Transcript mass spectroscopy

J.J. Thomson
Discoverer of the Electron
Background Information
•
•
•
•
Cathode Rays
Form when high voltage is applied across
electrodes in a partially evacuated tube.
Originate at the cathode (negative electrode)
and move to the anode (positive electrode)
Carry energy and can do work
Travel in straight lines in the absence of an
external field
A Cathode Ray Tube
Source of
Electrical
Potential
Stream of negative
particles (electrons)
Metal Plate
Gas-filled
glass tube
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58
Metal plate
Cathode Ray Experiment
1897 Experimentation
• Using a cathode ray tube, Thomson
was able to deflect cathode rays with an
electrical field.
• The rays bent towards the positive pole,
indicating that they are negatively
charged.
The Effect of an Obstruction on
Cathode Rays
High
voltage
source of
high voltage
shadow
cathode
yellow-green
fluorescence
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 117
The Effect of an Electric Field on
Cathode Rays
source of
high voltage
High
voltage
cathode
negative
plate
_
+
anode
positive
plate
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 117
Cathode Ray Experiment
Displacement
Volts
Anodes / collimators
Cathode
+
Deflection
region
Drift region
Thomson’s Calculations
Cathode Ray Experiment
• Thomson used magnetic and electric fields to measure and
calculate the ratio of the cathode ray’s mass to its charge.
Electric
=
deflection
Magnetic
=
deflection
charge of
ray particle
x
charge of
ray particle
x
length of
electric
x
deflection region x
field
mass of ray
velocity of 2
x
particle
ray particle
length of
drift region
length of
magnetic
x
deflection region x
field
mass of ray
velocity of
x
particle
ray particle
length of
drift region
magnetic deflection
electric deflection
magnetic field
=
electric field
x
velocity
Conclusions
• He compared the value with the mass/ charge ratio for the
lightest charged particle.
• By comparison, Thomson estimated that the cathode ray particle
weighed 1/1000 as much as hydrogen, the lightest atom.
• He concluded that atoms do contain subatomic particles - atoms
are divisible into smaller particles.
• This conclusion contradicted Dalton’s postulate and was not
widely accepted by fellow physicists and chemists of his day.
• Since any electrode material produces an identical ray, cathode
ray particles are present in all types of matter - a universal
negatively charged subatomic particle later named the electron
So what does J.J. Thomson have to do with mass spec?
• Just as J.J. Thomson used a magnetic field
to affect charged particles, so does a mass
spectrometer.
source of
High
voltage
_
negative
plate
high voltage
cathode
+
positive anode
plate
• The machine sorts ions according to their
mass to charge ratio, something Thomson
was able to calculate for the electron using
the results of his cathode ray experiments.
What is mass spectrometry?
Mass spectrometry is a technique used to separate a
substance into ions based on their mass.
Molecules are bombarded by high energy particles
that cause them to lose one electron and carry a +1
charge. These ions undergo further fragmentation
producing smaller positive ions.
The spectrum produced plots intensity (abundance of
ions) against the ions’ mass-to-charge ratio.
Substances can be identified by their characteristic
fragment ions represented on a mass spectrum
Detector
plate
Ion-accelerating
electric field
Least massive ions
accelerated
Ion beam
Positive ions
Sample
Most
massive
ions
Electron beam
Slits
Magnetic field
Heating device
to vaporize sample
Mass spectrometers that break up molecules into fragments
that can be characterized by electrical methods. [image link]
Mass Spectrophotometer
magnetic field
heaviest
ions
stream
of ions of
different
masses
electron
beam
gas
Dorin, Demmin, Gabel, Chemistry The Study of Matter 3rd Edition, page 138
lightest
ions
Components of a Mass Spectrometer
Signal
processor
Inlet
Source
Analyzer
Detector
Vacuum
Inlet - ensures that the sample enters the machine with minimal loss
Source - sample components are ionized (the method by which this is done depends on the
specific mass spectrometer being used.)
Analyzer - accelerates ion and separates them
Detector - records the charge induced when an ion passes by or hits a surface.
Signal Processor - produces a mass spectrum, a record of the m/z's at which ions are present.
*A vacuum must be used to maintain a low pressure. A low pressure reduces the collisions among the ions.
