Final Exam Instructions: This exam is worth 100 points. Some questions allow a choice as to which parts are answered. Do not answer more parts than are requested. velocity of light in a vacuum: 3.0x10 8 m/sec Planck's constant: 6.626x10-34 Joule-sec electron charge: 1.602x10-19 Coulomb 1 Joule = 1 Volt-Coulomb Avogadro's constant: 6.022x10 23 atoms/mole 1 nm = 10-9 m 1 cm = 10-2 m 1 µg = 10-6 g?? = c E = h? E = ev sin r = n?/d - sin I dr/d? = n/(d cos r) sin I / sin r = n 2 / n 1 R =? /?? R = S / HA S = S final (1 - e -t/t ) A = ebc A = -log 10 T T = P/P 0 A total = A 1 +... + A n A = log 10 ((P 0 ' + P 0 ")/(P 0 '10 -e b + P 0 "10 -e bc )) A = -log 10 ((P + P s )/(P 0 + P s )) F = kp 0 (1-10 -ebc ) F? kp 0 (2.303)ebC I(d) = 0.5 I(?)(1 + cos(2pd/?) I(t) = B(?) + B(?)cos(2p?2Vt) f = 2?V I(d) =?B(?)cos(2p?d) d? B(?) =?I(d)cos(2p?d) dd m/z = B 2 r 2 e/2v R = m /? m Physical Constants and Conversion Factors Equations
1. (15 points) For each of the following cases, indicate whether (1) flame atomic absorption, (2) furnace atomic absorption, or (3) inductively coupled plasma atomic emission is the preferred technique and explain your reasoning. a. Sample size is limited. b. Elements with high boiling points are to be analyzed. c. Multiple elements in the sample are to be determined. 2. (10 points) Answer one of the following. a. Describe the operation of a magnetic sector mass analyzer. b. Compare unimolecular and bimolecular methods of ionization in mass spectrometry.
3. (35 points) Answer five of the following. a. What is the most common broadband light source used in (1) absorption spectroscopy and (2) luminescence spectroscopy in the ultraviolet spectral region? b. Use appropriate equations to compare the sensitivity of luminescence spectroscopy to that of absorption spectroscopy? Which is greater? Why? c. Name three reasons why signal averaging is more important in infrared spectroscopy than in UV-visible spectroscopy. d. How is the resolution changed in (1) a dispersive spectrometer and (2) a Fourier transform spectrometer? e. Name three experimental changes in luminescence spectroscopy that can lead to an increase in quantum yield. f. For an analysis in which the signal is in microamps (µa) and concentration is in moles/liter, what are the units of (1) sensitivity, (2) selectivity, and (3) limit of detection.
4. (30 points) You have the following instrumental configurations: A. Double-beam luminescence spectrometer with Xe arc lamp and photomultiplier tube detectors. B. Double-beam luminescence spectrometer with Ar + laser and photomultiplier tube detectors. C. Fourier transform spectrometer with Globar source and TGS pyroelectric detector. D. Double-beam dispersive absorption spectrometer with Globar source and TGS pyroelectric detector E. Double-beam dispersive absorption spectrometer with 400 W tungsten-halogen source and phototube detector. F. Single-beam dispersive absorption spectrometer with 100 W tungsten-halogen source and photodiode array detector. G. Double-beam dispersive absorption spectrometer with 400 W tungsten-halogen source and photomultiplier tube detector. H. Inductively coupled plasma atomic emission spectrometer. I. Double-beam flame atomic absorption spectrometer. J. Double-beam furnace atomic absorption spectrometer. K. Double-focusing mass spectrometer equipped with fast atom bombardment ionization. L. Double-focusing mass spectrometer equipped with electron-impact ionization. M. Mass spectrometer equipped with chemical ionization and quadrupole mass analyzer. N. Mass spectrometer equipped with electron impact ionization and magnetic sector mass analyzer. For each of the following measurement scenarios, indicate the best instrument configuration to use by specifying the appropriate letter from the list above. a. You wish to determine the molecular weight of an organic compound to the highest precision. b. You wish to determine Cu in a water sample with low cost and fast analysis time. c. You wish to detect the effluent from a liquid chromatograph in a situation in which incomplete separation has taken place. You know the compounds of interest have chromophores, but you have no information about their volatility. d. You wish to differentiate alkane species in the effluent of a gas chromatograph. e. You wish to determine non-volatile polycyclic aromatic compounds in a water sample with the greatest sensitivity and without performing any sample manipulation or pretreatment.
5. (10 points) You are analyzing a mineral sample for Cu (AW=63.55) and Mg (AW =24.31) by absorption spectroscopy. You dissolve a 0.5120 g sample in acid, add a complexing reagent in excess and dilute to 100.0 ml in a volumetric flask. You measure the absorbance of the resulting solution at 440 and 550 nm in a 1.000 cm path length cell. You know the following absorptivity values: Species Molar absorptivity (L/mole-cm) 440 nm 550 nm Cu 32000 18000 Mg 12000 7000 The absorbance at 440 nm is 0.3312 and the absorbance at 550 nm is 0.1876. Calculate the Cu and Mg content in the original solid sample and express as µg/g (ppm).