Mass Spectrometry

Mass spectrometry.
PRESENTED BY--
MR. PUJAN SASMAL.
M.PHARM 1ST YEAR STUDENT.
DEPT.OF PHARMACEUTICAL CHEMISTRY.
UNDER THE GUIDANCE OF—
DR. RAJESH R.
ACHARYA AND BM REDDY COLLEGE OF PHARMACY.

COURSE CONTENT:
Principle.
Theory.
Instrumentation of Mass Spectroscopy.
Different types of ionization like electron impact, chemical, field,
FAB and MALDI, APCI, ESI, APPI Analyzers of Quadrupole and
Time of Flight.
Mass fragmentation and its rules.
Meta stable ions.
Isotopic peaks.
Applications of Mass Spectroscopy.
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PUJANSASMAL,MASSSPECTROMETRY

TOPICDISCUSSED:
 Principle of Mass Spectroscopy.
 Theory of Mass Spectroscopy.
 Electron Impact Ionizer.
 Chemical ionization.
 Quadrupole Analyzer.
3

INTRODUCTION:
 At the beginning of the 20th century, mass spectrometers were invented to help
physicists and physical chemists prove the existence of isotopes of the elements.
 As this era of mass spectrometry reached maturity in the 1940s, some
physicists announced that there would no longer be any need for mass
spectrometry because virtually all of the elements had been discovered and
characterized.
 While mass spectrometers were being used for the purification of fissionable
material for atomic weapons as part of the Manhattan Project of World War-II,
organic mass spectrometry was being invented for the analysis and quality control
of aviation fuel. In 1945, the application of mass spectrometry to organic
chemistry emerged as a productive new area of research and discovery.
 Toward the late 1950s, organic mass spectrometers began to be used for the
analysis of a wider variety of organic molecules and eventually became a
fundamental analytical tool for the characterization of synthetic organic
compounds.
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PUJAN SASMAL, MASS SPECTROMETRY

HISTORY:
PUJAN SASMAL, MASS SPECTROMETRY 5
 In 1886, Eugen Goldstein observed rays in gas discharges under low pressure that
traveled away from the anode and through channels in a perforated cathode, opposite to
the direction of negatively charged cathode rays (which travel from cathode to anode).
Goldstein called these positively charged anode rays "Kanalstrahlen"; the standard
translation of this term into English is "canal rays".
 Wilhelm Wien found that strong electric or magnetic fields deflected the canal rays
and, in 1899, constructed a device with perpendicular electric and magnetic fields that
separated the positive rays according to their charge-to-mass ratio (Q/m).
 Wien found that the charge-to-mass ratio depended on the nature of the gas in the
discharge tube.
 English scientist J. J. Thomson later improved on the work of Wien by reducing the
pressure to create the mass spectrograph.

HISTORY:
PUJANSASMAL,MASSSPECTROMETRY 6
Modern techniques of mass spectrometry were
devised by Arthur Jeffrey Dempster and F.W. Aston
in 1918 and 1919 respectively.
Sector mass spectrometers known
as calutrons were developed by Ernest O.
Lawrence and used for separating the isotopes of
uranium during the Manhattan Project. Calutron
mass spectrometers were used for uranium
enrichment at the Oak Ridge, Tennessee Y-12
plant established during World War II.
In 1989, half of the Nobel Prize in Physics was
awarded to Hans Dehmelt and Wolfgang Paul for
the development of the ion trap technique in the
1950s and 1960s.
In 2002, the Nobel Prize in Chemistry was
awarded to John Bennett Fenn for the development
of electrospray ionization (ESI) and Koichi
Tanaka for the development of soft laser
desorption (SLD) and their application to the
ionization of biological macromolecules, especially
proteins.

PRINCIPLE:
When compound is bombarded with an electron, the compound has tendency to
lose one electron and form the metastable ions as represented in the following
equation:
M M+ + e-
e-
The increase in energy will lead
to the production of cations
which are determined by the
mass spectrometer which is
based on their m/e of positive
ions.
e-
7

PRINCIPLE:
Mass to charge ratio (m/e) is the mass of samples divided by the charge
of the sample.
Ions give information about the structure and nature of the sample molecule.
The ions which has high mass to charge ratio, they are heavier isotopes and
vice versa.
The molecular mass of these separated ions is also determined. The separated
ions on the basis of m/e ratio are determined in proportion to their
abundance. The result will be obtained in the form of mass spectrum.
The mass spectrum is plotted between the ion’s abundance and m/e ratio.
By this way, a mass spectrum of the molecule is thus produced. It displays
the result in the form of a plot of ion abundance versus m/e ratio.
8

The mass spectrum for toluene.

