Transcript No Slide Title

Analysis Techniques
• Chapter 5: Organic Analysis
• The theory of Light
• Spectrophotometry
• Mass Spectoscopy
• Chapter 6: Inorganic Analysis
• Atomic Emission Spectroscopy (AES)
• Atomic Absorption Spectroscopy (AAS)
• Neutron Emission Analysis (NAA)
• X-Ray Diffraction (XDS)
• Chapter 7: Microscopes
• Compound
• Comparison
• Stereoscopic
• Polarizing
• Microspectrophotometry
• SEM
How Will I Approach These Topics?
• The Theory of Light
• The Structure of Matter
• Analysis of Gross Features and Morphology
• microscopes
• Analysis of Compounds (Molecules and Crystals)
• Photometry, GCMS, XDS
• Analysis of Trace Elements
• AES, AAS, NAA, SEM
The Theory of Light
Graphic representation of a Wave:
• Crest
Traveling
waves
• Trough
• Amplitude (A)
• Wave Length (l)
l
y(x)
Crest
A
x(m)
Trough
Standing
• Nodes
waves
• Anti-nodes
• Period = T
• frequency = f = 1/T
Velocity = v = lf
y(t)
T
A
f
Speed of Light = c = 3.00 x 108 m/s
t
The Wave Nature of Light:
• 1801 Thomas Young performed the double slit experiment with
light => demonstrated the Wave nature of light.
• 1864 James Clerk Maxwell developed a unified theory of electromagnetism (includes theory of electromagnetic waves).
• 1887 Heinrich Hertz experimentally confirms that light is
electromagnetic waves.
• Electomagnetic Spectrum:
max frequency (Hz) wavelength
• 103
• 107
• 107
• 1010
• 1012
• 3.9x1014
• 7.5x1014
• 1016
• 1019
RF
AM
TV,FM
microwave
infra-red
visible
ultra-violet
x-rays
gamma-rays
106
107
109
1012
3.9x1014
7.5x1014
1017
1020
…
> 1 km
500 m - 50 m
10 m - 1 m
10 cm - 1mm
1 mm - 720 nm
720 nm - 390 nm
390 nm - 1 nm
10 nm - .5 pm
< 10 pm
Now, let’s look a little closer at a few regions of
the Electromagnetic spectrum….
One can use specific absorption or transmission
to identify a compound… Forensic investigation
The Visible Spectrum: Why are there three colors of Light?
Blue
Green
Red
yellow
Wavelength (l)
Visible
Analysis is
easy. You
just look at
the sample..
IR Bands:
Wavelength (l)
IR light is not energetic enough to break molecular bounds,
but it will excite specific vibrational modes in large complex
molecules – can be used to identify specific organic
molecules.
UV Bands:
UV is energetic
enough to break
covalent bonds.
Causes sunburn.
Wavelength (l)
Quantization of Light:
• Photo-electric effect is creation of an electric charge with light.
• 1905 Einstein explains the photo-electric effect (Won the Nobel
prize for this in 1921).
Ephoton = hf
When photons eject electrons from a metallic surface:
hf = KEmax + W0
photon energy
of the electron
Binding Energy of the
outer electrons
“Work Function” of the metal
g
Main Concept => Light is quantized! => photon, particle nature of light
Intensity of the light does not effect KEmax or W0.
Quantization of Energy:
Visible Range
6000 K
“White Hot”
1900 Max Planck explained the
black-body curves assuming atomic
oscillations with discrete Energy values
4000 K
“Red Hot”
E = 0, hf, 2hf, 3hf, ….
500
En = nhf
h = Planck’s Constant
1000
l (nm)
1500
h = 6.626 x10-34 Js
Main Concept: => Energy is Quantized!  Introduce the Photon
2000
Absorption and Emission of Light
• Gamma rays  Nuclear Levels
• x-rays  inner atomic levels
• UV  Molecular Bonds
• Visible
• IR  vibrational modes
• microwaves  rotational modes
Covalent
Ionic
van der Waals
Scattering - Probe wavelength
and Size of Subject
Visible - 500 nm
x-rays - 10-8 to 10-12 m
Atoms - 10-10 m
The Structure of Matter
What is matter?  It has mass and volume
The Basis Building Blocks of Matter:
Particle
Mass
Proton
1.67x10-27 kg
neutron
1.67x10-27 kg
electron
9.11x10-31 kg
Charge
+1
0
-1
The Fundamental unit of Matter – the Atom:
Atom
Proton
Neutron
Electron
Size
1 fm
1 fm
0
(1 fm = 10-15m)
Nucleus
The Nucleus:
Discovered by James Chadwick, 1932
Made up of neutrons and protons,
collectively called ‘nucleons’
A X
Z
X = Element name
A = Atomic mass number
Z = Atomic number
Same Z, different A => ‘Isotopes’
Atomic wt (12C) = 12.0 amu
Example: 12C and 14C
Atomic Number (Z) = number of protons  Determines which chemical element
Atomic Mass Number (A) = # of n’s + # of p’s  Determines which isotope
How is the nucleus held together? => Strong Force!
PE
• Strong
• Short range attraction
• Hard-core Repulsion
r
The Valley of Stability:
Light Elements
N=Z
Example: 16O
(N = 8, Z = 8)
Strong -DPE ~ A
Electric +DPE ~ Z2
(other corrections from
surface area, and from
neutron+ proton ratio)
Heavy Elements
N>Z
Example: 197Au
(N = 118, Z = 79)
Most light nuclei have the same number of neutrons as protons.
Binding Energy per Nucleon: (In the valley of stability)
A
56
DPE/A
One of the Most
Important Plots
in the Universe!
Too Much
Surface
Too Much Coulomb PE
8
Best Bound Nuclei:
D PE / A   aV A  a S A
2/3
 aC
Z
A
2
1/ 3
 aA
Light Elements  Fusion
56Fe, 62Ni
(Z  N )
A
2
 A
3 / 4
aV=15.68 aS=18.56 aC=0.717
aA=28.1  = 34
Fusion: combining light nuclei to build up toward
56
DPE/A
A
56
The universe is
94% H, 6%He,
traces of 7Li
Fe
Energy release in the fusion process
is 8.70 MeV/nucleon. (It does, however,
take several steps to release all that energy)
Thermonuclear weapons are 103 more powerful than A-bombs
8
“Abridged” Stellar Burning:

