UMD CHEM 425 - sample_3_v15 (6 pages)

Midterm II CHEM 425 1 Explain why the gas solid chromatography is not used for the analysis of moderatemolecular weight polar molecules such as methanol ethanol and propanol 5pt 2 Polyethylene glycol PEG is one of the popular liquid stationary phases for GC column 5pt 2pt for a and 3pt for b a Draw its chemical structure b Generally PEG stationary phase is good for the separation of polar nonpolar analyte Pick one 3 Assume that you use a very nonpolar stationary phase in GC Please rank elution time from the shortest to the longest for the following three analyte species 3pt Isopropanol Methanol Methylbenzoate Shortest Middle Longest 4 Choose one of three detectors flame ionization detector FID thermal conductivity detector TCD electron capture detector ECD 15pt 3pt each a most insensitive detector in general b good for hydrocarbon analysis of petroleum c smallest dynamic range d not sensitive to H2O CO2 SO2 e most sensitive to polychlorinated biphenyls 5 Use the retention data given below to calculate the retention index of 1 hexene 10pt Show your work Sample Air n pentain n hexane 1 hexene Retention time min 0 5 2 5 5 0 3 5 6 Ions in a bare silica capillary move as a result of electrophoretic migration and electroosmotic flow EOF True or False 18pt a Electric field across the capillary affects the electrophoretic migration but does not affect electroosmotic flow b Charge of analyte ions affects more the electroosmotic flow than electrophoretic migration c Ionic strength of solution affects both the electrophoretic migration and electroosmotic flow d Electrophoretic migration of analyte molecule is dependent on the molecular weight but not dependent on the shape of the analyte molecule e The inner surface of the bare silica capillary has a negative surface charge at pH 7 f EOF can be shut down by removing the inner surface charge of the capillary with a certain surface chemical modification 7 In capillary zone electrophoresis with an unmodified silica capillary at pH 7 the correct elution order fastest first slowest last for the Ca2 A acetone B Cl C SO42 D and Na E is 5pt a A E B D C b A E B C D c D C A E B d D C B E A e None of the above 8 In capillary zone electrophoresis CZE for the separation of anionic analytes you would want to have positive negative charges on the inner wall surface of capillary in order to have the same EOF electroosmotic flow direction with the electrophoretic movement direction of anionic analytes Pick one 5pt 9 A certain cation has an electrophoretic mobility e of 4 31 x 10 4 cm2 s 1 V 1 This same ion has a diffusion coefficient of 9 8 x 10 6 cm2 s 1 This cation was separated by CZE in a 50 0 cm capillary at 10 0 kV Under the separation conditions the electroosmotic flow rate was 0 85 mm s 1 toward the cathode If the detector were placed 40 0 cm from the injection end of the capillary how long would it take in minutes for the analyte cation to reach the detector after the field is applied Hint Electrophoretic migration rate eE E electric field 13pt 10 In mass spec there are a variety of ionization techniques Let s compare three of them Electron impact ionization EI Electrospray ESI Fast atom bombardment FAB and MALDI Check x one for a b c d e f and g 21pt 3pt each Characteristic a Most compatible with HPLC b Most compatible with GC c Causes most extensive fragmentation d Yields most multiple charged ions of the same mass but different m z e Causes least fragmentation EI ESI f best for the analysis of industrial polymers g best for the analysis of thermally unstable biomolecules MALDI 11 Which of the following is not a variable to acquire mass spectra intensity vs m z in the magnetic sector analyzer 5pt a M a g n et ic field b Ion acceleration potential c Radius of magnetic sector d Diameter of metal analyzer tube 12 When a magnetic sector analyzer was operated with a constant magnetic field an accelerating voltage of 3000V was required to focus the CH4 on the detector What accelerating voltage would be required to focus a doubly charged ion with mass 500 10pt 13 Calculate the ratio of the M 4 to M peak height for C3H7ClBr Please consider 35 only the isotope effect of Cl and Br Cl 37Cl 100 32 5 79Br 81Br 100 98 10pt 14 With TOF time of flight mass analyzer the flight time of molecular ion t is given t L m 2 zeV by When L distance from source to detector is 1 m and V electric field pulse for ion acceleration is 10 kV which of the following is the most closely estimated flight time of a singly charged molecular ion MW 120 000 5pt a 1 s b 100 s c 10 ms d 1 s e 100 s