electron ionization

7/21/2019 Electron Ionization

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Electron ionization

Electron ionization (EI, formerly known as electron impact) is an ionization method

in which energetic electrons interact with gas phase atoms or molecules to produce ions.

This technique is widely used in mass spectrometry, particularly for volatile organic

molecules.

The following gas phase reaction describes the electron ionization process:

where M is the analyte molecule being ionized, e - is the electron and M+ is the resulting

ion.

In an EI ion source, electrons are produced

through thermionic emissionby heating a wirefilament that has electric current running through

it. The electrons are accelerated to 70 eV in the

region between the filament and the entrance to

the ion source block. The accelerated electrons are

then concentrated into a beam by being attracted

to the trap electrode. A sample vapour containing

the neutral molecules is introduced to the ion

source in a perpendicular direction to the e- beam.

Upon interaction with the e- beam, the analyte

molecules ionize to radical cations which are then

directed towards the mass analyzer by a repellerelectrode. Due to the high energy electrons and

the initial thermal distribution of the neutral

molecules, the ionization process frequently

causes cleavage reactions that give rise to

fragment ions, which can convey structural

information about the analyte.

The ionization efficiency and production of fragment ions depends strongly on the

chemistry of the analyte and the energy of the electrons. At low energies (around 20

eV), the interactions between the electrons and the analyte molecules do not transferenough energy to cause ionization. At around 70 eV, the de Brogliewavelength of the

electrons matches the length of typical bonds in organic molecules (about 0.14 nm) and

energy transfer to organic analyte molecules is maximized, leading to the strongest

possible ionization and fragmentation. Under these conditions, about 1 in 1000 analyte

molecules in the source are ionized. At higher energies, the de Broglie wavelength of

the electrons becomes smaller than the bond lengths in typical analytes; the molecules

then become "transparent" to the electrons and ionization efficiency decreases.
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Chemical ionization

Chemical ionization (CI) is an ionization technique used in mass spectrometry.

Chemical ionization is a lower energy process than electron ionization. The lower

energy yields less fragmentation, and usually a simplerspectrum. A typical CI spectra

has an easily identifiable molecular ion.

Mechanism

In a CI experiment, ions are produced through the collision of the analyte with ions of a

reagent gas that are present in the ion source. Some common reagent gases include:

methane, ammonia, andisobutane. Inside the ion source, the reagent gas is present in

large excess compared to the analyte. Electrons entering the source will preferentially

ionize the reagent gas. The resultant collisions with other reagent gas molecules will

create an ionization plasma. Positive and negative ions of the analyte are formed by

reactions with this plasma.

Primary Ion Formation:

Secondary Reagent Ions:

Product Ion Formation:

(protonation)

(H abstraction)

(adduct formation)

(charge exchange)

Self chemical ionization occurs when the reagent ion is an ionized form of the analyte. [5]

Variations

Negative chemical ionization (NCI)

Chemical ionization for gas phase analysis is either positive or negative. Almost all

neutral analytes can form positive ions through the reactions described above.

In order to see a response by negative chemical ionization, the analyte must be capable

of producing a negative ion (stabilize a negative charge) for example by electron

capture ionization. Because not all analytes can do this, using NCI provides a certain

degree of selectivity that is not available with other, more universal ionization
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techniques (EI, PCI). NCI can be used for the analysis of compounds containing acidic

groups or electronegative elements (especially halogens).[4]

Because of the high electronegativity of halogenatoms, NCI is a common choice for

their analysis. This includes many groups of compounds, such as PCBs[6] , pesticides[7] ,

and fire retardants.[8]

Most of these compounds are environmental contaminants, thusmuch of the NCI analysis that takes place is done under the auspices of environmental

analysis.

Atmospheric pressure chemical ionization (APCI)

Chemical ionization in an atmospheric pressure electric discharge is called atmospheric

pressure chemical ionization. The analyte is aa gas or liquid spray and ionization is

accomplished using an atmospheric pressure corona discharge. This ionization method

is often coupled withhigh performance liquid chromatography where the mobile phase

containing eluting analyte sprayed with high flow rates of nitrogen and the aerosol spray

is subjected to a corona discharge to create ions.

