Instrumental Chemistry
Chapter 11
Atomic Mass
Spectrometry
Atomic mass
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The mass of a single atom, usually
expressed in atomic mass units (amu)
Most of the mass of an atom is concentrated
in the protons and neutrons contained in the
nucleus
Each proton or neutron weighs about 1 amu,
and thus the atomic mass is always very
close to the mass number (total number of
protons and neutrons in the nucleus)
Mass Spectrometers
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Mass spectrometers use the difference
in mass-to-charge ratio (m/e) of ionized
atoms or molecules to separate them
from each other
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In general a mass spectrometer
consists of an ion source, a massselective analyzer, and an ion
detector
Diagram of a Mass Spectrometer
Stage 1: Ionization
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The atom is ionized by knocking one or
more electrons off to give a 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
Ionization (cont.)
Stage 2: Acceleration
The ions are accelerated so that they
all have the same kinetic energy
 All the ions are accelerated into a finely
focused beam
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Acceleration (cont.)
Stage 3: Deflection
• The ions are then deflected by a
magnetic field according to their
masses
 The lighter they are, the more they are
deflected
• The more the ion is charged, the more
it gets deflected
Deflection (cont.)
Stage 4: Detection
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The beam of ions passing through the
machine is detected electrically
When an ion hits the metal box, its charge is
neutralized by an electron jumping from the
metal on to the ion
That leaves a space amongst the electrons
in the metal, and the electrons in the wire
shuffle along to fill it
Detection (cont.)
Fourier-Transform MS
Magnetic-sector MS
Quadrupole MS
Time-of-flight MS
ICP-MS
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Inductively Coupled Plasma Mass
Spectrometry
In ICP-MS, the plasma is formed from Argon
gas
Plasma is defined as a gas consisting of
ions, electrons, and neutral particles
SSMS
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Spark Source Mass Spectrometry
Semi-quantitative trace element technique
for the analysis of solids and liquid materials
Advantages include total simultaneous
elemental coverage, low detection limits,
semi-quantitative accuracy (+2-3x), and high
resolution capabilities to eliminate many
spectral interferences
GDMS
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Glow-Discharge Mass Spectrometry
Analytical technique for the bulk elemental
analysis of inorganic solid samples
The most comprehensive and sensitive
technique available for the analysis of solids
Glow Discharge Ion Source
Useful Websites Dealing With Mass Spectrometry
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http://www.anachem.umu.se/jumpstation.htm
http://userwww.service.emory.edu/~kmurray/mslist.html
http://www.chemcenter/org
http://www.sciencemag.org
http://reference.allrefer.com/encyclopedia/A/atomMas.html
http://www.webref.org/geology/a/atomic_mass.htm
http://www.chemguide.co.uk/analysis/masspec/howitworks.html
http://www.northernanalyticallab.com/tech2.htm
http://www.shivatec.com/new/gdmsdesc.php4