Conventional and advance ionization techniques in MS. It gives an oversight to the concept.

1. Ionization Techniques in
Mass Spectrometry
• PAPER CODE: CHEM-457
• COURSE INSTRUCTOR: PROFESSOR ARSHAD
• SUBMITTED BY: ALI HAMZA (074403)
• SUBMITTED TO: PROFESSOR ARSHAD
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2. Types of
ionization
Gas phase Desorption Evaporation
Electronic
Ionization
Chemical
ionization
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4. Gas Phase Ionization
The oldest and popular technique in which sample is vaporized first.
Electron Ionization
It is also known as ‘’Hard ionization technique’’ due to use of high energy beam that cause high
fragmentation.
Instrumentation:
It comprises ionization chamber in which sample is added by inlet system from there it will reach
towards the chamber. The heated tungsten filament which is heated by electron current. In MS
we usually discuss organic compounds and I.E of organic compounds are 8-15eV but for
fragmentation we use 50-70eV. The neutral molecules are removed by vaccum pump and
electron beam is captured on anode. There we use repeller that capture negative ion and repel
positive ions & speed up & push them into the chamber. After chamber there are accelerating
plates that have potential difference of 8 KV & plates contain –ve charge.
Potential difference increases with every plate then we pass electron beam from a slit and finally
a beam generated and passed to analyzer.
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5. +
+
Pump Accelerating
Plates
Fig: Schematic representation
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6. Advantages
• High Fragmentation
EI produces highly fragmented ions, providing rich structural information about the molecule.
Example: In the EI mass spectrum of ethanol (CH3CH2OH), the molecular ion peak (M+) at m/z 46 is followed by peaks
corresponding to fragment ions such as CH3CH2O+ (m/z 45) and CH3+ (m/z 15), aiding in structural elucidation.
• Reproducibility
EI is highly reproducible, making it suitable for quantitative analysis. Reproducible fragmentation due to 70eV
Example: In the analysis of a mixture containing known concentrations of different compounds, EI mass spectrometry
yields consistent ion intensities, allowing accurate quantification.
• Wide Applicability
EI can analyze a wide range of organic compounds, from small molecules to larger biomolecules.
Example: EI is commonly used in the identification of organic pollutants in environmental samples, such as pesticides
and hydrocarbons.
• Library Matching
EI mass spectra can be compared with extensive spectral libraries for compound identification.
Example: A student can match the EI mass spectrum of an unknown compound with spectra in a library database to
determine its identity, aiding in structure elucidation
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7. Disadvantages
• High Fragmentation:
The high degree of fragmentation can make interpretation of mass spectra complex, especially for larger molecules.
Example: In the EI mass spectrum of cholesterol, numerous fragment peaks are observed, complicating the identification
of the molecular ion peak.
• Limited Sensitivity for Polar Compounds:
EI may have limited sensitivity for polar compounds, leading to lower ionization efficiency.
Example: Some polar compounds, such as amino acids and sugars, may exhibit poor ionization efficiency in EI,
necessitating the use of alternative ionization techniques.
• Thermal Decomposition:
EI can induce thermal decomposition of thermally labile compounds, leading to artifacts in the mass spectrum.
Example: In the EI analysis of certain unstable compounds, such as peroxides or highly reactive intermediates, thermal
decomposition may occur, affecting the accuracy of the mass spectral data.
• Limited Soft Ionization:
EI is considered a "hard" ionization technique, leading to extensive fragmentation and limited formation of intact
molecular ions.
Example: Compounds containing labile functional groups, such as ethers or esters, may undergo extensive fragmentation
in EI, hindering the detection of intact molecular ions.
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8. Chemical Ionization
Chemical ionization (CI) is a soft ionization technique used in mass spectrometry This was first introduced by
Burnaby Munson and Frank H. Field in 1966.This technique is a branch of gaseous ion-molecule
chemistry. Reagent gas molecules (often methane or ammonia) are ionized by electron ionization to form
reagent ions, which subsequently react with analyte molecules in the gas phase to create analyte ions for
analysis by mass spectrometry. Negative chemical ionization (NCI), charge-exchange chemical
ionization, atmospheric-pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI)
are some of the common variants of the technique. CI mass spectrometry finds general application in the
identification, structure elucidation and quantitation of organic compounds as well as some utility in
biochemical analysis. Samples to be analyzed must be in vapour form, or else (in the case of liquids or solids),
must be vapourized before introduction into the source.
Less fragmentation occurs due to which intense molecular peak will be formed and mainly used to detect
molecular mass, usually produce Quasi molecular ion peaks that are even electron species e.g. [M+H+]+. First of
all ionize the reagent gas and mix it with sample (reagent+sample). The ionizing gas can be ammonia, methane
or methanol. Ionization occurs by:
 Proton transfer
 Electrophilic formation (addition)
 Gas can accept electron or donate electron to analyte.
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9. Principles of working
The chemical ionization process generally imparts less energy to an analyte molecule than does electron impact (EI)
ionization, resulting in less fragmentation and usually a simpler spectrum. The amount of fragmentation, and
therefore the amount of structural information produced by the process can be controlled to some degree by
selection of the reagent ion. In addition to some characteristic fragment ion peaks, a CI spectrum usually has an
identifiable protonated molecular ion peak [M+1]+, allowing determination of the molecular mass. CI is thus useful
as an alternative technique in cases where EI produces excessive fragmentation of the analyte, causing the
molecular-ion peak to be weak or completely absent.