Atmospheric pressure chemical ionization

Atmospheric pressure chemical ionization (APCI) is an ionization method used in

mass spectrometry. It is a form ofchemical ionization which takes place at atmospheric

pressure.[1]

How it works

APCI allows for the high flow rates typical of standard bore HPLC to be used directly,

often without diverting the larger fraction of volume to waste. Typically the mobile

phase containing eluting analyte is heated to relatively high temperatures (above 400

degrees Celsius), sprayed with

high flow rates of nitrogen and

the entire aerosol cloud is

subjected to a corona discharge

that creates ions. Often APCI

can be performed in a modifiedESI source. This is basically a

gas phase ionisation, unlike

ESI which is a liquid phase

ionisation process. Also, we

can use nonpolar solvent for

solution making instead of polar solvent for supporting ions in solution as gaseous state

conversion of solvent before reaching to corona discharge pin is carried out here, which

well supports the ions formed. Typically, APCI is a less "soft" ionization technique than

ESI, i.e. it generates more fragment ions relative to the parent on
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Desorption atmospheric pressure photoionization

Desorption atmospheric pressure photoionization (DAPPI) is an atmospheric-

pressure ionization technique formass spectrometry. DAPPI enables direct analysis of

solid samples without pretreatment and analysis of samples deposited on surfaces by

means of a jet of hot solvent vapour and vacuum ultraviolet light. The hot jet thermally

desorbs the sample from a surface and the vaporized sample is ionized by the vacuum

ultraviolet light and consequently sampled into a mass spectrometer.

Ionization mechanisms

The ionization mechanism depends on the analyte and solvent used and for example thefollowing analyte (M) ions may be formed: [M + H]+, [M - H]-, M+, M-.

Applications

DAPPI has the potential to analyze both polar (e.g. verapamil) and nonpolar (e.g.

anthracene) compounds. Performance of DAPPI has also been demonstrated on direct

analysis of illicit drugs

Electrospray ionization

Electrospray ionization (ESI) is a technique used in mass spectrometry to produce

ions. It is especially useful in producing ions from macromolecules because it

overcomes the propensity of these molecules to fragment when ionized. The

development of electrospray ionization for the analysis of biological macromolecules[1]

was rewarded with the attribution of theNobel Prize in Chemistry to John Bennett Fenn

in 2002.[2]

Mass spectrometry using ESI is called electrospray ionization mass spectrometry

(ESI-MS) or, less commonly, electrospray mass spectrometry (ES-MS).

Ionization mechanism

The liquid containing the analyte(s) of interest is dispersed by electrospray into a fine

aerosol. Because the ion formation involves extensive solvent evaporation, the typical

solvents for electrospray ionization are prepared by mixing water with volatile organic

compounds (e.g. methanol, acetonitrile). To decrease the initial droplet size, compounds

that increase the conductivity (e.g. acetic acid) are customary added to the solution.

Large-flow electrosprays can benefit from additional nebulization by an inert gas such

as nitrogen. The aerosol is sampled into the first vacuum stage of a mass spectrometer

through a capillary, which can be heated to aid further solvent evaporation from thecharged droplets. The solvent evaporates from a charged droplet until it becomes
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unstable upon reaching its Rayleigh limit. At this point, the droplet deforms and emits

charged jets in a process known as Rayleigh fission. During the fission, the droplet loses

a small percentage of its mass along with a relatively large percentage of its charge.[6]

There are two major theories that explain the final production of gas-phase ions:

The Ion Evaporation Model (IEM)[7] suggests that as the droplet reaches a

certain radius the field strength at the surface of the droplet becomes large

enough to assist the field desorption of solvated ions.

The Charged Residue Model (CRM)[8] suggests that electrospray droplets

undergo evaporation and fission cycles, eventually leading progeny droplets that

contain on average one analyte ion or less. The gas-phase ions form after the

remaining solvent molecules evaporate, leaving the analyte with the charges that

the droplet carried.