THEORY:
12
The mass spectrometer is an instrument which help in separating the individual atoms or
molecules because of the difference in their masses.
Consider a molecule M, which is bombarded with a beam of electrons Suppose this is
ionised as follows:
M M+ + 2e-
where M is an ionised molecule and is an electron. The ions are then accelerated in an
electric field at voltage V. If this is the condition, the energy given to each particle is eV
and this is equal to the kinetic energy which is equal to 1/2mv2.
This can be expressed as---
1/2mv2 = eV -------------------(1)
e-
Where, v is the velocity of the particle of mass m. e is the charge on an electron and V
is the accelerating voltage.
v = √(2eV/m) --------------------(2)

THEORY:
All the particles possess the same energy eV. Also, all particles have the same kinetic
energy mv. As the value of m varies from particle to particle, the velocity v also
changes such that 1/2mv2 remains a constant.
For a particle of mass m1 and velocity v1 relation becomes as-
1/2m1v1
2 = eV
Similarly for a particle of mass m2 and velocity v2 relation becomes
1/2m2v2
2 = eV
For particles m3, m4........of velocities v3, v4 respectively, equation becomes as-
1/2m3v3
2 = eV
1/2m4v4
2 = eV…..
From this relation it follows that the velocity of different particles will vary,
depending on the mass of the particles.

THEORY:
After the charged particles have been accelerated by an applied voltage, they
enter a magnetic field H. This field attracts the particles and they move in a
circle around it.
This attractive force, due to magnet is Hev, whereas the balancing centrifugal
force of the particle is mv2/r. When the particle starts moving uniformly around
the circular path, the two forces become equal.
mv2/r = Hev, --------------------(3)
or, 1/r = Hev/mv2,
or, r = mv/eH, ---------------------(4)
Where r is the radius of the circle path of the particle.
Put the value of v from equation (2)-----
r = (m/eH)*(2eV/m)1/2,
Rearranging the equation and squaring both sides,
m/e = (H2r2/2V)

INSTRUMENTATION:
The four main parts of mass spectrometry are discussed below:
Ionizer:
The bombarding of the sample is done by the electrons. These electrons move
between cathode and anode. When the sample passes through the electron
stream between the cathode and anode, electrons with high energy knock
electrons out of the sample and form ions.
Accelerator:
The ions placed between a set of charged parallel plates get attracted to one
plate and repel from the other plate. The acceleration speed can be controlled
by adjusting the charge on the plates.
Deflector or Analyzer:
Magnetic field deflects ions based on its charge and mass. If an ion is heavy or
has two or more positive charges, then it is least deflected. If an ion is light or
has one positive charge, then it is deflected the most.
Detector:
The ions with correct charge and mass move to the detector. the ratio of mass to
charge is analyzed through the ion that hits the detector.

01.
• IONIZER
• Production of ions in gaseous phase.
02.
• ACCELARATOR
• Accelerates the ions in magnetic field.
03.
• MASS ANALYZER
• Separation of ions according to m/e ratio.
04.
• DETECTOR.
• Detection of ions.

INSTRUMENTATION:

Electron Impact Ionizer:
 Electron ionization (EI, formerly known as electron impact
ionization and electron bombardment ionization) is an ionization
method in which energetic electrons interact with solid or gas phase
atoms or molecules to produce ions.
 EI was one of the first ionization techniques developed for mass
spectrometry. However, this method is still a popular ionization
technique.
 This technique is considered a hard (high fragmentation) ionization
method, since it uses highly energetic electrons to produce ions.
 This leads to extensive fragmentation, which can be helpful for
structure determination of unknown compounds.
 EI is the most useful for organic compounds which have a molecular
weight below 600. Also, several other thermally stable
and volatile compounds in solid, liquid and gas states can be detected
with the use of this technique when coupled with various separation
methods.

 As the electron source, the cathode, which can be
a thin filament of tungsten or rhenium wire, is
inserted through a slit to the source block. Then it is
heated up to an incandescent temperature to emit
electrons. A potential of 70 V is applied between the
cathode and source block to accelerate them to 70
eV kinetic energy to produce positive ions.
 The potential of the anode (electron trap) is
slightly positive and it is placed on the outside of the
ionization chamber, directly opposite to the cathode.
The unused electrons are collected by this
electron trap.
 To increase the ionization process, a weak magnetic field is applied parallel to the direction
of the electrons' travel. Because of this, electrons travel in a narrow helical path, which
increases their path length.
 The positive ions that are generated are accelerated by the repeller electrode into the
accelerating region through the slit in the source block. To avoid the condensation of the
sample, the source block is heated to approximately 300 °C.

