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Chemistry
8152
: Analytical Spectroscopy


Fall
2012


4
Credits


Smith
111
: MWF
9
:
05


9
:
55


Instructor
:



Christy

Haynes




243

Smith

Hall




626
-
1096




chaynes@umn
.
edu

Text:

No required text.


Course notes and hand
-
outs will be available on the class blog

(
http://blog.lib.umn.edu/chaynes/
8152
/
)
.



If you know from experience that you learn best when you have a
book, consider buying a copy of “Ingle and Crouch”.





Other sources that may be useful:



James D. Ingle Jr. and Stanley R. Crouch,
“Spectrochemical Analysis”
, Prentice
Hall, New Jersey,
1988
.


Eugene Hecht,
“Optics”
, Addison Wesley, New York,
2002
.


Douglas A. Skoog and James J. Leary,
“ Principles of Instrumental Analysis”,

Harcourt Brace College Publishing, New York,
1997
.


Janet D. Dodd,
“The ACS Style Guide

A Manual for Authors and Editors”,
American Chemical Society, Washington DC,
1997
.

Class Description:




Spectroscopy describes the interaction of electromagnetic
radiation and matter.


In analytical spectroscopy, one applies spectroscopic
techniques to both analyte mixtures and trace samples.



In this class, we’ll cover fundamental principles as well as a
wide range of contemporary techniques.


Learning Objectives:


Critically consume scientific literature and talks.


Identify appropriate techniques for the analysis of any sample.
Recognize strengths/weaknesses of each method.



Final Grade



2 Exams @ 80 points each

160

4 Problem Sets @ 15 points each

60

Proposal White Paper and Outline

60

Proposal Presentation

60

10 Minute papers @ 6 points each

60


Total Points Possible


400

Exams


*Two

hour

exams

during

the

semester
.

2
nd

exam

is

NOT

cumulative

(though

it

will

be

in

the

time

slot

reserved

for

the

course

final

exam)
.



*No

equations

will

be

provided
.

Bring

a

calculator
.


*You

may

bring

one

8
.
5


x

11


page

of

equations

and

notes

to

each

midterm

exam
.



Problem Sets


Each problem set will receive equal weighting in
calculating the final grade (total of
60
points).


You may work in groups but each person must submit
their own unique solutions.


All problem sets are due by
5
pm in my mailbox (A
14
) or
email (chaynes@umn.edu).


Assignments submitted late without a valid excuse will
not be graded.


Original Proposal


*You will work on this assignment individually.


*Each person will identify an unexplored analytical chemistry
research question and choose appropriate spectroscopic
methods to explore this question.



*There will be in
-
class peer review of the written materials
before you turn in a white paper describing your proposed
research as well as an outline of the experiments to be done
(
60
points).


*During the final week of class, each person will present a
12
minute talk about their proposed research to the class (
60
points).


Minute Papers


The purpose of the "Minute Paper" assignments is to promote
exposure to the scientific literature.



Each week, you will choose an article from the ASAP alerts or
a departmental seminar that is relevant to this class to analyze
critically. It should be posted as a "comment" under that
week's minute paper blog post.


The minute paper should be grammatically correct, written in
your own words, and no longer than
500
words. You should
emphasize the technique that was used, the major findings of
the work, and your ideas about what should be done next
(stated as a
testable hypothesis
where possible).



Each week, a minute paper is due by Friday at
5
pm. You
must complete at least
10
of the
13
minute papers on time in
order to receive full credit. At least
2
of the minute papers
must be based on seminars.

Spectroscopy Vocabulary…

spectro
-
:
light

-
scope:

looking, examining, seeing


-
graph:

recording

-
meter,
-
metry
: measuring


Spectroscopy:

Science dealing with interaction of
electromagnetic radiation and matter.


Spectrometry:

Quantitative measurement based on
information from a spectrum.


Spectrum:

Display of the intensity of radiation emitted,
absorbed, or scattered by a sample versus a quantity related
to photon energy (e.g. wavelength or frequency).


Spectrophotometer:

Instrument used to provide input light and
determine the output light intensity at various wavelengths in
the spectrum.