1
H  H  H  e 
2
H  H  He  energy
4
He  He  Be
8
Be  He  C
1
2
2
Hydrogen
burning
4
4
4
8
12
12
C  He  O
16
O  C  Si
28
Si  Si  Fe
4
12
28
16
28
56
Helium
burning
C+O burning
Silicon burning
Where do the Heavy Elements Come from? => SuperNovae!
Consider a main sequence star approachng the end of it’s life….
Gravity is trying to collapse the star. Thermal pressure and electrostatic repulsion of the iron
nucleii support the core. When enough inert iron builds up in the core, the core cools, the iron
nuclei are forced together, … they touch => BOOM!
• The strong force is ‘turned on’
• The core becomes one huge nucleus => neutron star
• Recoil => mantle of the star is blown off
• R-process of nucleosnythesis => forms the heavy nuclei, rapid neutron capture and b decay
Formation of the Planets: and their elemental compositions
• Start with heavy elements coalescing… coffee talk… gravitationally capturing atoms
• Mass of the planet determines the mass of the molecules which can be captured
• lighter molcules
• Earth captures AMU~18 O2=32, N2=28, Si=28, Al=27, H2O=18, Ne=20
(Most terrestial rock is composed of Si and Al)
• Mars captures CO2 (mass 44), rocks are mostly iron ores.
• Jupiter captures Hydrogen => protostar… No fusion.
• Heavy Elements in the core
• Radioactive decays heat the core => this is where ALL He gas comes from
• Moon… We know the moon did not form independently => lunar rocks are SiO and
aluminates.
Relative Abundances of the Elements in the Earth’s Crust:
Trace Elements  Trace Element Analysis
The Wave Nature of Matter:
1923 Arthur Compton describes photon scattering (Compton Scattering)
Must conserve momentum:
g’
g
Relativistically
p e  g mv
e-
E e  g mc
P
For a photon:
•
•
E

1
c

Pg
hf
 Pg 
hf
c

P
2
E

v
c
2
h
l
1923 Louis deBroglie proposed that le = h/p
1927 Davisson and Germer demonstrated electron diffraction
on a crystal of nickel. Main Concept => Wave nature of Matter
Quantum Mechanics (Basics):
Consider a particle in a 1D “box”
Like a standing wave! Wavefunction MUST go to zero at the boudaries.
Etot = KE = (1/2)mv2
= n2h2/(8mL2)
E1 = h2/(8mL2)
E2 = 4 E1
l = h/p = h/mv
v = h/ml = 2nh/mL
E3
E3 = 9 E1
• Quantized energy levels.
• Need energy to change levels.
nl/2 = L
g
E2
E1
ground state
The Nature of the Atom:
• 1911 Ernest Rutherford observed backscattered alphas, which
indicted that there was a concentrated positively charged nucleus.
• 1913 Niels Bohr --- Model of the Atom
r
e
Fcent  Felec
mv
+Z
r
2

