Aspartic Acid and Arginine in RP-HPLC- POSSIBLE?

[hplc.chem]

Dear colleagues:

Is there any way to convince Aspartic Acid and Arginine to spend few minutes in RP-HPLC column?
I tried C18, C8 and AQ columns but none of them worked!

I tried acetonitrile/water acidified with 0.1% phosphoric acid, ACN/0.1TFA, and Water 100% with no success!

In most of the cases the two compounds are not retained at all!

Any IDEAS?

Thanks in advance.

[bunnahabhain]

You will find a way, I am sure. But first you will have to convince a circle to transform itself into a square

But seriously: You will have to use some kind of derivatisation (see this method by Agilent, or another type of column. Primesep columns come to my mind at first.

Cheers,
Jörg

[hplc.chem]

first you will have to convince a circle to transform itself into a square

Is it that difficult!

Thanks for the suggestions.
I am trying to avoid the derivatisation and also trying to work with the available equipment and columns because I need the method for only few runs!

Can anyone add more help in the negotiations with the two acids?

[HPLCaddict]

My first choice would be HILIC, I'd suppose that should be quite straight forward.
If you MUST use a C18, I'd think you might have a chance by using a low-pH mobile phase (in order to protonate the carboxylic and amine groups, so that both analytes are in their cationic form) in combination with an anionic ion-pairing reagent such as SDS.

[hplc.chem]

If you MUST use a C18, I'd think you might have a chance by using a low-pH mobile phase

I used a C8 column with 0.1% TFA in both ACN and Water with a pH of 2 but I got no retention at all!

I hope I am not going to the wrong direction!

[HPLCaddict]

The catch ist to use an ion-pairing reagent. At low pH your analytes will be positively charged, therefore interaction with your hydrophobic stationary phase ist next to nothing. TFA actually has some ion-pairing properties, but not as strong as a "true" ion-pairing agent like SDS or some sort of long-chain alkyl sulfonate.
Mixed-mode columns like the Primesep mentioned by Jörg would be a much more convenient choice...

[Vlad Orlovsky]

your answer is RP/mixed-mode or HILIC/mixed-mode:
http://www.sielc.com/Compound-Aspartic-Acid.html

for RP/mixed-mode you retain compounds by weak RP and stronger cation-exchange
For HILIC/mixed-mode you repaint compounds by HILIC and cation-exchange

You will need a gradinet of ions for your two compounds as one contains one basic/two acidic groups and second one contains one acidic and two basic groups.