SCHA 312Problem Set #2100 Points

Due June 20 (Thursday) by the end of Class.

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 20.0 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)
  1. What size column would you use to separate 2500 mg of the mixture?
  1. If the flow rate is 0.5 mL/min on the small column, what volume flow rate should be used on the large column?
  1. Calculate the linear flow rate for the both small column and the large column.
  1. The following data were obtained for four compounds separated on a 20-m capillary column. (15)

Compound / tr (min) / Width (min)
A / 8.04 / 0.15
B / 8.26 / 0.15
C / 8.43 / 0.16
  1. Calculate the number of theoretical plates for each compound and the average number of theoretical plates for the column.
  1. Calculate the average height of a theoretical plate.
  1. Explain why it is possible for each compound to have a different number of theoretical plates.
  1. Consider the peaks for n-heptane and p-difluorobenzene in the chromatogram in Problem 43 in Chapter 23. The elution time for unretained solute is 1.06 min. (20)
  1. Measuring w1/2, find the number of theoretical plates (N) and plate height (H) for each peak.
  1. Calculate the observed (from the data above) resolution between the two peaks using the chromatograms.
  1. 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.
  1. What fraction of time does p-difluorobenzene spend in the stationary phase?

  1. 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)
  1. How long does Y spend in the mobile phase?
  1. How long does Y spend in the stationary phase?
  1. What are the values of k’ (capacity factor) for X and Y?
  1. What is the resolution between X and Y?
  1. What is the retention volume, VR, for Y?
  1. What is VM?
  1. 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 105plates 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)
  1. The following data apply to a column for liquid chromatography: (15)

L (column length) 25.0 cm

Flow rate 0.50 mL/min

VM1.30 mL

VS0.164 mL

A chromatogram of a mixture of species A, B and C provided the following data:

Species / tr (min) / w (min)
Nonretained / 3.1 / -
A / 5.4 / 0.41
B / 13.3 / 1.07
C / 14.1 / 1.16
  1. Calculate the relative retention of B and C.
  1. Calculate the resolution of B and C.
  1. Calculate the length of column required to separate B and C with a resolution of 1.5.