Chemistry 564 - Instrumental Analysis
2^nd Take-home Examination
February 23, 1998

Due Monday, March 2, at 11:30 a.m.

Instructions: Answer the following 4 equal-valued questions on your own. Answers may be turned in either neatly hand written or typed (printed). Be sure to put your name on your response. Most everything required for successful performance is in the book or on the handouts received in class. You may not share your results with other students prior to the examination being graded and returned. You may ask me questions, either in person, or by e-mail (sbialkow@cc.usu.edu). If I respond with important information, this information may be shared with the class, by me only, either verbally or by posting on the Chemistry 564 internet home page (http://www.chem.usu.edu/~sbialkow/Classes/564/index.html).

1. Optical Spectroscopy Instrumentation
A. Refer to problems 7-3 and 7-4 in the textbook;
- Calculate the maximum emission wavelengths for a tungsten filament bulb light source operating at a temperature of 2800 K and 3050 K.
- Calculate the total energy output of the bulb in W/cm² at these two temperatures.
B. Why is glass better than a fused silica as a prism material for a monochromator to be used in the 400 to 800 nm region?
C. A monochromator with a focal length of 0.5 m is equipped with a 1200 blaze/mm echellette grating;
- Calculate the reciprocal linear dispersion of the instrument for first-order spectra.
- What is the first-order resolving power of the monochromator if 4 cm of grating is illuminated?
- In theory, what minimum wavelength difference could be resolved at 500 nm by the instrument?
D. Describe the basis for radiation detection with both the silicon and vacuum photodiode transducers. How does the photomultiplier tube improve upon the response of the vacuum photodiode?

2. Atomic Spectroscopy
A. A chemist attempts to determine strontium with an atomic absorption instrument equipped with a nitrous oxide-acetylene burner. The sensitivity associated with the 460.7 nm atomic line is not satisfactory. Suggest at least three things that might be tried to increase sensitivity.
B. In the concentration range of 500 to 2000 ppm of U, a linear relationship is found between absorbance at 351.5 nm and concentration. At lower concentrations, the relationship is nonlinear unless about 2000 ppm of an alkali metals salt is introduced into the sample. Explain.
C. Why are ion interferences sometimes less severe in ICP than flame emission spectroscopy?
D. What is an internal standard? Describing how one might use Li internal standards to obtain more accurate Na and K measurements in a flame emission spectrometer.

3. Visible/Ultraviolet Spectrophotometry
A. Describe the differences between the following and list any particular advantages possessed by one over the other.
- Hydrogen- and deuterium-discharge lamps as sources for ultraviolet radiation.
- Filters and monochromators as wavelength selectors.
- Photovoltaic cells and phototubes as detectors for electromagnetic radiation.
- Phototubes and photomultiplier tubes.
- Spectrophotometers and photometers.
- Single- and double-beam instruments for absorbance measurements.
- Conventional and diode-array spectrophotometers.
B. List and describe 3 sources of nonlinear (e.g., non-Beer's law) behavior in spectrophotometry.
C. How does optical bandwidth affect the measurement of absorbance? What gives a more accurate reading of absorbance; an interference filter with an optical bandwidth 10 nm, or one with a bandwidth of 0.1 nm?
D. The following questions refer to molecular luminescence spectrometry:
- Why is fluorescence spectrometry generally much more sensitive than absorption spectrophotometry? What type of noise (e.g., shot, thermal, or flicker) is reduced in luminescence spectrometry versus absorption spectrophotometry?
- What is the difference between fluorescence and phosphorescence?
- List two spectroscopic "features" of phosphorescence that distinguishing it from fluorescence.

4. Infrared and Fourier-Transform Infrared Spectroscopy
A. Fourier-Transform infrared spectrophotometers (FTIR)
- What are the main difference between the scanning-dispersive infrared spectrophotometer and the FTIR?
- Which instrument has higher light throughput?
- What are some of the advantages to using the FTIR?
- When is the twin-beam scanning disperse instrument advantageous?
B. Define the terms: fundamental; hot-band; overtone; combination band, as used in infrared spectroscopy.
C. The majority of molecules are in the ground vibrational state at room temperature. Use the Boltzmann equation (8-1) to calculate the N (v=1)/N (v=0) and N (v=2)/N (v=0) excited-to-ground state population ratios for the 650 cm^-1 band of CO₂ at 300 K. Use these results to predict the intensity of the v=1 to v=2 and v=2 to v=3 transitions relative to the intensity of v=0 to v=1 transition.
D. Using Equation 16-9 calculate the spring constant for H³⁵Cl. (v=0 to v=1 occurs at 2885 cm^-1) Based on this spring constant, predict vibrational frequencies for H¹⁹F, H⁷⁹Br, and H¹²⁷I. The experimental vibrational frequencies for H¹⁹F, H⁷⁹Br, and H¹²⁷I are 4138.32, 2649.67, and 2308.60 cm^-1, respectively. Are the predictions based on constant spring constant accurate? Why or why not?