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Chemistry 8152: Analytical Spectroscopy Fall 2012 â 4 Credits Smith 111: MWF 9:05 â 9:55

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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

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.

- 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.

*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 one8.5” x 11” page of equations and notes to each midterm exam.

- 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@umn.edu).
- Assignments submitted late without a valid excuse will not be graded.

*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).

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.

A Typical Spectrochemical Measurement

Photomultiplier Tube

(Detector/Transducer)

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.

Photons

- Photons are discrete packets of electromagnetic (EM) radiation energy.
- E = hn = (hc) = hcn
- l
- 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 108 m/s)
- = wavelength (m)

n = wavenumber (m-1)

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/encyclopedia/E/emspec.html

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

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)

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

in radians

Y(x,t)|t=0 = Y(x) = Asinkx

amplitude

www.wikipedia.org

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

Y(x) = Asink(x+vt)

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

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

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

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?

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

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

Constructive Interference

2 – 1 = = m 2

where m is an integer

Destructive Interference

2 – 1 = = (2m+1)

where m is an integer

Figure 3-4 – Ingle and Crouch, Spectrochemical Analysis

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.

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 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.

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 = e0c<E2>r

The irradiance from a point source a 1/r2

r

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

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 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:

<P(t)>T = I/c (units = N/m2)

If the average irradiance from the Sun impinging normally on a surface just outside the Earth’s atmosphere is 1400 W/m2, what is the resulting pressure (assuming complete absorption)? How does this pressure compare with atmospheric pressure (~ 105 N/m2)?

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)

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

organic molecule.

Skoog and Leary, Principles of Instrumental Analysis, 1992.

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?

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