kZe
r
2
2
Assume the states are quantized => Standing waves around the orbit
circumference = nle
2 r  n
h
p
n
h
mev
Quantization leads to
discrete energy levels.
The exact energies of
the transitions are
unique to each given
element.
Energy Levels for hydrogen
The Periodic table is defined by the electronic structure of the atoms.
Alakine Metals
Alkai Earths
Group determines
the chemistry
Transistion Metals
Rare Earth Metals
Noble Gases
Halogens
Classification of elements
•periodic table compactly shows relationships between elements features of the periodic table
•Periods are horizontal rows on the table.
•Groups (or families) are columns on the table. All have similar chemical properties.
•Blocks are regions on the table.
•important groups:
•alkali metals (Group IA, first column )
•soft, extremely reactive metals react with cold water to form hydrogen gas form +1 ions
•alkaline earth metals (Group IIA, second column):
•soft, reactive metals, compounds are a major component of earth's crust, form +2 ions
•halogens (Group VIIA, next-to-last column):
•poisonous and extremely reactive nonmetals, all form -1 ions
•fluorine and chlorine are yellow-green gases
•bromine is a volatile red-brown liquid
•iodine is a volatile blue black solid
•noble gases (Group 0, last column)
•all are monatomic gases, a. k. a. inert gases; almost completely unreactive
•Important blocks:
•transition metals are the elements in the region from the third to twelfth columns.
•hard, dense metals, less reactive than Group IA and IIA
•rare earth metals are the elements in the annex at the bottom of the table.
•lanthanides (annex, top row), actinides (annex, bottom row)
•main group elements are all elements except the transition and rare earth metals.
•group numbers end with "A"
•metals, nonmetals, and metalloids (semimetals)
Bonds: Molecules and Crystals
• Ionic versus Covalent Bonds
• Molecule - simple vs. complex
• Crystals
• Solids and Liquids
• Organic vs. Inorganic
Microscopes
Lens: Converging (+f) and Diverging (-f)
“Projector”
o>f => Real Image, Inverted
M = -i/o
“Magnifying Glass”
o<f => Virtual Image, upright
M = -i/o
Condenser
Concave Lens (diverging, -f)
Always Virtual Image, upright
M = -i/o
Thin Lens Equation:
Lens are used to bend light. The refractive index of glass is 1.3.
A single lens can give you a magnification of 10 to 40 without
distortion. A compound scope uses two lens to get much higher
magnification. You can multiple M1 by M2 to get Mtot.
1
o

