Molecular Mass
Spectroscopy
Chem. 331
Introduction
• In Mass Spectroscopy (MS), atomic and
molecular weights are generally expressed
in terms of atomic mass units (amu). The
atomic mass unit is based on upon a
relative scale in which the reference is the
carbon isotope 126C, which is assigned a
mass of exactly 12 amu. Thus the amu is
defined as 1/12 of the mass of one neutral
carbon atom.
Mass Spectroscopy
• Mass spectroscopy is perhaps one of the
most widely applicable of all the analytical
tools available to the analytical chemist in
the sense that this technique is capable of
providing information about
Technique is capable of providing
information about
• the qualitative and quantitative composition of
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both organic and inorganic analytes in complex
mixtures
this instrument measures compounds with
molecular masses up to 200, 000 Daltons.
the structures of a wide variety of complex
molecular species
isotopic ratios of atoms in samples and
the structure and composition of solid surfaces.
The four main components of a
molecular mass spectrometer
The Sample Inlet System
• Batch Inlet Systems:
• The Direct Probe Inlet:
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• These systems are used for
These systems are the simplest
and simply involve the
volatilization of the sample
externally and then the gradual
leakage of the volatilized sample
into the evacuated ionization
chamber. For gases, the sample is
introduced into the metering
volume container and then
expanded into the reservoir flask
where it is then leaked into the
ionization chamber. For liquids, a
small quantity of sample is
introduced into the reservoir and
the pressure of the system is
reduced to about 10-5 torr. The
inlet system is lined with glass to
avoid losses of polar analytes by
adsorption.
solids and non-volatile liquids
and in these systems the
sample is introduced into the
ionization region by means of
a sample holder, or probe,
which is inserted through a
vacuum lock. Probes are also
used when the amount of the
sample to be analyzed is small.
With a probe, the sample is
generally held on the surface
of a glass or aluminum
capillary tube and positioned
within a few meters of the
ionization source.
Magnetic Sector Analyzers
• Magnetic sector analyzers employ a permanent magnet or
electromagnet to cause the beam from the ion source to travel in a
circular path of 180, 90, or 60 degrees. Here, ions are formed by
electron impact.
• The translational energy of an ion of mass m and charge z upon
exciting slit B is given by
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K. = Zev = ½ mv2
Equation 1
where V is the voltage between A and B, v is the velocity of the ion
after acceleration, and e is the charge of the ion.
The path in the sector described by the ions of a given mass and
charge represents a balance between two forces acting upon them.
These two forces are the centripetal force and the magnetic force
and equating these two forces yields:
Bzev = mv2/rwhich rearranges to v = Bzer/m
Substituting the above equation into (1) gives m/z = B2r2e/2V
The last equation shows how mass spectra can be obtained by
varying one of the three variables B, V, or r.
Double Focusing Instruments
• These type of instruments, unlike single-
focusing which simply minimize directional
errors, are designed to limit both the
errors introduced because ions are initially
moving in different directions and also the
errors introduced due to the fact that ions
of the same mass-to-charge ratio may
have different translational energies.
A schematic of a doublefocusing instrument is shown
Time-of-Flight Analyzers
• In time-of-flight instruments, positive ions are
produced periodically by bombardment of the
sample with brief pulses of electrons, secondary
ions or laser generated photons. The ions
produced are then accelerated by an electric
field and then made to pass into a field-free drift
tube about a meter long. Because all ions
entering the tube ideally have the same kinetic
energies, their velocities in the tube must vary
inversely with their masses, with the lighter
particles arriving at the detector earlier than the
heavier ones.
Isotopes
• By definition isotopes are atoms having
the same atomic number but different
mass numbers this technique is
advantageous because in mass
spectroscopy isotopes are easily
differentiated. As depicted below: a
mixture which contains isotopes are
differentiated and shown in a spectrum.
Gas-Phase Sources
• Gas-phase sources require volatilization of the sample
before ionization and thus are limited to thermally stable
compounds that have boiling points less than about
500°C.
• Electron Impact Source : In the sources, electrons
emitted from a filament are accelerated by a potential of
about 70 V and made to collide with gaseous atoms or
molecules of the sample causing ionization. Electronimpact ionization is not very efficient and only about one
molecule in a million undergoes the primary reaction
M + e- = M.+ + 2e• Electron Impact spectra are very complex due to the
high energies possessed by the accelerated electrons
which collide with the sample and lead to fragmentation.
These complex spectra are very useful for compound
identification.
Advantages of Electron Impact
sources
• They are convenient and produce
high ion currents.
• Extensive fragmentation can lead to
unambiguous identification of
analytes.
Disadvantages of Electron
Impact sources
• The need to volatilize the sample limits
this method since it excludes analysis of
thermally unstable compounds.
• Excessive fragmentation can lead to the
disappearance of the molecular ion peak
therefore preventing the molecular mass
of the analyte to be determined.
Identification of Pure Compounds
by Mass Spectroscopy
• Mass spectroscopy can be used to determine the molecular weight
of a compound but this involves an identification of a molecular
peak and a comprehensive study of a spectrum.
• TANDEM MASS SPECTROSCOPY: This type of spectroscopy simply
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involves the coupling of one mass spectrometer to another and this
hyphenated technique has resulted in dramatic progress in the
analysis of complex mixtures.
SECONDARY ION MASS SPECTROSCOPY: This is one of the most
highly developed of the mass spectrometric surface methods, with
several manufacturers offering instruments for this technique. It
involves the bombarding of a surface with a beam of ions formed in
an ion gun. The ions generated from the surface layer are then
drawn into a spectrometer for mass analysis.
References
• TSRI Mass Spectrometry Home Page:
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http://masspec.scripps.edu/hist.html
American Chemical Society:
http://www.acs.org
Chemical Abstracts Service:
http://www.cas.org
Chemical Center Home Page:
http://www.chemcenter/org
Science Magazine: http://www.sciencemag.org
Journal of Chemistry and Spectroscopy:
http://www.kerouac.pharm.uky.edu/asrg/wave/wavehp.html
http://www.uis.edu/trammell/che425/ms-2
http://www.usf.edu/~sl/mass_spec/MS_instrumentation.html
http://minyos.its.rmit.edu.au/~rcmfa/mstheory.html
http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/MassSpec/
masspec1.htm
http://www.abrf.org/ABRFNews/1996/September1996/sep96iontrap.
html