First photo is the first part of the question and it is asking to find the reten

Question

First photo is the first part of the question and it is asking to find the retention factor (k') for compound 3. The second photo is the continuation of the first problem, however it is asking to explain why we would expect to either an increased or decreased theoretical plate number (N).
Please look at the both photos because they go together.
Assume you are using a wall coated open tubular GC column (50 meter long, 2 diameter, 90 um thick liquid stationary phase) to analyze pesticides in a sample. An isothermal method (90 °C) was used for this analysis. Helium is used as the mobile phase Assume you obtain a chromatogram of 4 peaks, with the first one being the dead time peak. Some information of the peaks is listed in the table below. Use this information to answer the following questions: Retention time Peak width (min) min Species 0.5 0Co Compound 1 5.1 0.38 Compound 2 5.3 0.52 Compound 3 220 32.5 1.06

Explanation / Answer

Part 1:

First we calculate the retention factor i.e. k for compound 3 as given in question.

The retention factor is the measure of retention of analyte on the chromatographic column.We got 4 peaks in GC

chromatogram.At retention time 0.5 min is the dead time peak i.e.we show it as tM.

The compound 3 is having tR 32.5 min.

We substitute the above values in following equation,

k=tR-tM/tM

k=32.5-0.5/0.5=64

The retention factor for compound 3 is 64.

Part 2:

We know the Van Deemter equation,

H=A+B/u+Cu

where H is the hieght of theoretical plate,

A is eddy diffusion curve,

B is longitudinal diffusion term

u is average linear velocity of mobile phase and

C is mass transfer term

Out of these the C term is refers to the mass transfer between statinary and mobile phase.

If film thickness increases the C term in Van Deemter is more and the plate height H is also increases.

We know the relation between H and N as shown follows

N=L/H

Therefore,as the thickness of liquid stationary phase increases the theoretical plate number decrases.

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