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UMD CHEM 425 - midterm_I_sample_2

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INSTRUMENTAL METHODS OF ANALYSISMID-TERM EXAM IXXX 125 Points1. (25 points; 5 pts each) From distribution studies, species M and N are known to have partition (distribution) coefficients between water and hexane of 5.81 and 6.20 (eg., KM=[M]H2O/[M]hex).The two species are to be separated by elution with hexane in a column packed with silica gel containing absorbed water. The ratio VS/VM for the packing is known to be 0.422.A) Calculate the retention (capacity) factors for M and N.B) Calculate the selectivity factor.C) How many theoretical plates will be needed to provide a resolution of 1.5?Page 1 of 8D) How long a column is needed if the plate height of the packing is 2.2x10-3 cm?E) If a linear flow velocity of 7.10 cm/min is employed, what time will be required to elute the two species?2. (10 points) A specific chromatographic medium is characterized as achieving up to 400 theoretical plates. When operated under these optimal conditions, what will be the peak width,W (in seconds), of a component eluting at a retention time of 729 s?Page 2 of 83. (10 Points) Provide a diagram of a high performance liquid chromatograph (HPLC) that employs low pressure gradient mixing, clearly labeling each of the major components.4. (10 Points) Provide a diagram of a capillary gas chromatograph, clearly labeling each of themajor components.Page 3 of 85. (9 points) Describe three general ways for improving resolution in a chromatographic separation.6. (15 points; 3 pts each) Explain how each of the following would affect plate height (increase, decrease, leave unchanged). Briefly justify each of your answers by indicating which terms in the van Deemter equation (A, B, C) would be affected and how. A. smaller particle size of stationary phasePage 4 of 8B. larger film thickness on stationary phaseC. a distribution of particles sizes in the stationary phase packingD. small diffusion coefficient of the analyteE. lower solvent viscosity (LC)7. (5 points) How is plate height affected (increased, decreased, unchanged) by each of the following, briefly justifying your answer in each case:A. very high linear flow velocityPage 5 of 8B. 1onger column length8. (6 points) Name three detectors employed in liquid chromatography and three detectors employed in gas chromatography.9. (A) (10 points) Name three different types of stationary phases used in liquid chromatographic separations. Briefly describe the physical principle behind analyte separation for each of these types of stationary phases.Page 6 of 8(B) (5 points) What stationary phase do we employ for LC separations in the CHEM 425 laboratory?10. (A) (5 points) How is the retention (capacity) factor manipulated in liquid chromatography? in gas chromatography?(B) (5 points) How is the selectivity factor manipulated in liquid chromatography? in gas chromatography?11. (A) (5 pts) Briefly describe the general elution problem.Page 7 of 8(B) (5 pts) How is this problem solved in LC? in GC?END of EXAMPage 8 of


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