While there is no definite scientific proof, a large body of indirect evidence suggests

that small ions are liberated into the gas phase through the ion evaporation mechanism,while larger ions form by charged residue mechanism.

The ions observed by mass spectrometry may be quasimolecular ions created by the

addition of aproton(a hydrogen ion) and denoted [M+ H]+, or of anothercation such as

sodium ion, [M+ Na]+, or the removal of a proton, [M H]. Multiply-charged ions

such as [M+ nH]n+ are often observed. For large macromolecules, there can be many

charge states, resulting in a characteristic charge state envelope. All these are even-

electron ion species: electrons (alone) are not added or removed, unlike in some other

ionization sources. The analytes are sometimes involved in electrochemical processes,

leading to shifts of the corresponding peaks in the mass spectrum.

Variants

The electrosprays operated a low flow rates generate much smaller initial droplets,

which ensure improved ionization efficiency. In 1994, two research groups coined the

name micro-electrospray (microspray) for electrosprays working at low flow rates.

Emmett and Caprioli demonstrated improved performance for HPLC-MS analyses

when the electrospray was operated at 300-800 nL/min. [9] Wilm and Mann demonstrated

that a capillary flow of ~ 25 nL/min can sustain an electrospray at the tip of emitters

fabricated by pulling glass capillaries to a few micrometers.[10]The latter was renamed

nano-electrospray (nanospray) in 1996.[11]

Currently the name nanospray is also in usefor electrosprays fed by pumps at low flow rates, not only for self-fed electrosprays.

Unfortunately, there are no clear flow rate boundaries between electrosprays,

microsprays, and nanosprays.

Applications

Liquid chromatographymass spectrometry (LC-MS)

Electrospray ionization is the ion source of choice to couple liquid chromatography with

mass spectrometry. The analysis can be performed online, by feeding the liquid eluting

from the LC column directly to an electrospray, or offline, by collecting fractions to belater analyzed in a classical nanoelectrospray-mass spectrometry setup.
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Noncovalent gas phase interactions

Electrospray ionization is also ideal in studying noncovalent gas phase interactions. The

electrospray process is capable of transferring liquid-phase noncovalent complexes into

the gas phase without disrupting the noncovalent interaction. This means that a cluster

of two molecules can be studied in the gas phase by other mass spectrometrytechniques. An interesting example of this is studying the interactions between enzymes

and drugs which are inhibitors of the enzyme. Because inhibitors generally work by

noncovalently binding to its target enzyme with reasonable affinity the noncovalent

complex can be studied in this way. Competition studies have been done in this way to

screen for potential new drug candidates.

Matrix-assisted laser desorption/ionization

Matrix-assisted laser desorption/ionization (MALDI) is a soft ionization technique

used in mass spectrometry, allowing the analysis ofbiomolecules (biopolymers such asproteins, peptides and sugars) and large organic molecules (such as polymers,

dendrimers and other macromolecules), which tend to be fragile and fragment when

ionized by more conventional ionization methods. It is most similar in character to

electrospray ionization both in relative softness and the ions produced (although it

causes many fewer multiply charged ions).

The ionization is triggered by a laser beam (normally a nitrogen laser). A matrix is used

to protect the biomolecule from being destroyed by direct laser beam and to facilitate

vaporization and ionization.

Matrix

The matrix consists ofcrystallized molecules, of which the three most commonly used

are 3,5-dimethoxy-4-hydroxycinnamic acid (sinapinic acid), -cyano-4-

hydroxycinnamic acid (alpha-cyano or alpha-matrix) and 2,5-dihydroxybenzoic acid

(DHB). A solution of one of these molecules is made, often in a mixture of highly

purified water and an organic solvent (normally acetonitrile (ACN) or ethanol).

Trifluoroacetic acid (TFA) may also be added. A good example of a matrix-solution

would be 20 mg/mL sinapinic acid in ACN:water:TFA (50:50:0.1).