Spectrometer:

Instrument used determine the output light
intensity at various wavelengths in the spectrum.

The Fluorescence Experiment:

A Typical Spectrochemical Measurement

Photomultiplier Tube

(Detector/Transducer)

Photons

Douglas A. Skoog and James J. Leary, Principles of Instrumental
Analysis, Saunders College Publishing, Fort Worth,
1992
.

n

= frequency is number of waves/unit time



= wavelength is number of units of length/wave

n

= wavenumber is number of cycles/unit length


Photons

Photons are discrete packets of electromagnetic
(EM) radiation energy.


E = h
n

= (hc) = hc
n





E = energy of photon (joules)

h = Planck’s constant (
6.63
x
10
-
34

Js)

n

= frequency (s
-
1
)

c = speed of light (
3.00
x
10
8

m/s)



= wavelength (m)

n

= wavenumber (m
-
1
)


Electromagnetic Spectrum

Image Source: http://www.daviddarling.info/encyclopedia/E/emspec.html

Primary focus in this class: UV, visible, IR


E units

= joules or electron volts (
1
eV =
1.6
x
10
-
19

J)



units

= nanometers (
10
-
9

m), micrometers (
10
-
6

m), or
angstroms (
1
Å

=
10
-
10

m)


1
eV of photon energy = radiation with


of
1240
nm

Are you getting the concept?

Calculate the energy of (a) a
5.30
Å

X
-
ray photon (in
eVs) and (b) a
530
-
nm photon of visible radiation (in
kJ/mole).




Electromagnetic Spectrum

The energy of the photon determines the type of
transition or interaction that occurs.

Table
1
-
1


Ingle and Crouch,
Spectrochemical Analysis

EM Radiation Sources


1
. Fundamentals of EM Radiation

2
. Light Sources

3
. Lasers

Wavefunctions (
Y)


x
y = f(x-vt)
0
f(0)
vt
x
y = f(x)
0
f(0)
Assume wave moves with speed v.

Assume shape remains constant.


y = f(x) at initial time t=
0

At later time, t, the wave will have

traveled a distance vt to the right.


y = f(x
-
vt) at later time t

Similarly, wave traveling to the left:
y = f(x+vt)

Harmonic Waves

(a.k.a. Sinusoidal or Simple Harmonic Waves)

www.wikipedia.org

“Although the energy
-
carrying disturbance advances through
the medium, the individual participating atoms remain in the
vicinity of their equilibrium positions.”

-
Hecht,
Optics
,
2002

Y
(x,t)
|
t=
0

=
Y
(x) = Asinkx

amplitude

in radians

Y
(x) = Asink(x+vt)

traveling in

x direction

Y
(x) = Asink(x
-
vt)
traveling
in +x direction

Spatial Period
-

Harmonic Waves

If this wave is traveling at speed v in the + x
-
direction:




Y
(x,t) = Asink(x
-
vt)

The wave is periodic in space and time.

The spatial period


is the number of length units/wave

Y
(x,t) =
Y
(x
±


,t)


With harmonic
Y
,
|k

| =
2
p
so k =
2
p/

Usually use
f
to represent the argument of the sine function.
f

describes the
phase

of harmonic wave.







Y
(x) =
0
whenever sin
f

=
0

(when
f

=
0
,
p,
2
p,
3
p
, etc. or x =
0
,
/
2
, ,
3
/
2
, etc.)

Hecht, Figure
2.6

Temporal Period


Harmonic Waves


The temporal period (
t
) is the time for one

wave to pass a stationary observer.




Y
(x,t) =
Y
(x, t
±

t
)

We can derive the expression:



t = /
v

Units of
t

= # units of time/wave.


Often use
1
/
t

→ frequency,
n

(the # waves/unit time)
.


Angular temporal frequency (
w
)

in radians/second:



w=
2
p/ =
2
pn


Hecht, Figure
2.7

Harmonic Wavefunction Interaction

Variation in the electric field for a plane
-
polarized wave:



E

=
E
m

sin (
w
t +
f
)


When two wavefunctions interact, consider the similarity or
difference in:


*amplitude (E
m
)


*frequency (
w
)


*phase (
f
)


How do these characteristics influence the electric field
resulting from wavefunction interaction?

