Desorption Electrospray Ionization DESI

A series of technologies has been developed during the last 5 years (with an alphabet soup of acronyms) for the identification of compounds adsorbed on surfaces. The most common methods are desorption electrospray ionization (DESI) (and related techniques) and direct analysis in real time (DART). Applications range from the identification of explosives and drugs in forensics to biological arenas, such as the characterization of tissue sections. 

Charged solvent droplets are directed at a surface, and protonated analyte, [M + H]+, ions are desorbed. 

FIGURE 2.17 Desorption electrospray ionization (DESI) sonrce. 

The DESI mass spectra of biopolymers include multiply charged ions and resemble those obtained in ESI. Ionization efficiency depends on several experimental parameters related to surface effects, the chemical composition and flow rate of the spraying solvent, the applied voltage, and importantly, the angle of the spray. 

Several other ionization methods have been developed based on DESI, including desorption atmospheric pressure chemical ionization (DAPCI), desorption atmospheric pressure photo-ionization (DAPPI), laser ablation electrospray ionization (LAESI), and extractive electrospray ionization (EESI). Each technique uses variations of the solvent, how the charged beam is formed, and how the beam is nsed to facilitate the prodnction of analyte ions. Because these are surface methods (except EESI), they are incompatible with LC. 

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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. 

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... 

Z. Takats, J. M. Wiseman and R. G. Cooks, Ambient mass spectrometry using desorption electrospray ionization (DESI) instmmentation, mechanisms and applications in forensics, chemistry, and biology, J. Mass Spectrom., 40, 1261 1275 (2005). 

J. M. Wiseman, D. R. Ifa, Q. Song and R. G. Cooks, Tissue imaging at atmospheric pressure using Desorption Electrospray Ionization (DESI) mass spectrometry, Angew. Chem. Int. Edn, 45, 7188 7192 (2006). 

Desorption electrospray ionization DESI Spray Nonvolatile molecular ions Direct, preparation-free analysis of samples... 

Direct analysis of solid samples or analytes present on solid surfaces without any sample preparation has always been a topic of interest. Desorption electrospray ionization (DESI) is an atmospheric pressure desorption ionization method introduced by Cooks et al., producing ions directly from the surface to be analyzed, which are then sampled with the mass spectrometer [22, 37]. DESI is based on charged liquid droplets that are directed by a high velocity gas jet (in the order of 300 m s ) to the surface to be analyzed. Analytes are desorbed from the surface and analyzed by mass spectrometer (Eig. 1.15). 

R. G. Ambient mass spectrometry using desorption electrospray ionization (DESI) instrumentation, mechanisms and applications in forensics, chemistry, and biology. 

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 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. 

Ifa D, Wiseman J, Song Q, Cooks G (2007) Development of capabilities for imaging mass spectrometry under ambient conditions with desorption electrospray ionization (DESI). Int J Mass Spectrom 259 8-15. doi 10.106/j.ijms.2006.08.003... 

Desorption electrospray ionization (DESI) uses an aqueous spray directed at an analyte deposited on an insulating surface (Fig. 8) [14,15], The sample is in the solid form at atmospheric pressure. The spray of charged droplets is produced, as in electrospray ionization (ESI), by passing an aqueous solution (i.e., methanol-water, containing some... 

Fig. 8. Schematic diagram of the desorption electrospray ionization (DESI) source. Reproduced with permission from Cotte-Rodriguez et al. [14]. Copyright 2005 American Chemical Society.

Fig. 11. Positive-ion desorption electrospray ionization (DESI) mass spectrum of (a) 10 ng of RDX and (b) 500pg of RDX. Reproduced with permission from Mulligan et al. [23].

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. 

MALDI), and, more recently, Atmospheric Pressure Photo Ionization (APPI) and Desorption Electrospray Ionization (DESI). For the majority of analytical tasks in hpidomics, ESI and nano-ESI are the most common choices. 

Recently, a new family of ionization techniques that are distinguished by their ability to ionize analytes from surfaces under ambient conditions have been developed.61 These methods are also characterized by the fact that no prior separation or extraction of the sample is required. Of these methods two have so far been well characterized, desorption electrospray ionization (DESI)62 and direct analysis in real time (DART).63... 

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. 

The development of ambient MS was initiated with the introduction of desorption electrospray ionization (DESI) by Cooks in 2004 (35). Since then, a variety of possibilities combining different desorption and ionization... 

Seemingly, the next frontier for fast bioanalysis is the removal of the column altogether. Techniques such as direct analysis in real time (DART) and desorption electrospray ionization (DESI) have shown great potential. These techniques are discussed in more detail in Chapter 13. 

One of the most significant developments in mass spectrometry in the recent years is the introduction of a new class of ionization methods where samples in either solid or liquid state can be directly ionized in their native environment under ambient conditions (rather than inside a mass spectrometer) without any sample preparation. This new class of ionization methods is often referred to as ambient ionization methods [1,2], Because these methods generally ionize analytes on the surface or near the surface of the samples at atmospheric pressure, they have also been called atmospheric pressure surface sampling/ionization methods or direct/open air ionization methods [3], Since the first reports on ambient ionization with desorption electrospray ionization (DESI) [4] and direct analysis in real time (DART) [5], numerous reports have been published on the applications of these new ionization methods as well as the introduction of many related ambient ionization methods such as desorption atmospheric pressure chemical ionization (DAPCI) [6], atmospheric solid analysis probe (ASAP) [7], and electrospray-assisted laser desorption/ionization (ELDI) [8], Recently, two reviews of the various established and emerging ambient ionization methods have been published [2,3],... 

A new generation of mass spectrometer inlets allow for direct sampling of a substrate under ambient conditions. Theoretically, this eliminates the need for any sample preparation. Examples include direct analysis in real time (DART) and desorption electrospray ionization (DESI), as well as desorption atmospheric-pressure chemical ionization (DAPCI) and atmospheric solids analysis probe (ASAP). These techniques utilize a source of energy interacting directly with a sample surface at ambient pressure, causing molecules of interest to desorb, ionize, and be sampled by a mass spectrometer. 

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