The identity of suitable matrix compounds isdetermined to some extent by trial and error,

but they are based on some specific molecular

design considerations:

They are of a fairly low molecular

weight (to allow facile vaporization),

but are large enough (with a low

enough vapor pressure) not to

evaporate during sample preparation or

while standing in the spectrometer.

They are acidic, therefore act as a proton source to encourage ionization of the

analyte.
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They have a strong optical absorption in the UV, so that they rapidly and

efficiently absorb the laser irradiation.

They are functionalized with polar groups, allowing their use in aqueous

solutions.

The matrix solution is mixed with the analyte (e.g. protein-sample). The organic solvent

allows hydrophobic molecules to dissolve into the solution, while the water allows for

water-soluble (hydrophilic) molecules to do the same. This solution is spotted onto a

MALDI plate (usually a metal plate designed for this purpose). The solvents vaporize,

leaving only the recrystallized matrix, but now with analyte molecules spread

throughout the crystals. The matrix and the analyte are said to be co-crystallized in a

MALDI spot.

Laser

The laser is fired at the crystals in theMALDI spot. The matrix absorbs the

laser energy and it is thought that

primarily the matrix is ionized by this

event. The matrix is then thought to

transfer part of its charge to the analyte

molecules (e.g. protein), thus ionizing

them while still protecting them from the

disruptive energy of the laser. Ions

observed after this process consist of a

neutral molecule [M] and an added or removed ion. Together, they form a

quasimolecular ion, for example [M+H]+ in the case of an added proton, [M+Na]+ in the

case of an added sodium ion, or [M-H]- in the case of a removed proton. MALDI is

capable of creating singly-charged ions, but multiply charged ions ([M+nH]n+) can also

be created, as a function of the matrix, the laser intensity and/or the voltage used. Note

that these are all even-electron species. Ion signals of radical cations can be observed

eg. in case of matrix molecules and other stable molecules.

Atmospheric pressure matrix-assisted laser desorption/ionization

Atmospheric pressure (AP) matrix-assisted laser desorption/ionization (MALDI) is an

ionization technique (ion source) that in contrast to vacuum MALDI operates at normal

atmospheric environment. The main difference between vacuum MALDI and AP-

MALDI is the pressure in which the ions are created. In vacuum MALDI, ions are

typically produced at 10 mTorr or less while in AP-MALDI ions are formed in

atmospheric pressure. Disadvantage of the AP MALDI source is the limited sensitivity

observed and the limited mass range.

AP-MALDI is used in mass spectrometry (MS) in a variety of applications ranging fromproteomics to drug discovery fields. Popular topics that are addressed by AP-MALDI
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mass spectrometry include: proteomics, DNA/RNA/PNA, lipids, oligosaccharides,

phosphopeptides, bacteria, small molecules and synthetic polymers, similar applications

as available also for vacuum MALDI instruments.

The AP-MALDI ion source is easily coupled to an ion trap mass spectrometer[12] or any

other MS system equipped with ESI (electrospray ionization) or nanoESI source.

Mass spectrometer

The type of a mass spectrometer most widely used with MALDI is the TOF (time-of-

flight mass spectrometer), mainly due to its large mass range. The TOF measurement

procedure is also ideally suited to the MALDI ionization process since the pulsed laser

takes individual 'shots' rather than working in continuous operation. MALDI-TOF

instruments are typically equipped with an "ion mirror", deflecting ions with an electric

field, thereby doubling the ion flight path and increasing the resolution. Today,

commercial reflectron TOF instruments reach a resolving power m/m of well above20'000 FWHM (full-width half-maximum, m defined as the peak width at 50% of

peak height).

