Problematic separation

[Svante]

I'd agree with UN. I compared a few of those for polar compunds a while beck and had the most luck with Phenomenex Polar-RP (Phenyl column with embedded groups that tolerate 100% water).

As for the ZIC HILIC colum you mention I definitly think there are good possibilities to achieve good separation and retention on that column.

[HW Mueller]

Danko,
irrespective of Sadilek´s coming results: I didn´t specify attraction, nor repulsion, but any dipole has the opposite formal charge to make it a dipole. The overall result seems to be an attraction, see as an example: RS Drago, Physical Methods in inorganic Chemistry, Reinhold, 1965, page 66.
But this is a mute argument here as these interactions seem to be washed out (overwhelmed) by mobile phase interactions.

[danko]

Hans, forget the dipole-ion interaction. Think hydrogen bonding between protonated silanol (hydrogen donor) and the double bonded oxygen (acceptor). OK?

Best Regards

[Jackus]

So, here is the result:

http://img187.imagevenue.com/img.php?im ... _158lo.JPG

As you can see, there is no difference between buffered phase and phase without buffer. I did two injections of sample for each phase, chromatograms are identical. I tested 2 columns (originally used Supelco and Symmetry Shield C18) without significant change.

[danko]

Hi Sadilek,

Thanks for data. Shame it didn’t work, but I still think it was worth trying – for the sake of the knowledge.
Additional question: How did the chromatography look like with buffer at pH 3 – 4 and higher? Also, was the buffer concentration the same – both at low pH (i.e. 2.3) and at higher pH values?

Something else, not related, is intriguing me though: With the described column (250x4.6mm) and 1 mL/min flow rate, it shouldn’t be possible to see peaks eluting before 2.7 min (approx.) – dead volume. Nevertheless, there is a distinct peak eluting at 1.2 min, most distinct on the chromatogram bellow (just water/MeOH). Do you start collecting data a minute or so after injection?

Best Regards

And thanks for sharing with us.

[Jackus]

Hi danko,

... How did the chromatography look like with buffer at pH 3 – 4 and higher? Also, was the buffer concentration the same – both at low pH (i.e. 2.3) and at higher pH values?

If I can remember, RT of principal peak was the same, buffer concentration approx. 15-20mM. Now I ordered recommended Atlantis T3 and Primesep B2 columns for additional testing.

... With the described column (250x4.6mm) and 1 mL/min flow rate, it shouldn’t be possible to see peaks eluting before 2.7 min (approx.) – dead volume. ... Do you start collecting data a minute or so after injection?

No, data collection is started immediatelly after start. Maybe used analytical column is well packed. As I can say from my experience, dead volume of 250x4.6;5um columns is usually about 1.8 - 2 mins.

Regards

[Bryan Evans]

For neutral compounds - you can try using H2O:THF (or a combination
of MeOH:THF). Get rid of the buffer.

Another column to try for neutral compounds is Cadenza CL-C18.
It excels at shape selectivity.

Below are steroid applications. Changes in selectivity were obtained by changing the solvent type:
http://www.silvertonesciences.com/files/TI324E.pdf

[HW Mueller]

OK, this agrees with what we have seen: No effect. The hydrogen bonding is als swamped, apparently.

[danko]

The hydrogen bonding is als swamped, apparently

Hi Hans,

“Swampedâ€

[HW Mueller]

Danko, I didn´t say that there is no hydrogen bonding, I said that it is obliterated, if there is any, by mobile phase interactions. I am talking about equilibrium shifts toward the mobile phase. In analogy you can eliminate retention due to hydrophobic interaction by increasing the organic content of the mobile phase.... so "swamping" doesn´t eliminate hydrophobic interaction (or hydrogen bonding, etc.), it just shifts it around.

[danko]

Hans, OK

Best Regards

[Jackus]

Thank you very much for stimulative hints.

Regards