Chem 7595
Exam 2
November 9th, 2004
Name:
1. (10 points) Describe some advantages and disadvantages of the electron capture
detector (ECD) as a GC detection system.
2. (25 points) Draw a typical Van Deemter curve include the A, B and C terms that make
up the equation (H = A + B/u + Cu) make sure you label the axis.
Explain each term in this equation and why some are influenced by flow and some are
not.
Show graphically the effect of particle size on the Van Deemter curve and explain why?
(you can explain in terms of the equations that go along with each part of the Van
Deemter curve)
3. (15 points) The following data apply to a column for liquid chromatography:
The length of column is 35.7 cm, the volume of the mobile phase and stationary phase is 1.6ml
and 0.134 ml respectively. The column was run at a flow rate of 0.5 ml/min
A mixture of 4 components (A,B,C,D) provided the following data:
non-retained
A
B
C
D
Retention time
(min)
2.1
5.4
10.3
12.1
20.6
Width of Peak Base
(min)
0.51
1.03
1.25
1.62
a. Using peak C, calculate the number of plates (N) in this column and the plate height (H) for
the column.
b. For peak D, calculate the capacity factor (k') and the partition coefficient (K).
c. For peak B and C, calculate the resolution (R) and the relative retention () .
4. (10 points) List 4 advantages and disadvantages of HPLC
5. (5 points) List some (5) basic detector requirements of HPLC
6. (5 points) Which one of these compounds would have the greatest electophoretic
mobility (circle one) and explain why.
COOH
HOOC
HOOC
COO-
-
OOC
HOOC
COOH
HOOC
COO
-
7. (10 points) Explain the difference between the two processes of electrophoresis and
electroosmosis. If one wanted all compounds (positive, negative and neutral molecules)
to elute off the CE column what process must dominate?
8. (5 points) Explain one of the new technologies that we discussed in class and why it is
significant
9. (10 points) What is the most popular mobile phase used in supercritical fluid
chromatography? Give two reasons why it is superior to other mobile phases. How can
its solvating power be easily changed, and how can its polarity be modified?
10. (5 points) Describe (pictorially) the basic concept of simulated moving bed
chromatography, including injection and fraction collection, as compared to traditional
“batch” chromatography (like that used in HPLC and GC analyses).
u =L/tM
VM = tMF
k' = (tR – tM)/tM
K = k' VM
VS
 = (tR)B – tM
(tR)A – tM
Rs= 2[(tR)B - (tR)A]
WA + WB
N = 16(tR/W)2
H=L/N
(tR)B = 16Rs2H ( / ( - 1) )2 (1+ k‘B)3
u
(k' B)2
Answers:
1. ECD is selective towards compounds that contain an electronegative group (e.g. a
halogen). This can be both an advantage and a disadvantage. ECD detectors use a
radioactive beta-emitter (usually 63Ni) to ionize GC effluent gases, causing free
electrons and allowing a current to be set up by an appropriate potential. This current
is “quenched” when an compound containing an electronegative group passes through
the detector because of “electron capture”. Other advantages of ECD include its
sensitivity, simplicity, portability, and the fact that it is mostly non-destructive (at
least compared to an FID, which totally destroys the analyte).
2.
A Term:
Eddy diffusion
• molecules may travel unequal distances
• particles (if present) cause eddies and turbulence
• A depends on size of stationary particles (want small)
and their packing (want uniform)
Band broadening is caused by differing flow velocities through the column,
B Term:
Longitudinal Diffusion-
the concentration of analyte is less at the edges of the band than at the center.
The analyte diffuses out from the center to the edges. This causes band broadening.
If the velocity of the mobile phase is high than the analyte has less time on the
column which decreases the effect of logitudinal diffusion.
C Term :
Resistance to Mass Transfer:
the analyte takes a certain amount of time to equilibrate between the stationary
phase and the mobile phase. If the velocity of the mobile phase is high, and an
analyte has a strong affinity for the stationary phase, then the analyte in the mobile
phase will move ahead of the analyte in the stationary phase. The band of analyte is
broadened. The higher the velocity of the mobile phase, the worse the broadening
becomes.
3.
4. Advantages
Speed (minutes)
High resolution
Sensitivity
Reproducibility
Accuracy
Automation
Disadvantages
Cost
Complexity
Low sensitivity for some compounds
Irreversibly adsorbed compounds not detected
Coelution difficult to detect
5. (I will take any 5 of these)
Low drift and noise level
High sensitivity (ability to discriminate between small differences
in analyte concentration)
Fast response
Wide linear dynamic range
Low dead Volume
Cell design that eliminates remixing of separated bands
Insensitivity to changes in types of solvent, flow rate, temp
Operational simplicity and reliability
Non destructive
6. Mobility is dependant on charge and size. Since all the compounds have close to
the same size, then mobility is dependant on charge the largest charge = highest
mobility. So the answer is the middle molecule. The questions is not which would
elute faster- if that was the question I would have had to give you the CE conditions.
7. Electrophoresis
8. New technologies question
9. CO2 is by far the most popular SFC mobile phase (over NH3 and many others). It
is safe, easy to use, has a favorable critical temperature and pressure, and can be
easily disposed of/removed. It is an excellent solvent for non-polar molecules. Its
solvating power can be modified by changing its density (i.e. changing pressure). Its
polarity can be altered by adding a co-solvent, most commonly 1-10% methanol
(which increases polarity).