History

The term matrix-assisted laser desorption ionization (MALDI) was coined in 1985 by

Franz Hillenkamp, Michael Karas and their colleagues.[13] These researchers found that

the amino acidalanine could be ionized more easily if it was mixed with the amino acid

tryptophan and irradiated with a pulsed 266 nm laser. The tryptophan was absorbing the

laser energy and helping to ionize the non-absorbing alanine. Peptides up to the 2843

Da peptide melittin could be ionized when mixed with this kind of matrix. [14] The

breakthrough for large molecule laser desorption ionization came in 1987 when Koichi

Tanaka of Shimadzu Corp. and his co-workers used what they called the ultra fine

metal plus liquid matrix method that combined 30 nm cobalt particles in glycerol with

a 337 nm nitrogen laser for ionization.[15] Using this laser and matrix combination,

Tanaka was able to ionize biomolecules as large as the 34,472 Da protein

carboxypeptidase-A. Tanaka received one-quarter of the 2002Nobel Prize in Chemistry

for demonstrating that, with the proper combination of laser wavelength and matrix, a

protein can be ionized.[16]

Karas and Hillenkamp were subsequently able to ionize the 67kDa protein albumin using a nicotinic acid matrix and a 266 nm laser.[17] Further

improvements were realized through the use of a 355 nm laser and the cinnamic acid

derivatives ferulic acid, caffeic acid and sinapinic acid as the matrix.[18] The availability

of small and relatively inexpensive nitrogen lasers operating at 337 nm wavelength and

the first commercial instruments introduced in the early 1990s brought MALDI to an

increasing number of researchers.[19] Today, mostly organic matrices are used for

MALDI mass spectrometry.

Use

In Biochemistry
http://en.wikipedia.org/wiki/MALDI-TOF#cite_note-11http://en.wikipedia.org/wiki/Time-of-flight_mass_spectrometryhttp://en.wikipedia.org/wiki/Reflectronhttp://en.wikipedia.org/wiki/Franz_Hillenkamphttp://en.wikipedia.org/wiki/Michael_Karashttp://en.wikipedia.org/wiki/MALDI-TOF#cite_note-12http://en.wikipedia.org/wiki/Amino_acidhttp://en.wikipedia.org/wiki/Alaninehttp://en.wikipedia.org/wiki/Tryptophanhttp://en.wikipedia.org/wiki/Melittinhttp://en.wikipedia.org/wiki/MALDI-TOF#cite_note-13http://en.wikipedia.org/wiki/Koichi_Tanakahttp://en.wikipedia.org/wiki/Koichi_Tanakahttp://en.wikipedia.org/wiki/Cobalthttp://en.wikipedia.org/wiki/Glycerolhttp://en.wikipedia.org/wiki/Nitrogen_laserhttp://en.wikipedia.org/wiki/MALDI-TOF#cite_note-14http://en.wikipedia.org/wiki/Nobel_Prize_in_Chemistryhttp://en.wikipedia.org/wiki/MALDI-TOF#cite_note-15http://en.wikipedia.org/wiki/MALDI-TOF#cite_note-15http://en.wikipedia.org/wiki/MALDI-TOF#cite_note-16http://en.wikipedia.org/wiki/Cinnamic_acidhttp://en.wikipedia.org/wiki/Ferulic_acidhttp://en.wikipedia.org/wiki/Caffeic_acidhttp://en.wikipedia.org/wiki/Sinapinic_acidhttp://en.wikipedia.org/wiki/MALDI-TOF#cite_note-17http://en.wikipedia.org/wiki/MALDI-TOF#cite_note-18http://en.wikipedia.org/wiki/MALDI-TOF#cite_note-11http://en.wikipedia.org/wiki/Time-of-flight_mass_spectrometryhttp://en.wikipedia.org/wiki/Reflectronhttp://en.wikipedia.org/wiki/Franz_Hillenkamphttp://en.wikipedia.org/wiki/Michael_Karashttp://en.wikipedia.org/wiki/MALDI-TOF#cite_note-12http://en.wikipedia.org/wiki/Amino_acidhttp://en.wikipedia.org/wiki/Alaninehttp://en.wikipedia.org/wiki/Tryptophanhttp://en.wikipedia.org/wiki/Melittinhttp://en.wikipedia.org/wiki/MALDI-TOF#cite_note-13http://en.wikipedia.org/wiki/Koichi_Tanakahttp://en.wikipedia.org/wiki/Koichi_Tanakahttp://en.wikipedia.org/wiki/Cobalthttp://en.wikipedia.org/wiki/Glycerolhttp://en.wikipedia.org/wiki/Nitrogen_laserhttp://en.wikipedia.org/wiki/MALDI-TOF#cite_note-14http://en.wikipedia.org/wiki/Nobel_Prize_in_Chemistryhttp://en.wikipedia.org/wiki/MALDI-TOF#cite_note-15http://en.wikipedia.org/wiki/MALDI-TOF#cite_note-16http://en.wikipedia.org/wiki/Cinnamic_acidhttp://en.wikipedia.org/wiki/Ferulic_acidhttp://en.wikipedia.org/wiki/Caffeic_acidhttp://en.wikipedia.org/wiki/Sinapinic_acidhttp://en.wikipedia.org/wiki/MALDI-TOF#cite_note-17http://en.wikipedia.org/wiki/MALDI-TOF#cite_note-18


