Question

The van Deemter equation describes plate height in terms of constants A, B, C, and the linear velocity, ux. Which statement is NOT true for the van Deemter equation?
A) A takes into account multiple pathways through the column. The value of A is column specific and independent of linear flow.

B) B takes into account longitudinal diffusion of the analyte in the mobile phase. The value of B is column specific and the impact of B on the plate height is inversely proportional to the linear velocity. x x + Cu u B H » A +

C) C takes into account equilibrium time between the stationary and mobile phase. The value of C is column specific and the impact of C on plate height is proportional to the linear velocity.

D) To minimize plate height, the optimal flow rate is the maxima for the plot of versus flow rate.

E) Linear velocity is the flow rate of the mobile phase.

Answer 1

Answer:

D) To minimize plate height, the optimal flow rate is the maxima for the plot of plate height versus flow rate.

Explanation:

Van Deemter equation in chromatography, relates the variance per unit length of a separation column to the linear mobile phase velocity by considering physical, kinetic, and thermodynamic properties of a separation.

The formula is:

H = A + B/u + C×u

Where:

H = HETP (plate height)

A = Eddy diffusion term .  

B = Longitudinal diffusion.

C = Resistance against mass transfer.

u = Linear velocity .

A) A takes into account multiple pathways through the column. The value of A is column specific and independent of linear flow.  TRUE. Eddy difussion term depends of column packing

B) B takes into account longitudinal diffusion of the analyte in the mobile phase. The value of B is column specific and the impact of B on the plate height is inversely proportional to the linear velocity. TRUE. Longitudinal difussion is another term that is specific to a column. Also, as the formula is H = A + B/u + C×u, the impact of B on the plate height is inversely proportional to the linear velocity.

C) C takes into account equilibrium time between the stationary and mobile phase. The value of C is column specific and the impact of C on plate height is proportional to the linear velocity.  TRUE. C is the time that system needs to equilibirum. Based on the formula, the impact of C on plate height is proportional to the linear velocity

D) To minimize plate height, the optimal flow rate is the maxima for the plot of plate height versus flow rate.  FALSE. The optimal flow is the minimum of the graph

E) Linear velocity is the flow rate of the mobile phase. TRUE. The Van Deemter equation describes u as the linear velocity of the mobile phase.

I hope it helps!