Chem 113, Prof. J.T. Spencer Spectroscopy and Spectrometry Forensic Science Copyright © James T.
Download ReportTranscript Chem 113, Prof. J.T. Spencer Spectroscopy and Spectrometry Forensic Science Copyright © James T.
Slide 1
Chem 113, Prof. J.T. Spencer
Spectroscopy and Spectrometry
1
Forensic Science
Copyright © James T. Spencer 2003 All Rights Reserved
Slide 2
Chem 113, Prof. J.T. Spencer
2
Chemical Analysis
So How On Earth Did We Get To Where
We Are Today?
Slide 3
Chem 113, Prof. J.T. Spencer
Atoms, Molecules and Ions
3
• Science: Atomic Theory
– from a fundamental understanding of the
macroscopic behavior of substances comes
an understanding the microscopic behavior
of atoms and molecules (Baseball rules from Baseball
Game?)
Macroscopic
Substances
Mixtures
Physical Properties and Changes
Microscopic
Atomic theory
Question: Can matter be infinitely divided?
Most Greek Philosophers - Yes
Democritus (460 BC) and John Dalton (1800s) - No (“atomos”means indivisible”)
Slide 4
Chem 113, Prof. J.T. Spencer
Atoms, Molecules and Ions
4
• History of Atomic Theory and Scientific Inquiry
– Aristotle - “metaphysics”,
thought experiments and
no experimental observations
necessary to substantiate ideas.
– Archimedes (287 - 212 BC) - Scientific Method,
determined composition of the King of Syracuse’s
crown by measuring density through water
displacement.
– Roger Bacon (1214 - 1294) - Experimental Science “ It
is the credo of free men - the opportunity to try, the
privilege to err, the courage to experiment anew.
...experiment, experiment, ever experiment”.
Slide 5
Chem 113, Prof. J.T. Spencer
Aristotle (384-322 BC)
• All of the sciences (epistêmai,
literally "knowledges") can be
divided into three branches:
theoretical, practical, and
productive. Whereas practical
sciences, such as ethics and
politics, are concerned with
human action, and productive
sciences with making things,
theoretical sciences, such as
theology, mathematics, and the
natural sciences, aim at truth
and are pursued for their own
sake.
5
Slide 6
Chem 113, Prof. J.T. Spencer
Archimedes (287-212BC)
6
• Archimedes was a native of Syracuse
(not NY). Stories from Plutarch, Livy,
and others describe machines invented
by Archimedes for the defence of
Syracuse (These include the catapult,
the compound pulley and a burningmirror).
• Archimedes discovered fundamental
theorems concerning the center of
gravity of plane figures and solids. His
most famous theorem gives the weight
of a body immersed in a liquid, called
Archimedes' principal.
His methods anticipated integral calculus 2,000 years before
Newton and Leibniz.
Slide 7
Chem 113, Prof. J.T. Spencer
Archimedes (287-212BC)
7
Slide 8
Chem 113, Prof. J.T. Spencer
Archimedes (287-212BC)
8
Suspecting that a goldsmith might have replaced some of the gold by silver in
making a crown, Hiero II, the king of Syracuse, asked Archimedes to determine
whether the wreath was pure gold. The wreath could not be harmed since it was a
holy object.
The solution which occurred when he stepped into his bath and caused it to overflow
was to put a weight of gold equal to the crown, and known to be pure, into a bowl
which was filled with water to the brim. Then the gold would be removed and the
king's crown put in, in its place. An alloy of lighter silver would increase the bulk of
the crown and cause the bowl to overflow.
Pure Gold?
Equal Weight of Gold
Crown Displaced More Water
Slide 9
Chem 113, Prof. J.T. Spencer
Archimedes
(287-212BC)9
Slide 10
Chem 113, Prof. J.T. Spencer
Mass Spectrometry
Background - Stoichiometry
10
• Antoine Lavoisier (1734 - 1794)
– Law of Conservation of Mass - atoms are neither
created nor destroyed in chemical reactions
– total number of atoms = total number of atoms after reaction
before reaction
– Stoichiometry - quantitative study of chemical
formulas and reactions
(Greek; “stoichion”= element, “metron” = measure)
• Chemical Equations - used to describe chemical
reactions in an accurate and convenient fashion
2H2 + O2
reactants
2 H2O
products
Slide 11
Chem 113, Prof. J.T. Spencer
Antoine Lavoisier
11
Antoine Lavoisier was born in
Paris, and although Lavoisier's
father wanted him to be a
lawyer, Lavoisier was
fascinated by science. From the
beginning of his scientific
career, Lavoisier recognized the
importance of accurate
measurements. He wrote the first modern chemistry (1789)
textbook so that it is not surprising that Lavoisier is often called the
father of modern chemistry. To help support his scientific work,
Lavoisier invested in a private tax-collecting firm and married the
daughter of one of the company executives. Guillotined for his tax
work in 1794.
Slide 12
Chem 113, Prof. J.T. Spencer
“Chemical” Family Trees
12
James T. Spencer
(1984, Iowa State University)
John G. Verkade
(Harvard University,1960)
Harry Julius Emeleus
(Imperial College London, 1926)
Theron Standish Piper
(Harvard University, 1956)
Russell N. Grimes
(University of Minesota)
Geoffrey Wilkinson
(Imperial College London, 1941)
William N. Lipscomb
(Caltech., 1945)
Alfred E. Stock
(Univ. of Berlin ca 1900
Henri Moissan
(University of Paris, 1879)
Edmond Fremy
(University of Paris, 1856)
Emil Fisher
(University of Strassbourg, 1874)
Joseph L. Gay Lussac
(University of Paris, 1800)
Claude L. Berthollet
(University of Paris, 1778)
Jean Bucquet
(University of Paris, 1770)
Red borders indicate Nobel
Laureates (first award 1901)
Linus Pauling
(Caltech, 1925)
Antoine Lavoisier
(University of Paris, 1764)
Adolf von Baeyer
(University of Berlin, 1858)
August Kekule
(University of Gressen, 1852)
Justus Liebig
(University of Erlangen, 1822)
Slide 13
Chem 113, Prof. J.T. Spencer
Forensic Chemical Analysis
Typical Chemical Problems
13
• Problem - An unknown sample of a white powered
compound is brought into the lab after a routine
traffic stop. What is the compound?
• Problem - A murder is committed with a lead pipe
(in the conservatory) that was removed from the
bathroom sink. Col. Mustard was found with a
deformed lead pipe. Were the two one unit in the
past?
• Problem - A fiber found on a hairbrush appears to
be from a wig. Did the fiber come from the wig of
the victim or from another source (possibly the
murderer)?
• Problem - Was Napoleon murdered?
Slide 14
Chem 113, Prof. J.T. Spencer
Analytical Methods
14
• Questions to consider in choosing an
analytical (chemical) method:
– Quantitative or qualitative required
– Sample size and sample preparation requirements
– What level of analysis is required (e.g., ± 1.0% or ±
0.001%)
– Detection levels and useful analytical concentration
ranges
– Destructive or non-destructive
– Availability of instrumentation
– Admissibility (e.g., are all lead pipes compositionally
the same or are there sufficient variations among
“known” Pb pipes of the world to link two samples)
Slide 15
Chem 113, Prof. J.T. Spencer
Spectroscopy and Spectrometry
15
• Mass Spectrometry (MS)
• Atomic Spectroscopy
– Atomic Absorption (AAS) and Emission Analysis (AES)
– Neutron Activation Analysis (NAA)
• Molecular Spectroscopy
– Electronic Spectroscopy
– Vibrational Spectroscopy
– Nuclear Magnetic Resonance (NMR or MRI)
• X-ray Methods
– X-ray Diffraction (XRD and CAT)
– Energy Dispersive X-ray Fluorescence (EDXRF)
Slide 16
Chem 113, Prof. J.T. Spencer
Comparison of Techniques
16
Technique
Qual.*
Sample Detection
or
Destructive
Size
levels
Quant.
Mass Spec.
Qual.
0.1 mL
to 10-8
mL
Infrared
Qual.
0.001 g
UV-visible
Qual.
0.001 g
Instr.
Avail.
*
Yes
Easy
*
No
Easy
No
Easy
AES
Quant.
10-4 g/L
Yes
Moderate
AAS
Quant.
10-4 g/L
Yes
Easy
NAA
Quant.
1 x 10-9 g
Possibly
Difficult
* Primary use is in qual. analysis, although it can be used quantitatively in some cases.
Slide 17
Chem 113, Prof. J.T. Spencer
Mass Spectrometry
17
• Chemical Background (mass scale, ave. atomic
masses, etc.)
• Instrumental Principles and Design
• Spectral Features
• Spectral Interpretation and Comparison
• GC-MS and LC-MS
Slide 18
Chem 113, Prof. J.T. Spencer
Mass Spectrometry
18
Underlying Ideas - Atomic and Molecular Weights
•Atomic Mass Scale - based upon 12C
isotope. This isotope is assigned a mass of
exactly 12 atomic mass units (amu) and the
masses of all other atoms are given relative
to this standard.
•Most elements in nature exist as mixtures
of isotopes.
Slide 19
Chem 113, Prof. J.T. Spencer
Mass Spectrometry
Underlying Ideas - Atomic Weights
19
• Average Atomic Mass (AW)- weighted average (by
% natural abundance) of the isotopes of an element.
•Example (1);
10B is 19.78% abundant with a mass of 10.013 amu
11B is 80.22% abundant with a mass of 11.009 amu
therefore the average atomic mass of boron is;
(0.1987)(10.013) + (0.8022)(11.009) = 10.82 amu
Although natural B does not actually contain any
B with mass 10.82, it is considered to be
composed entirely of mass 10.82 for stoich.
Slide 20
Chem 113, Prof. J.T. Spencer
Mass Spectrometry
Underlying Ideas - Atomic Weights
20
• Average Atomic Mass (AW)- weighted average (by
% natural abundance) of the isotopes of an element.
•Example (2):
194Pt
is 33.90% abundant with a mass of 193.963 amu
195Pt is 33.80% abundant with a mass of 194.965 amu
196Pt is 25.30% abundant with a mass of 195.965 amu
198Pt is 7.210% abundant with a mass of 197.968 amu
therefore the average atomic mass of platinum is;
(0.3390)(193.963) + 0.3380)(194.965) + (0.2530)(195.965 ) +
(0.07210)(197.968)= 195.504 amu
Slide 21
Chem 113, Prof. J.T. Spencer
Mass Spectrometry
Basic Ideas
21
A mass spectrometer (MS) creates charged particles (ions) from
gas phase molecules.
Electron Ionization (EI)- Uses electron impact to ionize a molecule.
Chemical Ionization (CI)- First ionizes a molecular gas (such as
methane) which in turn ionizes the molecule of interest.
A “gentler” method of ionization - often allows the
observation of a “sensitive” molecular ion as a P+1
peak.
Fast Atom Bombardment (FABS)- Mainly for involatile compounds very harsh.
The MS analyzes those ions to provide information about the
molecular weight of the compound and its chemical structure.
Slide 22
Chem 113, Prof. J.T. Spencer
Mass Spectrometry
Basic Ideas
Either move slit or
change deflecting force
to scan masses “across”
the detector
22
Slide 23
Chem 113, Prof. J.T. Spencer
Mass Spectrometer
Magnetic field deflection (quadrupole MS)23
• Direct methods of measuring (separating) mass.
• Sample molecules are ionized by e-beam to cations (+1 by
“knocking off” one electron) which are then deflected by
magnetic field - for ions of the same charge the angle of
deflection in proportional to the ion’s mass
vacuum chamber beam of pos. ions
accelerating grid (-)
N
sample
focusing slits
ionizing e- beam
Hg
S
magnetic field
Mass
Spectrum
200
Int.
mass number (amu)
detector
Slide 24
Chem 113, Prof. J.T. Spencer
Mass Spectrometer
Atomic Spectra
Mass
Spectrum
Cl
Int.
35
Mass
Spectrum
Int.
C
12
37
mass number (amu)
35Cl:
75% abundant
37Cl: 24% abundant
24
Mass
Spectrum
P
Int.
31
13
mass number (amu)
12Cl:
mass number (amu)
98.9% abundant 31P: 100% abundant
13Cl: 1.11% abundant
Slide 25
Chem 113, Prof. J.T. Spencer
Mass Spectrometry
Molecules
25
Slide 26
Chem 113, Prof. J.T. Spencer
Mass Spectrometry
26
Slide 27
Chem 113, Prof. J.T. Spencer
Mass Spectrometry
27
Ionization produces singly charged ions. The intact charged
molecule is the molecular ion. Energy from the electron impact
and instability in a molecular ion can cause that ion to break
into smaller pieces (fragments).
