91Ó°ÊÓ

The effect on resolution of increasing voltage from 28 to120kV.

In this work, you will be provided the figure which depicts the effect of increasing voltage on resolution.

(a)

Here's how we'll figure out what to expect ratio of migrating times in the two tests - increasing the voltage by 120kV / 28kV = 4.3 would raise the velocity by 4.3 resulting in a decrease in migration time, assuming peak 1 has the following:

$$\begin{gathered} migrationtim{e}_1=211.3\left(28\mathrm{kV}\right) \\ migrationtim{e}_2=54.36\min \left(120\mathrm{kV}\right) \end{gathered}$$

The ratio of the two is,

211.3min / 54.36min = 3.9

(b)

The predicted plate ratio $\left({\mathrm{N}}_{120}/{\mathrm{N}}_{28}\right)$will be determined next. Consider the following equation in the two experiments.

$N=\frac{{\mathrm{Ï\dots }}_{app}V{L}_d}{2D{L}_t}$

Since the number of plates (N) is proportional to the voltage (V) , increasing the voltage will increase the plate number by 4.3.

(c)

Calculate the predicted bandwidth ratio ${\mathrm{σ}}_{120}/{\mathrm{σ}}_{28}$ below.

Take into account that bandwidth is proportional to $1/\sqrt{N}$where

$N=L_d^2/{\mathrm{σ}}^2⇒\mathrm{σ}=L_d/\sqrt{N}$

With increase in the voltage by 4.3 increases the N by 4.3, reducing the bandwidth by

$1/\sqrt{4.3}=0.48$.

Also, bandwidth of 120 is percent more than bandwidth of 28.

(d)

Determine the physical reason why raising voltage will reduce bandwidth and increase resolution –

higher voltage = faster ions

faster ions = less time for diffusion

As a result, bandwidth would be reduced while resolution would be raised.