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Chemistry 454/545 Exam II

March 6, 2002

Answer TEN of the eleven questions (10 pts each) ; constants provided:

1. A 3.45 millimolar aqueous sample transmits 72.4 % of the incident light at 545 nm. Because of reflections, the same sample tube( 1 cm path length) transmits only 94.7% of the light when filled with water. What is the molar extinction coefficient of the sample?

clearly 0.724 is not T -- need to correct for the blank
think of I/Io as I(transmitted) / Io (transmitted w/o sample) = 0.724/0.947
so A=-log10(0.7xx) = 0.117
e= yyy/1/3.45e-3 = 33.9
- alternative, log10(.742)- log10(0.947) = 0.117 (subtract a blank reading)

2. If an X-ray tube uses an electron beam that has been accelerated by a 9500 Volt power supply. What wavelength X-rays could be produced from a target if the ionization potentials for inner core electrons are:

E(1s) 16250eV
E(2p) 4357 eV
E(3p) 1865 eV

We produce 9500 eV electrons (eV is an energy.)

not enough to remove the 1s electron, but we would remove 2p or 3p on our list
X-ray occurs when a 3p jumps into the now vacant 2p orbital
E difference = energy of the X-ray = (4357-1865) eV
wavelength is hc/E = h*c / [(2492 eV) * 1.602E-19 J/Ev]
=4.95e1- meter= 4.95 A
- might get other electrons to jump into vacant 3p (but lack information)
- note 1-- can't simply convert Ionization potential into X-ray wavelength
- note 2-- energy of electron only determined which orbitals are disrupted

3. How would you arrange an optical grating (1200 lines/mm) to select light at 587.2 nm.

basic equation is n (lambda)= d {sin i + sin r)
in the Bragg configuration, we set angles i and r equal, get n lambda= 2 d sin (theta)
d = 10^-3 mm/1200 lines per mm.
select the order (1 is common)
if you don't use Bragg form, select angle i now and solve for sin r
in Bragg form, angle is 20.6 degrees (position of the exit slit relative to the grating)
- of course you should also worry about an entrance slit, a concave mirror to collimate the beam, second mirror and exit slit

4. Compare flame (air/acetylene), graphite oven (or electrothermal source) and ICP as a means of
treating a sample for elemental analysis. (What does the source do to the sample?)

your answer should touch on most of these points
all three are hot and thermally decompose sample, create isolated atoms (in a vapor form)
graphite oven and flame mainly produce atoms in ground state
- can also produce small fraction of atoms in electronic excited states (emit light)
- limited to easily excited species (Na, K, Li, Ca, Sr...)
graphite is hotter, provides less reactive inert atmosphere
- graphite has wider range of elements
- generally lower detection limits
- can handle samples that are not solutions
ICP is much, much hotter
- sizable portion of atoms are electronically excited
- ICP acts as source of emission from excited states
- generally several lines per element

5. Discuss the role or significance of the hollow cathode (cold cathode) lamp and monochromator
in flame Atomic Absorption Spectrophotometry

AA needs very narrow bandwidth of light (to match the absorption spectrum)
- Cold cathode lamp contains element being studied
- electrical discharge sputters on into vapor and excites atoms
- hence lamp provides exactly the wavelength needed for the analysis
lamp produces other wavelengths (for same element, for Ar or Ne fill gas, for other elements in a multielement lamp); flame also produces some light
- monochrometer used to select only the line of interest (and to remove other wavelengths from the flame)

6. Lasers:
a. what is a population inversion and why is it necessary in a laser

population of lower energy state <population of excited state
otherwise emitted light absorbed efficiently by lower state and see net loss of light

b. describe two important processes used to excite laser medium

optical pumping (absorption of light from a lamp or another laser)
electrical discharge in a gaseous medium
(rare) chemical reaction producing excited state products

c. what is the difference between stimulated and spontaneous emission

spontaneous it just that-- each excited state acts independently
stimulated-- rate of emission depends on both population of excited state and on the intensity of radiation at that particular frequency
- consequences-- lasers beam is coherent, emission in same direction, in phase

7. Sketch either a double beam UV-Visible spectrophotometer or a modern ICP Spectrometer. Label and briefly describe the most important components and how a measurement is made.

8 . A. Identify one instrument we studied that can perform a nondestructive elemental analysis of a sample

The X-ray methods qualify best-- (AA, AES, ICP generally require solution-- need to dissolve sample, although we might only need a tiny sample) Sample in XES can be unchanged. SEM might qualify, but only for relatively small samples and they need to be safe in vacuum.
If you qualify your answer many spectra methods (NMR, UV, IR, fluorescence) do not damage a sample so you could make a solution, measure, evaporate and recover the sample. That's nondestructive. Of course this wouldn't leave the form intact. Perhaps it would have been better to talk about nondestructive in-situ (in place) analysis.

B. Identify one instrument we studied that can provide specific information about the surface of a sample.

SEM is probably the best example, since the electron beam only penetrates into a thin layer of surface.
Other X-ray methods go deeper. AA and AES typically dissolve entire sample.
Some spectral reflectance methods will also qualify.

9. Briefly compare the light coming from each of the following lamps or light sources:

deuterium lamp,-- continuum, rich in UV+ some lines, intense
tungsten lamp, tungsten halide lamp-- true continuum but mostly in visible (halide is richer in blue end, brighter)
, xenon or Hg arc lamp-- typically strong emission lines, pressure broadened with a continuum, rich in UV\; arc lamps are high pressure and are not narrow line sources
hollow cathode lamp-- discrete line spectra , typically UV

10. How does a Scanning Electron Microscope determine the chemical composition at specific points on the surface of a very tiny sample?

basically, the electron beam ionizes inner electrons from sample's atoms
result is production of X-rays
a solid state detector determines the energy of each of the X-ray photons
specific wavelengths correspond to specific elements
focused electron beam scans across sample, so one knows the location where X-ray is produced (to high resolution)

11. define five of the terms below:
Nebulizer-- creates a mist (droplets) from a solution
optical cavity (in a laser)-- needs the mirrors (don't confuse with the optical medium itself)
Bremstrahlen -- in X-rays, source of continuum
Doppler broadening-- related to Doppler shift (velocity change as emitting or absorbing species moves.) Broadening refers to increased spectral line width of light emitted (or absorbed) becuase of random thermal motion
matrix (in the sense of AA spectrometry)
Resolution (in an optical spectrometer)-- one sense is qualitative, how well we can separate (separately measure) two closely spaced spectral line. Quantitatively it's acually a ratio of delta-lambda / lambda; a numerical value.

Spectral Order (from a grating)-- each order arrises from interference ... first order occurs when paths differ by a single wavelength, second order differs by two wavelengths, etc. The dispersion is proportional to n and the different spectra produced overlap