- Ion source
An ion source is an electro-magnetic device that is used to create charged particles. These are used primarily within mass spectrometers or
mass spectrometry, an ionsource is a piece of equipment used to ionize analyte molecules and, if necessary, free them from the solid or liquid phase. Once the analyte ions are free to move electric fields will direct them into the mass analyzer.
where M is the atom of molecule being ionized, is the electron, and is the resulting ion.
Spark ionization is used to produce gas phase
ions from a solid sample. When incorporated with a mass spectrometer the complete instrument is referred to as a spark ionization mass spectrometer or as a spark source mass spectrometer (SSMS). [cite journal | author = H. E. Beske, A. Hurrle and K. P. Jochum | title = Part I. Principles of spark source mass spectrometry (SSMS) | year = 1981 | journal = Fresenius' Journal of Analytical Chemistry| volume = 309 | issue = 4 | pages = 258–261 | doi = 10.1007/BF00488596]
Chemical ionization is a lower energy process than
electron ionization. [Munson, M.S.B.; Field, F.H. "J. Am. Chem. Soc." 1966, "88", 2621-2630. [http://dx.doi.org/10.1021/ja00964a001 Chemical Ionization Mass Spectrometry. I. General Introduction] .] The lower energy yields less fragmentation, and usually a simpler spectra. A typical CI spectra has an easily identifiable molecular ion.cite book|last=de Hoffmann|first=Edmond|coauthors=Vincent Stroobant|title=Mass Spectrometry: Principles and Applications|publisher=John Wiley & Sons, Ltd.|location=Toronto|date=2003|edition=Second|pages=14| isbn = 0471485667 ]
In a CI experiment, ions are produced through the collision of the analyte with ions of a reagent gas in the ion source. Some common reagent gases include:
methane, ammonia, and isobutane. 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. For example, protonation occurs by
: (primary ion formation),
: (reagent ion formation),
: (product ion formation, e.g. protonation).
Ion-attachment ionization is similar to
chemical ionizationin which a cation is attached to the analyte molecule in a reactive collision::
Where M is the analyte molecule, X+ is the cation and A is a non-reacting collision partner. [ cite journal|title=Lithium ion attachment mass spectrometry: Instrumentation and features|journal=Review of Scientific Instruments|date=|first=|last=|coauthors=|volume=|issue=|pages=|id= |url=|format=|accessdate=2007-11-01 ]
Fast atom bombardment
In fast atom bombardment the analytes is mixed with a non-volatile chemical protection environment called a matrix and is bombarded under vacuum with a high energy (4000 to 10,000
electron volts) beam of atoms.cite journal | author = Morris HR, Panico M, Barber M, Bordoli RS, Sedgwick RD, Tyler A | title = Fast atom bombardment: a new mass spectrometric method for peptide sequence analysis | journal = Biochem. Biophys. Res. Commun. | volume = 101 | issue = 2 | pages = 623–31 | year = 1981 | pmid = 7306100 | doi = ] The atoms are typically from an inert gas such as argonor xenon. Common matrices include glycerol, thioglycerol, 3-nitrobenzyl alcohol(3-NBA), 18-Crown-6ether, 2-nitrophenyloctyl ether, sulfolane, diethanolamine, and triethanolamine. This technique is similar to secondary ion mass spectrometryand plasma desorption mass spectrometry.
Field desorption refers to an ion source in which a high-potential electric field is applied to an "emitter" with a sharp surface, such as a razor blade, or more commonly, a filament from which tiny "whiskers" have formed. [Beckey H.D. "Field ionization mass spectrometry." Research/Development, "1969", 20(11), 26] This results in a very high electric field which can result in ionization of gaseous molecules of the analyte. Mass spectra produced by FI have little or no fragmentation. They are dominated by molecular radical cations M+. and less often, protonated molecules .