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In proteomics, MALDI is used for the identification of proteins isolated through gel

electrophoresis: SDS-PAGE, size exclusion chromatography, and two-dimensional gel

electrophoresis. One method used is peptide mass fingerprinting by MALDI-MS, or

with post ionisation decay or collision-induced dissociation (further use see mass

spectrometry).

In Organic Chemistry

Some synthetic macromolecules, such as catenanes and rotaxanes, dendrimers and

hyperbranched polymers, and other assemblies, have molecular weights extending into

the thousands or tens of thousands, where most ionization techniques have difficulty

producing molecular ions. MALDI is a simple and rapid analytical method that can

allow chemists to analyze the results of such syntheses and verify their results.

In polymer chemistry

In polymer chemistry MALDI can be used to determine the molar mass distribution.[20]

Polymers withpolydispersity greater than 1.2 are difficult to characterize with MALDI

due to the signal intensity discrimination against higher mass oligomers. [

Reproducibility and performance

The sample preparation for MALDI is important for the result. Inorganic salts which are

also part of protein extracts interfere with the ionization process. The salts are removed

by solid phase extraction or washing the final target spots with water. Both methods can

also remove other substances from the sample. The matrix protein mixture is not

homogenous because the polarity difference leads to a separation of the two substancesduring crystallization. The spot diameter of the target is much larger than that of the

laser, which makes it necessary to do several laser shots at different places of the target,

to get the statistical average of the substance concentration within the target spot. The

matrix composition, the addition of trifluoroacetic acid and formic acid, delay between

laser pulses, delay time of the acceleration power, laser wavelength, energy density of

the laser and the impact angle of the laser on the target are among others the critical

values for the quality and reproducibility of the method.

Desorption electrospray ionization

Desorption electrospray ionization (DESI) is an ambient ionization technique that can

be used in mass spectrometry forchemical analysis. It is an atmospheric pressure ion

source that ionizes gases, liquids and solids in open air under ambient conditions. It was

developed in 2004 by Professor Graham Cooks et al. from Purdue University and is

now available commercially by Prosolia Inc. [1]DESI is a similar ionization technique to

