Chromatography Forum

The -NH function is part of the phosphoamide. It is not an amine.

While I do not know and did not look up the pKa of such structures, I expect an acidity in between a sulfonamide and standard amide. As you know, amide functions are not ionizable over the normal pH range and are neutral (pKa >14). Sulfonamides have a weak acidity, typically with a pKa around 11. Based on this, I guess that the pKa of the phosphoamide is around 13, which means that from the standpoint of chromatography I do not consider this to be an ionizable function.

Vlad may have better info.

Thanks Uwe,

That clears my question. Your explanation is perfectly reasonable.

In this most weird of recent weird chains it is finally acknowledged that it is "reasonable" that amides are not amines. But, it is quite disconcerting to have some questions still unanswered, foremost those of JA.
Another one for Mohan_2008: Why do you suspect that there might be a useful wavelength with MeOH in the mobile phase?

I would not spend time trying to figure out what eluets at 1 minute. There is absolutely no indication that it is target compound. It can be anything.....
I think all questions were answered considering the facts how haizy the task and approach are

I think that weirdness of this thread comes from lack of undestanding of basic principles: what is ionized compound, presense or absense of UV active groups in compound, absorbance by common solvents (ACN, MeOH, water, buffre additives) and use of the wrong approach (column, mobile phase, detection)

Hi Mueller:

If we run a PDA scan of the absorption spectrum for the compound (assuming it has some chromophore) specifically while doing a HPLC run in the 20% Methanol: water mobile phase - maybe there is a wavelength over 220 nm or above.

I am not sure and cannot guarantee that there is a wavelength available over the 190 nm. But, with the PDA scan we can figure out if any.

If Vikmurush can take a scan of a pure standard (known) instead of the sample in the mentioned mobile phase - there might be a possibility.

Again, I am not arguing if there is a chromophore.

Typically we approach our method development problems through obtaining a PDA scan of the pure standard in the relevant mobile phase.

Thanks everybody for your input. This compound is one of the degradant/impurity from the amide group compound. This impurity supplied by the vendor and the vendor submitted paper work for identity of the structure. I had injected this compound on C18 25cm column using 25% acetonitrile in water by UV detector at 195nm and found out that it elutes at about 1 min. The main active amide group compound also had less chromophore and it has max abs at 195nm. Once again I am thankful to everybody for all of your input.