Chemistry 332
Second Hour Exam
February 17, 2016
1. Which of the following is not an advantage of Fourier transform spectroscopy?
(a) Fourier transform spectroscopy offers improved signal-to-noise ratio.
(b) Fourier transform spectroscopy allows for the acquisition of spectra in a very short
time.
(c) Fourier transform spectroscopy is applicable to all important regions of the
electromagnetic spectrum.
2. In GC, separation between two different solutes occurs because
(a) the solutes have different solubilities in the mobile phase.
(b) the solutes volatilize at different rates in the injector.
(c) the solutes spend different amounts of time on the stationary phase.
3. Typical GC detectors include
(a) flame ionization, thermal conductivity, electron capture, and mass spectrometer.
(b) ultraviolet, fluorescence, amperometric, and conductivity.
(c) spectrophotometer, refractive index, light scattering, and electrochemical.
4. A TCD or thermal conductivity detector measures the
(a) ability of a substance to transport heat from a cold region to a hot region.
(b) ability of a substance to transport heat from a hot region to a cold region.
(c) ability of the detector to transport heat from a hot region to a cold region.
5. Normal-phase liquid chromatography refers to
(a) the use of a nonpolar solvent as the eluent in HPLC.
(b) the use of a polar material as the stationary phase in a chromatographic column.
(c) the use of a nonpolar material as the stationary phase in a chromatographic column.
6. Reversed-phase HPLC of a multi-component solute usually uses
(a) an isocratic polar solvent.
(b) a gradient from more polar to less polar solvent.
(c) a gradient from less polar to more polar solvent.
7. The use of smaller stationary phase particles in liquid chromatography
(a) increases relative surface area, and therefore, improves resolution.
(b) increases the magnitude of irregular flow paths.
(c) decreases the resistance to solvent flow.
8. Which of the following statements is true for a refractive index detector in HPLC?
(a) It is more sensitive than a UV detector.
(b) It can only be used for isocratic elutions.
(c) It does not respond to many solutes.
Longer Answers
9. A single extraction with 100 mL of CHCl3 extracts or removes 88.5% of the weak
acid, HA (Ka = 3.5 x 10-5), from 50 mL of aqueous solution at a pH = 3.00? If a student
carries out the same extraction at a pH = 7.00 and reports that fraction remaining in the
aqueous phase is 0.01, is this result is reasonable, explain?
pKa = 4.5, 11.5% remain after extraction into a nonpolar solvent. At pH 7 the
weak acid will be dissociated and should extract to a lesser extent. Therefore,
having 10% remaining in the aqueous phase does not make sense.
10. You have just received a call from Rainin Instrument Company saying that
they want to hire you as an instrument design specialist because of you extensive
instrumental background. You accept the job, and on your first day you are
asked to design an isocratic HPLC system with an IR detector. The IR detector
should be able to rapidly scan the IR spectrum (4000-1000 cm-1) of the column
effluent.
a) Sketch the system you would build, describing each component.
b) Please justify your selection of components.
Things you will need: solvent, pump, injector, column, detector. The detector
works in the IR so you will need a solvent that transmits IR light. I would use
a ATR cell connected to the HPLC inserted into a FTIR. You will need an FT
instrument in order to scan from 400-1000 wavenumbers.
11. A chromatogram of a standard test mixture produced the following results:
’
tr (min)
W (min)
b
Peak 1 Peak 2
2.00 2.50
0.113 0.141
a) Calculate the effective number of plates for the column (Neff) from
data for peak 1.
b) Calculate the resolution factor (Rs) for the two peaks.
Using the convenient chromatography equation sheet.
a) N=16(tr/W)^2 = 5012
b) Rs=(delta tr)/(Wavg) = 3.9
12. a) Describe how kinetic and thermodynamic processes influence
chromatographic separation.
Using the second form of the equation describing chromatographic resolution,
recognize that N is the kinetic term and kb and alpha are thermodynamic
terms.
b) Provide an example of a how you could change the kinetic properties of a
chromatographic system.
Detail steps to change N
c) Provide an example of a how you could change the thermodynamic properties
of a chromatographic system.
Detail steps to change kB or alpha
13. How is it possible to use Raman spectroscopy to measure the vibrational
energy levels of a molecule using high-energy, visible light (a 632 nm laser in our
case).
Describe the energy diagram that produces Stokes emission. Recognize that
we are plotting intensity as a function of energy difference from the excitation
source. This energy difference is the vibrational energy.
14. Describe how a GC thermal conductivity detector works. What are the
advantages and disadvantages of this detector?
See details on page 542 of Harris. The advantage is a general response to most
analytes. The disadvantage is poor sensitivity.