Secondary electrospray ionization

M. Tam and H.H. HiU Jr., Secondary electrospray ionization-ion mobility spectrometry for explosive vapor detection. Analytical Chemistry 76(10) (2004) 2741—2747. 

FIGURE 3.7 Graphical representation of CGC retention time (in minutes), mobility drift time (in milliseconds), and total ion intensity (in arbitrary nnits) of lavender oil. The highlighted peaks (in white boxes) show separation of analyte peaks by GC only, IMS only, and both CGC and IMS this dnal CGC and IMS techniqne enables a greater degree of separation for a complex mixture than with either technique alone. (From Crawford et al.. The novel use of gas chromatography-ion mobility-time of flight mass spectrometry with secondary electrospray ionization for complex mixtnre analysis, Int. J. Ion Mobil. Spectrom. 2010, 14, 23-30. With permission.)... 

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. 

IMS for illicit drug analysis has been known since the 1970s (see Chapter 14), but expansion to pharmaceuticals developed slowly and is still only in its infancy of being accepted by the pharmaceutical industry. For volatile or semivolatile pharmaceutical compounds, Ni or secondary electrospray ionization (SESI) sources may be used, but the availability of electrospray ionization (ESI) IMS has expanded the number of applications possible for the pharmacy industry. While this process of evaluating the response and measuring the mobilities of individual pharmaceutical standards is ongoing, applications for real-world analyses in the pharmaceutical industry are being developed. 

Wu, C. Siems, W.F. Hill, H.H. Secondary electrospray ionization ion mobility spec-trometry/mass spectrometry of illicit drugs. Anal. Chem. 2000, 72, 396 03. 

LC-MS Liquid chromatography-mass SESl Secondary electrospray ionization... 

Figure 10.16 IMS spectra of 100 ng of (a) RDX, (b) NC, and (c) PETN. Spectra were obtained at 200°C with methylene chloride as dopant Reprinted with permission from Tam, M., and H. H. Hill, "Secondary Electrospray Ionization-Ion Mobility Spectrometry for Explosive Vapor Detection," Analytical Chemistry 76 (2004), 2741-2747. Copyright 2004 American Chemical Society. 

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. 

The ionization methods reported for IMS included MALDI [41,76-80], Secondary Ion Mass Spectrometry (SIMS) [19, 81-86], Matrix-enhanced (ME)-SIMS [87, 88], Desorption Electrospray Ionization (DESI) [89-99], Nanostructure Initiator Mass Spectrometry (NIMS) [100-102], Atmospheric Pressure Infrared MALDI Mass Spectrometry (AP-IR-MALDI-MS) [103], Laser Ablation-inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) [104-106], Laser Desorption Postionization (LDPI) [107], Laser Ablation Electrospray Ionization Mass Spectrometry (LAESI) [108, 109], and Surface-assisted Laser Desorption/ioniza-tion Mass Spectrometry (SALDI) [110-112], Another method was called probe electrospray ionization (PESI) that was used for both liquid solution and the direct sampling on wet samples. 

Methods such as static secondary ion mass spectrometry and desorption electrospray ionization are cited in recent technical literature for detection of chemical agent. These and other recent advances in analytical techniques are reported in NRC, 2005. 

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. 

Recent advances in protein analysis by MS are due to the introduction of electrospray ionization (ESI), matrix-assisted laser desorption ionization (MALDI), MSN scan modes, as well as improvements in instrument sensitivity, resolution, and mass accuracy. With these improved techniques, researchers will continue to use MS to help elucidate primary, secondary, and to a lesser extent, tertiary structure of proteins. 

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