Are you getting the concept?

Sketch the sum wavefunction of the red and blue waves.

0.5
-0.5
1.0
-1.0

kx
0.5
-0.5
1.0
-1.0

kx
y

y

If
f
1



f
2
, the phase changes:

Eugene Hecht,
Optics
, Addison
-
Wesley,
Reading, MA,
1998
.

Superposition Principle

Figure
3
-
4


Ingle and Crouch,
Spectrochemical Analysis

Constructive Interference:

If two plane
-
polarized waves
overlap in space, the resulting
electromagnetic disturbance is
the algebraic sum of the two
waves.

Destructive Interference:

The
interaction of two or more light
waves yielding an irradiance
that is not equal to the sum of
the irradiances.

Optical Interference

Constructive Interference

f
2



f
1

=


=

m
2
p


where m is an integer

Destructive Interference

f
2



f
1

=


=
(
2
m+
1
)
p


where m is an integer


Figure
3
-
4


Ingle and Crouch,
Spectrochemical Analysis

Electromagnetic Radiation

www.ieee
-
virtual
-
museum.org

Seminal work by: Faraday, Gauss, Amp
è
re, and Maxwell

A time
-
varying electric field has an
associated magnetic field.

A time
-
varying magnetic field has an
associated electric field.

The electric field due to point
charges.

A closed surface in a magnetic field
has a net flux of zero.

Implies a mathematical and physical symmetry

between electric and magnetic fields.

Electromagnetic Radiation

Consider:


-

the general perpendicular relationship between
E

and B


-

the symmetry of Maxwell’s Equations


-

the interdependence of E and B

Use Maxwell’s Equations to calculate the speed of EM radiation
in free space: c =
2.99792458
x
10
8

m/sec

Skoog and Leary,
Principles of Instrumental Analysis
,
1992
.

E x B points in propagation direction

Moment
-
to
-
moment direction of E is
the polarization

Energy and Momentum

EM waves transport energy and momentum. The energy
streaming through space in the form of an EM wave is shared
equally between the electric and magnetic fields.

Irradiance (I) quantifies the amount of light illuminating a
surface.

I =
e
0
c
<
E
2
>
r

The irradiance from a point source
a

1
/r
2

The time rate of flow of radiant energy = optical power (P)
measured in watts

r

Photon Force

When an EM wave impinges on a material, it interacts with the
charges that constitute bulk matter. It exerts a force on that
material.

(Newton’s
2
nd

Law suggest that waves carry
momentum.)


Maxwell wrote, “In a medium in which waves are propagated,
there is a pressure in the direction normal to the waves, and
numerically equal to the energy in a unit of volume.”
The radiation
pressure (
P
) is the energy density of the EM wave.


Assume that the E and B fields are varying rapidly, calculate the
average radiation pressure:

<
P
(t)>
T

= I/c (units = N/m
2
)

Are you getting the concept?

If the average irradiance from the Sun impinging normally on a
surface just outside the Earth’s atmosphere is
1400
W/m
2
, what is
the resulting pressure (assuming complete absorption)? How
does this pressure compare with atmospheric pressure (~
10
5

N/m
2
)?

Photon Emission

E. Hecht,
Optics
,
1998
.


atom in ground state


atom excited by high T or collision, stays in excited quantum
state for
10
-
8

or
10
-
9

sec


atom returns to ground state, emitting a photon


Frequency of emitted light is associated with the quantized
atomic transition (
D
E = h
n
)

Photon Radiation

Figure
5
-
16
Partial energy
-
level diagram for a fluorescent

organic molecule.

Skoog and Leary,
Principles of Instrumental Analysis
,
1992
.

Are you getting the concept?

Many streetlights are sodium discharge lamps. The emitted
orange light is due to the sodium D
-
line transition:

What is the energy level spacing (in eV) for
the
3
p

3
s transition?