Chemistry 8152: Analytical Spectroscopy

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Chemistry 8152 Analytical Spectroscopy Fall
2012 4 Credits  Smith 111 MWF 905 955
Instructor Christy Haynes 243 Smith
Hall 626-1096 chaynes_at_umn.edu
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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 GuideA
Manual for Authors and Editors, American
Chemical Society, Washington DC, 1997.
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• 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, well 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.

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Final Grade
2 Exams _at_ 80 points each 160
4 Problem Sets _at_ 15 points each 60
Proposal White Paper and Outline 60
Proposal Presentation 60
10 Minute papers _at_ 6 points each 60
Total Points Possible 400
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Exams Two hour exams during the semester. 2nd
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.
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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 (A14) or email (chaynes_at_umn.edu). Assi
gnments submitted late without a valid excuse
will not be graded.
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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).
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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.
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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.
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The Fluorescence Experiment A Typical
Spectrochemical Measurement
Photomultiplier Tube (Detector/Transducer)
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Photons

• n frequency is number of waves/unit time
• wavelength is number of units of length/wave
• n wavenumber is number of cycles/unit length

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

• Photons are discrete packets of electromagnetic
  (EM) radiation energy.
• E hn (hc) hcn
• l
• E energy of photon (joules)
• h Plancks constant (6.63 x 10-34 Js)
• n frequency (s-1)
• c speed of light (3.00 x 108 m/s)
• wavelength (m)
• n wavenumber (m-1)

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Electromagnetic Spectrum

• 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 l of 1240
  nm

Image Source http//www.daviddarling.info/encycl
opedia/E/emspec.html
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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).
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Electromagnetic Spectrum
The energy of the photon determines the type of
transition or interaction that occurs.
Table 1-1 Ingle and Crouch, Spectrochemical
Analysis
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EM Radiation Sources
1. Fundamentals of EM Radiation
2. Light Sources
3. Lasers
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Wavefunctions (Y)
Assume wave moves with speed v. Assume shape
remains constant. y f(x) at initial time t0
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(xvt)
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Harmonic Waves (a.k.a. Sinusoidal or Simple
Harmonic Waves)
in radians
Y(x,t)t0 Y(x) Asinkx
amplitude
www.wikipedia.org
Y(x) Asink(x-vt) traveling in x direction
Y(x) Asink(xvt) traveling in x direction
Although the energy-carrying disturbance
advances through the medium, the individual
participating atoms remain in the vicinity of
their equilibrium positions. -Hecht, Optics, 2002
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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 l is the number of length
units/wave Y(x,t) Y(x l,t) With harmonic Y,
kl 2p so k 2p/l Usually use f to represent
the argument of the sine function. f describes
the phase of harmonic wave. Y(x) 0
whenever sinf 0 (when f 0, p, 2p, 3p, etc. or
x 0, l/2, l, 3l/2, etc.)
Hecht, Figure 2.6
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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
l/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 2p/l 2pn
Hecht, Figure 2.7
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Harmonic Wavefunction Interaction
Variation in the electric field for a
plane-polarized wave E Em sin (wt
f) When two wavefunctions interact, consider the
similarity or difference in amplitude
(Em) frequency (w) phase (f) How do these
characteristics influence the electric field
resulting from wavefunction interaction?
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Are you getting the concept?
Sketch the sum wavefunction of the red and blue
waves.
y
y
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If ?1 ? ?2, the phase changes
Eugene Hecht, Optics, Addison-Wesley, Reading,
MA, 1998.
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Superposition Principle
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.
Figure 3-4 Ingle and Crouch, Spectrochemical
Analysis
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Optical Interference
Constructive Interference ?2 ?1 ? ?m
2? where m is an integer
Destructive Interference ?2 ?1 ?
(2m1)? where m is an integer
Figure 3-4 Ingle and Crouch, Spectrochemical
Analysis
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Electromagnetic Radiation
Seminal work by Faraday, Gauss, Ampère, and
Maxwell
A time-varying electric field has an associated
magnetic field.
A time-varying magnetic field has an associated
electric field.
www.ieee-virtual-museum.org
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.
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Electromagnetic Radiation
Consider - the general perpendicular
relationship between E and B - the symmetry of
Maxwells Equations - the interdependence of E
and B
Use Maxwells Equations to calculate the speed of
EM radiation in free space c 2.99792458 x 108
m/sec
E x B points in propagation direction
Moment-to-moment direction of E is the
polarization
Skoog and Leary, Principles of Instrumental
Analysis, 1992.
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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 e0cltE2gtr
The irradiance from a point source a 1/r2
r
The time rate of flow of radiant energy optical
power (P) measured in watts
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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.
(Newtons 2nd 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 ltP(t)gtT I/c (units N/m2)
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Are you getting the concept?
If the average irradiance from the Sun impinging
normally on a surface just outside the Earths
atmosphere is 1400 W/m2, what is the resulting
pressure (assuming complete absorption)? How
does this pressure compare with atmospheric
pressure ( 105 N/m2)?
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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 (DE hn)

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Photon Radiation
Figure 5-16 Partial energy-level diagram for a
fluorescent organic molecule.
Skoog and Leary, Principles of Instrumental
Analysis, 1992.
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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
3p ? 3s transition?