DESI interface

Fig. 13.1. Schematic of a DESI interface. A jet of gas and charged microdroplets is created by means of a standard pneumatic ESI sprayer and directed onto a sample surface at angle a. As a result, charged microdroplets containing ions of the surface material are created and transported away due to the action of the reflected gas stream and electric repulsion at angle p. A portion of the secondary ESI spray may be taken up by the atmospheric pressure interface of the mass spectrometer. Although at the expense of optimum sensitivity, an extended ion transfer line is normally employed to bridge the gap from surface to interface sampling orifice [1,12].

Developed around the same time as the DART technique (Section 3.3B), DESI interfaces with a mass analyzer using a heated ion transfer tube that is in some cases flexible and can be held in the researcher s hand directly above the sample surface. 

Offline-surface mediated approach Collect effluent from multiple LCs on surface place surface in MS interface use fast, surface-mediated ionization methods such as MALDI, DESI, etc. to sample effluents... 

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. 

DESI is performed by directing electrosprayed-charged droplets onto a surface for analysis under atmospheric conditions. The collision of the charged droplets with the surface leads to the ionization and desorption of the analyte (35). Then, the ions produced in the gas phase are sampled by an atmospheric interfaced mass analyzer. DESI has been used to create two-dimensional images related to the distribution of lipid species in human tissues (36). 

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. 

Atmospheric pressure ionization (API). The need to analyze polar componnds and the necessity to interface LC with MS led to the development of techniqnes where the ionization occurs at atmospheric pressure outside the vacuum chamber, and the resulting ions are transferred directly into the mass analyzer. Electrospray ionization (ESI) is the most successful of the API methods because of the range of molecular masses to which it can be applied, from small molecules to proteins. Other API methods include atmospheric pressure chemical ionization (APCI) and atmospheric pressure photo-ionization (APPI), and also the recently developed surface ionization methods such as desorption electrospray ionization (DESI) and direct analysis in real time (DART) (see below and Sections 2.2.2 and 2.2.3). 

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. 

Critical Parameters The limitations of ESI discussed in Section 46.2.2.1 are also experienced with DESI and ESSI.The reliability of measured molecular weights and molecular weight distributions, especially for polydis-persed polymers, is also affected by mass discrimination, changes in the cone potential in the atmospheric interface, and acconamodation of multiple charges by large molecules. Loss of resolution and an increase in baseline may occur with the increase in number of possible types... 

Silicon nitride coated CNT-templated UTLC plates were employed for analyte detection via DESI-MSI and direct analysis in real-time mass spectrometry (DART-MS) after chromatography. Rhodamine B (m/z 443.2 M+) and Basic Blue 7 (m/z 478.3 M ) were desorbed with methanol as spray solvent at 0.003 mL min within 45 min for scanning an 85-mm plate section (Figure 9.9). Dimethyl Yellow (m/z 226 [M-i-H]+), Oracet Red G (m/z 238 [M-i-H]+), Solvent Blue 35 (m/z 351 [M-t-H]+), Sudan Red G (m/z 279 [M-i-H]+), and Solvent Blue 22 (m/z 305 [M-i-H]+) were detected within 3.5 min with a snbstantially optimized interface for HPTLC-DART-MS applying a helium flow of 3 L min at 500°C (Figure 9.10) [2]. 

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. 

The electrocharged aerosol particles interact with the surface, and as a result of their impact, the analyte is desorbed from the surface and ionized. Then, ions are transferred to the mass spectrometer by an ambient pressure sample transfer line, and mass spectra are recorded using an unmodified commercial mass spectrometer equipped with an atmospheric pressure interface (see Fig. 2). DESI ionize both small and large molecules (up to small proteins), and the efficiency of the desorption/ionization depends mainly on three factors (i) physicochemical properties of the analytes (ii) the spray mixture, i.e., the solvent composition, pH, viscosity, volatility, etc. and finally (iii) DESI surface on which the analyte is deposited. Indeed, the chemical composition, roughness, surface energy, and conductivity of the surface are all factors that determine the behavior of a DESI surface. 

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. 13.1. Photograph of a 4000 QTRAP mass spectrometer atmospheric sampling interface region equipped with a PDI during imaging of a mouse thin tissue section showing the DESI emitter, surface, and sampling inlet. (Reproduced from ref. (13) with permission from American Chemical Society.)...

All these methods have one inportant characteristic in common they direct a stream of ionizing or at least ion-desorbing fluid medium onto a sarrple surface from which analyte ions are withdrawn and transported through air into the mass analyzer via a standard API interface. The beauty of this approach lies in the fact that a sarrple needs just to be exposed to the ionizing medium imder ambient conditions. In other words, DESI, DART and those numerous related methods enable the detection of surface materials like waxes, alkaloids, flavors, or pesticides from plants as well as explosives, pharmaceuticals, or drugs of abuse from luggage or banknotes. These and many more analytical apphcations are readily accessible by... 

Note In principle, any mass spectrometer equipped with an ESI source can be modified for DESI operation by mounting the sprayer on an adjustable frame and placing ajc,y-movable sample stage between sprayer and entrance of the interface (Chap. 13.1.3) [15]. For safety, a GQ-resistor should be welded in the high-voltage supply cord. However, for successful analytical application, only sensitive modem instruments are suitable. 

Fig. 13.8. Fully computer-controlled TLC-DESI unit attached to an ESI interface using the curtain gas design. Reprinted with permission from Ref. [23]. The American Chemical Society, 2005.

In closing this chapter, the below short table summarizes ambient MS methods. They are listed in alphabetical order of their acronyms together with a key reference (Table 13.2). It should be noted however, that only the truly simple-to-operate yet effective and rugged interfaces will survive over the years. Most probably, DESI and DART are the ones to persist in the long term. 

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