The methanol ion may fragment in various ways, with one
fragment carrying the charge and one fragment remaining
uncharged. For example:
CH3OH+. (molecular ion)
(or) CH3OH+.(molecular ion)
CH2OH+(fragment ion) + . H
CH3+(fragment ion) + .OH
Slide 28
Chem 113, Prof. J.T. Spencer
Mass Spectrometry
28
Slide 29
Chem 113, Prof. J.T. Spencer
Mass Spectrometry
29
Slide 30
Chem 113, Prof. J.T. Spencer
Mass Spectrometer
30
Unknown white powdery substance ingested by
unconscious patient.
What do you do? Is it Heroin, Cocaine, Caffeine?
In ten sity
Mass Spectrum of Unknown Compound
Mass
25
50
75
100
125 150
175
200
225
250
275
300
Slide 31
Chem 113, Prof. J.T. Spencer
Mass Spectrometer
In ten sity
MS Library
43
25
50
75
215
146
100 125 150 175 200 225 250 275
194
In ten s ity
67
Mass
55
25
50
75
300
Caffeine
109
82
42
other peaks at
327 and 369
268
204
94
Mass
Heroin
Heroin
31
MS of Unknown
100 125 150 175 200 225 250 275
300
Slide 32
Chem 113, Prof. J.T. Spencer
In ten sity
Mass Spectrometer
MS
Library
82
Cocaine
Cocaine
182
303
42
122
Mass
25
50
75
272
150
100 125 150 175 200 225
194
67
In ten s ity
Mass
32
55
25
50
75
300
Caffeine
109
MS of Unknown
82
42
250 275
100 125 150
175 200 225
250 275
300
Slide 33
Chem 113, Prof. J.T. Spencer
Mass Spectrometer
MS Library
In ten s ity
67
Mass
55
50
109
75
100 125 150
175 200 225
194
67
In ten s ity
Mass
55
50
75
250 275
300
Caffeine
109
MS of Unknown
82
42
25
Caffeine
Caffeine
82
42
25
194
33
100 125 150
175 200 225
250 275
300
Slide 34
Chem 113, Prof. J.T. Spencer
Mass Spectrometer
O
Unknown white powdery
substance ingested by
unconscious patient.
What do you do?
H 3C
N
N
O
Mass Spectrum
N
N
CH 3
Caffeine
In ten sity
Mass
CH 3
25
50
75
100
125 150
175
200
225
250
275
300
34
Mol. Wgt
= 194
Slide 35
Chem 113, Prof. J.T. Spencer
GC-Mass Spectrometry
35
A mixture of compounds to be analyzed is injected into the gas
chromatograph (GC) where the mixture is vaporized in a heated
chamber. The gas mixture travels through a GC column, where
the compounds become separated as they interact with the
column. Those separated compounds then immediately enter the
mass spectrometer.
Slide 36
Chem 113, Prof. J.T. Spencer
GC-Mass Spectrometry
36
Slide 37
Chem 113, Prof. J.T. Spencer
Atomic and Molecular Spectroscopy
37
• Science: Atomic Theory
– “The strength of a science is that its conclusions are
derived by logical arguments from facts that result
from well-designed experiments. Science has
produced a picture of the microscopic structure of the
atom so detailed and subtle of something so far
removed from our immediate experience that it is
difficult to see how its many features were
constructed. This is because so many experiments
have contributed to our ideas about the atom.”
B. Mahan from University Chemistry
Slide 38
Chem 113, Prof. J.T. Spencer
Atomic and Molecular Spectroscopy
38
•
•
•
•
•
•
Electromagnetic Radiation
Atomic Electronic Structure
Quantization of Energy Levels
Absorption, Transmission and Emission Spectra
Atomic Spectroscopy
Molecular Spectroscopy
Slide 39
Chem 113, Prof. J.T. Spencer
39
364.6 nm
410.2 nm
434.0 nm
486.1 nm
656.3 nm
Hydrogen Emission
Ultraviolet
Red
Blue
A Swiss schoolteacher in 1885 (J. Balmer) derived
a simple formula to calculate the wavelengths of
the emission lines (purely a mathematical feat
with no understanding of why this formula
worked)
frequency = C ( 1 - 1 ) where n = 1, 2, 3, etc...
22 n2 C = constant
Slide 40
Chem 113, Prof. J.T. Spencer
Spectroscopy
Background - Electromagnetic Radiation 40
= c
where = wavelength,
= frequency,
c = light speed
amplitude
1 cycle per sec = 1 hertz
wavelength ()
QuickTime™ and a
Gra phics d ecomp re sso r
are n eed ed to see this p ictu re.
Slide 41
Chem 113, Prof. J.T. Spencer
Electromagnetic Radiation
41
= c
where = wavelength, = frequency, c = light speed
Gamma
UV/Vis Infrared Microwave Radio
X-ray
Wavelength (m)
10-11m
10 m
Slide 42
Chem 113, Prof. J.T. Spencer
Electromagnetic Radiation
42
Magnetic and Electronic Parts
mutually perpendicular
QuickTime™ and a
Graphics decompressor
are needed to see this picture.
Slide 43
Chem 113, Prof. J.T. Spencer
Spectroscopy
Electronic Structure - Background
43
• Prior to 1926, Many experiments in the structure of matter
showed several important relationships:
– Light has BOTH wavelike and particulate (solid
particle-like) properties.
– Even solid particles display BOTH wavelike and
particulate properties.
– Whether the wavelike or particulate properties are
predominantly observed depends upon the nature of
the experiment (what is being measured).
Slide 44
Chem 113, Prof. J.T. Spencer
Wave Properties of Matter
44
• De Broglie - particles behave under some circumstances
as if they are waves (just as light behaves as particles
under some circumstances). Determines relationship:
= h/mv
= wavelength
h = Planck’s const.
m = mass
v = velocity
Particle
electron
He atom (a)
Baseball
fast ball
slow ball
mass (kg)
9 x 10-31
7 x 10-27
v (m/sec)
1 x 105
1000
(pm)
7000
90
0.1
0.1
20
0.1
3 x 10-22
7 x 10-20
Slide 45
Chem 113, Prof. J.T. Spencer
Niels Bohr (Denmark)
• Built upon Planck,
Einstein and others work
to propose explanation of
line spectra and atomic
structure.
• Nobel Prize 1922
• Worked on Manhattan
Project
• Advocate for peaceful
nuclear applications
45
Slide 46
Chem 113, Prof. J.T. Spencer
Bohr’s Model
46
• Continuous Spectra vs. Line Spectra
Wave-like
Behavior
Sunlight
Wave-like
Behavior
Hydrogen
Dispersion by Prism
Dispersion by Prism
Slide 47
Chem 113, Prof. J.T. Spencer
Bohr’s Model
“Microscopic Solar System”
• Electrons in circular orbits
around nucleus with
quantized (allowed) energy
states
• When in a state, no energy is
radiated but when it changes
states, energy is emitted or
gained equal to the energy
difference between the states
• Emission from higher to
lower, absorption from lower
to higher
47
n=∞
n=4
n=3
n=2
electronic
transitions
n=1
Slide 48
Chem 113, Prof. J.T. Spencer
48
364.6 nm
410.2 nm
434.0 nm
486.1 nm
656.3 nm
Hydrogen Emission
Ultraviolet
Red
Blue
A Swiss schoolteacher in 1885 (J. Balmer) derived
a simple formula to calculate the wavelengths of
the emission lines (purely a mathematical feat
with no understanding of why this formula
worked)
frequency = C ( 1 - 1 ) where n = 1, 2, 3, etc...
22 n2 C = constant
Slide 49
Chem 113, Prof. J.T. Spencer
Bohr’s Model
“Microscopic Solar System”
49
Slide 50
Chem 113, Prof. J.T. Spencer
Microscopic Properties
50
• Light energy may behave as waves or as small
particles (photons).
• Particles may also behave as waves or as small
particles.
• Both matter and energy (light) occur only in discrete
units (quantized).
Quantized
(can stand only on steps)
Non-Quantized
(can stand at any position on the ramp)
Slide 51
Chem 113, Prof. J.T. Spencer
What is Quantization
51
• Examples of quantization (when only discrete and
defined quantities or states are possible):
Quantized
Non-Quantized
Piano
Stair Steps
Typewriter
Dollar Bills
Football Game Score
Light Switch (On/Off)
Energy
Matter
Violin or Guitar
Ramp
Pencil and Paper
Exchange rates
Long Jump Distance
Dimmer Switch
Slide 52
Chem 113, Prof. J.T. Spencer
Quantum Numbers
52
• Quantum Numbers also specify energy of the occupying
electrons,
0
E
N
E
R
G
Y
n=∞
n=4
n=3
n=2
n=1
l=0
4s
3s
l=1
4p
3p
2s
2p
1s
l=2
4d
3d
l=3
4f
32
electrons
max
18
electrons
max
8
electrons
max
2
l
electrons
max
n
Slide 53
Chem 113, Prof. J.T. Spencer
Many Electron Atoms
0
n=1
n=2
n=3
3d
n=4
4p
4s
53
n=5
5s
3p
E
N
E
R
G
Y
3s
2p
2s
1s
s (l = 0)
p (l = 1)
d (l = 2)
Slide 54
Chem 113, Prof. J.T. Spencer
Red
Blue
54
364.6 nm
410.2 nm
434.0 nm
486.1 nm
656.3 nm
Hydrogen Emission
Ultraviolet
No Just Emission - molecules (and atoms) can
also absorb energy.
Slide 55
Chem 113, Prof. J.T. Spencer
Spectroscopy
55
• When electromagnetic radiation passes through a
substance, it can either be absorbed or transmitted,
depending upon the structure of the substance.
• When a molecule absorbs radiation it gains energy as it
undergoes a quantum transition from one energy state
(Einitial) to another (Efinal).
The frequency of the
absorbed radiation is related
to the energy of the
transition by Planck's law:
Efinal - Einitial = E = h = hc/ .
Slide 56
Chem 113, Prof. J.T. Spencer
Atomic Spectroscopy
56
• Atomic Absorption and Emission- Techniques that
involve the determination and measurement of
atomic energy levels (spectrometry) and chemical
identification based on how atoms absorb or emit
electromagnetic radiation.
• Neutron Activation Analysis - Quantitative multielement analysis of major, minor, trace (ppb) and rare
elements. The sample is placed in a flux of neutrons
and after removal the emissions of the radionuclides
produced are measured. Forensic applications include
gunshot residues, bullet lead, glass, paint, hair, etc.
Slide 57
Chem 113, Prof. J.T. Spencer
Atomic Spectroscopy
57
• Ground state - the lowest energy state of an atom or molecule
(most stable state) with regard to the position of the electrons
around the nucleus
• Excited state – results when ground state electrons are excited
by energy to higher energy states. Excited states are unstable
and an atom in the excited state immediately returns to the
ground state
• Emission - When an electric current is passed through a gas,
the gas emits light. This is due to the change of energy of the
gas. The electrons in the atoms of the gas become excited to a
higher energy state (the “excited state”) and when they return
to the original, low-energy state (“the ground state”), the atoms
of the gas emit the excess energy as light.
• Absorption - This is due to the change of energy of the gas.
The electrons in the atoms of the gas become excited by
absorbing energy (light).
Slide 58
Chem 113, Prof. J.T. Spencer
Atomic Spectroscopy
58
Slide 59
Chem 113, Prof. J.T. Spencer
Flame Tests
59
Atomic Emission
Slide 60
Chem 113, Prof. J.T. Spencer
Atomic Emission
60
Slide 61
Chem 113, Prof. J.T. Spencer
Atomic Spectroscopy
61
Slide 62
Chem 113, Prof. J.T. Spencer
Atomic Emission
AES
62
• Atomic Emission (AE) - uses quantitative measurement of
the optical emission from excited atoms to determine analyte
concentration. Analyte atoms in solution are aspirated into the
excitation region where they are atomized by a flame, discharge,
or plasma. These high-temperature atomization sources provide
sufficient energy to promote the atoms into high energy levels.
The atoms decay back to lower levels by emitting light. Since
the transitions are between distinct atomic energy levels, the
emission lines in the spectra are narrow.
Slide 63
Chem 113, Prof. J.T. Spencer
Atomic Emission Spectroscopy
AES
63
• Advantages of Inductively coupled plasma (ICP-AES):
– Multielement analyses
– Determination of low concentration, difficult to
atomize elements
– Less chemical interference due to the high
temperature in the plasma employed
– Determination of many elements (e.g., Zn, Cu)
– Great linear detection range
– Supplementary to AAS
Slide 64
Chem 113, Prof. J.T. Spencer
Atomic Emission
AES
64
• Zr-content of flame resist-treated wool (Low-Smoke
Zirpro finishing)
Slide 65
Chem 113, Prof. J.T. Spencer
Atomic Emission
AES
• Russian Icon of St. Nicholas - The
pigments present on this mid19th Century painting were
characterized by AES
spectroscopy (laser-induced
breakdown spectroscopy, LIBS)
and Raman microscopy. The
identification of pigments on the
original work along with those
applied in restoration of cracks
in the varnish and painting
surface were analyzed.
