DESI ion source

Figure 2.23. Schematic of a prototype DESI ion source A, surface holder block B, sprayer C, three-dimensional moving stage for surface assembly D, three-dimensional moving stage for sprayer alignment E, rotating stage for sprayer F, mass spectrometer inlet. (Reproduced from ref. 91 by permission of Wiley-Interscience, copyright 2005.)...

Mass spectrometer equipped with desorption electrospray (DESI) ion source is capable of detection and identification of different substances from the surface. Moreover, the information about spatial distribution of those substances is retained. Connection of this technique with TLC not only allows for the measurement of retention times for separated chemicals (spatial distribution), but also for their unambiguous identification, based on the molecular weight of certain substances and their fragmentation spectra. Additionally, because DESI works under ambient conditions, there is no need to apply a high vacuum system for the sample introduction. Moreover, samples analyzed by DESI practically do not require any kind of preparation (e.g., covering with matrix prior to MALDI analysis), thus connection of those two techniques is relatively easy. Certainly, not all the substances, due to their chemical features, may be detected with this technique. Only compounds, which are able to ionize in this type of ion source, may be analyzed. 

In the case of combining DESI ion source with TLC separation technique for its analysis, it is advisable to use good quality TLC plates. During the ionization process, the stationary phase might be detached from the plate surface and, in extreme cases, may clog the MS inlet. TLC plates with aluminum support seem to be more prone to such process than are those with glass support. 

Like DART, DESI has received widespread acceptance as evidenced by more than 750 papers and conference presentations till mid-2014, referring to the technique since its introduction in 2004 by Cooks and coworkers [195, 196]. The technique makes use of electrospray ionization (ESI) that is widely used in the mass spec-trometiy of larger molecules in which a solution is nebulized to create a fine spray of very small droplets. In DESI, a standard electrospray of charged droplets hits the surface where the molecules of interest are present or adsorbed (including larger biomolecules), detaches them from the siuface, and delivers them as desolvated ions in the mass spectrometer. DESI is thus similar to DART (Sect. 8.4) where the gaseous plasma of ions from the ion source desorbs molecules from a surface. A schematic diagram of the main aspects of a DESI ion source is shown in Fig. 8.9. 

Fig. 8.9 DESI ion source showing an electrospray of charged droplets impacting the surface where the molecules of interest are present. Once detached, they enter the ion source shown of the mass spectrometer. (Used with permission of Prosolia, Inc., Indianapolis, IN, USA)...

Fig. 13.4. A commercial DESI ion source. This one here is the ProsoUa Omni Spray 2D source coupled to a Thermo Fisher LTQ mass spectrometer. The surface shown is a 96-spot Omni Slide HC having the standard microtiter plate dimensions. Note the extended desolvation capillary and the adjustable sprayer. Courtesy ProsoUa, Inc., IndianapoUs, In USA.

Two new independently developed techniques called Dart ° (direct analysis in real time) and Desi (desorption electrospray ionisation) are making a huge impact on mass spectrometry. Together they remove the need for sample preparation and vacuum, speed up analysis time and can work in the open air. The sample is held in a gas or liquid stream at room temperature and the impact induces the surface desorption of ions. The ions then continue into the vacuum interface of the MS for analysis. Samples can be hard, soft or even liquid in nature. Ifa et al. have used Desi to image biological samples in two dimensions, recording images of tissue sections and the relative concentrations of molecules therein. Jeol have launched a commercial Dart ion source for non-contact analysis of materials in open air under ambient conditions. 

A somewhat similar but mechanistically different ion source from DART is DESI. DESI ionization is accomplished by directing electrosprayed charged droplets and ions of the solvent onto a surface of the sample. Eigure 3.15 provides a schanatic of the DESI process. 

The IMS had a resolving power of about 25 in this design. One primary advantage of MALDI over DART and DESI is that the last two ion sources are continuous, and... 

FIGURE 4.4 A schematic of desorption electrospray ionization (DESI). The eleetrosprayed droplets from an ESI source impinge on the surface of the sample, and the ions desorbed from the surface are analyzed by MS or IMS. (From Wikipedia, http //en.wikipedia.org/wiki/ File DESl ion source.jpg)... 

The preparation of samples for mass spectrometric analysis almost always requires recovery of the analyte from a matrix, often followed by further preparative steps prior to introduction of the sample into the ion source. Such destructive processes are unacceptable when the objective is to determine the specific location of a particular compound within a biological sample, e.g., the location of a certain lipid in a tissue, and led to the development of imaging mass spectrometry with MALDI as the ionization method, although DESI is also applicable. 

Schematics of a DESI experiment and a prototype source are shown in Figures 2.22 and 2.23, respectively. The ion source consists of a sprayer assembly and a surface assembly. The former is a pneumatically assisted microelectrospray source that is a part of a vertical rotating stage and a three-dimensional linear moving stage. The surface assembly is mounted on a separate three-dimensional moving stage. For high-throughput applications, the surface assembly can be replaced by a moving belt system.

Rgure9.15 Prosolia DESI Omni Spray ion source geometry and operation diagram. The family of different (Manual, 1D and 2D) ion sources can be used on MS equipment from Agilent, Thermo, Waters, ABSciex, Bmker, and LEGO. (Used with permission from Prosolia, Inc. www.prosolia.com.)... 

