[confusedstud]

In a normal phase HPLC setup the sample components are generally polar in nature, stationary phase (SP) is polar as well while the mobile phase (MP) is non-polar. So we if had 3 sample components A=least polar, B=intermediate polarity and C=most polar. Sample A is least attracted to the SP compared to B and C so it will be eluted out first, followed by B then C.

Then if we were to run the same mixture with the same column and same operating conditions, but a more polar mobile phase how will the elution order change?

I thought that if the MP is more polar, the interaction between C-MP becomes stronger than B-MP or A-MP. Essentially interaction of analyte and mobile phase is as follows: C-MP>B-MP>A-MP. So using this trend C should be eluted out first instead. However, the interaction of the component and the stationary phase is as follows: C-SP>B-SP>A-SP and using this trend alone, A should eluted out first.

So generally the MP will be less polar than the stationary phase no matter what. So the C-SP>C-MP and B-SP>B-MP and A-SP>A-MP still. I feel like this should tell me the elution order but I can't seem to get it. Can anyone explain this to me?

[Babcock_Hall]

As a rough first-approximation, I like to think of solvent versus analyte competing for binding to the stationary phase.

[confusedstud]

As a rough first-approximation, I like to think of solvent versus analyte competing for binding to the stationary phase.

Hmm how would that tell us which would be eluted out first? I always determine the elution order by comparing which component has the strongest interaction with the stationary phase and mobile phase. But now for a normal phase HPLC when I increased the polarity of the mobile phase i can't seem to compare them very well.

Can you explain their elution order?

Thanks

[Babcock_Hall]

I would try ignoring their interaction between the analyses and the mobile phase altogether, and then I would make a prediction accordingly and check against the data.  See if that works.

[confusedstud]

I would try ignoring their interaction between the analyses and the mobile phase altogether, and then I would make a prediction accordingly and check against the data.  See if that works.

Hmm why should we ignore the interaction between the analytes and the mobile phase?

Thanks again

[Babcock_Hall]

I have observed many beginning students making the same mistake you did, namely assuming that the order of R_f values of the analytes reverses when one increases the polarity of the solvent (mobile phase).  Empirically, one usually observes that the order stays the same but the R_f values increase.  One makes models to explain the data, and models are simplifications of the real world.