Transcript Document

Mass Spectroscopy
Mass Spectroscopy (Mass Spec)
Applying Atomic Structure
Knowledge to Chemical Analysis
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Mass Spectroscopy
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.
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Mass Spectroscopy
Mass Spectroscopy
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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
path.
•
Positive ions of different masses and charges are
deflected differently in the field.
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Mass Spectroscopy
Mass Spectroscopy Analysis
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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.
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Mass Spectroscopy
Mass Spectrometer Basics
(An ions deflection
depends on their mass
and charge, but most all
have a +1 charge)
++
+
+
+ +
+ +
++++++++
Ionization by
electron beams
Accelerating
voltage applied
(This must be done in
vacuum so the ions can
move about freely without
hitting air molecules)
(Ions are accelerated
through a series of slits
with decreasing voltages)
Selection by
magnetic field
velocity
+
velocity
Deflection in
a magnetic
field
----------------
amplifier
recorder
or PC
5
Detector
+
+
+
Mass Spectroscopy
Schematic of a Mass Spectrometer
ion-accelerating
electric field
positive ions
accelerated ion beam
least massive ions
heating
device to
vaporize
sample
sample
most massive ions
slits
electron beam
magnetic field
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Mass Spectroscopy
Mass Spectrometry Competing Phenomena
1) Molecular formation by atomic collisions
Carbon atom with 4
electrons in its outer orbit
C
O
Note:
Carbon has 12 units of mass
C
2) Ion Formation
Oxygen atom with 6
electrons in its outer orbit
Note:
Oxygen has 16 units of mass
O
Carbon Monoxide
Note: Carbon Monoxide has 28 units of mass
Alternate Drawing
Six electrons being shared by oxygen and carbon
C
O
(or three covalent bonds between oxygen and carbon)
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Mass Spectroscopy
Mass Spectrometry Competing Phenomena
1) Molecular Formation
2) Ion Formation
2) Ions formation by electron collisions with atoms and/or
molecules
C
Energy
+
O
Carbon Monoxide
(collection of positive ions)
C
++
O
C
Note: This ion has a mass
of 28 units per unit of charge
C
O
Note: This ion has a mass
of 14 units per unit of charge
O
+
+
Note: This ion has a mass
16 unit per unit charge
Note: This ion has a mass
of 12 units per unit charge
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Mass Spectroscopy
Mass Spectrometry
Carbon Monoxide sample
from a vacuum process chamber
O
a detector that counts the ions
that pass through the quadrupoles
O
C
Ionization, acceleration and selection
C
++
C
C
O
O
C
C
C
+
+
O
C
e
+
O
O
e
e
e
e
e
e
Cold
wire
Hot wire
Quadrupole rods direct select mass to charge
ions down middle path depending on magnetic
fields applied to pole pairs.
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O
Mass Spectroscopy
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.
ionizing filaments
the quadrupoles
electronics for quadrupole
mass spec
quadrupole mass
spectrometer installed in
a vacuum application
4 steel rods inside
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Mass Spectroscopy
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.
Relative abundance
Schematic of a Typical Mass Spectra
90
92
94
96
98
100
Mass to charge ratio, m/z
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102
Mass Spectroscopy
Relative abundance, %
Mass Spectra – the Data
• The y-axis reflects the relative
abundance of a particular ion
hitting the detector.
100
• The ions hitting the detector at
each location on the detector
produce an electrical current.
80
60
40
• This current is proportional to the
number of ions hitting the
detector.
20
0
90
92
94
96
98
100
Mass to charge ratio, m/z
102
• 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.
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Mass Spectroscopy
Mass Spectra – the Analysis
Relative abundance, %
• 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 charge.
100
80
60
40
20
0
90
92
94
96
98
100
Mass to charge ratio, m/z
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• 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.
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Mass Spectroscopy
Mass Spectra – the Data
Relative abundance
•This is the mass spectra of
molybdenum, Mo.
•Each of the 7 peaks reflect a
different isotope of Mo.
90
92
94
96
98
100
•If they all have +1 charge,
they all have masses of 92,
94, 95, 96, 98, and 100.
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Mass to charge ratio, m/z
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
analyzed.
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Mass Spectroscopy
Relative abundance
Mass Spectra – the Data
• Mo is element 42 in the periodic
table, and has an average atomic
weight of 95.94.
90
92
94
96
98
100
Mass to charge ratio, m/z
102
• If you read the relative abundance
of each isotope’s 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
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Mass Spectroscopy
Mass Spec Applications
Isotopic masses are used:
• to determine average atomic mass of elements.
• to identify a compound’s composition and
structure.
• for archaeological dating.
• to identify particulates in space (when mounted on
a satellite or the space station).
• to assure safe environments in nuclear powered
vessels.
• to monitor process conditions when fabricating
computer microchips.
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Mass Spectroscopy
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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Mass Spectroscopy
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