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Mass Spectroscopy (Mass Spec)


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Title: Mass Spectroscopy (Mass Spec)

Mass Spectroscopy (Mass Spec) Applying Atomic
Structure Knowledge to Chemical Analysis
Mass Spectroscopy
  • Spectroscopy is the study of the interaction of
    electromagnetic radiation with matter.
  • In mass spectroscopy, atoms and/or molecules are
    exposed to a beam of high-speed electrons.
  • The electron beam knocks electrons off the atoms
    or molecules and thereby changes them into
    positively charged ions.

Mass Spectroscopy
  • If the sample is an atom, the mass spectra will
    reveal the different isotopes of the element.
  • If the sample is a molecule, it is broken into
    several fragments, each of which becomes ionized
    in the electron beam.
  • After ionization, an applied electric field
    accelerates the positive ions into a chamber
    where an applied magnetic field deflects their
  • Positive ions of different masses and charges are
    deflected differently in the field.

Mass Spectroscopy Analysis
  • The amount of deflection in the magnetic field
    for each ion depends on its mass and charge.
  • The most massive, singly charged ions are
    deflected the smallest amount.
  • The locations where different ions hit the
    detector plate can be correlated to their atomic
    masses and charges.

Mass Spectrometer Basics
Schematic of a Mass Spectrometer
least massive ions
ion-accelerating electric field
positive ions
accelerated ion beam
heating device to vaporize sample
most massive ions
magnetic field
electron beam
Mass Spectrometry Competing Phenomena
1) Molecular formation by atomic collisions
2) Ion Formation
Carbon atom with 4 electrons in its outer orbit
Oxygen atom with 6 electrons in its outer orbit
Carbon Monoxide
Alternate Drawing
Six electrons being shared by oxygen and carbon
(or three covalent bonds between oxygen and
Mass Spectrometry Competing Phenomena
1) Molecular Formation
2) Ion Formation
2) Ions formation by electron collisions with
atoms and/or molecules
Carbon Monoxide
(collection of positive ions)
Mass Spectrometry Ionization, acceleration and
Quadrupole rods direct select mass to charge ions
down middle path depending on magnetic fields
applied to pole pairs.
Hot wire
Mass Spectra the Quadrupole
A quadrupole (4 rods) is one configuration used
for deflecting ions to separate them by mass. A
magnetic field is created by the 4 rods inside
the steel tube and can be adjusted to cause
different ions to reach the detector as the
applied magnetic field is changed.
Mass Spectra the Data
A mass spectra is sometimes referred to as a
stick diagram which shows the relative amounts
of the different ions as a function of their
mass, expressed as the ratio of mass to charge,
m/z. Most of the ions formed in the electric
field have a 1 charge.
Schematic of a Typical Mass Spectra
Mass Spectra the Data
  • The y-axis reflects the relative abundance of a
    particular ion hitting the detector.
  • The ions hitting the detector at each location on
    the detector produce an electrical current.
  • This current is proportional to the number of
    ions hitting the detector.
  • The more ions of a given size that reach the
    detector, the larger the signal for that ion.
  • In this application, the detector acts something
    like a counter.

100 80 60 40 20 0
Mass Spectra the Analysis
  • The x-axis reflects the mass (m) of each ion as a
    ratio to its charge (z).
  • Most ions formed in mass spectrometers have a 1
  • For samples that are atoms, the different mass to
    charge ratios reflect different isotopes.
    Isotopes of an element have different numbers of
    neutrons but the same number of protons.
  • For samples that are molecules, the ion with the
    highest m/z ratio is called the parent ion.
    This ion is the original molecule with one less
    electron, and thus has the same mass weight as
    the original species.

100 80 60 40 20 0
Mass Spectra the Data
  • This is the mass spectra of molybdenum, Mo.
  • Each of the 7 peaks reflect a different isotope
    of Mo.
  • If they all have 1 charge, they all have masses
    of 92, 94, 95, 96, 98, and 100.

Using the spectra, which isotope of Mo is the
most abundant in this particular sample?
The tallest stick is the one at m/z 98. This
isotope of Mo is the most abundant in the sample
Mass Spectra the Data
  • Mo is element 42 in the periodic table, and has
    an average atomic weight of 95.94.
  • If you read the relative abundance of each
    isotopes peak, you could calculate the average
    atomic weight from this spectra and compare the
    value you get to the published value of 95.94.

Knowing the atomic number of molybdenum is 42,
how may neutrons are in the Mo isotope
represented by the peak that is most abundant in
the sample?
98 (protons neutrons) 42 protons 56 neutrons
How many neutrons are in the peak with the
smallest abundance?
97 (protons neutrons) 42 protons 55 neutrons
Mass Spec Applications
  • Isotopic masses are used
  • to determine average atomic mass of elements.
  • to identify a compounds composition and
  • for archaeological dating.
  • to identify particulates in space (when mounted
    on a satellite or the space station).
  • to assure safe environments in nuclear powered
  • to monitor process conditions when fabricating
    computer microchips.

Mass Spec Applications
Many disciplines use mass spectroscopy for
chemical identification. Astronomy analysis of
astronomical components of the solar
system Electronics analysis of
microchips Environmental detection of toxic
chemical, monitoring of nuclear facilities,
analysis of petroleum products, etc. Forensics
toxicology, trace metals, biological materials,
etc. Medical drug abuse diagnosis, analysis of
pharmaceuticals and products of genetic
engineering Military mobile mass spectrometers
are used to detect liquid chemical warfare agents
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