65
Slide 66
Chem 113, Prof. J.T. Spencer
Atomic Emission
AES
66
• LIBS depth profile measurements leave a minute crater in the
surface of the art object being studied. This allows
stratagraphic information to be collected. A typical cross
section of the icon is shown.
Slide 67
Chem 113, Prof. J.T. Spencer
Atomic Emission
AES
67
• Several areas of the icon, where white paint was
used, were analyzed. The LIBS spectrum showed
strong peaks characteristic of lead. This was
confirmed by the Raman spectrum, which verified
the presence of lead carbonate, [2PbCO3·Pb(OH)2].
LIBS
Raman
Slide 68
Chem 113, Prof. J.T. Spencer
Atomic Emission
AES
The brown pigment was
characterized as an iron-based
pigment mixed with lead
white. The pigment scattered
poorly and so did not produce
a Raman spectrum. The LIBS
spectrum showed the presence
of Fe and Al, corresponding to
an iron oxide and an earth such
as clay. Also present are
emissions characteristic of
magnesium, lead and calcium.
The peak corresponding to iron
at ~275nm is characteristic of
iron that has been observed in
studies on pure iron oxide
pigments (for example, Mars
black, Fe3O4).
68
Slide 69
Chem 113, Prof. J.T. Spencer
Atomic Absorption
AAS
• Atomic Absorption - Atomic-absorption (AA)
69
spectroscopy uses the absorption of light to measure the
concentration of gas-phase atoms. Since samples are
usually liquids or solids, the analyte atoms or ions must be
vaporized in a flame or graphite furnace. The atoms absorb
ultraviolet or visible light and make transitions to higher
electronic energy levels. The analyte concentration is
determined from the amount of absorption.
Slide 70
Chem 113, Prof. J.T. Spencer
Atomic Absorption
AAS
70
Typical Problem - A child becomes quite ill and is
taken to the hospital. It is found that the child is
suffering from lead poisoning. A forensic laboratory
is contacted and asked if it can determine the source
of the lead which the child has ingested. No crime has
been committed, per se, but the source must be
eliminated to prevent future danger to the child. Paint
samples from a number of objects with which the
child has had repeated contact are collected. Paint on
the child's crib, paint from his toys, and paint from
the child's swing, to name a few, are sent to the
laboratory. AA is the best method for these analyses.
Slide 71
Chem 113, Prof. J.T. Spencer
Neutron Activation Analysis
NAA
71
Neutrons interact with a target nucleus to form a compound nucleus in
an excited state. The compound nucleus will decay into a more stable
configuration through emission of one or more gamma rays. This new
configuration may yields a radioactive nucleus which also decays by
emission of delayed gamma rays, but at a much slower rate according to
the unique half-life of the radioactive nucleus.
Slide 72
Chem 113, Prof. J.T. Spencer
Neutron Activation Analysis
NAA
72
•NAA falls into two categories: (1) prompt gamma-ray neutron activation
analysis (PGNAA), where measurements take place during irradiation, or
(2) delayed gamma-ray neutron activation analysis (DGNAA), where the
measurements follow radioactive decay (most common). About 70% of
the elements have properties suitable for measurement by NAA.
•Parts per billion
or better.
Gamma-ray spectrum
showing medium- and
long-lived elements
measured in a sample of
pottery irradiated for 24
hours, decayed for 9 days,
and counted for 30 minutes
on a HPGe detector.
Slide 73
Chem 113, Prof. J.T. Spencer
Neutron Activation Analysis
NAA
73
An example of the gamma-ray spectrum from the activation of a
human nail used as a biological monitor of trace-element status.
Slide 74
Chem 113, Prof. J.T. Spencer
Neutron Activation Analysis
Arsenic in Hair
Napoleon Bonaparte
One of the most brilliant individuals in
history, Napoleon Bonaparte was a masterful
soldier, grand tactician, sublime statesman and
exceedingly capable administrator. After an
extraordinary career, he was finally defeated
and exiled to Elba. He returned from Elba to
be ultimately defeated at
Waterloo. He was finally
exiled to the remote tiny
volcanic island of St. Helena,
south of the Equator. The
nearest land is Ascension
Island, 700 miles to the north.
74
Slide 75
Chem 113, Prof. J.T. Spencer
Neutron Activation Analysis
Arsenic in Hair
Murdered or Not?
For years a controversy has raged about
Napoleon being killed on St. Helena - either
by French Royalists, persons in his exiled
entourage or the British - and all have
pointed to the high levels of arsenic in the
emperor's body as being evidence of such
behavior. The emperor's body contained
some 15 parts per million of the poison,
where the maximum safe limit is only three
parts per million. The determination was
by neutron activation analysis of his hair.
75
Slide 76
Chem 113, Prof. J.T. Spencer
Neutron Activation Analysis
Arsenic in Hair
“So Who Done It?”
(if it was done at all)
British Authorities - The Allied heads of
state had no greater wish than to ensure that
Napoleon was permanently “out of the
way”. Strong hatred by British local
commander.
Royalists - Revenge and insurance against
Napoleon for declaring himself Emperor
and dismantling the aristocracy.
Exiled Entourage - Jealousy (romantic
triangles), intrigue, revenge.
76
Slide 77
Chem 113, Prof. J.T. Spencer
Neutron Activation Analysis
Arsenic in Hair
77
NAA of Napoleon’s Hair
From the old tradition of keeping hair locks, many sample of
Napoleon’s hair are known. NAA showed high concentrations of As at
various locations along hair shafts. The As, however, was determined
not to have been taken orally.
So how did he die and why did he have such high As
concentrations?
Slide 78
Chem 113, Prof. J.T. Spencer
Neutron Activation Analysis
Arsenic in Hair
The wallpaper in his room was dyed with
Scheele's Green (Paris Green), a coloring
pigment that had been used in fabrics and
wallpapers from around 1770. Named
after the Swedish chemist who invented
it, the dye contained copper arsenite. It
was discovered that if wallpaper
containing Scheele’s Green became
damp, the mould converted the copper
arsenite to a poisonous vapor form of
arsenic. Breathing the arsenic on its own
might not have been enough to kill
Napoleon, but he was ill already with a
stomach ulcer/cancer. On the 5 May
1821, the arsenic tipped the scale against
"the little corporal."
78
Slide 79
Chem 113, Prof. J.T. Spencer
Salem Witch Trials
79
From June through September of 1692,
nineteen men and women, all having been
convicted of witchcraft, were carted to
Gallows Hill, a barren slope near Salem
Village, for hanging. Another man of over
eighty years was pressed to death under
heavy stones for refusing to submit to a
trial on witchcraft charges. Hundreds of
others faced accusations of witchcraft.
Dozens languished in jail for months
without trials. Then, almost as soon as it
had begun, the hysteria that swept through
Puritan Massachusetts ended.
Slide 80
Chem 113, Prof. J.T. Spencer
Salem Witch Trials
80
In February of the exceptionally cold winter of 1692, young Betty Parris
became strangely ill. She dashed about, dove under furniture, contorted
in pain, and complained of fever. Cotton Mather had recently
published a popular book, "Memorable Providences," describing the
suspected witchcraft of an Irish washerwoman in Boston, and Betty's
behavior mirrored that of the afflicted person described in Mather's
widely read and discussed book. It was easy to believe in 1692 in Salem,
with an Indian war raging less than seventy miles away (and many
refugees from the war in the area) that the devil was close at hand.
Sudden and violent death occupied minds. Talk of witchcraft increased
when other playmates of Betty, including eleven-year-old Ann Putnam,
seventeen-year-old Mercy Lewis, and Mary Walcott, began to exhibit
similar unusual behavior. When his own nostrums failed to effect a
cure, William Griggs, a doctor called to examine the girls, suggested
that the girls' problems might have a supernatural origin. The
widespread belief that witches targeted children made the doctor's
diagnosis seem increasing likely.
Slide 81
Chem 113, Prof. J.T. Spencer
Salem Witch Trials
81
“Trial of George Jacobs”
(1692)
“Examination of a Witch”
Slide 82
Chem 113, Prof. J.T. Spencer
St. Anthony’s Fire - Bosch
82
Slide 83
Chem 113, Prof. J.T. Spencer
Ergot
83
Ergot - A toxic fungus, ( Claviceps purpurea ) found as a parasite
on grains of rye. One form is hallucinogenic ergotism, in which
people often experience symptoms of one of the other forms of
ergotism (gangrenous ergotism - people experience nausea, and
pains in the limbs, bodily extremities turn black, dry and
become mummified, makingit possible for infected limbs to
spontaneously break off at the joints, or convulsive ergotism)
along with vivid hallucinations. The other symptoms are very
much like those of modern psychedelic drugs such as
nervousness, physical and mental excitement, insomnia and
disorientation. People with this form of ergotism were also
observed to perform strange dances with wild, jerky movements
accompanied by hopping, leaping and screaming. They would
dance compulsively until exhaustion lead them to collapse
unconscious.
Slide 84
Chem 113, Prof. J.T. Spencer
Ergotism?
84
St. Christopher Carrying the Christ Child through a Sinful World, Bosch, c1520
Slide 85
Chem 113, Prof. J.T. Spencer
Ergotism
85
Ergotamine tartrate
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Ergot on grains of rye
Slide 86
Chem 113, Prof. J.T. Spencer
Ergotism
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86
lysergic acid
diethylamide
Slide 87
Chem 113, Prof. J.T. Spencer
Molecular Spectroscopy
87
• Electronic Spectroscopy
• Vibrational Spectroscopy
• Nuclear Magnetic Resonance Spectroscopy
(NMR or MRI)
Slide 88
Chem 113, Prof. J.T. Spencer
Electronic Spectroscopy
UV-visible
88
When white light passes through or is reflected by a
colored substance, a characteristic portion of the
total wavelengths is absorbed. The remaining light
will then assume the complementary color to the
wavelength(s) absorbed.
The remaining light will
then assume the
complementary color to the
wavelength(s) absorbed.
Slide 89
Chem 113, Prof. J.T. Spencer
Electronic Spectroscopy
UV-visible
89
Visible region of the spectrum has photon energies of 36 to 72
kcal/mole, and the near ultraviolet region 72 to 143 kcal/mole (200 nm).
Sufficient E to excite a molecular electron to a higher energy orbital.
Of the six transitions outlined, only the two lowest energy ones (leftmost, colored blue) are achieved by these energies (200-800 nm).
Energetically favored electron promotion will be from the highest
occupied molecular orbital (HOMO) to the lowest unoccupied
molecular orbital (LUMO).
Slide 90
Chem 113, Prof. J.T. Spencer
Electronic Spectroscopy
UV-visible
90
When sample molecules are exposed to light having an
energy that matches a possible electronic transition within the
molecule, some of the light energy will be absorbed as the
electron is promoted to a higher energy orbital. An optical
spectrometer records the wavelengths at which absorption
occurs, together with the degree of absorption at each
wavelength.
Slide 91
Chem 113, Prof. J.T. Spencer
Electronic Spectroscopy
UV-visible
91
Effect of Conjugation
Slide 92
Chem 113, Prof. J.T. Spencer
Electronic Spectroscopy
UV-visible
UV-vis. instrument
92
Slide 93
Chem 113, Prof. J.T. Spencer
Vibrational Spectroscopy
Infrared and Raman Spectroscopy 93
• Radiation from 500 to 4000 cm-1 (vibrational
transitions in the molecules).
• Vibrational “mode” must have a change in dipole
moment in the transition. Energy of the transition is
dependent upon the strengths of the bonds and
geometric structure.
Slide 94
Chem 113, Prof. J.T. Spencer
Vibrational Spectroscopy
Infrared and Raman Spectroscopy 94
• For the water molecule, for which there are three
vibrational modes, there are consequently three sets
of energy levels within which transitions may occur
(shown ).
The spacing between energy
levels depends upon the
particular vibration being
considered. Each spacing
requires a photon of
different energy to cause the
transition, so we expect
photons of three different
energies to be absorbed by
H2O.
Slide 95
Chem 113, Prof. J.T. Spencer
Vibrational Spectroscopy
Infrared and Raman Spectroscopy 95
• In order for a particular vibrational mode to directly absorb
infrared electromagnetic radiation, the vibrational motion
associated with that mode must produce a change in the dipole
moment of the molecule.
