Chromatography Forum

Hello all,

The van deemter equation desribes how flow rates can affect efficiency. How can you obtain optimum flow rates in HPLC using this equation?

In my experiment I have changed my mobile phase from 11% ACN to 15% ACN and I noticed the components eluted quicker because the solvent strength has increased. This resulted in reduced retention time and reduced resolution.

I need explain why this happens in my assignment and I have no clue!

Thank you all.

Hello all,

The van deemter equation desribes how flow rates can affect efficiency. How can you obtain optimum flow rates in HPLC using this equation?

In my experiment I have changed my mobile phase from 11% ACN to 15% ACN and I noticed the components eluted quicker because the solvent strength has increased. This resulted in reduced retention time and reduced resolution.

I need explain why this happens in my assignment and I have no clue!

Thank you all.

unclear what you want to proof?

You now that van Deemter is about flow rate vs. plate height
but then you vary the solvent composition which is something completely different...?!

for a vD plot you need to acquire chromatograms with the same mobile phase but at different flow rates and then calculate the according plate numbers of a peak (same peak in all runs).
then calculate the height of a single plate (by dividing the column lenght by the plate number) and plot this versus the flow rate on the x-axis (effectively the linear flow rate should be used but if you just want to know the optimum of a particular column, this could be neglected)

Ok, i think i understand ur question...
bare this in mind... each time u changed the condition (instrument set-up).. the Van Demter plot will be differ... (Van Demter plot will be used to determine the most optimal flow rate)
So, in order to obtain the optimum flow rate (for one condition), u've to run several flow rate test... 20 to 50 run with different flow rate.. each of flow rate will give different peak area.. by using peak area, u can obtain the H value...
refer back -----> H= A + B/u + Cu
this value will be plot in the graph of H against flow rate..
From this graph, the optimum flow rate will be the lowest point of Van Demter plot...

Yes, when u changed the mobile phase composition, u've change the condition of instrument set-up... that's why it give u the different retention time and resolution...

take note that in Van Demter equation, mobile phase composition is refer to diffusion coefficients in B-term and C-term..

i juz want to know why optimizing flowrate is one of the important steps in chromatographic analysis based van deemter plot.

i juz want to know why optimizing flowrate is one of the important steps in chromatographic analysis based van deemter plot.

Try refer back to VanDeemter formula..
H = A + B/u + Cu

The value H there will be used in H = L/N
That formula shows that value of H is inversely proportional to N (number of theoretical plates)

Then, this N will be used in resolution formula (check it back) where the resolution Rs value is
proportional to the N value..

When this Rs is proportional to N value, it mean Rs also inversely proportional to the H..
Increasing the H value will decrease the resolution, Rs and will give bad separation.

Based on Van Deemter, B and C term is affected by velocity greatly where when the velocity is high,
B term will reduce while C term will increase.. When, the velocity is low, the C term will decrease while
the B term will increase.. take note the the decrease of B and C term will affect the value of H..

So, that's why we have Van deemter plot, in order to obtain the fastest speed for B term and the slowest
speed for C term and at the same time to obtain the lowest H value for the analysis.. lower H give high resolution value, Rs (better separation)