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UMD CHEM 425 - sample_1_o15

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XXX 125 PointsINSTRUMENTAL METHODS OF ANALYSISMID-TERM EXAM IIXXX 125 PointsCALCULATIONS (60 Points)1. (30 pts) The chelate CuA22- exhibits maximum absorption at 480 nm. When the chelating reagent is present in at least a 10-fold excess, the absorbance is dependent only upon the analytical concentration of Cu(II) and conforms to Beer’s law over a wide range. A solution in which the analytical concentration of Cu2+ is 2.30x10-4 M and that for A2- is 8.60x10-3 M has an absorbance of 0.690 when measured with a 1.00-cm cell at 480 nm. A solution in which the analytical concentrations of Cu2+ and A2- are 2.3x10-4 M and 5.00x10-4 M, respectively, has an absorbance of 0.540 when measured under the same conditions. Use this information to calculate the formation constant, K, for the reaction,Cu2+ + 2 A2-  CuA22- 2. (20 pts) The fluorescence lifetime of a compound without quenching is 6.0 ns, while with aquencher concentration of 1.00x10-2 M, the fluorescence lifetime is 2.0 ns. The rate constantPage 1 of 10for intersystem crossing is 1.00x108 s-1, while that for internal conversion from S1 to S0 isnegligible. Calculate: A) the rate constant for fluorescence (kr): B) the quenching rate constant (kq): C) the Stern-Volmer quenching constant, K=τ●kq (units of M-1): D) the fluorescence quantum yield of the quenched compound:3. (10 pts) Calculate the resolution required for a mass analyzer to resolve peaks for,a) CH2N (MW=28.0187) and N2+ (MW=28.0061)Page 2 of 10b) C2H4+ (MW=28.0313) and CO+ (MW=27.9949)INSTRUMENTS (25 points)4. A. (5 pts) Provide a block diagram of a photodiode array UV/Vis absorption spectrophotometer clearly labeling each of the components.Page 3 of 10B. (5 pts) Provide a diagram of a monochromator, clearly labeling each of the components. C. (5 pts) Provide a block diagram of a scanning spectrofluorometer clearly labeling each of the components.Page 4 of 10D. (5 pts) Provide a block diagram of a capillary electrophoresis system clearly labeling eachof the components.E. (5 pts) Provide a block diagram of a double beam scanning UV/Vis spectrophotometer, clearly labeling each of the components.Page 5 of 10SHORT ANSWERS (40 points)Mass Spectrometry (10 points)5. A. (2 pts) What ionization source is used in the mass spectrometer in the 425 laboratory?Page 6 of 10B. (2 pts) Name the two ionization sources that are used most commonly for the analysis of high molecular weight polar compounds such as proteins.__________________________________________________________________________________________________C. (2 pts) What mass analyzer is employed in the GC/MS in the 425 laboratory? D. (2 pts) For what reason is this mass analyzer employed in the GC/MS in the 425 laboratory?E. (2 pts) Name three other mass analyzers.____________________________________________________________________________________________________________________________________Ultraviolet/Visible Absorption Spectrophotometry (10 points)6. A. (2 pts) Name two instrumental factors that can produce deviations from Beer’s Law.________________________________________________________________________________________________________________________Page 7 of 10B. (2 pts) Name two light sources that are commonly employed in UV/Vis spectrophotometryto provide light in:UV-Vis range: _____________________________________________________Visible range: _________________________________________________C. (2 pts) Name two types of electronic transitions that have very low molar absorptivities:__________________________________________________________________________________________________________________________D. (2 pts) What detector is employed in the spectrophotometer in the 425 lab?E. (2 pts) Name one advantage of using a photodiode array spectrophotometer, and one advantage of using a scanning spectrophotometer.Advantage of a photodiode array spectrophotometer: _______________________________________________________________________Advantage of using a scanning spectrophotometer: _______________________________________________________________________Fluorescence Spectroscopy (10 pts)7. A. (2 pts) Under what condition is fluorescence intensity proportional to concentration?Page 8 of 10B. (3 pts) What is the “ratio mode” in spectrofluorometry and why is it employed?C. (2 pts) Explain the difference between a fluorescence emission spectrum and a fluorescence excitation spectrum. Which resembles an absorption spectrum and why?D. (3 pts) Name three possible relaxation pathways for a molecule in its first excited singlet state.___________________________________________________________________________________________________________________________________________________Capillary Electrophoresis (10 Points) 8. A. (4 pts) Provide the order of elution of cations, anions and neutral species using capillary zone electrophoresis:Can neutral species be separated by this technique? Why or why not? Page 9 of 10B. (3 pts) What is electroosmotic flow and why does it occur? Suggest a way in which electroosmotic flow can be: 1) suppressed: 2) reversed:C. (3 points) Name two advantages of using capillaries in electrophoretic separations. Name one major disadvantage.Advantages: ______________________________________________________________________________________________________________________Disadvantage: ___________________________________________________________END of EXAMPage 10 of


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