Download Mass Spectrometer

Chemistry
Topic: Atomic theory
Subtopic : Mass Spectrometer
What is mass spectrometer
• An instrument that measures the masses
of individual molecules that have been
converted into ions, i.e., molecules that
have been electrically charged.
• An instrument which separates particles
according to their masses & records
relative proportions of these.
Basic principle
• Samples are analyzed by introducing them
as gas into the analyzer tube where the
constituent molecules are ionized by
subjecting them to bombardment by a
controlled beam of electrons originating
from a hot-wire filament.
Simple diagram
& Stages of Operation
VAPORIZATION
Stage 1- Vaporization
• If sample is solid or liquid, it must be
heated to produce the vapor.
• This is connected to rest of the mass
spectrometer by a fine tube or
capillary so that the transfer of
material into the body of instrument
occurs very slowly (as MS must be
kept at high vacuum).
Stage 2
Ionization
The atom is ionized by knocking
one or more electrons off to give
a positive ion, by subjecting it to
bombardment by a controlled
beam of electrons originating from
a hot-wire filament.
Ionization
• The electrically heated metal coil
gives off electrons which collide with
electrons in the sample.
• Some of the collisions are energetic
enough to knock one or more
electrons out of the sample particles
to make positive ions
The need for a vacuum
It's important that the ions produced in the
ionization chamber have a free run
through the machine without hitting air
molecules.
Why +1
• Most of the positive ions formed will carry
a charge of +1 because it is much more
difficult to remove further electrons from
an already positive ion.
• These positive ions are persuaded out into
the rest of the machine by the ion repeller
which is another metal plate carrying a
slight positive charge.
Stage 3
Acceleration
• The positive ions are accelerated by
an electric field in electrostatic
analyzer.
• The ions are accelerated so that they
all have the same kinetic energy.
Stage 4-Deflection
The ions are then deflected by a magnetic field by
different amounts. The amount of deflection depends
on:
• the mass of the ion- Lighter ions are deflected more
than heavier ones.
• the charge on the ion- Ions with 2 (or more) positive
charges are deflected more than ones with only 1
positive charge.
These two factors are combined into the mass/charge
ratio. Mass/charge ratio is given the symbol m/z (or
sometimes m/e).
Deflection
• In this diagram, ion stream A is most
deflected - it will contain ions with the
smallest mass/charge ratio.
• Ion stream C is the least deflected - it
contains ions with the greatest
mass/charge ratio.
• A & C ions collide with the walls where
they will pick up electrons and be
neutralized.
Stage 5
Detection
• Only ions of a certain mass
(depending on the field strength) are
detected by detector
• In the diagram, only ion stream B
makes it right through the machine to
the ion detector.
• How might the other ions be
detected - those in streams A
and C which have been lost in
the machine?
• Remember that stream A was most deflected - it has the
smallest value of m/z (the lightest ions if the charge is
1+).
• Remember that stream C was least deflected - it has
larger m/z value (the heavier ions if the charge is 1+).
Mass spectrum
A mass spectrum is a graph of ion
intensity as a function of mass-tocharge ratio. Mass spectra are often
depicted as simple histograms
X-axis –Mass/Charge
Y-axis – Relative abundance
Base peak
• Each ion formed during ionization has a
specific value of m/z therefore each ion
will produce a signal whose intensity
depends on its relative amount.
• The peak of highest intensity is called a
base peak
• When 2 or more ions have same m/z, their
peaks superimpose & peak intensity is due
to combined abundance of all these ions.
Atomic masses & isotopes
Relative molecular masses
Applications
Mass spectrometry is a powerful analytical technique that is
used
• To identify unknown compounds,
• To quantify known compounds, and
• To elucidate the structure and chemical properties of
molecules.
• Detection of compounds can be accomplished with very
minute quantities. This means that compounds can be
identified at very low concentrations (one part in 1012) in
chemically complex mixtures.
• Mass spectrometry provides valuable information to a
wide range of professionals: physicians, astronomers,
and biologists, to name a few.
Mass spectrometry is used to...
• Detect and identify the use of steroids in athletes
• Monitor the breath of patients by anesthesiologists
during surgery
• Determine the composition of molecular species found in
space
• Determine whether honey is adulterated with corn syrup
• Locate oil deposits by measuring petroleum precursors
in rock
• Monitor fermentation processes for the biotechnology
industry
• Detect dioxins in contaminated fish
• Determine gene damage from environmental causes
• Establish the elemental composition of semiconductor
materials