j.j. thomson discoverer of the electron. background information cathode rays form when high voltage...
TRANSCRIPT
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
Source ofElectricalPotential
Metal Plate
Gas-filledglass tube Metal plate
Stream of negativeparticles (electrons)
A Cathode Ray Tube
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58
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
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 117
Highvoltage
cathode
source ofhigh voltage
yellow-greenfluorescence
shadow
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 117
The Effect of an Electric Field on Cathode Rays
Highvoltage
cathode
source ofhigh voltage
positiveplate
negative plate
anode
_
+
Cathode Ray Experiment
Deflectionregion
Drift region
Displacement
+
-Anodes / collimators
Cathode
Volts
Thomson’s CalculationsCathode Ray Experiment
• Thomson used magnetic and electric fields to measure and calculate the ratio of the cathode ray’s mass to its charge.
Magneticdeflection
charge ofray particle
magneticfield
length ofdeflection region
length of drift region
mass of rayparticle
velocity ofray particle
x x x
x=
Electricdeflection
charge ofray particle
electricfield
length ofdeflection region
length of drift region
mass of rayparticle
velocity ofray particle
x x x
x=
2
magnetic deflection
electric deflection
magnetic field
electric fieldx 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.
• 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.
Highvoltage
cathode
source of
high voltage
positiveplate
negative plate
anode
_
+
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
Mass spectrometers that break up molecules into fragments that can be characterized by electrical methods. [image link]
Detectorplate
Least massive ionsIon-accelerating
electric field
Magnetic field
Heating device to vaporize sample
Positive ions
Sample
Electron beam
acceleratedIon beam
Most massive
ions
Slits
Mass Spectrophotometer
electron beam
magnetic field
gas
stream of ions of differentmasses lightest
ions
heaviest ions
Dorin, Demmin, Gabel, Chemistry The Study of Matter 3rd Edition, page 138
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.
Components of a Mass Spectrometer
InletSignal
processor
Source Analyzer Detector
Vacuum
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
Ion AcceleratingArray
MolecularSource
Magnetic FieldBends Path of Charged
Particles
Collector Exit Slit
Ho
Mass Spectrometry
- +
Photographic plate
196 199 201 204
198 200 202
Mass spectrum of mercury vaporMass spectrum of mercury vapor
Hill, Petrucci, General Chemistry An Integrated Approach1999, page 320
Stream of positive ionsStream of positive ions
Mass Spectrum for Mercury
196 197 198 199 200 201 202 203 204
Mass numberMass number
Rel
ativ
e n
umb
er o
f at
oms
Rel
ativ
e n
umb
er o
f at
oms
30
25
20
15
10
5
196 199 201 204
198 200 202
Mass spectrum of mercury vaporMass spectrum of mercury vapor
The percent natural abundances The percent natural abundances for mercury isotopes are:for mercury isotopes are:
Hg-196 0.146%Hg-196 0.146% Hg-198 10.02%Hg-198 10.02% Hg-199 16.84%Hg-199 16.84% Hg-200 23.13%Hg-200 23.13% Hg-201 13.22%Hg-201 13.22% Hg-202 29.80%Hg-202 29.80% Hg-204 6.85%Hg-204 6.85%
(The photographic record has been converted to a scale of relative number of atoms)
The percent natural abundances The percent natural abundances for mercury isotopes are:for mercury isotopes are:
Hg-196 0.146%Hg-196 0.146% Hg-198 10.02%Hg-198 10.02% Hg-199 16.84%Hg-199 16.84% Hg-200 23.13%Hg-200 23.13% Hg-201 13.22%Hg-201 13.22% Hg-202 29.80%Hg-202 29.80% Hg-204 6.85%Hg-204 6.85%
(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
Hg200.59
80
(% "A")(mass "A") + (% "B")(mass "B") + (% "C")(mass "C") + (% "D")(mass "D") + (% "E")(mass "E") + (% F)(mass F) + (% G)(mass G) = AAM
ABCDEFG
• 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
Cl35.453
17
Using our estimated % abundance data
55% Cl-35 and 45% Cl-36
calculate an average atomic mass for chlorine.
Cl35.453
17
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
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’s mass got to do with it?
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).
40
30
20
10
50
90
80
70
60
100
0
5 10 15 20 25 30 35 40 45 50m/z
% R
ELA
TIV
E I
NT
EN
SIT
Y
Mass spectrum of carbon dioxide, CO2 molecular ion is seen at m/z 44.
12 16 28C+ O+
CO+
CO2+ M+
Mass spectrums reflect the abundance of naturally occurring isotopes.
Hydrogen
Carbon
Nitrogen
Oxygen
Sulfur
Chlorine
Bromine
1H = 99.985% 2H = 0.015%
12C = 98.90% 13C = 1.10%
14N = 99.63% 15N = 0.37%
16O = 99.762% 17O = 0.038% 18O = 0.200%
32S = 95.02% 33S = 0.75%
34S = 4.21% 36S = 0.02%
35Cl = 75.77% 37Cl = 24.23%
79Br = 50.69% 81Br = 49.31%
Natural Abundance of Common Elements
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
M +1 = 17[C13H4]+.
1.11
M+ = 15C12H3
+
86100 Base peak
M+ = 16Molecular ion
[C12H4]+.
12 13 14 15 16 17m/z
The Mass Spectrum of Methane
16
83
[C12H2]+.
[C12]+.
C12H+
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.
• http://www.aist.go.jp/RIODB/SDBS/• http://www.infochembio.ethz.ch/links/en/
spectrosc_mass_lehr.html • http://dbhs.wvusd.k12.ca.us/AtomicStructure/Disc-of-
Electron-Intro.html• http://wps.prenhall.com/wps/media/objects/
340/348272/Instructor_Resources/Chapter_12/47
References