UMD CHEM 425 - midterm2-2017key (10 pages)

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midterm2-2017key



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midterm2-2017key

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Pages:
10
School:
University of Maryland, College Park
Course:
Chem 425 - Instrumental Methods of Analysis
Instrumental Methods of Analysis Documents
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INSTRUMENTAL METHODS OF ANALYSIS MID TERM EXAM II xxx semester 125 Points CALCULATION 55 Points 1 25 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 Page 1 of 10 2 15 pts In UV Vis spectrophotometry what is the relative error in concentration from 1 stray light for a sample with a molar absorptivity of 5 0x104 M 1cm 1 and a concentration of 2 x10 5 M held in a 1 00 cm cell What is the relative error if the concentration is five times higher 1 0x10 4 M 3 5 pts If a mass analyzer had a resolution of 5000 at an m z 600 would you be able to resolve masses at 600 00 and 600 05 You must show your calculations to obtain credit for your answer Page 2 of 10 4 10 pts If the fluorescence quantum yield and lifetime of compound X were 0 5 and 100 nanoseconds respectively under anaerobic conditions what would the quantum yield and lifetime be under aerated conditions assuming that O2 quenches the excited singlet state with a rate constant of 1x1010 M 1s 1 and the solution concentration of O2 is 2 5x10 4 M Page 3 of 10 INSTRUMENTS 25 points 5 A 5 pts Provide a block diagram of an ultraviolet visible UV VIS diode array spectrophotometer clearly labeling each of the components B 5 pts Provide a block diagram of a scanning dual beam UV VIS spectrophotometer clearly labeling each of the components Page 4 of 10 C 5 pts Provide a block diagram of a scanning spectrofluorometer clearly labeling each of the components D 5 points Provide a block diagram of a mass spectrometer clearly labeling each of the components E 5 points Provide a block diagram of a capillary electrophoresis system clearly labeling each of the components Page 5 of 10 SHORT ANSWERS 45 points 6 Mass Spectrometry 10 points A 2 points Name two gas phase ionization sources B 2 points What ionization source is used in the mass spectrometer in the 425 laboratory C 2 pts What is the purpose of the solvent cut time feature in the GC tab of the method development in the 425 laboratory The solvent cut time keeps the mass spectrometer off during this time while the solvent peak elutes D 2 pts Name two ionization sources that are used most commonly for the analysis of high molecular weight polar compounds such as proteins E 2 pts 0 5 pts each Name four mass analyzers Page 6 of 10 7 Capillary Electrophoresis 10 points A 2 pts Provide the order of elution of cations anions and neutral species using capillary zone electrophoresis B 2 pts Name a capillary electrophoretic method that can be used to separate neutral species on the basis of their polarity C 2 pts Which capillary length length to the detector or the total length is used in calculating the electrophoretic velocity of the species D 2 pts In isoelectric focusing would the species with the highest pI focus closer to the cathode or the anode E 2 pts Indicate a way in which electroosmotic flow can be a repressed and b reversed Page 7 of 10 8 Ultraviolet Visible Absorption Spectrophotometry 10 points A 2 pts Provide an equation that relates absorbance to concentration What is this equation called B 3 pts Name three factors that can produce deviations from the relationship provided in Question A C 2 pts Name two types of electronic transitions that have very low molar absorptivities D 2 pts What two light sources are commonly employed in UV Vis spectrophotometry to provide light in the ultraviolet and visible wavelength ranges respectively Ultraviolet Visible E 1 pt Why are absorbance readings taken at the peak wavelength of maximum absorbance Page 8 of 10 9 Fluorescence Spectroscopy 15 points A 2 pts Under what condition is fluorescence intensity proportional to concentration B 3 pts What is the ratio mode in spectrofluorometry and why is it employed C 2 pts Define fluorescence quantum yield in either words or an equation D 2 pts Define triplet quantum yield in either words or an equation E 6 pts Explain the difference between a fluorescence emission spectrum and a fluorescence excitation spectrum Which resembles an absorption spectrum and why Page 9 of 10 END of EXAM Page 10 of 10


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