Consider a gas chromatogram containing peaks obtained from a mixture of n-heptan
ID: 1058798 • Letter: C
Question
Consider a gas chromatogram containing peaks obtained from a mixture of n-heptane and p-difluorobenzenc. The elution time for unretained solute is 1.06 min. The open tubular column is 30.0 m in length and 0.530 mm in diameter, with a 3.0 pm thick layer of stationary phase on the inner wall. Find the adjusted retention times and retention factors for both compounds. (t_r = 14.56 min for heptane, t_r = 14.77 min for p-difluorobenzene) Find the relative retention (alpha) Find the number of plates (N_1 and N_2) and the plate height (H) for these two compounds (w = 0.216 min for heptane, w = 0.196 min for p-difluorobenzene) Calculate the resolution between the two peaks. Given that the linear flow rate is 30.0 m/1.06 min, find the volume flow rate. (in mL/min)Explanation / Answer
Answer 9(a):
Adjusted retention time = Retention time for compound - retention time for unretained solute
Adjacent retention time for n-heptane = 14.56 -1.06 = 13.5 min.
Adjacent retention time for p-diflurobenzene (DFB) = 14.77 - 1.06 = 13.71 min.
Retention factor of n-heptane = 13.5 / 1.06 = 12.7 min.
Retention factor of DFB = 13.71 / 1.06 = 12.9 min.
Answer 9 (b):
Relative retention of two peks = retention facton of DFB / retention factor of n-heptane = 12.9 /12.7 = 1.02
Answer 9 (c):
Number of plates = 16 ( adjusted retention time / W )2
Number of plates for n-heptane = 16 * (12.7 / 0.216)2 = 5.53 * 104
Number of plates for DFB = 16 * (12.9 / 0.196)2 = 6.9 * 104.
Plate Height = Coloumn length (in mm)/ Number of theoretical plates.
Lenght of coloumn = 30 m = 30m * 1000mm/1m = 30000 mm
Plate height for n-heptane = 30000 / 55300 = 0.542 mm.
Plate height for DFB = 30000 / 69000 =0.435 mm
Answer 9 (d):
Resolution between two peaks = Difference between retention time of two solutes / 0.5 (W1 +W2)
= 0.21/0.206 = 1.02. /
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