft ionization technique for bioorganic samples, especially large biomolecules. Fenn and co-workers68 first published the successful soft ionization technique for... 

Due to the increasing significance of soft ionization techniques for the analysis of phosphorus-, metal- and metalloid-containing biomolecules, matrix-assisted laser-induced desorption/ionization (MALDI) and electrospray ion sources (ESI) will be discussed briefly at the end of this chapter. 

FAB and PD have been replaced by electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) in the analytical mass spectrometry laboratory, because both of these newer techniques have a wider mass range of analysis and have lower detection limits. ESI and MALDI have become invaluable ionization techniques for nonvolatile components. This is particularly true for a wide range of biological molecules including proteins, peptides, nucleic acids, etc. Samples can be analyzed by ESI using either direct injection or introduction through liquid chromatography. 

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). 

The use and development of high-resolving separation techniques as well as highly accurate mass spectrometers is nowadays essential to solve the proteome complexity. Currently, more than a single electrophoretic or chromatographic step is used to separate the thousands of proteins found in a biological sample. This separation step is followed by analysis of the isolated proteins (or peptides) by mass spectrometry (MS) via the so-called soft ionization techniques, such as electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) combined with the everyday more powerful mass spectrometers. Two fundamental analytical strategies can be employed the bottom-up and the top-down approach. 

Mass spectrometry must be used with caution, as the weakness of the E-M bonding framework in some cases leads to fragmentation of the parent ion, which is thus not observed. For the smaller, more volatile molecules, conventional electron-impact techniques can be valuable,42 but softer ionization techniques may be required for these as well. Chemical ionization (Cl) has been successfully employed for the characterization of Fe3 (CO)10(PR).192 Electrospray techniques have proven useful for characterizing even nonvolatile, ionic complexes.231 Positive ion-laser desorption methods were employed successfully to characterize (CO)4W PW(CO)5 4.461... 

Any MS experiment begins with ionization of molecules of analyte. Numerous ionization techniques (electron ionization, fast atom bombardment, plasma desorption, electrospray ionization, etc.) allow MS analysis of a wide range of organic molecules. In most cases the characterization of combinatorial libraries means analysis of crude compounds i.e. one can expect not only the intended compound to be present in the analyte, but also products of side... 

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