HPLC analysis of acetic,fumaric,cytric , glyoxalic formaic acids

4 posts / 0 new
Last post
PRAVEEN PALIWAL
PRAVEEN PALIWAL's picture
HPLC analysis of acetic,fumaric,cytric , glyoxalic formaic acids

I am praveen I want to analysis of orgaic acids on HPLC. Presently we do not have such type of column which are comatable with aqueous phase , so we wan to try the analysis on ION-pair buffer. I wan to know that if we use C-18 column along with tetrabutylammonium hydrogen sulphate then we can got probable retention or not. the pH of mobile phase is near to 6-7
thanks and rergards
praveen

Dr. Analytical
Dr. Analytical's picture
Yes, you should be able to

Yes, you should be able to get some retention of these acids.  However, detection may be a problem if you have small concentrations.
Make sure the mobile phase is properly buffered; a 10 - 20 mM solution should be adequate.  Start with about 10 mM of the TBA solution.
If you need more retention, reduce the acetonitrile concentration first, then try increasing the TBA concentration.
These systems come to equilibrium slowly, so you will have to wait longer than with other columns.
Please write back to share your results.
 

PRAVEEN PALIWAL
PRAVEEN PALIWAL's picture
Thanks Dr. Analytical. Can

Thanks Dr. Analytical. Can you tell me more about ion pair chromatogarphy and its releven application's on HPLC. Some more litrature's as well as application also.
thanks
praveen

Dr. Analytical
Dr. Analytical's picture
Ion pairing chromatography

Ion pairing chromatography (IPC) is a special variation of reversed phase (RP) LC.  In RPLC, neutral compounds are retained because they are chemically similar to the neutral, non-polar stationary phase (e.g., C18).  However, ionized compounds (acids or bases in their dissociated forms) are not attracted to the stationary phase (the charge makes the molecule much more polar).  As a result, ionized compounds often show very little retention.
If you could change the pH, so that the compound is in its neutral form, then you would get more retention.  For example, for an ionized acid (A-), making the mobile phase more acidic will put the molecule in it's neutral form.
A-           +        H+    -->         HA
(less retention)     (more retention)
But sometimes this still does not produce enough retention, because the neutral molecule is still too polar.  Your small acids are good examples of this problem.
Another approach is to select a more hydrophobic molecule that has the opposite charge from your analyte.  Tetrabutyl ammonium salts (N(C4H9)4+) are one example.  When you add these molecules to the mobile phase, some of them will stick to the surface of the stationary phase, and create an ion-exchange type of surface (the surface will have a positive charge).  Your negatively charged acid is now attracted to the positively charged surface; when it "sticks" to the surface, you have retention.
If you add more TBA to the mobile phase, this puts more charge on the surface, and retention increases.  But this is still a reversed phase system, so if you increase the organic solvent component, retention will decrease.
A google search will help you find many excellent tutorials.  You should also consider purchasing a textbook.