• There are many molecules which, although possessing no
permanent dipole moment, still undergo vibrations which
cause changes in the value of the dipole moment from 0 to
some non-zero value. Consider the CO2 molecule:
Slide 96
Chem 113, Prof. J.T. Spencer
IR Spectrum of CO2
96
C alculated spectrum o f co 2 B 3L Y P /6-31G (d)
595
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495
a b so rb a n c e
395
O=C=O
295
O=C=O
195
95
-5
0
500
1000
1500
2000
w avenum ber/cm -1
2500
3000
3500
4000
Slide 97
Chem 113, Prof. J.T. Spencer
Vibrational Spectroscopy
Infrared and Raman Spectroscopy 97
• Different types of
bonds have
characteristic
regions of the
spectrum where
they absorb
Slide 98
Chem 113, Prof. J.T. Spencer
Vibrational Spectroscopy
Infrared and Raman Spectroscopy 98
• Forensic Applications of Infrared Spectroscopy
• Use of computer databases of IR’s of known compounds
»
Analyzing Alcohol - The breath is tested
with a mechanism similar to a breathalyzer
(chemical oxidation) but uses the infrared
absorptions of alcohol.
»
Slide 99
Chem 113, Prof. J.T. Spencer
Vibrational Spectroscopy
Infrared and Raman Spectroscopy 99
• Forensic Applications of Infrared Spectroscopy
• Use of computer databases of IR’s of known compounds
»
Analyzing Drugs - The drug's various
chemical components absorb infrared light.
The absorptions are compared to known
samples using a database.
Slide 100
Chem 113, Prof. J.T. Spencer
Vibrational Spectroscopy
Infrared and Raman Spectroscopy100
• Forensic Applications of Infrared Spectroscopy
• Use of computer databases of IR’s of known compounds
»
Analyzing Fibers - The expected identity of
the fiber has been established by observing it
under a microscope. Its IR spectrum can
confirm the suspected identity.
•
Slide 101
Chem 113, Prof. J.T. Spencer
Vibrational Spectroscopy
Infrared and Raman Spectroscopy101
• Forensic Applications of Infrared Spectroscopy
• Use of computer databases of IR’s of known compounds
»
Analyzing Paint - Paint has been recovered
from a crime scene. Since there is a limited
amount of paint, the first tests to be done
should be nondestructive. Colors, layers,
texture, and other physical properties are
recorded. The individual layers of paint are
analyzed by infrared spectroscopy. The
results can be compared to IR results of
known paint samples.
Slide 102
Chem 113, Prof. J.T. Spencer
Infrared Spectroscopy
102
• “With infrared radiation, forensic scientists can determine the exact ink
type and pen that a death threat was written in, or the very model and
year of a suspect's automobile in a hit-and-run accident. Using a
technique known as infrared spectromicroscopy, forensic investigators
have been able to identify and analyze a broad range of samples—from
inks and paint chips to fibers and drugs. The procedure uses infrared
light to study the properties of molecules at an atomic resolution.
Researchers at Lawrence Berkeley National Laboratory have now
expanded the boundaries of infrared forensics with the use of
synchrotron radiation from the Lab's Advanced Light Source (ALS)
facility…..” (Daily Californian, Wednesday, September 18, 2002)
Slide 103
Chem 113, Prof. J.T. Spencer
Magnetic Resonance Spectroscopy
NMR (MRI)
103
• Visualize soft tissue by measuring proton (nuclear)
magnetic alignments relative to an external
magnetic field.
Review Electron Spin Properties First.
Slide 104
Chem 113, Prof. J.T. Spencer
Electron Spin
104
• Electrons have spin properties (spin along axis).
N
-
-
N
Electron spin is
quantized
ms = + 1/2 or - 1/2
Magnetic Fields
Slide 105
Chem 113, Prof. J.T. Spencer
Experimental Electron Spin
105
• Passing an atomic beam (neutral atoms) which contained
an odd number of electrons (1 unpaired electron, see later)
through a magnetic field caused the beam to split into two
spots.
• Showed the possible states of the single (unpaired)
electron as quantized into ms = +1/2 or - 1/2.
Atom
Beam
Generator
Slits
Magnetic
Field
N
S
Viewing
Screen
two
electron
spin
states
Slide 106
Chem 113, Prof. J.T. Spencer
Nuclear Spin
106
• Like electrons, nuclei spin and because of this
spinning of a charged particle (positively charged),
it generates a magnetic field. Two states are
possible for the proton (1H).
N
S
+
+
S
N
Slide 107
Chem 113, Prof. J.T. Spencer
Nuclear Spin
Similar to a canoe paddling
either upstream or
downstream
107
S
Antiparallel
Degenerate
E
N
N
N
S
Parallel
S
N
External Magnetic Field
S
Slide 108
Chem 113, Prof. J.T. Spencer
Magnetic Resonance Imaging MRI
108
• Hydrogen atom has two nuclear spin quantum numbers
possible (+1/2 and -1/2).
• When placed in an external magnetic field, 1H can either
align with the field (“parallel” - lower energy) or against
the field (“antiparallel” - higher energy).
• Energy added (E) can raise the energy level of an electron
from parallel to antiparallel orientation (by absorbing
radio frequency irradiation).
• Electrons (also “magnets”) in “neighborhood” affect the
value of E (i.e., rocks in stream).
• By detecting the E values as a function of position within
a body, an image of a body’s hydrogen atoms may be
obtained.
Slide 109
Chem 113, Prof. J.T. Spencer
MRI
109
• Advantages (first three are not really important for
forensics)
– non-invasive.
– no ionizing or other “dangerous” radiation (such as Xrays of positrons).
– Can be done frequently to monitor progress of
treatment.
– images soft tissues (only those with hydrogen atoms
(almost all “soft” tissues).
– images function through the use of contrast media.
• Disadvantages
– Relatively expensive equipment.
Slide 110
Chem 113, Prof. J.T. Spencer
MRI; Hardware
110
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Chem 113, Prof. J.T. Spencer
MRI
111
Slide 112
Chem 113, Prof. J.T. Spencer
MRI
112
Slide 113
Chem 113, Prof. J.T. Spencer
MRI
113
Slide 114
Chem 113, Prof. J.T. Spencer
MRI
114
Slide 115
Chem 113, Prof. J.T. Spencer
MRI
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115
Slide 116
Chem 113, Prof. J.T. Spencer
MRI
116
Slide 117
Chem 113, Prof. J.T. Spencer
Forensic MRI/CT
117
• Used to reconstruct facial images from skulls. Use
for ancient mummies to modern skulls.
• Allows a very fine discrimination between materials
with different densities providing an enormous
amount of information about the mummy and its
skeleton.
• The level of automation reached in building models
from CT data, reconstruction, texture application
and visualization allow to the user to complete
whole process in 2-3 hours on a PC or graphic
workstation.
Slide 118
Chem 113, Prof. J.T. Spencer
Forensic MRI and CT
118
• The “Virtopsy” focuses on four goals:
• radiological digital imaging methods as main diagnostic tools in
forensic pathology, ultimately leading to "minimally invasive
autopsy" analogous to "keyhole surgery" in clinical medicine.
• three-dimensional optical measuring techniques - a reliable,
accurate geometric presentation of all forensic findings (the body
surface as well as the interior).
• 3D surface scanning in forensic reconstruction.
• Producing and validating of a post-mortem biochemical profile to
estimate the time of death.
• The implementation of an imaging database as a technical basis of a
"center for competence in virtual autopsy”.
Slide 119
Chem 113, Prof. J.T. Spencer
Forensic MRI
119
Virtopsy, a new imaging horizon in forensic pathology:
virtual autopsy by postmortem multislice computed
tomography (MSCT) and magnetic resonance imaging (MRI)
- 40 forensic cases were examined and findings were verified by
subsequent autopsy. Results were classified as follows: (I) cause of death,
(II) relevant traumatological and pathological findings, (III) vital
reactions, (IV) reconstruction of injuries, (V) visualization. In these 40
forensic cases, 47 partly combined causes of death were diagnosed at
autopsy, 26 (55%) causes of death were found independently using only
radiological image data. Radiology was superior to autopsy in revealing
certain cases of cranial, skeletal, or tissue trauma. Some forensic vital
reactions were diagnosed equally well or better using MSCT/MRI.
Radiological imaging techniques are particularly beneficial for
reconstruction and visualization of forensic cases.
(J Forensic Sci. 2003, 48, 386-403)
Slide 120
Chem 113, Prof. J.T. Spencer
Forensic MRI
Validating of a post-mortem analysis
120
Complex scull fracture
system following motor
vehicle accident (victim was
overrun by automobile).
3D reconstructed MSCT image.
Slide 121
Chem 113, Prof. J.T. Spencer
Forensic MRI
Validating of a post-mortem analysis
121
Injury due to vehicle impact in a motor vehicle accident
(pedestrian). (right) finding at autopsy; right lower leg showing
fracture of the fibula. (left) 3 D reconstructed MSCT;
Slide 122
Chem 113, Prof. J.T. Spencer
Forensic MRI and CT
Facial Reconstructions
Egyptian Mummy Head
122
The method uses the tables combined with the warping of a
3D model of a reference scanned head, until the relevant
surface to bone distances are correct. Texture mapping is
used to provide colors and aesthetic features.
Slide 123
Chem 113, Prof. J.T. Spencer
Forensic MRI and CT
Mummy Facial Reconstruction
123
Model skin (blue) and
mummy skull (white)
Face shape
generated
Slide 124
Chem 113, Prof. J.T. Spencer
Forensic MRI and CT
Mummy Facial Reconstruction
124
Texturized model of reconstructed soft
tissues of the mummy
Slide 125
Chem 113, Prof. J.T. Spencer
X-ray Methods
• X-ray Diffraction (XRD and CT)
• Energy Dispersive X-ray Fluorescence
125
Slide 126
Chem 113, Prof. J.T. Spencer
Bragg’s Law and X-ray Diffraction
126
incoming
light
E
D
B
d
lattice in a
crystal
C
Since BCD = 2d sin is the limiting condition for
observing a reflection then because of wave addition
and cancellation;
Bragg’s Law:
n = 2d sin
where n = 1, 2, 3, etc...
Slide 127
Chem 113, Prof. J.T. Spencer
Diffraction
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Chem 113, Prof. J.T. Spencer
Energy-Dispersive X-ray
Fluorescence (EDXRF)
•Did your luxury purchase
originate in a mine deep in the heart
of Central America, or the bottom of
a silty river tributary in Africa, or
perhaps even a flask in a laboratory
in Chicago or Minsk?
•Metal ions such as V3+, Cr3+, Mn2+,
Mn3+, Fe2+, Fe3+, Ni2+, Cu2+, and
UO22+ are responsible for the colors
of most common gemstones and
minerals.
•U.S. Federal Trade Commission
says consumers must be informed
of alterations in gemstones.
128
Slide 129
Chem 113, Prof. J.T. Spencer
Energy-Dispersive X-ray
Fluorescence (EDXRF)
129
•Among the most sensitive and
popular of the nondestructive
spectroscopic techniques used for
trace-metal determination is
EDXRF. In this technique, X-rays
excite the gemstone to fluoresce and
the fluorescent line spectrum
indicates which chemical elements
are present.
EDXRF can also be used to differentiate freshwater from saltwater pearls on the
basis of the greater concentration of magnesium present in the former.
Slide 130
Chem 113, Prof. J.T. Spencer
Energy-Dispersive X-ray
Fluorescence (EDXRF)
130
•EDXRF has been called 'the curator's dream instrument'
because measurements are non-destructive and usually
the whole object can be analyzed, rather than a sample
removed from one. The technique involves aiming an Xray beam at the surface of an object; this beam is about 2
mm in diameter.
•The interaction of X-rays with an object causes
secondary (fluorescent) X-rays to be generated. Each
element present in the object produces X-rays with
different energies. These X-rays can be detected and
displayed as a spectrum of intensity against energy: the
positions of the peaks identify which elements are
present and the peak heights identify how much of each
element is present.
Slide 131
Chem 113, Prof. J.T. Spencer
Energy-Dispersive X-ray
Fluorescence (EDXRF)
131
An incoming X-ray ejects a K-shell electron from an atom of the target. An
electron in the M or L-shell loses energy as it transitions to the vacant K-shell. It
given off energy in the form of fluorescence.
Slide 132
Chem 113, Prof. J.T. Spencer
Energy-Dispersive X-ray
Fluorescence (EDXRF)
http://www.thebritishmuseum.ac.uk/science/techniques/sr-tech-xrf.html
132
Slide 133
Chem 113, Prof. J.T. Spencer
Analytical Methods
133
• Questions to consider in choosing an analytical
(chemical) method:
–
–
–
–
–
–
–
Quantitative or qualitative required
Sample size and sample preparation requirements
What level of analysis is required (e.g., ± 1.0% or ± 0.001%)
Detection levels and useful analytical concentration ranges
Destructive or non-destructive
Availability of instrumentation
Admissibility (e.g., are all lead pipes compositionally the
same or are there sufficient variations among “known” Pb
pipes of the world to link two samples)
Slide 134
Chem 113, Prof. J.T. Spencer
Finis
134
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Chem 113, Prof. J.T. Spencer
135
Chem 113, Prof. J.T. Spencer
Spectroscopy and Spectrometry
1
Forensic Science
Copyright © James T. Spencer 2003 All Rights Reserved
Slide 2
Chem 113, Prof. J.T. Spencer
2
Chemical Analysis
So How On Earth Did We Get To Where
We Are Today?