Matrix-assisted laser desorption/ionization
MALDI is a soft
ionizationtechnique allowing the analysis of biomolecules ( biopolymers such as proteins, peptidesand sugars) and large organic molecules(such as polymers, dendrimersand other macromolecules), which tend to be fragile and fragment when ionized by more conventional ionization methods. It is most similar in character to electrospray ionizationboth 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 laserbeam and to facilitate vaporization and ionization.
Atmospheric pressure chemical ionization
Atmospheric pressure chemical ionization is a form of
chemical ionizationwhich takes place at atmospheric pressure.cite journal |author=Prakash C, Shaffer CL, Nedderman A |title=Analytical strategies for identifying drug metabolites |journal=Mass spectrometry reviews |volume=26 |issue=3 |pages=340–69 |year=2007 |pmid=17405144 |doi=10.1002/mas.20128] A spray of solvent 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 dischargethat creates ions. APCI is not as "soft" an ionization technique as ESI.cite journal |author=Zaikin VG, Halket JM |title=Derivatization in mass spectrometry--8. Soft ionization mass spectrometry of small molecules |journal=European journal of mass spectrometry (Chichester, England) |volume=12 |issue=2 |pages=79–115 |year=2006 |pmid=16723751 |doi=10.1255/ejms.798]
In electrospray ionization, a
liquidis pushed through a very small, charged and usually metal, capillary. [cite journal | doi = 10.1002/mas.1280090103 | author = Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S. F.; Whitehouse, C. M. | title = Electrospray Ionization-Principles and Practice | journal = Mass Spectrometry Reviews| date = 1990 | volume = 9 | issue = 1 | pages = 37–70] This liquid contains the substance to be studied, the analyte, dissolved in a large amount of solvent, which is usually much more volatile than the analyte. Volatile acids, bases or buffers are often added to this solution too. The analyte exists as an ionin solution either in its anion or cation form. Because like charges repel, the liquid pushes itself out of the capillary and forms an aerosol, a mist of small droplets about 10 μm across. The aerosol is at least partially produced by a process involving the formation of a Taylor coneand a jet from the tip of this cone. An uncharged carrier gas such as nitrogenis sometimes used to help nebulize the liquid and to help evaporatethe neutral solvent in the droplets. As the solvent evaporates, the analyte molecules are forced closer together, repel each other and break up the droplets. This process is called Coulombic fission because it is driven by repulsive Coulombic forces between charged molecules. The process repeats until the analyte is free of solvent and is a bare ion. The ions observed may are created by the addition of a proton(a hydrogen ion) and denoted , or of another cationsuch as sodiumion, , or the removal of a proton, . Multiply-charged ions such as are often observed. For large macromolecules, there can be many charge states, occurring with different frequencies; the charge can be as great as , for example.
Desorption electrospray ionization
Desorption electrospray ionization uses an
electrospraysource to create charged droplets that are directed at a solid sample a few millimetersto a few centimetersaway. The charged droplets pick up the sample through interaction with the surface and then form highly charged ions that can be sampled into a mass spectrometer.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]
Ions can be created in an electric glow discharge.
Inductively coupled plasma
Ions can be created in an inductively coupled plasma, which is a plasma source in which the
energyis supplied by electrical currents which are produced by electromagnetic induction, that is, by time-varying magnetic fields. [A. Montaser and D. W. Golightly, eds. Inductively Coupled Plasmas in Analytical Atomic Spectrometry, VCH Publishers, Inc., New York, 1992.]
Microwave induced plasma
Microwaves are capable of exciting electrodeless gas discharges to create ions.
Thermospray is a form of atmospheric pressure ionization in
mass spectrometry. It transfers ions from the liquid phase to the gas phase for analysis. It is particularly useful in liquid chromatography-mass spectrometry. [Blakley, C. R.; Carmody, J. J.; Vestal, M. L. "Liquid Chromatograph-Mass Spectrometer for Analysis of Nonvolatile Samples" Analytical Chemistry 1980, 52, 1636-1641.]
