Types of ionazation in Mass spectroscopy

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Mass spectrometry:
 A technique for measuring and analyzing
molecules,that involves introducing enough energy
into a target molecule to cause its ionizationand
disintegration.The resulting primary ions and their
fragments are then analyzed, based on their mass/
charge ratios, to produce a “molecular fingerprint.”

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Electron impact Ionization (EI) :
 In the EI process,the sample is vaporizedinto the
mass spectrometer Ion source,where it is impacted
by the a beam of electrons with Sufficientenergy to
ionize the molecule.
 Used for volatile and thermally stable compounds.
e.g. M (g) + e- M+(g) + 2 e-

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 This method is suitable for volatile organic
compounds
e.g. = hydrocarbon, oils, flavors’,
 Probability of breaking Bond is maximum at 70eV
but less than 70eV energy electionpass through
the molecule without breaking bond .
 Disadvantage ; produce abdunt fragmention.

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Chemical ionization (CI) :
 Chemical ionizationinvolves ion-moleculereactions
to produce ions fromthe analyte.
 A high reagent gas pressure (60 Pa), results in ion-
molecule reactions between the reagent gas ions
and reagent gas neutrals
 The products of these ion-molecule reactions can
react with the analyte molecules to produce
analyte ions

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Field ionization (FI) :
 The sample is evaporated froma direct insertion
probe, gas , or gas inlet. As the gas molecules pass
near the Emitter, they are ionizedby electron
tunnelling.( Quantum tunnelling)
 The sample must be thermally volatile.
 If compound is non-volatile or thermolabile soln
Prepared in Volatile solvent applied to fieldion
emitter the sample then Subjected to high electric
field this modificationis called field Desorption.
 Field desorption(FD) is a method of ion formation
used in mass spectrometry (MS) in which a high-
potential electricfield is applied to an emitter with
a sharp surface, such as a razor blade, or more
commonly,a filamentfromwhich tiny "whiskers"
have formed.
 This results in a high electricfieldwhich can result
in ionizationof gaseous molecules of the analyte.

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Fast atom bombardment ionization(FAB):
 The sample is deposited on a target that is
bombarded with atoms, Neutrals, or ions.
 Dissolve the analyte in a liquid matrix with low
volatility and to use a Relatively high current of
bombarding particles (4000 to 10,000 eV)
 m-nitrobenzylicalcohol (MNBA) is a good liquid
matrix for non-polar compounds
 It is useful for non-volatile,polar and thermally
unstable compounds And also high molecular
weight compounds like insulin, vitamins.

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Matrix assisted laser desorption ionization
(MALDI) :
 The analyte is dissolvedin a solution containing an
excess of a matrix such as sinapinic acid or
dihydroxybenzoicacid that has a chromophore that
absorbs at the laser wavelengthA small amount of
this solutionis placed on the laser target
 The matrix absorbs the energy fromthe laser pulse
and produces a plasma that results in vaporization
and ionizationof the analyte.
 It is used as non-volatile,polar and thermally
unstable compounds. Good for large molecules
(proteins, polymers,carbohydrates)

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Atmospheric Pressure Chemical Ionization
(APCI):
• gas phase chemical ionization(CI) process where the
vapourizedLC Mobile phase acts as the CI reagent gas
to ionize the sample
• Mobile phase and analyte are first nebulized (N2) and
vapourisedby Heating to 350-550⁰C
• The resulting vapour is ionizedusing a corona
discharge (source of Electrons)
• Subsequent ion/molecule reactions (CI) then cause
ionizationof the Analyte
• Unlike ESI, analyte ions do not need to exist in
solution
• Unlike ESI, best sensitivity is achieved at high liquid
flowrates i.e. 200μL–1mL/min therefore easily
interfaced to conventional HPLC
 Samples containing heteroatoms:urea’s,
benzodiazepines,Carbamates

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Electrospray ionization (ESI):
 The sample solutionis sprayed across a high potential
difference (3_4 KV) froma needle into an orifice in
the interface.Heat and gas flows are used to
desolvate the ions existing in the sample solution.
 Production of charged droplets at the electrospray
capillary tip;
 an evolutionof the charged Droplets by repeated
droplet disintegrationand formationof very small
highly charged droplets Capable of producing gas-
phase ions.

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Atmospheric pressure photoionization(APPI):
• Experimentally,you can viewAPPI as an APCI source
where the coronadischarge has been replaced with a uv
lamp
• The 1st step is complete vapourizationof the mobile
phase used in the LC separation employing nebulization
(N2) and heating to 350-550⁰C
• gas phase photoionizationprocess
• where the vapourizedmobile phase may be
photoionizedto form a CI plasma
• or a modifier (dopant = e.g. toluene and acetone. ) is
added to aid the photoionization
process and formationof the CI plasma
• or the analyte can be directly photoionizedby
photons from the uv lamp

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