Ionization desorption electrospray

DESI is an ambient ionization technique that can be used in MS for analysis of compounds with little sample preparation. This technique was developed in 2004 

DESI-MS has been used to systematically evaluate the characterization of GPLs and sphingolipids [106]. The effects of surface and solvents on DESI-MS analysis of these lipids were extensively investigated [106]. In this study, a total lipid extract from porcine brain was subjected to the analyses in the positive- and negative-ion modes. The ions such as deprotonated PS, PI, and ST species dominate the spectmm in the negative-ion mode. Similar to ESI-MS, PC species are predominant in the positive-ion mode. The identities of these detected ions in the negative- and positive-ion modes were performed after CID [106]. 

DESI offers numerous advantages for lipid analysis including little or no sample preparation, not requiring the addition of a matrix, atmospheric pressure ionization outside the mass spectrometer, and readily ionizing many lipid species. Thus, this technology can be conveniently used for the direct analysis and imaging of those ionizable lipid species present in biological samples such as tissue [105, 107-109]. This topic is extensively described in Chapter 12. 

Murphy, R.C. (1993) Mass Spectrometry of Lipids. Plenum Press, New York, pp 290. 

Christie, W.W. and Han, X. (2010) Lipid Analysis Isolation, Separation, Identification and Lipidomic Analysis. The Oily Press, Bridgwater, England, pp 448. 

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. 

FIGURE 3.15 Schematic of a typical DESI experiment. (From Weston et al. Direct analysis of pharmaceutical drug formulations using ion mobility spectrometry/quadrupole-time-of-flight mass spectrometry combined with desorption electrospray ionization Anal. Chem. 2005, 77(23), 7572-7580. 

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 

The sample may be exposed as such or deposited on a sample holder that may be made of any material, conducting or non-conducting, provided it does not produce background noise. Furthermore, the sample can be freely moved or manipulated during the experiment. An interesting feature is that reactants can be introduced in the spray solution to react with the sample. Another interesting feature of DESI is the ability to map the position of analytes of the native surfaces, such as plant or animal tissues. It has also been demonstrated that DESI can be used with thin-layer chromatography [87]. 

A pneumatically assisted ion spray source is oriented at 45° towards a sample. The nitrogen spray is adjusted so as to have a linear speed of the droplets of about 35 m s-1. From Takats Z., Wiseman J.M., Gologan B. and Cooks R.G., Science, 306, 471-473, 2004. Reprinted, with permission. 

The DESI process can be observed with conducting and insniating snrfaces. DESI is applicable to polar and nonpolar molecules as well as to condensed-phase 

Note Although it appears that the results of DESI analyses are almost instantaneously available, DESI spectra tend to require more thorough examination than standard ESI spectra in order to draw the right analytical conclusions. 

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. 

DESI parameters can be grouped in a compound class-specific manner. It was found that proteins, for example, yield stronger signals when the spray is pointed onto the sample from close to vertical at about 1 mm distance while small molecules such as caffeine work best at an angle around 45° and several millimeters distance from the spray needle (Fig. 13.3) [12]. 

Early on, commercial DESI sources have become available and have meanwhile reached a level of maturity. They feature ESI sprayers adjustable in both angle and distance towards sample plates that can be operated under data system control (Fig. 13.4). 

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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...

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). 

A new family of ionization techniques allows ions to be created under ambient conditions and then collected and analyzed by MS. They can be divided into two major classes desorption electrospray ionization (DESI) and direct analysis in real time (DART). 

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... 

MALDESI matrix-assisted laser desorption electrospray ionization... 

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

Figure 2.7. Schematic of a desorption electrospray ionization (DESI) source.

There is a recent hybrid between AP-MALDI and ESI, matrix-assisted laser desorption electrospray ionization (MALDESI) [202], where species desorbed from a MALDI target are subjected to an electrospray before entering the mass spectrometer. The method is similar to ELDI except that the analyte is embedded in a matrix as in MALDI. 

