[cowgalT]

Hey guys, im writing up an HPLC practical that I did a few weeks ago and theres a qestion that seems pretty basic but has completely thrown me;
Why does the pump pressure vary when the composition of the mobile phase is changed? This is using isocratic elution.
Any help would be great,
Thanks 

[Arkcon]

Some fluids are simply more viscous than others.  Others are more compressible than others.  Alcohols, like methanol and ethanol are surprisingly viscous -- you'll notice they pour from a beaker in a "funny" way compared to water, and tend to run down a vessel.  When sent through narrow passages in HPLC tubing, you will really notice a pressure difference.

[fledarmus]

Why does the pump pressure vary when the composition of the mobile phase is changed? This is using isocratic elution.

These two sentences don't seem to agree - isocratic means constant composition, so the composition of the mobile phase isn't changing...

[DrCMS]

Why does the pump pressure vary when the composition of the mobile phase is changed? This is using isocratic elution.

These two sentences don't seem to agree - isocratic means constant composition, so the composition of the mobile phase isn't changing...

It does if it means from mobile phase mix 1 to another analysis using mobile phase mix 2 on the same column using the same flow rate.

[fledarmus]

Why does the pump pressure vary when the composition of the mobile phase is changed? This is using isocratic elution.

These two sentences don't seem to agree - isocratic means constant composition, so the composition of the mobile phase isn't changing...

It does if it means from mobile phase mix 1 to another analysis using mobile phase mix 2 on the same column using the same flow rate.

Ah - thanks, that would explain it. Then I concur with Arkcon - different mixtures of solvents have different viscosities, and thus will require different pressures to achieve the same flow rate.

[cowgalT]

Thanks for your help, yes I did mean from one mobile phase mix to another analysis using a different mobile phase mix (same column, flow rate) Sorry if that was unclear!