Desorption ESI, DESI

Desorption ESI (DESI) was introduced by Takatz et al. [102]. The phenomenon actually was observed earlier but was discarded as a nuisance (e.g., an analyte or calibration mixture that coated the entrance of the transfer capillary and contributed to undesired peaks in the spectra). The idea of using the electrospray for desorption is as clever as it is simple. The method is sensitive and large species such as proteins can be detected. The ions observed are more or less the same as with regular ESI. 

Two new ionization methods to appear recently are desorption ESI (DESI) and direct analysis in real time (DART). They are the first in a new area of ionization mechanisms that are collectively referred to as open-air ionization. Both DART and DESI form ions in an open atmosphere and sample the resulting plume through an entrance cone into the mass spectrometer. 

Although this chapter covers certain aspects of the ESI and MALDI sources, earlier chapters of this book are recommended for details about these ion sources and especially for coverage of the new development of desorption ESI (DESI) by Cooks and co-workers in 2004. 

Figure 9 Approximate ranges of analyte polarity and size that may be suited to different ionization techniques. With respect to the surface desorption techniques, DESI and DART, they are comparable in their range of application to ESI and APCI, respectively.

Figure 2.1 Mass spectrometric approach. Dl, direct inlet GC, gas chromatography HPLC, high performance liquid chromatography CZE, capillary zone electrophoresis El, electron ionization Cl, chemical ionization ESI, electrospray ionization DESI, desorption electrospray ionization APCI, atmospheric pressure chemical ionization MALDI, matrix assisted laser desorption ionization B, magnetic analyzer E, electrostatic analyzer...

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... 

DGE a AC AMS APCI API AP-MALDI APPI ASAP BIRD c CAD CE CF CF-FAB Cl CID cw CZE Da DAPCI DART DC DE DESI DIOS DTIMS EC ECD El ELDI EM ESI ETD eV f FAB FAIMS FD FI FT FTICR two-dimensional gel electrophoresis atto, 10 18 alternating current accelerator mass spectrometry atmospheric pressure chemical ionization atmospheric pressure ionization atmospheric pressure matrix-assisted laser desorption/ionization atmospheric pressure photoionization atmospheric-pressure solids analysis probe blackbody infrared radiative dissociation centi, 10-2 collision-activated dissociation capillary electrophoresis continuous flow continuous flow fast atom bombardment chemical ionization collision-induced dissociation continuous wave capillary zone electrophoresis dalton desorption atmospheric pressure chemical ionization direct analysis in real time direct current delayed extraction desorption electrospray ionization desorption/ionization on silicon drift tube ion mobility spectrometry electrochromatography electron capture dissociation electron ionization electrospray-assisted laser desorption/ionization electron multiplier electrospray ionization electron transfer dissociation electron volt femto, 1CT15 fast atom bombardment field asymmetric waveform ion mobility spectrometry field desorption field ionization Fourier transform Fourier transform ion cyclotron resonance... 

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... 

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. 

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)... 

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). 

Methods Ambient ionization methods, of which there are now over 20, e.g., desorption electrospray ionization (DESI), desorption atmospheric pressure chemical ionization (DAPC), desorption atmospheric pressme photo-ionization (DAPPI), and direct analysis in real time (DART), are now joined by paper spray, a method where ESI is initiated at the pointed tip of a piece of filter paper. A drop of blood ( 15 pi) is dried on the paper, and then the paper is moistened with 25 pi of a solvent suited to both the extraction of the analytes from the blood and the ESI process (e.g., 90% methanol 10% water with either 100 ppm acetic acid or 200 ppm sodium acetate). When the paper is exposed to high voltage (3-5 kV) while held close ( 5 mm) to the entrance of the mass analyzer, a spray (similar to electrospray) is induced at the tip of the paper as capillary action carries extracted compounds through the paper (Figure 4.5). The spray is maintained for 30-90 s at a flow rate comparable to that used in nano-electrospray. 

More than a decade ago, the group of Graham Cooks introduced a modified version of ESI, which they called desorption electrospray ionization (DESI) [107] (see also Chapter 2). In this technique, the ESI plume is directed onto the sample surface. A very rapid chemical analysis of the sample surface is possible [5]. While the technique was originally presented as a way to analyze samples supported on solids, it has been modified to enable analysis of liquid samples [108]. This development facilitated implementations of DESI in TRMS. For example, in the work mentioned earlier in this chapter, Miao et al. [2] demonstrated the possibility to monitor reactions with sub-millisecond time resolution. Here, two reactant solutions mix rapidly to form a free liquid jet which is then ionized by... 

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). 

TATP + Na]+ or [TATP + K]" were not produced as has been previously reported for the analysis of TATP by ESI-MS and desorption electrospray ionization (DESI) [50,51]. A [TATP + H]" ion was also not observed by LEMS however, the sodium and potassium adduct ions of the TATP dimmer at m/z 467 [2TATP + Na]+ and m/z 483 [2TATP + K]", respectively, were observed [54],... 

Desorption electrospray ionization (DESI) is a variant of ESI [33]. Electrosprayed aqueous solvent, with charged droplets and solvent ions, is directed at the sample of interest at ambient conditions. The sample may be a native sample or an analyte placed on an insulating surface. The analyte is desorbed by the result of either sputtering or impact desolvation. Desorbed ions are transferred into the vacuum system via an ion transfer line and mass analyzed. Jackson et al. [33] used DESI to analyze PMMA, poly (alpha-methyl styrene), and other polymers. 

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. 

---