1
i

1
f
Compound Microscope Parts:
Eyepiece
Objectives
Fine Adjustment Knob
Power Switch
Stage
Diaphragm
Base
Body Tube
Stage Clips
Stage Stop
Coarse Adjustment Knob
Aperture
Arm
Light Source
Schematic view of
A compound scope.
Use transmitted light for a
transparent sample. The light
illuminates from below. The
condenser concentrates the
light. It takes light that is
dispersing and concentrates in
on the sample. The aperature
controls the total amount of
light.
The objective lens creates a
magnified real image. The
Ocular lens creates a
magnified virtual image that
your eye can see and interpret.
The typical illumination of
specimens in which light
passes through the specimen
and travels to you eye is called
Bright Field microscopy.
Elodea
Comparison Microscope
Has a split image. It consists of
two coupled compound scopes.
You are looking at two separate
objects that you want to compare
side-by-side. There is a bridge
that connects the two scopes. You
need to have the same
magnification and lighting on
both sides.
Primarily used to compare
bullets, fibers, or even to match
up jagged pieces of glass.
Stereomicroscope
Two Compound scopes – no
bridge to form a single image.
Two independent images  The
goals is to create depth
perception. The two scopes view
the sample from slightly
different angles.
Best for viewing samples that
have some shape to them – i.e.
not just looking at a flat surface.
Stereomicroscopes have
characteristics that are valuable in
situations where three-dimensional
observation and perception of depth
and contrast is critical to the
interpretation of specimen structure.
Polarizing Microscope
You put polarizers where
the condenser would be
on a normal compound
scope. The polarizers
select a specific
electromagnetic
orientation. You adjust
the polarizers to see how
the colors of the field
changes.
Best for identifying
certain types of polymers
and glasses.
Scanning Electron Microscope
• Totally different than a light microscope.
• It focuses a high energy scan of electrons at a sample. One observes
how the electrons scatter back.
• One can probe extremely small things with the electron beam.
• The beam scans across the surface and recreates the surface with
extremely good depth of field.
• One can do trace element analysis with the SEM.
• Can be coupled with an x-ray detector for elemental analysis.
Trace Element Analysis
We want to determine the distribution of trace
elements. These are elements heavier that iron.
The “trace element finger print” is unique to
materials that come from a single source. These
trace elements are on the ppb level. If you can
identify the trace elements, you can identify the
source of material
destructive
• Atomic Emission Spectroscopy
• Atomic Absorption Spectroscopy
• Neutron Activation Analysis
• SEM
Atomic Emission Spectroscopy
• AES is a destructive technique. One vaporizes the
sample by using an electrical arc. This ionizes the
sample causing it to emit light a frequencies
characteristic of the elements in the sample. You
can determine the elemental distribution in the
sample, but you do not know anything about the
molecular structure.
• The technique will tell us what the trace elements
used to be in the sample.
Atomic Absorption Spectroscopy
• AAS is more sensitive than AES as the
elements will not be obscured. As above,
the sample is vaporized. You shine a light
source with a known and calibrated
frequency into the vaporized sample. When
you want to look for a specific trace
element, expose the vapor to a specific
wavelength and determine the amount of
absorption.
Neutron Activation Analysis
• NAA is a non-destructive technique. The sample is
placed inside a region of high neutron flux (like
the core of a nuclear reactor). The bath the sample
in neutrons. The nuclei in the sample will absorb
some of the neutron, leaving unstable radioactive
elements which decay with characteristic gamma
ray emissions. Use of a high precision gamma
detector to acquire the spectrum will allow one to
determine the distribution of all trace elements in a
single shot
Scanning Electron Microscope
• SEM’s are becoming quite common. You can do a
lot with an SEM. SEM uses x-ray emission
spectrocopy. You bombard the sample with an
energetic electron beam (cheaper than a nuclear
reactor). The electrons excite the atoms in the
material. The atoms emit their characteristic xrays. A detector picks up the x-rays from which we
can determine the elemental composition.
• One can both ID the trace elements, and determine
their physical location on the surface of the
sample.
Analysis of Compounds
• UV Spectrophotometry
• IR Spectrophotometry
• GC Mass Spectroscopy
• X-ray Diffraction
UV Spectrophotometry
•
Spectrophotometry  the absorption of specific frequencies of light. You start
with a bright, broad frequency light source, then pass it through a prism; this
breaks the light into its component frequencies. You pass this dispersed light
through a narrow slit to get a particular frequency. You can then rotate the
prism to take a measurement at a different frequency and thereby determine
the absorption as a function of frequency.
•
UV spectrophotometry gives a gross absorption curve; you are not exciting
specific modes; you are degrading the material which you are looking at,
because your are breaking covalent bonds. The complex molecules of heroin,
sugar and cocaine are all white powders. You can do a gross ID with UV
spectrophometry, you should do a final ID using IR spectrophotometry.
•
UV spectra and Visible spectra can be used to identify an unknown compound
by a comparative analysis. One can compare the UV or Visible spectra of the
unknown with the spectra of known suspects. Those that match are evidence
that they could be one and the same. However using a match on UV or Visible
is not conclusive. Usually IR and Proton NMR spectra must nail down the
exact identification.
IR Spectrophometry
• The IR spectrophotometer uses IR light to
excite vibrational bands in complex
molecule. These absorption spectra provide
quick positive ID for complex organic
molecules whose “IR fingerprints” are
stored in its memory.
GC Mass Spectrometry
• Mass spectrometer is usually used in conjunction with gas
chromatography. The result of the GC goes through an
ionizer where it is bombarded by a high energy electron
beam. This beam breaks the complex molecules like heroin
into a standard set of chunks (fragments). The ionized
samples then go through a magnet that separates the
fragments based upon their mass. A detector picks up the
fragments of a certain mass. By adjusting the magnetic
field, one can determine the fragment mass spectrum. This
is usually done with complex organic molecules. Inorganic
molecules do not create complicated bonding structures.
X-ray Diffraction
• ID’s specific inorganic crystals. Can also be used
on organic molecules that one can crystalize (i.e.
you can crystalize DNA etc.)
• Use an x-ray in the nm range to illuminate the
sample. Reflections off the different layers will
yield bright spots at specific angles. The pattern of
spot from an x-ray diffraction measurement allows
one to ID specific substances  the pattern is
unique to each crystal.
END OF LECTURE
Radioactivity: the quest for a more stable state
1896 Discovered by Henri Bequerel => Researched by the Marie and Pierre Curie
a => 4He nuclei
b+/- => e+/g
=> high energy photons
bbb
aaaa
N(t) = N0e-lt
T1/2 = ln(2)/l
a and b decay change the nuclear species. g’s are emitted during
internal restructuring of the nucleus, NOT changing A or Z.
Fission: when a and b emission just isn’t fast enough!
235U
+ n => two neutron rich fragments and several neutrons
U
Kr
Ba
Average energy release ~ 1 MeV/nucleon
Chain Reaction… 235U is pretty stable. 236U is NOT. Add a neutron to 235U and it
immediately fissions, creating several extra neutrons...
Hiroshima: 104 Joules or 20 kilotonnes of TNT from 3 moles of 235U fissioning
rewrite
 nh
v  
 m e 2 r
Go back to
mv
2
2