DESI), Venturi easy ambient spray ionization (V-EASI), electron ionization (El), and photoionization. These canonical ion sources were adapted for specific purposes in TRMS. Much emphasis was put on mixing reactants, exposing reaction mixtures to fight (in the case of photochemical processes) or electric potentials (in the case of electrochemical processes) (cf. [10]). However, there also exist examples of more exotic systems constracted to enable measurements in the specific niche applications (see, e.g., [11]). Implementation of various interfaces in TRMS systems will be summarized in the following sections. Examples of their applications in different areas of chemistry and biochemistry will be discussed in Chapters 10-13. 

Ionization techniques derived from ESI, such as electrosonic spray ionization (ESSI), are also suitable for studies of fragile molecules. The ESSI-MS approach enables observation of non-covalent complexes of myoglobin, protein kinase A/ATP complex, and other proteins [47]. It can be used to study on-line deprotonation reactions on peptides and proteins [48,49]. Such analyses can be done by introducing volatile bases between the ion source and mass analyzer. This method is fast one reference base can be scanned in a time interval of 1 min. The desorption electrospray ionization (DESI) technique was also implemented in the study of protein conformation in solution [50]. The interaction time between the spray solvent and the protein was estimated to be 1 ms, and it was suggested that this timescale would be too short for the studied proteins to unfold [50]. 

In terms of the hardware, TRMS methods described in this book use most common types of ion sources and analyzers. Electrospray ionization (ESI), electron ionization (El), atmospheric pressure chemical ionization (APCI), or photoionization systems, and their modified versions, are all widely used in TRMS measurements. The newly developed atmospheric pressure ionization schemes such as desorption electrospray ionization (DESI) and Venturi easy ambient sonic-spray ionization (V-EASI) have already found applications in this area. Mass analyzers constitute the biggest and the most costly part of MS hardware. Few laboratories can afford purchasing different types of mass spectrometers for use in diverse applications. Therefore, the choice of mass spectrometer for TRMS is not always dictated by the optimum specifications of the instrument but its availability. Fortunately, many real-time measurements can be conducted using different mass analyzers equipped with atmospheric pressure inlets - with better or worse results. For example, triple quadrupole mass spectrometers excel at quantitative capabilities however, in many cases, popular ion trap (IT)-MS instruments can be used instead. On the other hand, applications of TRMS in fundamental studies often require a particular type of instrument (e.g., Fourier transform ion cyclotron resonance mass spectrometer for photodissociation studies on trapped ions). 

DESI, DART, APGD and ELDI, and most elution-based approaches do not require any plate pretreatment. With these ion sources, MS spectra were obtained directly from the plate within seconds. The spectra provide mostly the de-/protonated molecule ion and/or its adducts, making interpretation easy. This is a clear advantage over FAB, MALDI, SALDI, or SIMS, where a strict plate pretreatment protocol over several steps is an obligation, and the interpretation of the mass spectra, especially for MALDI or SIMS, is more challenging. 

Currently, there are at least three ionization techniques routinely used in IMS (1) secondary ion mass spectrometry (SIMS), (2) matrix-assisted laser desorption ionization (MALDI), and (3) desorption electrospray ionization (DESI). Each of these ion sources has their own advantages and drawbacks thus, they are usually selected in the context of the imaged sample. In the following text, we provide a concise description of these devices. 

In the case of typical TLC plates, DESI seems to be a very useful technique of visualization due to its ambient working conditions. The limited spatial resolution of this source is not important for a majority of the TLC plates, including high-performance TLC (HP-TLC). However, for some applications, it is recommended to apply another ion source providing better imaging capabilities, such as MALDI-IMS. 

Summarizing, DESI due to its ability to work under ambient conditions allows for convenient connection of simple and low-cost TLC separation and highly specific MS analysis. Of the currently popular MS imaging techniques, DESI seems to be the most comprehensive. This type of ion source does not cause extensive fragmentation of the sample as in the case of SIMS. Additionally, since there is no need to use any kind of matrix or high vacuum, as in the case of MALDI, the molecules of low molecular weight, such as drugs and their metabolites, may be easily analyzed. 

Another ionization technique that can be carried out under atmospheric conditions and is utilized for direct analysis in real time is DART-MS. Similar to DESI-MS, samples do not require pretreatment, and thus, the time required for DART-MS analysis decreases. The DART ion source has been used to analyze a number of substances such as drugs in seized materials and biological fluids, dyes and inks, food, and environmental compounds. All of these analytes can be ionized directly on surfaces such as glass, TEC plates, concrete, and paper [11]. 

Four of the most commonly used desorption/ionization methods for MSI are secondary ion mass spectrometry (SIMS), desorption electrospray ionization (DESI), matrix-assisted laser desorption/ionization (MALDI), and laser ablation (LA) with post-ionization. Other desorption/ionization approaches such as laser desorption/ionization (LDI) see Chapter 9, (12)), desorption/ionization on silicon (DIOS) (13), electrospray ionization (ESI) (14), and nanostructure-initiator mass spectrometry (NIMS) (15, 16) also have great potential in MSI. Importantly, many mass spectrometers equipped with a MALDI ion source can be used with related ionization processes such as LDI, DIOS, LA, and laser-NIMS. Erequently, a specific ion source arrangement is optimized for a specific mass analyzer for example, MALDI is often interfaced to a time-of-flight (TOF) mass analyzer (described below) although it can also be used with ICR-based instruments. 

Fig. 7.6 Schematic diagram of the setup and ion source for desorption electrospray ionization (DESI-MS). (Adapted from N. Talaty et al. Analyst, 130, 2005, 1624-1633 with permission of the PCCP Owner Societies)...

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