Direct analysis in real time
A Direct analysis in real time source operates by exposing the sample to a dry gas stream (typically helium or nitrogen) that contains long-lived electronically or vibronically excited neutral
atomsor molecules(or "metastables"). Excited states are typically formed in the DART source by creating a glow dischargein a chamber through which the gas flows.
onic spray ioniation
Sonic spray ionization is method for creating
ions from a liquid solution, for example, a mixture of methanoland water.cite journal |author=Hirabayashi A, Sakairi M, Koizumi H |title=Sonic spray mass spectrometry |journal=Anal. Chem. |volume=67 |issue=17 |pages=2878–82 |year=1995 |pmid=8779414|doi=10.1021/ac00113a023] A pneumatic nebulizeris used to turn the solution into a supersonicspray of small droplets. Ions are formed when the solvent evaporates and the statistically unbalanced charge distribution on the droplets leads to a net charge and complete desolvation results in the formation of ions.
particle accelerators an ion source creates a particle beamat the beginning of the machine, the "Source". The technology to create ion sources for particle accelerators depends strongly on the type of particle that needs to be generated: electrons, protons, H- ion or a heavy ion.
Electrons are generated with an
electron gun, and there are many varieties of these.
Protons are generated with a plasma-based device, like a
duoplasmatronor a magnetron.
H- ions are generated with a
magnetronor a Penning source. A magnetron consists of a central cylindrical cathode surrounded by an anode. The discharge voltage is typically greater than 150 V and the current drain is around 40 A. A magnetic fieldof about 0.2 tesla is parallel to the cathodeaxis. Hydrogen gas is introduced by a pulsed gas valve. Caesiumis often used to lower the work functionof the cathode, enhancing the amount of ions that are produced.
For a Penning source, a strong magnetic field parallel to the electric field of the sheath guides electrons and ions on cyclotron spirals from cathode to cathode. Fast H-minus ions are generated at the cathodes as in the magnetron. They are slowed down due to the charge exchange reaction as they migrate to the plasma aperture. This makes for a beam of ions that is colder than the ions obtained from a magnetron.
Heavy ions are generated with an electron cyclotron resonanceion source. The use of electron cyclotron resonance (ECR) ion sources for the production of intense beams of highly charged ions has immensely grown over the last decade. ECR ion sources are used as injectors into linear accelerators, Van-de-Graaff generators or cyclotrons in nuclear and elementary particle physics. In atomic and surface physics ECR ion sources deliver intense beams of highly charged ions for collision experiments or for the investigation of surfaces. For the highest charge states, however, Electron beam ion sources (EBIS) are needed. They can generate even bare ions of mid-heavy elements. The Electron beam ion trap(EBIT), based on the same principle, can produce up to bare uranium ions and can be used as an ion source as well.
Theory of Operation
Gas flows through the ion source between the
anodeand the cathode. A positive voltageis applied to the anode. This voltage, combined with the high magnetic fieldbetween the tips of the internal and external cathodes allow a plasma to start. Ionsfrom the plasma are repelled by the anode electric field. This creates an ion beam. [cite web | url = http://www.advanced-energy.com/en/upload/File/Sources/SL-ION-230-02.pdf | title = Ion Beam Sources | publisher = Advanced Energy| accessdate = 2006-12-14]
Ion source applications
*Surface cleaning and pretreatment for large area deposition
*Deposition of Thick
Diamond-like carbon(DLC) Films
*Surface roughening of
polymersfor improved adhesionand/or biocompatibility[cite web | url = http://www.advanced-energy.com/en/Ion.html | title = Ion Beam Source Technology | publisher = Advanced Energy | accessdate = 2006-12-14]
RF antenna ion source
* [http://www.advanced-energy.com/en/upload/File/Sources/SL-IONP-260-01.pdf Ion Source Precleaning for Large Area Deposition]
* [http://www.tectra-gmbh.com/sputter.htm filamentfree Ion Source e.g. for sputter cleaning]
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