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
are ionized by electron ionization, which subsequently react with analyte molecules in the
gas phase in order to achieve ionization.
 Negative chemical ionization (NCI), charge-exchange chemical ionization and
atmospheric-pressure chemical ionization (APCI) are some of the common variations of
this technique. CI has several important applications in identification, structure elucidation
and quantitation of organic compounds.

Chemical ionization:
 Chemical ionization requires a lower amount of energy compared to electron
ionization (EI), but this depends on the reactant material used.
 This low-energy ionization mechanism yields less or sometimes no
fragmentation, and usually a simpler spectrum.
 The lack of fragmentation limits the amount of structural information that can
be determined about the ionized species.
 However, a typical CI spectrum has an easily identifiable protonated molecular
ion peak [M+1]+, which allows easy determination of molecular mass.
 This technique requires the transfer of high-mass entities from the reagent gas
to the analyte, and therefore, the Franck-Condon principle does not govern the
process of ionization. CI is thus quite useful in cases where the energy of the
bombarding electrons in EI is high, resulting exclusively in fragmentation of the
analyte, causing the molecular-ion peak to be less detectable or completely
absent.

Mechanism of Chemical ionization:
 A Chemical Ionisation experiment involves the use of gas phase acid-base
reactions in the chamber.
 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, water and
isobutane. Inside the ion source, the reagent gas is present in large excess
compared to the analyte.
 Electrons entering the source with energy around 200-500 eV will
preferentially ionize the reagent gas. Then, the ion/molecule reactions produces
more stable reagent ions and 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.

Mechanism of Chemical ionization:
The following reactions are possible with methane as the reagent gas--
CHEMICAL
IONIZATION.
POSITIVECHEMICAL
IONIZATION.
NEGATIVECHEMICAL
IONIZATION.

Quadrupole AnalYzer:
 The quadrupole mass analyzer (QMS), also known as a transmission
quadrupole mass spectrometer, quadrupole mass filter, or quadrupole
mass spectrometer, is one type of mass analyzer used in mass spectrometry.
 As the name implies, it consists of four cylindrical rods, set parallel to each
other.
 In a quadrupole mass spectrometer the quadrupole is the mass analyzer - the
component of the instrument responsible for selecting sample ions based on
their mass-to-charge ratio (m/z). Ions are separated in a quadrupole based on the
stability of their trajectories in the oscillating electric fields that are applied to
the rods.

Quadrupole AnalYzer:
 The quadrupole consists of four parallel metal rods (molybdenum alloys).
 Each opposing rod pair is connected together electrically, and a radio
frequency (RF) voltage with a DC offset voltage is applied between one pair of
rods and the other. Ions travel down the quadrupole between the rods. Only ions
of a certain mass-to-charge ratio will reach the detector for a given ratio of
voltages: other ions have unstable trajectories and will collide with the rods.
If RF>DC then larger particle hit
detector first.
 If RF<DC then smaller particles
hit detector first.
 This permits selection of an ion
with a particular m/z or allows the
operator to scan for a range of m/z-
values by continuously varying the
applied voltage.

 Ideally, the rods are hyperbolic. Cylindrical rods with a specific ratio of rod
diameter-to-spacing provide an easier-to-manufacture adequate approximation
to hyperbolas. Small variations in the ratio have large effects on resolution and
peak shape. Different manufacturers choose slightly different ratios to fine-tune
operating characteristics in context of anticipated application requirements. In
recent decades some manufacturers have produced quadrupole mass
spectrometers with true hyperbolic rods.

REFERENCE:
1. “Instrumental Methods of Chemical Analysis” by Gurdeep R.
Chatwal and Sham K. Anand, 1st Edition-2017, Page No: 2.272-
2.302.
2. “ELEMENTARY ORGANIC SPECTROSCOPY” by Y.R.
SHARMA, 1st Eedition, Page No: 291-364.
3. BURGER’S Medicinal Chemistry & Drug Discovery, Vol-1,
SIXTH EDITION, Page No: 583-610.
4. https://en.wikipedia.org/wiki/Quadrupole_mass_analyzer.
5. https://en.wikipedia.org/wiki/Chemical_ionization.

THANK YOU.....
PUJAN SASMAL, MASS SPECTROMETRY 31

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