Direct Analysis in Real Time (DART) in its versatility, applications and analysis time.It is different in that some sample preparation is required; liquid samples have to be
http://en.wikipedia.org/wiki/Proteomicshttp://en.wikipedia.org/wiki/Gel_electrophoresishttp://en.wikipedia.org/wiki/Gel_electrophoresishttp://en.wikipedia.org/wiki/SDS-PAGEhttp://en.wikipedia.org/wiki/Size_exclusion_chromatographyhttp://en.wikipedia.org/wiki/Two-dimensional_gel_electrophoresishttp://en.wikipedia.org/wiki/Two-dimensional_gel_electrophoresishttp://en.wikipedia.org/wiki/Peptide_mass_fingerprintinghttp://en.wikipedia.org/w/index.php?title=Post_ionisation_decay&action=edit&redlink=1http://en.wikipedia.org/wiki/Collision-induced_dissociationhttp://en.wikipedia.org/wiki/Mass_spectrometryhttp://en.wikipedia.org/wiki/Mass_spectrometryhttp://en.wikipedia.org/wiki/Catenanehttp://en.wikipedia.org/wiki/Rotaxanehttp://en.wikipedia.org/wiki/Dendrimerhttp://en.wikipedia.org/w/index.php?title=Hyperbranched_polymer&action=edit&redlink=1http://en.wikipedia.org/wiki/Molar_mass_distributionhttp://en.wikipedia.org/wiki/MALDI-TOF#cite_note-isbn3-540-44259-6-19http://en.wikipedia.org/wiki/Polydispersityhttp://en.wikipedia.org/wiki/MALDI-TOF#cite_note-Nielen1997-20http://en.wikipedia.org/wiki/Trifluoroacetic_acidhttp://en.wikipedia.org/wiki/Formic_acidhttp://en.wikipedia.org/wiki/Ambient_ionizationhttp://en.wikipedia.org/wiki/Mass_spectrometryhttp://en.wikipedia.org/wiki/Chemical_analysishttp://en.wikipedia.org/wiki/Atmospheric_pressurehttp://en.wikipedia.org/wiki/Ion_sourcehttp://en.wikipedia.org/wiki/Ion_sourcehttp://en.wikipedia.org/wiki/Ambienthttp://en.wikipedia.org/wiki/Desorption_electrospray_ionization#cite_note-0http://en.wikipedia.org/wiki/Desorption_electrospray_ionization#cite_note-0http://en.wikipedia.org/wiki/DART_ion_sourcehttp://en.wikipedia.org/wiki/Proteomicshttp://en.wikipedia.org/wiki/Gel_electrophoresishttp://en.wikipedia.org/wiki/Gel_electrophoresishttp://en.wikipedia.org/wiki/SDS-PAGEhttp://en.wikipedia.org/wiki/Size_exclusion_chromatographyhttp://en.wikipedia.org/wiki/Two-dimensional_gel_electrophoresishttp://en.wikipedia.org/wiki/Two-dimensional_gel_electrophoresishttp://en.wikipedia.org/wiki/Peptide_mass_fingerprintinghttp://en.wikipedia.org/w/index.php?title=Post_ionisation_decay&action=edit&redlink=1http://en.wikipedia.org/wiki/Collision-induced_dissociationhttp://en.wikipedia.org/wiki/Mass_spectrometryhttp://en.wikipedia.org/wiki/Mass_spectrometryhttp://en.wikipedia.org/wiki/Catenanehttp://en.wikipedia.org/wiki/Rotaxanehttp://en.wikipedia.org/wiki/Dendrimerhttp://en.wikipedia.org/w/index.php?title=Hyperbranched_polymer&action=edit&redlink=1http://en.wikipedia.org/wiki/Molar_mass_distributionhttp://en.wikipedia.org/wiki/MALDI-TOF#cite_note-isbn3-540-44259-6-19http://en.wikipedia.org/wiki/Polydispersityhttp://en.wikipedia.org/wiki/MALDI-TOF#cite_note-Nielen1997-20http://en.wikipedia.org/wiki/Trifluoroacetic_acidhttp://en.wikipedia.org/wiki/Formic_acidhttp://en.wikipedia.org/wiki/Ambient_ionizationhttp://en.wikipedia.org/wiki/Mass_spectrometryhttp://en.wikipedia.org/wiki/Chemical_analysishttp://en.wikipedia.org/wiki/Atmospheric_pressurehttp://en.wikipedia.org/wiki/Ion_sourcehttp://en.wikipedia.org/wiki/Ion_sourcehttp://en.wikipedia.org/wiki/Ambienthttp://en.wikipedia.org/wiki/Desorption_electrospray_ionization#cite_note-0http://en.wikipedia.org/wiki/DART_ion_source


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deposited on a desired surface and allowed to dry and gases have to be adsorbed unto a

material.

Principle of operation

Schematic diagram of

the DESI ion source

DESI is a

combination of

electrospray (ESI) and

desorption (DI)

ionization methods.