Slide 3
Chem 113, Prof. J.T. Spencer
Atoms, Molecules and Ions
3
• Science: Atomic Theory
– from a fundamental understanding of the
macroscopic behavior of substances comes
an understanding the microscopic behavior
of atoms and molecules (Baseball rules from Baseball
Game?)
Macroscopic
Substances
Mixtures
Physical Properties and Changes
Microscopic
Atomic theory
Question: Can matter be infinitely divided?
Most Greek Philosophers - Yes
Democritus (460 BC) and John Dalton (1800s) - No (“atomos”means indivisible”)
Slide 4
Chem 113, Prof. J.T. Spencer
Atoms, Molecules and Ions
4
• History of Atomic Theory and Scientific Inquiry
– Aristotle - “metaphysics”,
thought experiments and
no experimental observations
necessary to substantiate ideas.
– Archimedes (287 - 212 BC) - Scientific Method,
determined composition of the King of Syracuse’s
crown by measuring density through water
displacement.
– Roger Bacon (1214 - 1294) - Experimental Science “ It
is the credo of free men - the opportunity to try, the
privilege to err, the courage to experiment anew.
...experiment, experiment, ever experiment”.
Slide 5
Chem 113, Prof. J.T. Spencer
Aristotle (384-322 BC)
• All of the sciences (epistêmai,
literally "knowledges") can be
divided into three branches:
theoretical, practical, and
productive. Whereas practical
sciences, such as ethics and
politics, are concerned with
human action, and productive
sciences with making things,
theoretical sciences, such as
theology, mathematics, and the
natural sciences, aim at truth
and are pursued for their own
sake.
5
Slide 6
Chem 113, Prof. J.T. Spencer
Archimedes (287-212BC)
6
• Archimedes was a native of Syracuse
(not NY). Stories from Plutarch, Livy,
and others describe machines invented
by Archimedes for the defence of
Syracuse (These include the catapult,
the compound pulley and a burningmirror).
• Archimedes discovered fundamental
theorems concerning the center of
gravity of plane figures and solids. His
most famous theorem gives the weight
of a body immersed in a liquid, called
Archimedes' principal.
His methods anticipated integral calculus 2,000 years before
Newton and Leibniz.
Slide 7
Chem 113, Prof. J.T. Spencer
Archimedes (287-212BC)
7
Slide 8
Chem 113, Prof. J.T. Spencer
Archimedes (287-212BC)
8
Suspecting that a goldsmith might have replaced some of the gold by silver in
making a crown, Hiero II, the king of Syracuse, asked Archimedes to determine
whether the wreath was pure gold. The wreath could not be harmed since it was a
holy object.
The solution which occurred when he stepped into his bath and caused it to overflow
was to put a weight of gold equal to the crown, and known to be pure, into a bowl
which was filled with water to the brim. Then the gold would be removed and the
king's crown put in, in its place. An alloy of lighter silver would increase the bulk of
the crown and cause the bowl to overflow.
Pure Gold?
Equal Weight of Gold
Crown Displaced More Water
Slide 9
Chem 113, Prof. J.T. Spencer
Archimedes
(287-212BC)9
Slide 10
Chem 113, Prof. J.T. Spencer
Mass Spectrometry
Background - Stoichiometry
10
• Antoine Lavoisier (1734 - 1794)
– Law of Conservation of Mass - atoms are neither
created nor destroyed in chemical reactions
– total number of atoms = total number of atoms after reaction
before reaction
– Stoichiometry - quantitative study of chemical
formulas and reactions
(Greek; “stoichion”= element, “metron” = measure)
• Chemical Equations - used to describe chemical
reactions in an accurate and convenient fashion
2H2 + O2
reactants
2 H2O
products
Slide 11
Chem 113, Prof. J.T. Spencer
Antoine Lavoisier
11
Antoine Lavoisier was born in
Paris, and although Lavoisier's
father wanted him to be a
lawyer, Lavoisier was
fascinated by science. From the
beginning of his scientific
career, Lavoisier recognized the
importance of accurate
measurements. He wrote the first modern chemistry (1789)
textbook so that it is not surprising that Lavoisier is often called the
father of modern chemistry. To help support his scientific work,
Lavoisier invested in a private tax-collecting firm and married the
daughter of one of the company executives. Guillotined for his tax
work in 1794.
Slide 12
Chem 113, Prof. J.T. Spencer
“Chemical” Family Trees
12
James T. Spencer
(1984, Iowa State University)
John G. Verkade
(Harvard University,1960)
Harry Julius Emeleus
(Imperial College London, 1926)
Theron Standish Piper
(Harvard University, 1956)
Russell N. Grimes
(University of Minesota)
Geoffrey Wilkinson
(Imperial College London, 1941)
William N. Lipscomb
(Caltech., 1945)
Alfred E. Stock
(Univ. of Berlin ca 1900
Henri Moissan
(University of Paris, 1879)
Edmond Fremy
(University of Paris, 1856)
Emil Fisher
(University of Strassbourg, 1874)
Joseph L. Gay Lussac
(University of Paris, 1800)
Claude L. Berthollet
(University of Paris, 1778)
Jean Bucquet
(University of Paris, 1770)
Red borders indicate Nobel
Laureates (first award 1901)
Linus Pauling
(Caltech, 1925)
Antoine Lavoisier
(University of Paris, 1764)
Adolf von Baeyer
(University of Berlin, 1858)
August Kekule
(University of Gressen, 1852)
Justus Liebig
(University of Erlangen, 1822)
Slide 13
Chem 113, Prof. J.T. Spencer
Forensic Chemical Analysis
Typical Chemical Problems
13
• Problem - An unknown sample of a white powered
compound is brought into the lab after a routine
traffic stop. What is the compound?
• Problem - A murder is committed with a lead pipe
(in the conservatory) that was removed from the
bathroom sink. Col. Mustard was found with a
deformed lead pipe. Were the two one unit in the
past?
• Problem - A fiber found on a hairbrush appears to
be from a wig. Did the fiber come from the wig of
the victim or from another source (possibly the
murderer)?
• Problem - Was Napoleon murdered?
Slide 14
Chem 113, Prof. J.T. Spencer
Analytical Methods
14
• Questions to consider in choosing an
analytical (chemical) method:
– Quantitative or qualitative required
– Sample size and sample preparation requirements
– What level of analysis is required (e.g., ± 1.0% or ±
0.001%)
– Detection levels and useful analytical concentration
ranges
– Destructive or non-destructive
– Availability of instrumentation
– Admissibility (e.g., are all lead pipes compositionally
the same or are there sufficient variations among
“known” Pb pipes of the world to link two samples)
Slide 15
Chem 113, Prof. J.T. Spencer
Spectroscopy and Spectrometry
15
• Mass Spectrometry (MS)
• Atomic Spectroscopy
– Atomic Absorption (AAS) and Emission Analysis (AES)
– Neutron Activation Analysis (NAA)
• Molecular Spectroscopy
– Electronic Spectroscopy
– Vibrational Spectroscopy
– Nuclear Magnetic Resonance (NMR or MRI)
• X-ray Methods
– X-ray Diffraction (XRD and CAT)
– Energy Dispersive X-ray Fluorescence (EDXRF)
Slide 16
Chem 113, Prof. J.T. Spencer
Comparison of Techniques
16
Technique
Qual.*
Sample Detection
or
Destructive
Size
levels
Quant.
Mass Spec.
Qual.
0.1 mL
to 10-8
mL
Infrared
Qual.
0.001 g
UV-visible
Qual.
0.001 g
Instr.
Avail.
*
Yes
Easy
*
No
Easy
No
Easy
AES
Quant.
10-4 g/L
Yes
Moderate
AAS
Quant.
10-4 g/L
Yes
Easy
NAA
Quant.
1 x 10-9 g
Possibly
Difficult
* Primary use is in qual. analysis, although it can be used quantitatively in some cases.
Slide 17
Chem 113, Prof. J.T. Spencer
Mass Spectrometry
17
• Chemical Background (mass scale, ave. atomic
masses, etc.)
• Instrumental Principles and Design
• Spectral Features
• Spectral Interpretation and Comparison
• GC-MS and LC-MS
Slide 18
Chem 113, Prof. J.T. Spencer
Mass Spectrometry
18
Underlying Ideas - Atomic and Molecular Weights
•Atomic Mass Scale - based upon 12C
isotope. This isotope is assigned a mass of
exactly 12 atomic mass units (amu) and the
masses of all other atoms are given relative
to this standard.
•Most elements in nature exist as mixtures
of isotopes.
Slide 19
Chem 113, Prof. J.T. Spencer
Mass Spectrometry
Underlying Ideas - Atomic Weights
19
• Average Atomic Mass (AW)- weighted average (by
% natural abundance) of the isotopes of an element.
•Example (1);
10B is 19.78% abundant with a mass of 10.013 amu
11B is 80.22% abundant with a mass of 11.009 amu
therefore the average atomic mass of boron is;
(0.1987)(10.013) + (0.8022)(11.009) = 10.82 amu
Although natural B does not actually contain any
B with mass 10.82, it is considered to be
composed entirely of mass 10.82 for stoich.
Slide 20
Chem 113, Prof. J.T. Spencer
Mass Spectrometry
Underlying Ideas - Atomic Weights
20
• Average Atomic Mass (AW)- weighted average (by
% natural abundance) of the isotopes of an element.
•Example (2):
194Pt
is 33.90% abundant with a mass of 193.963 amu
195Pt is 33.80% abundant with a mass of 194.965 amu
196Pt is 25.30% abundant with a mass of 195.965 amu
198Pt is 7.210% abundant with a mass of 197.968 amu
therefore the average atomic mass of platinum is;
(0.3390)(193.963) + 0.3380)(194.965) + (0.2530)(195.965 ) +
(0.07210)(197.968)= 195.504 amu
Slide 21
Chem 113, Prof. J.T. Spencer
Mass Spectrometry
Basic Ideas
21
A mass spectrometer (MS) creates charged particles (ions) from
gas phase molecules.
Electron Ionization (EI)- Uses electron impact to ionize a molecule.
Chemical Ionization (CI)- First ionizes a molecular gas (such as
methane) which in turn ionizes the molecule of interest.
A “gentler” method of ionization - often allows the
observation of a “sensitive” molecular ion as a P+1
peak.
Fast Atom Bombardment (FABS)- Mainly for involatile compounds very harsh.
The MS analyzes those ions to provide information about the
molecular weight of the compound and its chemical structure.
Slide 22
Chem 113, Prof. J.T. Spencer
Mass Spectrometry
Basic Ideas
Either move slit or
change deflecting force
to scan masses “across”
the detector
22
Slide 23
Chem 113, Prof. J.T. Spencer
Mass Spectrometer
Magnetic field deflection (quadrupole MS)23
• Direct methods of measuring (separating) mass.
• Sample molecules are ionized by e-beam to cations (+1 by
“knocking off” one electron) which are then deflected by
magnetic field - for ions of the same charge the angle of
deflection in proportional to the ion’s mass
vacuum chamber beam of pos. ions
accelerating grid (-)
N
sample
focusing slits
ionizing e- beam
Hg
S
magnetic field
Mass
Spectrum
200
Int.
mass number (amu)
detector
Slide 24
Chem 113, Prof. J.T. Spencer
Mass Spectrometer
Atomic Spectra
Mass
Spectrum
Cl
Int.
35
Mass
Spectrum
Int.
C
12
37
mass number (amu)
35Cl:
75% abundant
37Cl: 24% abundant
24
Mass
Spectrum
P
Int.
31
13
mass number (amu)
12Cl:
mass number (amu)
98.9% abundant 31P: 100% abundant
13Cl: 1.11% abundant
Slide 25
Chem 113, Prof. J.T. Spencer
Mass Spectrometry
Molecules
25
Slide 26
Chem 113, Prof. J.T. Spencer
Mass Spectrometry
26
Slide 27
Chem 113, Prof. J.T. Spencer
Mass Spectrometry
27
Ionization produces singly charged ions. The intact charged
molecule is the molecular ion. Energy from the electron impact
and instability in a molecular ion can cause that ion to break
into smaller pieces (fragments).
