Midterm - Solutions

1. Answer 2 of 3 (a)-(c).  For this question, you must supply specific details (instruments, sampling techniques, detectors, etc.) (4 points each, 8 total)

There are several options for each situation, I've supplied what I think is the simplest (but not necessarily the best) method.  In each case I'm looking for a comment on each of the following:
 - sample preparation and/or injection method
 - type of chromaography (because all are mixtures, must use one of these methods)
 - column type
 - detector type (including quantitation)

Also: somewhere you should make a comment about running standards and making a calibration line (all three of these are quantitative)
 
1. (a) A truck carrying several types of PCB waste has crashed into a river.  How would you determine (quantitatively) how much of each PCB is present in the river water at a point downstream of the spill?  Note: PCB = “polychlorinated biphenyl” - i.e. they each contain 2 benzene rings.  PCB's are non-volatile.

sampling: already in water, can be injected directly into machine (presumably dilute enough)
type of instrument: HPLC best choice (non-volatile organic)
column: non-polar compounds, so reverse-phase best choice
detector: conjugated, so UV-visible active; this is also quantitative by peak area on chromatogram
 
1. (b)
(b) How would you quantitatively analyse each of the several volatile hydrocarbons in a sample of gasoline?  These compounds are saturated (no double bonds), usually isomers of C7H16 and C8H18.

sampling: liquid, can probably be injected directly into machine (if diluted, in lighter hydrocarbon, e.g., hexane)
type of instrument: GC (volatile, not conjugated)
column: either type (packed or capillary), non polar so reverse-phase if packed
detector: FID best - quantitative and general
 
1. (c) A drug has 2 chiral forms, R and S.  The R-isomer is toxic.  Unfortunately, pure S-isomer can racemise (isomerise into the R form) over time.  It is expected that a patient has taken expired medicine that has recemised.  How would you analyse the patient's blood to determine how much of each isomer is present?  (The drug is a solid containing benzene rings.)

sampling: some comment on removing proteins and such from blood (don't want to inject blood in directly)
type of instrument: HPLC best choice (non-volatile organic), could also argue for capillary electrophoresis
column: must be chiral.  Based on information, normal-phase column best, but could also use revered-phase
detector: conjugated, so UV-visible active; this is also quantitative by peak area on chromatogram
 
2. Answer 3 of the following 4 (a) to (d) (2 points each, 6 total):
 
2. (a) Explain how ion-exchange chromatography works.

- stationary phase a gel with exchangable ions
- ions in mobile phase (analyte ions) can exchange with resin protons
- affinity of individual analyte ions for resin causes separation
- regenerate resin when finished by adding solution (of high concentration) of original ion
 
2. (b) Fluorescence does not follow Beer's Law (A = ebc).  Instead, under conditions of constant light source power, the formula F = 2.3K'ebc can be used.  Explain what this extra term K' represents.

- e represents amount of light analyte will absorb (whether photon is absorbs)
- several decay pathways are possible, fluorescence only one of them
- therefore, K' represents the chance that, if a photon has been absorbed, that it will be re-emitted as a photon (fluoresce) rather than decaying by another pathway
 
2. (c) A pyridine base has a distribution constant between ether and water of 3.25.  A water sample (40.0 mL) is 0.300 M in the acid.  What is the concentration remaining in the water after 5 extractions with ether if each extraction uses a volume of 10.00 mL?

 
2. (d) Explain why capillary electrophoresis is both faster and has better resolution than traditional gel electrophoresis.

- both N (number of plates) and v (velocity of electroosmotic flow) are directly proportional to V (voltage)
- a high voltage will cause heat to build up
- capillaries more efficient than plates at dispersing the heat, therefore higher voltage can be used
 
3. An HPLC experiment has the following parameters:
Length             24.7 cm
Flow rate         0.313 mL/min
VM                  1.37 mL
VS                   0.164 mL
  retention time (min) width of peak (W) (min)
solvent
PCB-A
PCB-B		 3.1
13.3
14.1		 -
1.07
1.16
Calculate the length of column required to completely resolve the two peaks unk-A and unk-B (i.e., a resolution of 1.5).

There are a number of ways (different formulae) to do this, but basically the steps are: (i) calculate number of plates in column, (ii) calculate number of plates needed to get desired resolution, (iii) calculate length required for that many plates.  The easiest method (I think) is shown below:



Therefore, average N = 2418
Plate height H = L/N = 24.7 cm / 2418 = 0.0102 cm
If , then,

Therefore, new column length = L = N H = (0.0102 cm)(10580) = 108 cm
 
4. Occasionally, the military has to clean up old ammunition dumps.  At one such ammo dump, it is known that a barrel contains a toluene-based explosive or precursor, but it is not known which one.  Infrared absorption was used to determine the identity of the unknown.
 
4. (a) The compound was initially a solid.  The solid could be shock-sensitive (i.e. explode if it is ground up).  How could you safely prepare and run the sample?

If you cannot grind the compound, that rules out both KBr discs and nujol mulls.  Therefore, either of the solution techniques is acceptable (thin film by evaporation of solvent, or running the IR of the solution itself).
 
4. (b) The compound is the carboxylic acid, as evidenced by the broad O-H peak (3100) accompanied by the carbonyl peak (C=O at 1690).  The other two peaks are likely a C-O and the O-H bend, further verifying the identity as the acid.


 

Last updated, March 5, 2003
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