SCHA 312
Problem Set #2
100 Points
Due April 11 by the end of the day.
Neatness matters - remember to show units on answers and you should report answers to
about 3 figures even if this might not be appropriate!! Show you work and return your
answers on this form.
1. A chromatographic procedure separates 10.0 mg of unknown mixture on a column
with a length of 15 cm and a diameter of 0.46 cm. For this problem assume that the
mobile phase occupies 60% of the column volume. (15)
a. What size column would you use to separate 2000 mg of the mixture?
b. If the flow rate is 0.5 mL/min on the small column, what volume flow rate should
be used on the large column?
c. Calculate the linear flow rate for the both small column and the large column.
2. The following data were obtained for three compounds separated on a 20-m capillary
column. (15)
Compound
A
B
C
tr (min)
10.04
10.26
10.43
Width (min)
0.18
0.18
0.20
a. Calculate the number of theoretical plates for each compound and the average
number of theoretical plates for the column.
b. Calculate the average height of a theoretical plate.
c. Explain why it is possible for each compound to have a different number of
theoretical plates.
3. Consider the peaks for n-heptane and p-difluorobenzene in the chromatogram in
Problem 44 in Chapter 23. The elution time for unretained solute is 1.06 min. (20)
a. Measuring w1/2, find the number of theoretical plates (N) and plate height (H) for
each peak.
b. Calculate the observed (from the data above) resolution between the two peaks
using the chromatograms.
c. Using the number of plates (N = (N1N2)1/2), with values from part a), the relative
retention and capacity factors, calculate what the resolution is and compare your
answer to the measured resolution in part b.
d. What fraction of time does p-difluorobenzene spend in the stationary phase?
4. You analyze a mixture of two compounds, X and Y, by gas chromatography. You
find that air elutes 3.0 min after injection. The retention time for substance X is 21.2
minutes; peak width is 0.75 min. The retention time for substance Y is 20.0 minutes;
peak width is 0.95 min. The volume flow rate of the mobile phase is 25 mL/min. (25)
a. How long does Y spend in the mobile phase?
b. How long does Y spend in the stationary phase?
c. What are the values of k’ (capacity factor) for X and Y?
d. What is the resolution between X and Y?
e. What is the retention volume, VR, for Y?
f. What is VM?
5. A 30-m long open tubular capillary column is used for GC having an optimum
velocity of 12 cm/s and exhibits 1.6 x 105 plates at this optimum velocity. Determine
the B and C parameters (from the van Deemter equation) for this column, assuming
that the A term is negligible. (Be sure to give units!) (10)
6. The following data apply to a column for liquid chromatography: (15)
L (column length)
Flow rate
VM
VS
25.0 cm
0.50 mL/min
1.30 mL
0.164 mL
A chromatogram of a mixture of species A, B and C provided the following data:
Species
Nonretained
A
B
C
tr (min)
3.1
5.4
13.3
14.1
w (min)
0.41
1.07
1.16
a. Calculate the relative retention of B and C.
b. Calculate the resolution of B and C.
c. Calculate the length of column required to separate B and C with a resolution of
1.5.