The methanol ion may fragment in various ways, with one
fragment carrying the charge and one fragment remaining
uncharged. For example:
CH3OH+. (molecular ion)
(or) CH3OH+.(molecular ion)
CH2OH+(fragment ion) + . H
CH3+(fragment ion) + .OH
Slide 28
Chem 113, Prof. J.T. Spencer
Mass Spectrometry
28
Slide 29
Chem 113, Prof. J.T. Spencer
Mass Spectrometry
29
Slide 30
Chem 113, Prof. J.T. Spencer
Mass Spectrometer
30
Unknown white powdery substance ingested by
unconscious patient.
What do you do? Is it Heroin, Cocaine, Caffeine?
In ten sity
Mass Spectrum of Unknown Compound
Mass
25
50
75
100
125 150
175
200
225
250
275
300
Slide 31
Chem 113, Prof. J.T. Spencer
Mass Spectrometer
In ten sity
MS Library
43
25
50
75
215
146
100 125 150 175 200 225 250 275
194
In ten s ity
67
Mass
55
25
50
75
300
Caffeine
109
82
42
other peaks at
327 and 369
268
204
94
Mass
Heroin
Heroin
31
MS of Unknown
100 125 150 175 200 225 250 275
300
Slide 32
Chem 113, Prof. J.T. Spencer
In ten sity
Mass Spectrometer
MS
Library
82
Cocaine
Cocaine
182
303
42
122
Mass
25
50
75
272
150
100 125 150 175 200 225
194
67
In ten s ity
Mass
32
55
25
50
75
300
Caffeine
109
MS of Unknown
82
42
250 275
100 125 150
175 200 225
250 275
300
Slide 33
Chem 113, Prof. J.T. Spencer
Mass Spectrometer
MS Library
In ten s ity
67
Mass
55
50
109
75
100 125 150
175 200 225
194
67
In ten s ity
Mass
55
50
75
250 275
300
Caffeine
109
MS of Unknown
82
42
25
Caffeine
Caffeine
82
42
25
194
33
100 125 150
175 200 225
250 275
300
Slide 34
Chem 113, Prof. J.T. Spencer
Mass Spectrometer
O
Unknown white powdery
substance ingested by
unconscious patient.
What do you do?
H 3C
N
N
O
Mass Spectrum
N
N
CH 3
Caffeine
In ten sity
Mass
CH 3
25
50
75
100
125 150
175
200
225
250
275
300
34
Mol. Wgt
= 194
Slide 35
Chem 113, Prof. J.T. Spencer
GC-Mass Spectrometry
35
A mixture of compounds to be analyzed is injected into the gas
chromatograph (GC) where the mixture is vaporized in a heated
chamber. The gas mixture travels through a GC column, where
the compounds become separated as they interact with the
column. Those separated compounds then immediately enter the
mass spectrometer.
Slide 36
Chem 113, Prof. J.T. Spencer
GC-Mass Spectrometry
36
Slide 37
Chem 113, Prof. J.T. Spencer
Atomic and Molecular Spectroscopy
37
• Science: Atomic Theory
– “The strength of a science is that its conclusions are
derived by logical arguments from facts that result
from well-designed experiments. Science has
produced a picture of the microscopic structure of the
atom so detailed and subtle of something so far
removed from our immediate experience that it is
difficult to see how its many features were
constructed. This is because so many experiments
have contributed to our ideas about the atom.”
B. Mahan from University Chemistry
Slide 38
Chem 113, Prof. J.T. Spencer
Atomic and Molecular Spectroscopy
38
•
•
•
•
•
•
Electromagnetic Radiation
Atomic Electronic Structure
Quantization of Energy Levels
Absorption, Transmission and Emission Spectra
Atomic Spectroscopy
Molecular Spectroscopy
Slide 39
Chem 113, Prof. J.T. Spencer
39
364.6 nm
410.2 nm
434.0 nm
486.1 nm
656.3 nm
Hydrogen Emission
Ultraviolet
Red
Blue
A Swiss schoolteacher in 1885 (J. Balmer) derived
a simple formula to calculate the wavelengths of
the emission lines (purely a mathematical feat
with no understanding of why this formula
worked)
frequency = C ( 1 - 1 ) where n = 1, 2, 3, etc...
22 n2 C = constant
Slide 40
Chem 113, Prof. J.T. Spencer
Spectroscopy
Background - Electromagnetic Radiation 40
= c
where = wavelength,
= frequency,
c = light speed
amplitude
1 cycle per sec = 1 hertz
wavelength ()
QuickTime™ and a
Gra phics d ecomp re sso r
are n eed ed to see this p ictu re.
Slide 41
Chem 113, Prof. J.T. Spencer
Electromagnetic Radiation
41
= c
where = wavelength, = frequency, c = light speed
Gamma
UV/Vis Infrared Microwave Radio
X-ray
Wavelength (m)
10-11m
10 m
Slide 42
Chem 113, Prof. J.T. Spencer
Electromagnetic Radiation
42
Magnetic and Electronic Parts
mutually perpendicular
QuickTime™ and a
Graphics decompressor
are needed to see this picture.
Slide 43
Chem 113, Prof. J.T. Spencer
Spectroscopy
Electronic Structure - Background
43
• Prior to 1926, Many experiments in the structure of matter
showed several important relationships:
– Light has BOTH wavelike and particulate (solid
particle-like) properties.
– Even solid particles display BOTH wavelike and
particulate properties.
– Whether the wavelike or particulate properties are
predominantly observed depends upon the nature of
the experiment (what is being measured).
Slide 44
Chem 113, Prof. J.T. Spencer
Wave Properties of Matter
44
• De Broglie - particles behave under some circumstances
as if they are waves (just as light behaves as particles
under some circumstances). Determines relationship:
= h/mv
= wavelength
h = Planck’s const.
m = mass
v = velocity
Particle
electron
He atom (a)
Baseball
fast ball
slow ball
mass (kg)
9 x 10-31
7 x 10-27
v (m/sec)
1 x 105
1000
(pm)
7000
90
0.1
0.1
20
0.1
3 x 10-22
7 x 10-20
Slide 45
Chem 113, Prof. J.T. Spencer
Niels Bohr (Denmark)
• Built upon Planck,
Einstein and others work
to propose explanation of
line spectra and atomic
structure.
• Nobel Prize 1922
• Worked on Manhattan
Project
• Advocate for peaceful
nuclear applications
45
Slide 46
Chem 113, Prof. J.T. Spencer
Bohr’s Model
46
• Continuous Spectra vs. Line Spectra
Wave-like
Behavior
Sunlight
Wave-like
Behavior
Hydrogen
Dispersion by Prism
Dispersion by Prism
Slide 47
Chem 113, Prof. J.T. Spencer
Bohr’s Model
“Microscopic Solar System”
• Electrons in circular orbits
around nucleus with
quantized (allowed) energy
states
• When in a state, no energy is
radiated but when it changes
states, energy is emitted or
gained equal to the energy
difference between the states
• Emission from higher to
lower, absorption from lower
to higher
47
n=∞
n=4
n=3
n=2
electronic
transitions
n=1
Slide 48
Chem 113, Prof. J.T. Spencer
48
364.6 nm
410.2 nm
434.0 nm
486.1 nm
656.3 nm
Hydrogen Emission
Ultraviolet
Red
Blue
A Swiss schoolteacher in 1885 (J. Balmer) derived
a simple formula to calculate the wavelengths of
the emission lines (purely a mathematical feat
with no understanding of why this formula
worked)
frequency = C ( 1 - 1 ) where n = 1, 2, 3, etc...
22 n2 C = constant
Slide 49
Chem 113, Prof. J.T. Spencer
Bohr’s Model
“Microscopic Solar System”
49
Slide 50
Chem 113, Prof. J.T. Spencer
Microscopic Properties
50
• Light energy may behave as waves or as small
particles (photons).
• Particles may also behave as waves or as small
particles.
• Both matter and energy (light) occur only in discrete
units (quantized).
Quantized
(can stand only on steps)
Non-Quantized
(can stand at any position on the ramp)
Slide 51
Chem 113, Prof. J.T. Spencer
What is Quantization
51
• Examples of quantization (when only discrete and
defined quantities or states are possible):
Quantized
Non-Quantized
Piano
Stair Steps
Typewriter
Dollar Bills
Football Game Score
Light Switch (On/Off)
Energy
Matter
Violin or Guitar
Ramp
Pencil and Paper
Exchange rates
Long Jump Distance
Dimmer Switch
Slide 52
Chem 113, Prof. J.T. Spencer
Quantum Numbers
52
• Quantum Numbers also specify energy of the occupying
electrons,
0
E
N
E
R
G
Y
n=∞
n=4
n=3
n=2
n=1
l=0
4s
3s
l=1
4p
3p
2s
2p
1s
l=2
4d
3d
l=3
4f
32
electrons
max
18
electrons
max
8
electrons
max
2
l
electrons
max
n
Slide 53
Chem 113, Prof. J.T. Spencer
Many Electron Atoms
0
n=1
n=2
n=3
3d
n=4
4p
4s
53
n=5
5s
3p
E
N
E
R
G
Y
3s
2p
2s
1s
s (l = 0)
p (l = 1)
d (l = 2)
Slide 54
Chem 113, Prof. J.T. Spencer
Red
Blue
54
364.6 nm
410.2 nm
434.0 nm
486.1 nm
656.3 nm
Hydrogen Emission
Ultraviolet
No Just Emission - molecules (and atoms) can
also absorb energy.
Slide 55
Chem 113, Prof. J.T. Spencer
Spectroscopy
55
• When electromagnetic radiation passes through a
substance, it can either be absorbed or transmitted,
depending upon the structure of the substance.
• When a molecule absorbs radiation it gains energy as it
undergoes a quantum transition from one energy state
(Einitial) to another (Efinal).
The frequency of the
absorbed radiation is related
to the energy of the
transition by Planck's law:
Efinal - Einitial = E = h = hc/ .
Slide 56
Chem 113, Prof. J.T. Spencer
Atomic Spectroscopy
56
• Atomic Absorption and Emission- Techniques that
involve the determination and measurement of
atomic energy levels (spectrometry) and chemical
identification based on how atoms absorb or emit
electromagnetic radiation.
• Neutron Activation Analysis - Quantitative multielement analysis of major, minor, trace (ppb) and rare
elements. The sample is placed in a flux of neutrons
and after removal the emissions of the radionuclides
produced are measured. Forensic applications include
gunshot residues, bullet lead, glass, paint, hair, etc.
Slide 57
Chem 113, Prof. J.T. Spencer
Atomic Spectroscopy
57
• Ground state - the lowest energy state of an atom or molecule
(most stable state) with regard to the position of the electrons
around the nucleus
• Excited state – results when ground state electrons are excited
by energy to higher energy states. Excited states are unstable
and an atom in the excited state immediately returns to the
ground state
• Emission - When an electric current is passed through a gas,
the gas emits light. This is due to the change of energy of the
gas. The electrons in the atoms of the gas become excited to a
higher energy state (the “excited state”) and when they return
to the original, low-energy state (“the ground state”), the atoms
of the gas emit the excess energy as light.
• Absorption - This is due to the change of energy of the gas.
The electrons in the atoms of the gas become excited by
absorbing energy (light).
Slide 58
Chem 113, Prof. J.T. Spencer
Atomic Spectroscopy
58
Slide 59
Chem 113, Prof. J.T. Spencer
Flame Tests
59
Atomic Emission
Slide 60
Chem 113, Prof. J.T. Spencer
Atomic Emission
60
Slide 61
Chem 113, Prof. J.T. Spencer
Atomic Spectroscopy
61
Slide 62
Chem 113, Prof. J.T. Spencer
Atomic Emission
AES
62
• Atomic Emission (AE) - uses quantitative measurement of
the optical emission from excited atoms to determine analyte
concentration. Analyte atoms in solution are aspirated into the
excitation region where they are atomized by a flame, discharge,
or plasma. These high-temperature atomization sources provide
sufficient energy to promote the atoms into high energy levels.
The atoms decay back to lower levels by emitting light. Since
the transitions are between distinct atomic energy levels, the
emission lines in the spectra are narrow.
Slide 63
Chem 113, Prof. J.T. Spencer
Atomic Emission Spectroscopy
AES
63
• Advantages of Inductively coupled plasma (ICP-AES):
– Multielement analyses
– Determination of low concentration, difficult to
atomize elements
– Less chemical interference due to the high
temperature in the plasma employed
– Determination of many elements (e.g., Zn, Cu)
– Great linear detection range
– Supplementary to AAS
Slide 64
Chem 113, Prof. J.T. Spencer
Atomic Emission
AES
64
• Zr-content of flame resist-treated wool (Low-Smoke
Zirpro finishing)
Slide 65
Chem 113, Prof. J.T. Spencer
Atomic Emission
AES
• Russian Icon of St. Nicholas - The
pigments present on this mid19th Century painting were
characterized by AES
spectroscopy (laser-induced
breakdown spectroscopy, LIBS)
and Raman microscopy. The
identification of pigments on the
original work along with those
applied in restoration of cracks
in the varnish and painting
surface were analyzed.
