final2004 - 1 CHEM*3440 FINAL EXAMINATION Fall 2004...

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CHEM*3440 FINAL EXAMINATION Fall 2004 Duration: 2 hours 1. (8 points) A certain molecule was analyzed by electrospray ionization mass spectrometry. There are a lot of overlapping bands, but a tetramer of the molecule appears with a residual charge of +7 in a region without interference at an apparent m/z = 791. (a) What is the mass of the monomer? (b) At what value of m/z would the +6 charged tetramer appear? 3. (10 points) Sucrose can be enzymatically broken down into fructose and glucose by the enzyme invertase. The following procedure was used to determine the sucrose content of sweet potatoes. A 82.3 g sample of sweet potato was chopped into a juice. The juice was carefully collected (lots of washings) along with a buffer solution to form a final solution volume of 500.0 mL. Several 3.00 mL aliquots were taken and analyzed by a spectroscopic technique for glucose. Samples without any added enzyme determined the free glucose concentration. Three trials of this gave the following concentrations 212 mg/dL, 204 mg/dL, and 207 mg/dL. Invertase was added to three more aliquots and after 20 minutes, they were also analyzed for glucose, giving the following results: 462 mg/dL, 457 mg/dL, and 454 mg/dL. (a) In units of mg/dL, what is the concentration of glucose arising from the decomposition of sucrose in sweet potato? (b) Given the molar mass of glucose (180.16 g/mol) and sucrose (342.3 g/mol), what is the percent sucrose (w/w) in the sweet potato? sucrose invertase glucose + fructose 2. (20 points) Describe the following instrumental components. Outline the physical principles behind its operation and specify any limitations to its operation. (a) A proportional counter detector of x-radiation. (b) A PbS photoconducting transducer for the detection of near-IR radiation. (c) The He:Ne laser source used in a Michelson Interferometer. (d) Time-of-flight mass analyzer. 4. (8 points) In hexane, acetone has an absorption maximum at 270 nm, where its molar absorptivity has a value of 12 M -1 cm -1 . Our spectrometer can reliably measure transmittance between 10% and 90%. What is the range of acetone concentration that can be measured in a 1.00 cm cell under these circumstances? 1
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5. (15 points) An ore is analyzed for its platinum content and nickel is used as an internal standard. X-ray fluorescence is employed as a measurement technique and the count rate (in counts per second) were recorded for the K α emission lines for both atoms. Wt% Pt 0.00 2.00 4.00 6.00 8.00 Ni (cps) 217 219 216 215 219 Pt (cps) 17 222 425 620 827 The calibration data are given here. A linear least squares fit to the data produced the following parameters. m = 0.4633 b = 0.09075 s m = 0.0034 s b = 0.01635 s y = 0.02111 x i = 20 ! x i 2 ! = 120 y i ! = 9.720 y i 2 ! = 27.481 An unknown ore was measured four times and the average measured Pt/Ni x-ray ratio was 2.656. (a) What is the weight% Pt in the unknown ore? (b) What is the standard deviation of this unknown concentration?
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final2004 - 1 CHEM*3440 FINAL EXAMINATION Fall 2004...

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