Ionization takes place

by directing anelectrically charged

mist to the sample

surface that is a few millimeters away. [2] 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. [2] 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: anelectron 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 suface. 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.
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The ionization mechanism of low molecular weight molecules in DESI is similar to

DARTs ionization mechansim, in that there is a charge transfer that occurs in the gas

phase.

Ionization efficiency

The ionization efficiency of DESI is

complex and depends on several

parameters such as, surface effects,

electrospray parameters, chemical

parameters and geometric parameters.[2] Surface effects include chemical

composition, temperature and electric

potential applied. Electrospray

parameters include electrospray

voltage, gas and liquid flow rates.

Chemical parameters refers to thesprayed solvent composition, e.g.

addition of NaCl. Geometric

parameters are , , d1 and d2 (see

figure on the right).

Furthermore, and d1 affect the ionization efficiency, while and d2 affect the

collection efficiency. Results of a test performed on a variety of molecules to determine

optimal 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 d1 distances (1-3 mm). The

optimal conditions for the low molecular weight group are the opposite, low incident

angles (35-50) and long d1 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: , , d1 and d2.

Sonic spray ionization

Sonic spray ionization (SSI) is method for creating ions from a liquid solution, for

example, a mixture of methanol and water. A pneumatic nebulizer is used to turn the

solution into a supersonic spray of small droplets. Ions are formed when the solvent

evaporates and the statistically unbalanced charge distribution on the droplets leads to a

net charge. Complete desolvation results in ions that can be detected using mass

spectrometry.[1]

Applications

Sonic spray ionization has been coupled with high performance liquid chromatographyfor the analysis of drugs. Oligonucleotides have been studied with this method. SSI has
http://en.wikipedia.org/wiki/DART_ion_sourcehttp://en.wikipedia.org/wiki/Desorption_electrospray_ionization#cite_note-pmid16237663-1http://en.wikipedia.org/wiki/Ionhttp://en.wikipedia.org/wiki/Solutionhttp://en.wikipedia.org/wiki/Methanolhttp://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/wiki/Pneumatichttp://en.wikipedia.org/wiki/Nebulizerhttp://en.wikipedia.org/wiki/Supersonichttp://en.wikipedia.org/wiki/Evaporatehttp://en.wikipedia.org/wiki/Mass_spectrometryhttp://en.wikipedia.org/wiki/Mass_spectrometryhttp://en.wikipedia.org/wiki/Sonic_spray_ionization#cite_note-pmid8779414-0http://en.wikipedia.org/wiki/High_performance_liquid_chromatographyhttp://en.wikipedia.org/wiki/DART_ion_sourcehttp://en.wikipedia.org/wiki/Desorption_electrospray_ionization#cite_note-pmid16237663-1http://en.wikipedia.org/wiki/Ionhttp://en.wikipedia.org/wiki/Solutionhttp://en.wikipedia.org/wiki/Methanolhttp://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/wiki/Pneumatichttp://en.wikipedia.org/wiki/Nebulizerhttp://en.wikipedia.org/wiki/Supersonichttp://en.wikipedia.org/wiki/Evaporatehttp://en.wikipedia.org/wiki/Mass_spectrometryhttp://en.wikipedia.org/wiki/Mass_spectrometryhttp://en.wikipedia.org/wiki/Sonic_spray_ionization#cite_note-pmid8779414-0http://en.wikipedia.org/wiki/High_performance_liquid_chromatography


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been used in a manner similar to desorption electrospray ionization for ambient

ionization and has been couplet with thin layer chromatography in this manner.
http://en.wikipedia.org/wiki/Desorption_electrospray_ionizationhttp://en.wikipedia.org/wiki/Ambient_ionizationhttp://en.wikipedia.org/wiki/Ambient_ionizationhttp://en.wikipedia.org/wiki/Thin_layer_chromatographyhttp://en.wikipedia.org/wiki/Desorption_electrospray_ionizationhttp://en.wikipedia.org/wiki/Ambient_ionizationhttp://en.wikipedia.org/wiki/Ambient_ionizationhttp://en.wikipedia.org/wiki/Thin_layer_chromatography

electron ionization

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