Question number 6 In the van Deemter equation, H = A + Bu + cu where u is the fl

Question

Question number 6

In the van Deemter equation, H = A + Bu + cu where u is the flow rate, A is the multiple path term, B is related to the longitudinal ion, and C is related to the equilibration time. Which of the following is true? The longitudinal diffusion can be minimized by slower linear flow. b) The parameters A, B, and C, depend on the stationary phase only c) The equilibration time C is related to the rates of mass transfer through the stationary phase and through the mobile phase. The multiple path term A is important for open tubular columns The multiple path term A 0 for packed columns. 7) Tailing, a distortion of the Gaussian band shape, occurs when: a) too much solute has been loaded to the column. b) the flow rate is inappropriate. c) the temperature is unstable. more strongly than other sites.

Explanation / Answer

A          The longitudinal diffusion can be minimized by slower linear flow

longitudinal diffusion, refers to the diffusion of individual analyte molecules in the mobile phase along the longitudinal direction of a column. Longitudinal diffusion contributes to peak broadening only at very low flow rates below the minimum (optimum) plate heigth.

B          The Parameter A, B, and C depend on the Stationary Phase only.

                        Resistance to mass transfer is dependent on the speed with which the partition equilibrium between mobile and stationary phase is obtained. Since the resistance to mass transfer in the mobile phase is not the same for all molecules of one type of analyte (it depends on the location in the column at a particular time and the distance they have to travel to partition between phases) this will also result in peak broadening of that analyte in the column.

C            The equilibrium time c is related to the rates of mass transfer through the stationary phase and through mobile phase.

Longitudinal diffusion is proportional to the molecular diffusion coefficient. It obeys Fick’s law and is therefore affected by viscosity, temperature and molecular size (weight). Increased temperature and diffusion coefficient increase the B term, while increased viscosity decreases it.

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