The general operation of a mass spectrometer is:
1. create gas-phase ions
2. separate the ions based on their mass-to-charge ratio
3. measure the quantity of ions of each mass-to-charge ratio
Electron Beam
Magnetic Field
Bends Path of Charged
Particles
Molecular
Source
Ion Accelerating
Array
Collector
Exit Slit
Ho
Mass Spectrometry
198
200
202
Photographic plate
196
-
+
Stream of positive ions
Hill, Petrucci, General Chemistry An Integrated Approach 1999, page 320
199
201
204
Mass spectrum of mercury vapor
Mass Spectrum for Mercury
(The photographic record has been converted to a scale of relative number of atoms)
The percent natural abundances
for mercury isotopes are:
Hg-196
Hg-198
Hg-199
Hg-200
Hg-201
Hg-202
Hg-204
Relative number of atoms
30
25
198
0.146%
10.02%
16.84%
23.13%
13.22%
29.80%
6.85%
196
200
199
15
10
5
197
198
201
204
Mass spectrum of mercury vapor
20
196
202
199
200
Mass number
201
202
203
204
80
Hg
200.59
The percent natural abundances
for mercury isotopes are:
A
B
C
D
E
F
G
Hg-196
Hg-198
Hg-199
Hg-200
Hg-201
Hg-202
Hg-204
0.146%
10.02%
16.84%
23.13%
13.22%
29.80%
6.85%
(% "A")(mass "A") + (% "B")(mass "B") + (% "C")(mass "C") + (% "D")(mass "D") + (% "E")(mass "E") + (% F)(mass F) + (% G)(mass G) = AAM
(0.00146)(196) + (0.1002)(198) + (0.1684)(199) + (0.2313)(200) + (0.1322)(201) + (0.2980)(202) + (0.0685)(204) = x
0.28616 + 19.8396 + 33.5116 + 46.2600 + 26.5722 + 60.1960 + 13.974 = x
x = 200.63956 amu
17
Cl
35.453
• Assume you have only two atoms of chlorine.
• One atom has a mass of 35 amu (Cl-35)
• The other atom has a mass of 36 amu (Cl-36)
• What is the average mass of these two isotopes?
35.5 amu
• Looking at the average atomic mass printed on the
periodic table...approximately what percentage is Cl-35
and Cl-36?
55% Cl-35 and 45% Cl-36 is a good approximation
17
Cl
35.453
Using our estimated % abundance data
55% Cl-35 and 45% Cl-36
calculate an average atomic mass for chlorine.
Average Atomic Mass = (% abundance of isotope "A")(mass "A") + (% "B")(mass "B") + ...
AAM = (% abundance of isotope Cl-35)(mass Cl-35) + (% abundance of Cl-36)(mass Cl-36)
AAM = (0.55)(35 amu) + (0.45)(36 amu)
AAM = (19.25 amu) + (16.2 amu)
AAM = 35.45 amu
What’s mass got to do with it?
An electric or magnetic field can deflect charged
particles.
 The particles have kinetic energy as they move through
a magnetic field (KE=1/2mv2).
The particles’ inertia depends on their mass.
 A mass analyzer can steer certain masses to the
detector based on their mass-to-charge ratios (m/z). by
varying the electrical or magnetic field.
 Typically ions in a mass spectrometer carry a +1 charge
so the m/z ratio is equivalent to the ion’s mass.
What does a mass spectrum look like?
Intensity or ion abundance is plotted on the
y-axis.
The m/z ratio is plotted on the x-axis.
The base beak is from the ion that is the
most abundant and is assigned an intensity
of 100%.
The molecular ion peak, M+, is the peak due
to the parent ion (the original molecule
minus one electron).
Mass spectrum of carbon dioxide, CO2
molecular ion is seen at m/z 44.
CO2+
M+
% RELATIVE INTENSITY
100
90
80
70
60
50
40
30
C+
12
20
10
O+
CO+
28
16
0
m/z
5
10
15
20
25
30
35
40
45
50
Mass spectrums reflect the abundance of
naturally occurring isotopes.
Natural Abundance of Common Elements
Hydrogen
1H
= 99.985%
2H
= 0.015%
Carbon
12C
= 98.90%
13C
= 1.10%
Nitrogen
14N
= 99.63%
15N
= 0.37%
Oxygen
16O
= 99.762%
17O
= 0.038%
Sulfur
32S
= 95.02%
33S
= 0.75%
34S
= 4.21%
36S
= 0.02%
Chlorine
35Cl
= 75.77%
37Cl
= 24.23%
Bromine
79Br
= 50.69%
81Br
= 49.31%
18O
= 0.200%
For example….Methane
For carbon 1 in approximately 90
atoms are carbon-13
The rest are carbon-12 the isotope that
is 98.9% abundant.
So, for approximately 90 methane
molecules…1 carbon is carbon-13
C-13
Where’s
Waldo?
Where’s
Waldo?
The Mass Spectrum of Methane
100 Base peak
86
M+ = 15
C12H3+
M+ = 16
Molecular ion
[C12H4]+.
[C12H2]+.
C12H+
[C12]+.
3 8
16
1.11
12 13 14 15 16 17
M +1 = 17
[C13H4]+.
m/z
Ethyl Bromide
M = 29
C2H5+
C2H5Br81
C2H5Br79
Why is the Mass Spectrometer an Important Analytical Instrument?
Mass Spectrometers have been used in:
1) Forensics
2) Organic synthesis laboratories
3) The analysis of large biomolecules:
proteins and nucleic acids
4) Drug Test
5) Determination of isotopic abundance
6) Identification of impurities in pharmaceutical
products
7) Diagnosis of certain diseases.
References
• http://www.aist.go.jp/RIODB/SDBS/
• http://www.infochembio.ethz.ch/links/en/spectrosc_m
ass_lehr.html
• http://dbhs.wvusd.k12.ca.us/AtomicStructure/Disc-ofElectron-Intro.html
• http://wps.prenhall.com/wps/media/objects/340/3482
72/Instructor_Resources/Chapter_12/47