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← Older revision		Revision as of 06:09, 20 December 2013
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	~~I would like~~ to ~~introduce myself~~ to ~~you~~, ~~I am Andrew~~ and ~~my spouse doesn't~~ like ~~it at all~~. As a ~~lady what she truly likes~~ is ~~style~~ and ~~she's been doing it~~ for ~~quite~~ a ~~while~~. ~~Distributing manufacturing~~ has ~~been his occupation for some time~~. ~~For years she'~~s ~~been living~~ in ~~Kentucky but her husband desires them~~ to transfer.<br><br>~~my web~~-~~site~~; [http://~~chungmuroresidence~~.~~com~~/xe/~~reservation_branch2~~/~~152663 psychic readings~~]		{{merge to\|Electrospray ionization \|discuss=Talk:Electrospray ionization #Proposed merge with Desorption electrospray ionization\|date=January 2014}}
			{{Infobox chemical analysis
			\| name = Desorption Electrospray Ionization (DESI)
			\| image =
			\| caption =
			\| acronym = DESI
			\| classification =[[Mass spectrometry]]
			\| analytes = [[Organic molecule]]s<br>[[Biomolecule]]s
			\| related = [[Electrospray ionization]]<br>[[Atmospheric pressure chemical ionization]]
			\| hyphenated = }}

			'''Desorption electrospray ionization (DESI)''' is an [[ambient ionization]] technique that can be used in [[mass spectrometry]] for [[chemical analysis]]. It is an [[atmospheric pressure]] [[ion source]] that ionizes gases, liquids and solids in open air under [[wikt:ambient\|ambient]] conditions. It was developed in 2004 by Professor Graham Cooks' group from Purdue University <ref>{{cite journal \|author=Z. Takáts, J.M. Wiseman, B. Gologan, R.G. Cooks\|title=Mass Spectrometry Sampling Under Ambient Conditions with Desorption Electrospray Ionization\|journal=Science\|volume=306\|pages=471–473 \|year=2004 \|doi=10.1126/science.1104404 \|pmid=15486296 \|issue=5695\|bibcode = 2004Sci...306..471T }}</ref> and is now available commercially by Prosolia Inc. DESI is a similar ionization technique to Direct Analysis in Real Time ([[DART ion source\|DART]]) in its versatility, applications and analysis time.

			==Principle of operation==
			[[Image:DESI ion source.jpg\|thumb\|right\|400 px\|Schematic diagram of the DESI ion source]]
			DESI is a combination of [[electrospray]] (ESI) and desorption (DI) ionization methods.
			Ionization takes place by directing an electrically charged mist to the sample surface that is a few millimeters away.<ref name="pmid16237663">{{cite journal \|author=Takáts Z, Wiseman JM, Cooks RG \|title=Ambient mass spectrometry using desorption electrospray ionization (DESI): instrumentation, mechanisms and applications in forensics, chemistry, and biology \|journal=Journal of mass spectrometry : JMS \|volume=40 \|issue=10 \|pages=1261–75 \|year=2005 \|pmid=16237663 \|doi=10.1002/jms.922}}</ref> The electrospray mist is attracted to the surface by applying a voltage on the sample holder. After ionization, the ions travel through air into the atmospheric pressure interface which is connected to the mass spectrometer. DESI is a technique that allows for ambient ionization of a trace sample at atmospheric pressure, with little sample preparation.

			===Ionization mechanism===

			In DESI there are two kinds of ionization mechanism, one that applies to low molecular weight molecules and another to high molecular weight molecules.<ref name="pmid16237663"/> High molecular weight molecules, such as proteins and peptides show electrospray like spectra where multiply charged ions are observed. This suggests desorption of the analyte, where multiple charges in the droplet can easily be transferred to the analyte. The charged droplet hits the sample, spreads over a diameter greater than its original diameter, dissolves the protein and rebounces. The droplets travel to the mass spectrometer inlet and are further desolvated. The solvent typically used for the electrospray is a combination of methanol and water.

			For the low molecular weight molecules, ionization occurs by charge transfer: an electron or a proton. There are three possibilities for the charge transfer. First, charge transfer between a solvent ion and an analyte on the surface. Second, charge transfer between a gas phase ion and analyte on the surface; in this case the solvent ion is evaporated before reaching the sample surface. This is achieved when the spray to surface distance is large. Third, charge transfer between a gas phase ion and a gas phase analyte molecule. This occurs when a sample has a high vapour pressure.

			:<math>A^- _{(aq)} + S_{(s)} \to A_{(aq)} + S^-_{(s)}\,\qquad(1)</math>

			:<math>A^- _{(g)} + S_{(s)} \to A_{(g)} + S^-_{(s)}\,\qquad(2)</math>

			:<math>A^- _{(g)} + S_{(g)} \to A_{(g)} + S^-_{(g)}\,\qquad(3)</math>

			The ionization mechanism of low molecular weight molecules in DESI is similar to [[DART ion source\|DART’s]] ionization mechanism, in that there is a charge transfer that occurs in the gas phase.

			==Ionization efficiency==
			[[Image:DESI side view.jpg\|thumb\|right\|300 px\|Side view of DESI ion source along with a table containing typical values for the geometric parameters]]
			The ionization efficiency of DESI is complex and depends on several parameters such as, surface effects, electrospray parameters, chemical parameters and geometric parameters.<ref name="pmid16237663"/> Surface effects include chemical composition, temperature and electric potential applied. Electrospray parameters include electrospray voltage, gas and liquid flow rates.
			Chemical parameters refers to the sprayed solvent composition, e.g. addition of NaCl. Geometric parameters are α, β, d<sub>1</sub> and d<sub>2</sub> (see figure on the right).

			Furthermore, α and d<sub>1</sub> affect the ionization efficiency, while β and d<sub>2</sub> affect the collection efficiency. Results of a test performed on a variety of molecules to determine optimal α and d1 values show that there are two sets of molecules: high molecular weight (proteins, peptides, oligosaccharide etc.) and low molecular weight (dizazo dye, stereoids, caffeine, nitroaromatics etc.). The optimal conditions for the high molecular weight group are high incident angles (70-90°) and short d<sub>1</sub> distances (1–3 mm). The optimal conditions for the low molecular weight group are the opposite, low incident angles (35-50°) and long d<sub>1</sub> distances (7–10 mm). These test results indicate that each group of molecules has a different ionization mechanism; described in detail in the Principle of operation section.

			The sprayer tip and the surface holder are both attached to a 3D moving stage which allow to select specific values for the four geometric parameters: α, β, d<sub>1</sub> and d<sub>2</sub>.

			==References==
			{{Reflist}}

			==External links==
			*[http://aston.chem.purdue.edu/research/ionization/desorption-electrospray-ionization-desi Purdue Aston Lab - DESI]
			*[http://www.prosolia.com Prosolia, Inc.] - Manufacturer of DESI sources
			* ''Method and System for Desorption Electrospray Ionization'' - {{US patent application\|20050230635}}
			*''Method and System for Desorption Electrospray Ionization'' - {{Cite patent\|WO\|2005094389}}
			*''Ionization by droplet impact'' - {{US patent application\|20060108539}}

			{{Mass spectrometry}}

			[[Category:Ion source]]
			[[Category:Mass spectrometry]]

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I would like to introduce myself to you, I am Andrew and my spouse doesn't like it at all. As a lady what she truly likes is style and she's been doing it for quite a while. Distributing manufacturing has been his occupation for some time. For years she's been living in Kentucky but her husband desires them to transfer.<br><br>my web-site; [http://chungmuroresidence.com/xe/reservation_branch2/152663 psychic readings]

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