Mechanism
A CI experiment involves the use of gas phase acid-base reactions in the chamber. Some common reagent gases
include: methane, ammonia, water and isobutane. Inside the ion source, the reagent gas is present in large excess
compared to the analyte. Electrons entering the source will mainly ionize the reagent gas because it is in large excess
compared to the analyte. The primary reagent ions then undergo secondary ion/molecule reactions (as below) to
produce more stable reagent ions which ultimately collide and react with the lower concentration analyte molecules
to form product ions. The collisions between reagent ions and analyte molecules occur at close to thermal energies,
so that the energy available to fragment the analyte ions is limited to the exothermicity of the ion-molecule
reaction. For a proton transfer reaction, this is just the difference in proton affinity between the neutral reagent
molecule and the neutral analyte molecule.[8] This results in significantly less fragmentation than does 70 eV
electron ionization (EI).
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11. Types of Chemical Ionization/Variants in mass spectrometry
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12. • Negative Chemical ionization (NCI)
In order to see a response by negative chemical ionization (NCI, also NICI), 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 techniques (EI,
PCI). NCI can be used for the analysis of compounds containing acidic groups or electronegative elements (especially
halogens).Moreover, negative chemical ionization is more selective and demonstrates a higher sensitivity toward oxidizing
agents and alkylating agents.
Because of the high electronegativity of halogen atoms, NCI is a common choice for their analysis. This includes many
groups of compounds, such as PCBs, pesticides, and fire retardants Most of these compounds are environmental
contaminants, thus much of the NCI analysis that takes place is done under the auspices of environmental analysis. In
cases where very low limits of detection are needed, environmental toxic substances such as halogenated species,
oxidizing and alkylating agents are frequently analyzed using an electron capture detector coupled to a gas
chromatograph.
Negative ions are formed by resonance capture of a near-thermal energy electron, dissociative capture of a low energy
electron and via ion-molecular interactions such as proton transfer, charge transfer and hydride transfer. Compared to the
other methods involving negative ion techniques, NCI is quite advantageous, as the reactivity of anions can be monitored
in the absence of a solvent. Electron affinities and energies of low-lying valencies can be determined by this technique as
well.
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13. • Charge-exchange chemical ionization
This is also similar to CI and the difference lies in the production of a radical cation with an odd number of electrons. The
reagent gas molecules are bombarded with high energy electrons and the product reagent gas ions abstract electrons from
the analyte to form radical cations. The common reagent gases used for this technique are toluene, benzene, NO, Xe, Ar and
He.
Careful control over the selection of reagent gases and the consideration toward the difference between the resonance
energy of the reagent gas radical cation and the ionization energy of the analyte can be used to control fragmentation.
• Atmospheric-pressure chemical ionization
Chemical ionization in an atmospheric pressure electric discharge is called atmospheric pressure chemical ionization (APCI),
which usually uses water as the reagent gas. An APCI source is composed of a liquid chromatography outlet, nebulizing the
eluent, a heated vaporizer tube, a corona discharge needle and a pinhole entrance to 10−3 torr vacuum. The analyte is a gas
or liquid spray and ionization is accomplished using an atmospheric pressure corona discharge. This ionization method is
often coupled with high performance liquid chromatography where the mobile phase containing eluting analyte sprayed
with high flow rates of nitrogen or helium and the aerosol spray is subjected to a corona discharge to create ions. It is
applicable to relatively less polar and thermally less stable compounds. The difference between APCI and CI is that APCI
functions under atmospheric pressure, where the frequency of collisions is higher. This enables the improvement in
sensitivity and ionization efficiency.
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14. • .
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15. Modern Ionization Techniques:
• Atmospheric Pressure Chemical Ionization (APCI)
Principle: Sample is ionized by chemical reactions with ions generated in a corona discharge or by other means at
atmospheric pressure.
Instruments: Widely used in LC-MS systems for analyzing compounds that are less polar or less amenable to ESI.
• Desorption Electrospray Ionization (DESI)
Principle: Solvent spray interacts with the sample surface, generating ions from the surface for mass analysis.
Instruments: Useful for ambient ionization, allowing direct analysis of surfaces, such as tissues in biological samples.
• Direct Analysis in Real Time (DART)
Principle: Sample is exposed to a stream of gas at high temperatures, generating ions for analysis.
Instruments: Provides rapid analysis of solid, liquid, and gaseous samples, often used for forensic and pharmaceutical
applications.
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• Paper Spray Ionization (PSI)
Principle: Analyte is introduced by spraying onto paper substrates, and ions are formed in the
presence of a high voltage.
Instruments: Particularly useful for rapid screening and analysis of samples, with potential
applications in clinical and environmental analysis.
• Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
Principle: Sample is introduced into an argon plasma, generating ions which are then analyzed
using mass spectrometry.
Instruments: Primarily used for elemental analysis, particularly in environmental, geological,
and biological research.

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18. Thank You
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