65
Slide 66
Chem 113, Prof. J.T. Spencer
Atomic Emission
AES
66
• LIBS depth profile measurements leave a minute crater in the
surface of the art object being studied. This allows
stratagraphic information to be collected. A typical cross
section of the icon is shown.
Slide 67
Chem 113, Prof. J.T. Spencer
Atomic Emission
AES
67
• Several areas of the icon, where white paint was
used, were analyzed. The LIBS spectrum showed
strong peaks characteristic of lead. This was
confirmed by the Raman spectrum, which verified
the presence of lead carbonate, [2PbCO3·Pb(OH)2].
LIBS
Raman
Slide 68
Chem 113, Prof. J.T. Spencer
Atomic Emission
AES
The brown pigment was
characterized as an iron-based
pigment mixed with lead
white. The pigment scattered
poorly and so did not produce
a Raman spectrum. The LIBS
spectrum showed the presence
of Fe and Al, corresponding to
an iron oxide and an earth such
as clay. Also present are
emissions characteristic of
magnesium, lead and calcium.
The peak corresponding to iron
at ~275nm is characteristic of
iron that has been observed in
studies on pure iron oxide
pigments (for example, Mars
black, Fe3O4).
68
Slide 69
Chem 113, Prof. J.T. Spencer
Atomic Absorption
AAS
• Atomic Absorption - Atomic-absorption (AA)
69
spectroscopy uses the absorption of light to measure the
concentration of gas-phase atoms. Since samples are
usually liquids or solids, the analyte atoms or ions must be
vaporized in a flame or graphite furnace. The atoms absorb
ultraviolet or visible light and make transitions to higher
electronic energy levels. The analyte concentration is
determined from the amount of absorption.
Slide 70
Chem 113, Prof. J.T. Spencer
Atomic Absorption
AAS
70
Typical Problem - A child becomes quite ill and is
taken to the hospital. It is found that the child is
suffering from lead poisoning. A forensic laboratory
is contacted and asked if it can determine the source
of the lead which the child has ingested. No crime has
been committed, per se, but the source must be
eliminated to prevent future danger to the child. Paint
samples from a number of objects with which the
child has had repeated contact are collected. Paint on
the child's crib, paint from his toys, and paint from
the child's swing, to name a few, are sent to the
laboratory. AA is the best method for these analyses.
Slide 71
Chem 113, Prof. J.T. Spencer
Neutron Activation Analysis
NAA
71
Neutrons interact with a target nucleus to form a compound nucleus in
an excited state. The compound nucleus will decay into a more stable
configuration through emission of one or more gamma rays. This new
configuration may yields a radioactive nucleus which also decays by
emission of delayed gamma rays, but at a much slower rate according to
the unique half-life of the radioactive nucleus.
Slide 72
Chem 113, Prof. J.T. Spencer
Neutron Activation Analysis
NAA
72
•NAA falls into two categories: (1) prompt gamma-ray neutron activation
analysis (PGNAA), where measurements take place during irradiation, or
(2) delayed gamma-ray neutron activation analysis (DGNAA), where the
measurements follow radioactive decay (most common). About 70% of
the elements have properties suitable for measurement by NAA.
•Parts per billion
or better.
Gamma-ray spectrum
showing medium- and
long-lived elements
measured in a sample of
pottery irradiated for 24
hours, decayed for 9 days,
and counted for 30 minutes
on a HPGe detector.
Slide 73
Chem 113, Prof. J.T. Spencer
Neutron Activation Analysis
NAA
73
An example of the gamma-ray spectrum from the activation of a
human nail used as a biological monitor of trace-element status.
Slide 74
Chem 113, Prof. J.T. Spencer
Neutron Activation Analysis
Arsenic in Hair
Napoleon Bonaparte
One of the most brilliant individuals in
history, Napoleon Bonaparte was a masterful
soldier, grand tactician, sublime statesman and
exceedingly capable administrator. After an
extraordinary career, he was finally defeated
and exiled to Elba. He returned from Elba to
be ultimately defeated at
Waterloo. He was finally
exiled to the remote tiny
volcanic island of St. Helena,
south of the Equator. The
nearest land is Ascension
Island, 700 miles to the north.
74
Slide 75
Chem 113, Prof. J.T. Spencer
Neutron Activation Analysis
Arsenic in Hair
Murdered or Not?
For years a controversy has raged about
Napoleon being killed on St. Helena - either
by French Royalists, persons in his exiled
entourage or the British - and all have
pointed to the high levels of arsenic in the
emperor's body as being evidence of such
behavior. The emperor's body contained
some 15 parts per million of the poison,
where the maximum safe limit is only three
parts per million. The determination was
by neutron activation analysis of his hair.
75
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Chem 113, Prof. J.T. Spencer
Neutron Activation Analysis
Arsenic in Hair
“So Who Done It?”
(if it was done at all)
British Authorities - The Allied heads of
state had no greater wish than to ensure that
Napoleon was permanently “out of the
way”. Strong hatred by British local
commander.
Royalists - Revenge and insurance against
Napoleon for declaring himself Emperor
and dismantling the aristocracy.
Exiled Entourage - Jealousy (romantic
triangles), intrigue, revenge.
76
Slide 77
Chem 113, Prof. J.T. Spencer
Neutron Activation Analysis
Arsenic in Hair
77
NAA of Napoleon’s Hair
From the old tradition of keeping hair locks, many sample of
Napoleon’s hair are known. NAA showed high concentrations of As at
various locations along hair shafts. The As, however, was determined
not to have been taken orally.
So how did he die and why did he have such high As
concentrations?
Slide 78
Chem 113, Prof. J.T. Spencer
Neutron Activation Analysis
Arsenic in Hair
The wallpaper in his room was dyed with
Scheele's Green (Paris Green), a coloring
pigment that had been used in fabrics and
wallpapers from around 1770. Named
after the Swedish chemist who invented
it, the dye contained copper arsenite. It
was discovered that if wallpaper
containing Scheele’s Green became
damp, the mould converted the copper
arsenite to a poisonous vapor form of
arsenic. Breathing the arsenic on its own
might not have been enough to kill
Napoleon, but he was ill already with a
stomach ulcer/cancer. On the 5 May
1821, the arsenic tipped the scale against
"the little corporal."
78
Slide 79
Chem 113, Prof. J.T. Spencer
Salem Witch Trials
79
From June through September of 1692,
nineteen men and women, all having been
convicted of witchcraft, were carted to
Gallows Hill, a barren slope near Salem
Village, for hanging. Another man of over
eighty years was pressed to death under
heavy stones for refusing to submit to a
trial on witchcraft charges. Hundreds of
others faced accusations of witchcraft.
Dozens languished in jail for months
without trials. Then, almost as soon as it
had begun, the hysteria that swept through
Puritan Massachusetts ended.
Slide 80
Chem 113, Prof. J.T. Spencer
Salem Witch Trials
80
In February of the exceptionally cold winter of 1692, young Betty Parris
became strangely ill. She dashed about, dove under furniture, contorted
in pain, and complained of fever. Cotton Mather had recently
published a popular book, "Memorable Providences," describing the
suspected witchcraft of an Irish washerwoman in Boston, and Betty's
behavior mirrored that of the afflicted person described in Mather's
widely read and discussed book. It was easy to believe in 1692 in Salem,
with an Indian war raging less than seventy miles away (and many
refugees from the war in the area) that the devil was close at hand.
Sudden and violent death occupied minds. Talk of witchcraft increased
when other playmates of Betty, including eleven-year-old Ann Putnam,
seventeen-year-old Mercy Lewis, and Mary Walcott, began to exhibit
similar unusual behavior. When his own nostrums failed to effect a
cure, William Griggs, a doctor called to examine the girls, suggested
that the girls' problems might have a supernatural origin. The
widespread belief that witches targeted children made the doctor's
diagnosis seem increasing likely.
Slide 81
Chem 113, Prof. J.T. Spencer
Salem Witch Trials
81
“Trial of George Jacobs”
(1692)
“Examination of a Witch”
Slide 82
Chem 113, Prof. J.T. Spencer
St. Anthony’s Fire - Bosch
82
Slide 83
Chem 113, Prof. J.T. Spencer
Ergot
83
Ergot - A toxic fungus, ( Claviceps purpurea ) found as a parasite
on grains of rye. One form is hallucinogenic ergotism, in which
people often experience symptoms of one of the other forms of
ergotism (gangrenous ergotism - people experience nausea, and
pains in the limbs, bodily extremities turn black, dry and
become mummified, makingit possible for infected limbs to
spontaneously break off at the joints, or convulsive ergotism)
along with vivid hallucinations. The other symptoms are very
much like those of modern psychedelic drugs such as
nervousness, physical and mental excitement, insomnia and
disorientation. People with this form of ergotism were also
observed to perform strange dances with wild, jerky movements
accompanied by hopping, leaping and screaming. They would
dance compulsively until exhaustion lead them to collapse
unconscious.
Slide 84
Chem 113, Prof. J.T. Spencer
Ergotism?
84
St. Christopher Carrying the Christ Child through a Sinful World, Bosch, c1520
Slide 85
Chem 113, Prof. J.T. Spencer
Ergotism
85
Ergotamine tartrate
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Ergot on grains of rye
Slide 86
Chem 113, Prof. J.T. Spencer
Ergotism
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86
lysergic acid
diethylamide
Slide 87
Chem 113, Prof. J.T. Spencer
Molecular Spectroscopy
87
• Electronic Spectroscopy
• Vibrational Spectroscopy
• Nuclear Magnetic Resonance Spectroscopy
(NMR or MRI)
Slide 88
Chem 113, Prof. J.T. Spencer
Electronic Spectroscopy
UV-visible
88
When white light passes through or is reflected by a
colored substance, a characteristic portion of the
total wavelengths is absorbed. The remaining light
will then assume the complementary color to the
wavelength(s) absorbed.
The remaining light will
then assume the
complementary color to the
wavelength(s) absorbed.
Slide 89
Chem 113, Prof. J.T. Spencer
Electronic Spectroscopy
UV-visible
89
Visible region of the spectrum has photon energies of 36 to 72
kcal/mole, and the near ultraviolet region 72 to 143 kcal/mole (200 nm).
Sufficient E to excite a molecular electron to a higher energy orbital.
Of the six transitions outlined, only the two lowest energy ones (leftmost, colored blue) are achieved by these energies (200-800 nm).
Energetically favored electron promotion will be from the highest
occupied molecular orbital (HOMO) to the lowest unoccupied
molecular orbital (LUMO).
Slide 90
Chem 113, Prof. J.T. Spencer
Electronic Spectroscopy
UV-visible
90
When sample molecules are exposed to light having an
energy that matches a possible electronic transition within the
molecule, some of the light energy will be absorbed as the
electron is promoted to a higher energy orbital. An optical
spectrometer records the wavelengths at which absorption
occurs, together with the degree of absorption at each
wavelength.
Slide 91
Chem 113, Prof. J.T. Spencer
Electronic Spectroscopy
UV-visible
91
Effect of Conjugation
Slide 92
Chem 113, Prof. J.T. Spencer
Electronic Spectroscopy
UV-visible
UV-vis. instrument
92
Slide 93
Chem 113, Prof. J.T. Spencer
Vibrational Spectroscopy
Infrared and Raman Spectroscopy 93
• Radiation from 500 to 4000 cm-1 (vibrational
transitions in the molecules).
• Vibrational “mode” must have a change in dipole
moment in the transition. Energy of the transition is
dependent upon the strengths of the bonds and
geometric structure.
Slide 94
Chem 113, Prof. J.T. Spencer
Vibrational Spectroscopy
Infrared and Raman Spectroscopy 94
• For the water molecule, for which there are three
vibrational modes, there are consequently three sets
of energy levels within which transitions may occur
(shown ).
The spacing between energy
levels depends upon the
particular vibration being
considered. Each spacing
requires a photon of
different energy to cause the
transition, so we expect
photons of three different
energies to be absorbed by
H2O.
Slide 95
Chem 113, Prof. J.T. Spencer
Vibrational Spectroscopy
Infrared and Raman Spectroscopy 95
• In order for a particular vibrational mode to directly absorb
infrared electromagnetic radiation, the vibrational motion
associated with that mode must produce a change in the dipole
moment of the molecule.
