UMD CHEM 425 - midterm_I_sample_2 (8 pages)

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midterm_I_sample_2



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midterm_I_sample_2

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Pages:
8
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 I XXX 125 Points 1 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 8 D 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 8 3 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 the major components Page 3 of 8 5 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 phase Page 4 of 8 B larger film thickness on stationary phase C a distribution of particles sizes in the stationary phase packing D small diffusion coefficient of the analyte E 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 velocity Page 5 of 8 B 1onger column length 8 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



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