Z. Takats, I. Cotte-Rodrfguez, N. Talaty, H. Chen, and R. G. Cooks. Direct, Trace Level Detection of Explosives on Ambient Surfaces by Desorption Electrospray Ionization Mass Spectrometry. Chem. Commun., no. 15 (2005) 1950-1952. 

J. S. Sampson, A. M. Hawkridge, and D. C. Muddiman. Generation and Detection of Multiply-Charged Peptides and Proteins by Matrix-Assisted Laser Desorption Electrospray Ionization (MALDESI) Fourier Transform Ion Cyclotron Resonance Mass Spectrometry. J. Am. Soc. Mass Spectrom., 17(2006) 1712-1716. 

TLC coupled with mass spectrometry employing desorption electrospray ionization has been used for the separation of synthetic dyes. The chemical structures of dyes included in the investigation are shown in Fig. 3.7. ODS HPTLC plates (10 X 10 cm) were used as the stationary phase the mobile phase consisted of methanol-tetrahydrofuran (60 40, v/v) containing 50-100 mM ammonium acetate for the positive-ion test and of methanol-water (70 30, v/v) for the negative-ion test. Test mixtures for negative- and positive-ion mode detection consisted of methyleneblue, crystal violet, rhodamine 6G... 

G.J. Van Berkel, M.J. Ford and M.A. Deibel, Thin-layer chromatography and mass spectrometry coupled using desorption electrospray ionization. Anal. Chem., 11 (2005)1207-1215. 

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). 

Fig. 1.15 Desorption electrospray ionization interface. The sample, in this case a pharmaceutical pill, is placed in front of the orifice and is hit by nebulized droplets. Desorbed ions are then sampled into the mass spectrometer.

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). 

Kauppila, T. J., Wiseman, J. M., Ketola, R. A., Kotiaho, T., Cooks, R. G., and Kostiainen, R. (2006). Desorption electrospray ionization mass spectrometry for the analysis of pharmaceuticals and metabolites. Rapid Commun. Mass Spectrom. 20 387-392. 

Kertesz, V., and Van Berkel, G. J. (2008). Improved imaging resolution in desorption electrospray ionization mass spectrometry. Rapid Commun. Mass Spectrom. 22 2639-2644. 

Chen, H., Li, M., Zhang, Y.-P., Yang, X., Lian, J.-J. and Chen, J.-M. (2008) Rapid analysis of SVOC in aerosols by desorption electrospray ionization mass spectrometry. Journal of the American Society for Mass Spectrometry, 19, 450-4. 

Takats, Z., J.M. Wiseman, B. Gologan, and R.G. Cooks. 2004. Mass spectrometry sampling under ambient conditions with desorption electrospray ionization. Science 306 471-473. 

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. 

Dill A, Ifa D, Manicke N, Zheng O, Cooks R (2009) Mass spectrometric imaging of lipids using desorption electrospray ionization. J Chromatogr B 877 2883-2889. doi 10.1016/j. jchromb.2008.12.058... 

Ellis S, Wu C, Deeley J, ZhuX, Truscott R, Panhuis J, Cooks R, Mitchell T, Blanksby S (2010) Imaging of human lens lipids by desorption electrospray ionization mass spectrometry. J Am Soc Mass Spectrom 21 2095-2104. doi 10.1016/j.jasms.2010.09.003... 

Girod M, Shi Y, Cheng J, Cooks G (2010) Desorption electrospray ionization imaging mass spectrometry of lipids in rat spinal cord. J Am Soc Mass Spectrom 21 1177-1189. doi 10.1016/j. jasms.2010.03.028... 

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... 

Kertesz V, van Berkel G, Vavrek M, Koeplinger K, Schneider B, Covey T (2008) Comparison of drug distribution images from whole-body thin tissue sections obtained using desorption electrospray ionization tandem mass spectrometry and autoradiography. Anal Chem 80 5168-5177. doi 10.1021/ac800546a... 

Manicke N, Dill A, Ifa D, Cooks R (2010) High-resolution tissue imaging on an orbitrap mass spectrometry by desorption electrospray ionization mass spectrometry. J Mass Spectrom 45 223-226. doi 10.1002/jms,1707... 

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