• There are many molecules which, although possessing no
permanent dipole moment, still undergo vibrations which
cause changes in the value of the dipole moment from 0 to
some non-zero value. Consider the CO2 molecule:
Slide 96
Chem 113, Prof. J.T. Spencer
IR Spectrum of CO2
96
C alculated spectrum o f co 2 B 3L Y P /6-31G (d)
595
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495
a b so rb a n c e
395
O=C=O
295
O=C=O
195
95
-5
0
500
1000
1500
2000
w avenum ber/cm -1
2500
3000
3500
4000
Slide 97
Chem 113, Prof. J.T. Spencer
Vibrational Spectroscopy
Infrared and Raman Spectroscopy 97
• Different types of
bonds have
characteristic
regions of the
spectrum where
they absorb
Slide 98
Chem 113, Prof. J.T. Spencer
Vibrational Spectroscopy
Infrared and Raman Spectroscopy 98
• Forensic Applications of Infrared Spectroscopy
• Use of computer databases of IR’s of known compounds
»
Analyzing Alcohol - The breath is tested
with a mechanism similar to a breathalyzer
(chemical oxidation) but uses the infrared
absorptions of alcohol.
»
Slide 99
Chem 113, Prof. J.T. Spencer
Vibrational Spectroscopy
Infrared and Raman Spectroscopy 99
• Forensic Applications of Infrared Spectroscopy
• Use of computer databases of IR’s of known compounds
»
Analyzing Drugs - The drug's various
chemical components absorb infrared light.
The absorptions are compared to known
samples using a database.
Slide 100
Chem 113, Prof. J.T. Spencer
Vibrational Spectroscopy
Infrared and Raman Spectroscopy100
• Forensic Applications of Infrared Spectroscopy
• Use of computer databases of IR’s of known compounds
»
Analyzing Fibers - The expected identity of
the fiber has been established by observing it
under a microscope. Its IR spectrum can
confirm the suspected identity.
•
Slide 101
Chem 113, Prof. J.T. Spencer
Vibrational Spectroscopy
Infrared and Raman Spectroscopy101
• Forensic Applications of Infrared Spectroscopy
• Use of computer databases of IR’s of known compounds
»
Analyzing Paint - Paint has been recovered
from a crime scene. Since there is a limited
amount of paint, the first tests to be done
should be nondestructive. Colors, layers,
texture, and other physical properties are
recorded. The individual layers of paint are
analyzed by infrared spectroscopy. The
results can be compared to IR results of
known paint samples.
Slide 102
Chem 113, Prof. J.T. Spencer
Infrared Spectroscopy
102
• “With infrared radiation, forensic scientists can determine the exact ink
type and pen that a death threat was written in, or the very model and
year of a suspect's automobile in a hit-and-run accident. Using a
technique known as infrared spectromicroscopy, forensic investigators
have been able to identify and analyze a broad range of samples—from
inks and paint chips to fibers and drugs. The procedure uses infrared
light to study the properties of molecules at an atomic resolution.
Researchers at Lawrence Berkeley National Laboratory have now
expanded the boundaries of infrared forensics with the use of
synchrotron radiation from the Lab's Advanced Light Source (ALS)
facility…..” (Daily Californian, Wednesday, September 18, 2002)
Slide 103
Chem 113, Prof. J.T. Spencer
Magnetic Resonance Spectroscopy
NMR (MRI)
103
• Visualize soft tissue by measuring proton (nuclear)
magnetic alignments relative to an external
magnetic field.
Review Electron Spin Properties First.
Slide 104
Chem 113, Prof. J.T. Spencer
Electron Spin
104
• Electrons have spin properties (spin along axis).
N
-
-
N
Electron spin is
quantized
ms = + 1/2 or - 1/2
Magnetic Fields
Slide 105
Chem 113, Prof. J.T. Spencer
Experimental Electron Spin
105
• Passing an atomic beam (neutral atoms) which contained
an odd number of electrons (1 unpaired electron, see later)
through a magnetic field caused the beam to split into two
spots.
• Showed the possible states of the single (unpaired)
electron as quantized into ms = +1/2 or - 1/2.
Atom
Beam
Generator
Slits
Magnetic
Field
N
S
Viewing
Screen
two
electron
spin
states
Slide 106
Chem 113, Prof. J.T. Spencer
Nuclear Spin
106
• Like electrons, nuclei spin and because of this
spinning of a charged particle (positively charged),
it generates a magnetic field. Two states are
possible for the proton (1H).
N
S
+
+
S
N
Slide 107
Chem 113, Prof. J.T. Spencer
Nuclear Spin
Similar to a canoe paddling
either upstream or
downstream
107
S
Antiparallel
Degenerate
E
N
N
N
S
Parallel
S
N
External Magnetic Field
S
Slide 108
Chem 113, Prof. J.T. Spencer
Magnetic Resonance Imaging MRI
108
• Hydrogen atom has two nuclear spin quantum numbers
possible (+1/2 and -1/2).
• When placed in an external magnetic field, 1H can either
align with the field (“parallel” - lower energy) or against
the field (“antiparallel” - higher energy).
• Energy added (E) can raise the energy level of an electron
from parallel to antiparallel orientation (by absorbing
radio frequency irradiation).
• Electrons (also “magnets”) in “neighborhood” affect the
value of E (i.e., rocks in stream).
• By detecting the E values as a function of position within
a body, an image of a body’s hydrogen atoms may be
obtained.
Slide 109
Chem 113, Prof. J.T. Spencer
MRI
109
• Advantages (first three are not really important for
forensics)
– non-invasive.
– no ionizing or other “dangerous” radiation (such as Xrays of positrons).
– Can be done frequently to monitor progress of
treatment.
– images soft tissues (only those with hydrogen atoms
(almost all “soft” tissues).
– images function through the use of contrast media.
• Disadvantages
– Relatively expensive equipment.
Slide 110
Chem 113, Prof. J.T. Spencer
MRI; Hardware
110
Slide 111
Chem 113, Prof. J.T. Spencer
MRI
111
Slide 112
Chem 113, Prof. J.T. Spencer
MRI
112
Slide 113
Chem 113, Prof. J.T. Spencer
MRI
113
Slide 114
Chem 113, Prof. J.T. Spencer
MRI
114
Slide 115
Chem 113, Prof. J.T. Spencer
MRI
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115
Slide 116
Chem 113, Prof. J.T. Spencer
MRI
116
Slide 117
Chem 113, Prof. J.T. Spencer
Forensic MRI/CT
117
• Used to reconstruct facial images from skulls. Use
for ancient mummies to modern skulls.
• Allows a very fine discrimination between materials
with different densities providing an enormous
amount of information about the mummy and its
skeleton.
• The level of automation reached in building models
from CT data, reconstruction, texture application
and visualization allow to the user to complete
whole process in 2-3 hours on a PC or graphic
workstation.
Slide 118
Chem 113, Prof. J.T. Spencer
Forensic MRI and CT
118
• The “Virtopsy” focuses on four goals:
• radiological digital imaging methods as main diagnostic tools in
forensic pathology, ultimately leading to "minimally invasive
autopsy" analogous to "keyhole surgery" in clinical medicine.
• three-dimensional optical measuring techniques - a reliable,
accurate geometric presentation of all forensic findings (the body
surface as well as the interior).
• 3D surface scanning in forensic reconstruction.
• Producing and validating of a post-mortem biochemical profile to
estimate the time of death.
• The implementation of an imaging database as a technical basis of a
"center for competence in virtual autopsy”.
Slide 119
Chem 113, Prof. J.T. Spencer
Forensic MRI
119
Virtopsy, a new imaging horizon in forensic pathology:
virtual autopsy by postmortem multislice computed
tomography (MSCT) and magnetic resonance imaging (MRI)
- 40 forensic cases were examined and findings were verified by
subsequent autopsy. Results were classified as follows: (I) cause of death,
(II) relevant traumatological and pathological findings, (III) vital
reactions, (IV) reconstruction of injuries, (V) visualization. In these 40
forensic cases, 47 partly combined causes of death were diagnosed at
autopsy, 26 (55%) causes of death were found independently using only
radiological image data. Radiology was superior to autopsy in revealing
certain cases of cranial, skeletal, or tissue trauma. Some forensic vital
reactions were diagnosed equally well or better using MSCT/MRI.
Radiological imaging techniques are particularly beneficial for
reconstruction and visualization of forensic cases.
(J Forensic Sci. 2003, 48, 386-403)
Slide 120
Chem 113, Prof. J.T. Spencer
Forensic MRI
Validating of a post-mortem analysis
120
Complex scull fracture
system following motor
vehicle accident (victim was
overrun by automobile).
3D reconstructed MSCT image.
Slide 121
Chem 113, Prof. J.T. Spencer
Forensic MRI
Validating of a post-mortem analysis
121
Injury due to vehicle impact in a motor vehicle accident
(pedestrian). (right) finding at autopsy; right lower leg showing
fracture of the fibula. (left) 3 D reconstructed MSCT;
Slide 122
Chem 113, Prof. J.T. Spencer
Forensic MRI and CT
Facial Reconstructions
Egyptian Mummy Head
122
The method uses the tables combined with the warping of a
3D model of a reference scanned head, until the relevant
surface to bone distances are correct. Texture mapping is
used to provide colors and aesthetic features.
Slide 123
Chem 113, Prof. J.T. Spencer
Forensic MRI and CT
Mummy Facial Reconstruction
123
Model skin (blue) and
mummy skull (white)
Face shape
generated
Slide 124
Chem 113, Prof. J.T. Spencer
Forensic MRI and CT
Mummy Facial Reconstruction
124
Texturized model of reconstructed soft
tissues of the mummy
Slide 125
Chem 113, Prof. J.T. Spencer
X-ray Methods
• X-ray Diffraction (XRD and CT)
• Energy Dispersive X-ray Fluorescence
125
Slide 126
Chem 113, Prof. J.T. Spencer
Bragg’s Law and X-ray Diffraction
126
incoming
light
E
D
B
d
lattice in a
crystal
C
Since BCD = 2d sin is the limiting condition for
observing a reflection then because of wave addition
and cancellation;
Bragg’s Law:
n = 2d sin
where n = 1, 2, 3, etc...
Slide 127
Chem 113, Prof. J.T. Spencer
Diffraction
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Chem 113, Prof. J.T. Spencer
Energy-Dispersive X-ray
Fluorescence (EDXRF)
•Did your luxury purchase
originate in a mine deep in the heart
of Central America, or the bottom of
a silty river tributary in Africa, or
perhaps even a flask in a laboratory
in Chicago or Minsk?
•Metal ions such as V3+, Cr3+, Mn2+,
Mn3+, Fe2+, Fe3+, Ni2+, Cu2+, and
UO22+ are responsible for the colors
of most common gemstones and
minerals.
•U.S. Federal Trade Commission
says consumers must be informed
of alterations in gemstones.
128
Slide 129
Chem 113, Prof. J.T. Spencer
Energy-Dispersive X-ray
Fluorescence (EDXRF)
129
•Among the most sensitive and
popular of the nondestructive
spectroscopic techniques used for
trace-metal determination is
EDXRF. In this technique, X-rays
excite the gemstone to fluoresce and
the fluorescent line spectrum
indicates which chemical elements
are present.
EDXRF can also be used to differentiate freshwater from saltwater pearls on the
basis of the greater concentration of magnesium present in the former.
Slide 130
Chem 113, Prof. J.T. Spencer
Energy-Dispersive X-ray
Fluorescence (EDXRF)
130
•EDXRF has been called 'the curator's dream instrument'
because measurements are non-destructive and usually
the whole object can be analyzed, rather than a sample
removed from one. The technique involves aiming an Xray beam at the surface of an object; this beam is about 2
mm in diameter.
•The interaction of X-rays with an object causes
secondary (fluorescent) X-rays to be generated. Each
element present in the object produces X-rays with
different energies. These X-rays can be detected and
displayed as a spectrum of intensity against energy: the
positions of the peaks identify which elements are
present and the peak heights identify how much of each
element is present.
Slide 131
Chem 113, Prof. J.T. Spencer
Energy-Dispersive X-ray
Fluorescence (EDXRF)
131
An incoming X-ray ejects a K-shell electron from an atom of the target. An
electron in the M or L-shell loses energy as it transitions to the vacant K-shell. It
given off energy in the form of fluorescence.
Slide 132
Chem 113, Prof. J.T. Spencer
Energy-Dispersive X-ray
Fluorescence (EDXRF)
http://www.thebritishmuseum.ac.uk/science/techniques/sr-tech-xrf.html
132
Slide 133
Chem 113, Prof. J.T. Spencer
Analytical Methods
133
• Questions to consider in choosing an analytical
(chemical) method:
–
–
–
–
–
–
–
Quantitative or qualitative required
Sample size and sample preparation requirements
What level of analysis is required (e.g., ± 1.0% or ± 0.001%)
Detection levels and useful analytical concentration ranges
Destructive or non-destructive
Availability of instrumentation
Admissibility (e.g., are all lead pipes compositionally the
same or are there sufficient variations among “known” Pb
pipes of the world to link two samples)
Slide 134
Chem 113, Prof. J.T. Spencer
Finis
134
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Chem 113, Prof. J.T. Spencer
135