kZe
r
r
2
2
2
2
 nh
m e 
 m e 2 r
rn  n h




2
2
2

kZe
 

r

4 kZe m e
2
2
E tot  KE  PE
E tot 
1
mv 
2
2
E tot  
kZe
2

r
1 me
n
2
2
2 kZe
1
mv  mv
2
2
2
h
2

1
mv
2
2

2

Z
2
n
2
13 . 6 eV
Main Concept:
Demonstrates quantization of energy,
wave nature of matter, and structure
of the atom.
Law of Reflection: qi=qr
Reflection:
“Specular Reflection”
from a smooth surface
Incident
ray
qi
qr
DEMO:
Optics Board
Reflected
ray
Surface
“Diffuse Reflection”
off a rough surface
Often, not all light is reflected from a surface
=> Reflectivity
- Dependent on frequency of light
- Dependent on angle of incidence
- Dependent on polarization
What happens to the Light that is NOT Reflected?
1) Absorbed
2) Refracted
opaque
transparent
Neutral filters
translucent
Varies w/ frequency
q1
Wavefronts, l apart
Incident
rays
n1
n2
l = v/f
v1=c/n1
v2=c/n2
q2
Snell’s Law:
n1sinq1 = n2sinq2
Nvacuum
nwater
nglass
ndiamond
= 1.000
= 1.329
= 1.500
= 2.417
Total Internal Reflection
q2
qi qr
Low n to high n => Bent towards normal
High n to low n => Bent away from normal
The Critical angle (qc):
n1sinqc = n2sin90
sinqc = n2/n1
qc = arcsin(n2/n1)
DEMO:
Optics Board
Optical Systems: (Mirrors and Lens)
Rays diverge
• Object
Converges
diverges, bends
reflects light
• Optical system
A point from
which rays
diverge or
appear to
diverge
• Image
You
• Observer
object
Your eye expects
to see diverging rays
from an object.
No optical
system. No
“image” rays
Observer
(focuses
diverging
rays only)
Now Add an Optical System:
Focus
Real
Image
Virtual
Image
Reflect
and
De-Focus
Optical
system
Defocus
Virtual
Image
Reflect
and
Focus
Real
Image
Images:
Our eyes focus diverging rays….
Our eyes can not distinguish:
an object, a real image, or a virtual image.
• Eye can
see after the
image
• Can Focus
on screen
• Eye can see
• Can NOT Focus
on screen
The Simplest Optical System -- The plane mirror
o
i
Reflected
virtual
image
Spherical (or Parabolic) Mirrors:
A parabolic mirror will focus
parallel light to a single point
= the “focal point”
f
focus
The mirror is characterized with
a given “focal length”, f.
From Pre-Calculus:
(x  f )  y
2
2
 x  f  y  4 fx
2
y
directorix
Incident para-axial rays
(-f,0)
focus
(f,0)
x
Ray Tracing:
1)
3)
2)
i
o
f
With Mirror or Lens:
Typically draw 3 Rays:
1) Parallel Ray reflected thru focus
2) Ray thru focus reflected parallel
3) Ray to center qi=qr for reflection
Thin Lens Equation:
In this case, o>f => real inverted image
1
Magnification = -i/o
o

1
i

1
f
Positives and Negatives:
1)
In this case, o<f, i must
be negative
=> virtual upright image
Object distance (o):
always positive
Image distance (i):
positive if same side as viewer
3)
f
Magnification = -i/o
o
i
Focal length (f):
positive if converging lens/mirror
negative if diverging lens/mirror