Question

The following conditions were established for a chromatographic separation of multiple hydrocarbons on a packed column with 1.1 g of liquid stationary phase: inlet pressure 826 torr, room pressure 756 torr, vapor pressure of water at room temperature 17.5 torr; column temperature 110ºC, room temperature 20°C; measured outlet flow rate 20.0 mL/min; retention time for nonretained species 25.3 s, retention time of the first eluting species 1.56 min. Based on these conditions, the pressure drop correction factor j is _______, the average flow rateF in mL/min is __________, and the specific retention volume for the first eluting species V_g in mL/g is _________. Report all values to 3 significant figures.

Answer 1

1. The pressure drop correction factor, j caluclated as follows.

j = 1.5 [(Pi/Po)²-1] / [(Pi/Po)³-1]

Pi : inlet pressure : 826 torr

Po : outlet pressure : 756 torr

j = 1.5 [1.194-1] / [ 1.305-1] = 0.955

Pressure drop correction factor is 0.955

2. The average flow rate F

Flow rate at ambient temperature F_a = 20 mL/min

F = F_a / (1 - Pw/Pa)

Pw = Vapour pressure of water at room temperature : 17.5 torr

Pa = Room pressure : 756 torr

Pw/Pa = 17.5/756 = 0.023

F = 20 / 0.977 = 20.471 mL/min

3. The specific retention volume (Vg) is the net retention volume per gram of stationary liquid.

V_R' = V_R - V_M = (1.56 x 20.471) - (0.42 x 20.471) = 23.337 mL

V_N is the adjusted retention volume = j x V_R' = 22.287 mL

Column temperature : 110°C = 383 K

Room temperature : 20°C = 293 K

wL = Weight of liquid stationary phase = 1.1 g

Specific retention volume, Vg = (293 x V_N) / (wL x 383) = 15.500 mL/g