I am using a Hamilton PRP X100 column, in the test chromatogram is indicated a phase with
p-hydroxibenzoic acid at pH 8.5.
My question is why?
As I am performing a anion exchange , I just need hydroxyl groups.
Theoretically I could use the organic anions as counter ions in my analysis , or I canīt ?

If I understand your question, you want to know if you can substitute some other organic acid for p-hydroxbenzoic acid. This depends entirely on your application and your detection mode. This eluent species was selected in order to achieve two critical objectives: low background conductivity and high elution strength. If you are using conductivity detection without a suppressor, then you can't randomly substitute other organic acids. You must consider the equivalent conductance of the acids. Ideally, your eluent species should have low equivalent conductance in this case. You can substitute a wide variety of other aromatic acids without problems but you probably would have difficulty with a highly conductive anion such as oxalate. Likewise, you must also consider the elution power of the organic acid if you are using nonsuppressed conductivity detection. If you substitute a weakly eluting organic acid such as acetate, you'll find that you need an excessive concentration to have similar retention times.

If you are planning on using this column for LC-MS, then you may find it difficult to identify a suitable volatile anion with the necessary elution power. You might be better off trying a more hydrophilic stationary phase in this case.

The p-hydoxibenzoic acid eluent allows to separate the standard anions including posphate. If phosphate is not required a phthalic acid eluent at a pH around 5 may be used.

Details may be found under: http://www.metrohm.com/docs/pub/brochures/icworldgate.html
As well as in Metrohm IC Application Notes

Thanks Chris and Markus,

I am using a nonsuppressed conductivity detection for sulphate, nitrate, nitrite and bromate.
In this case can I just remove the
P-hydroxybenzoic acid of my phase and use just the basic eluent?

Changing to just hydroxide, like KOH, will raise the background conductivity of your eluant. The real reason for using eluants like pHBA or phthalate, is that the background conductivity is fairly low and it is easy to see the signal from the ions of interest. Not saying that in your case hydroxide wouldn't work, but why raise the background conducitivity unless necessary? You get better signal to noise typically using inorganic eluants in a non-surpressed mode.

Regards,
Mark

Thanks Mark,

If I change to just organic ions, Theoretically I will low down my background noise, but will my selectivity keep the same level? I mean, It is worth to try without ajust the pH?

Fabiano

Fabiano,

As I mentioned in my earlier posting an aromatic acid such as p-hydroxybenzoic acid is chosen for two reasons: minimal equivalent conductivity and high elution potency. If we take a couple of examples, I think you will be better able to understand my point: I couldn't find equivalent conductance data on hydroxybenzoic acid but for benzoate acid this value is 32 and for salicylate the value is 36 while for an aliphatic divalent acid such as succinate the value is 59 and oxalate the value is 74. So, even if hydroxybenzoic acid and succinate had the identical elution strengths, your conductivity background would be roughly double if you switched from an aromatic acid eluent system to an aliphatic acid eluent system. Furthermore, the aromatic acid will be at least 2-3 times stronger eluent on your column so to get equivalent retention time you'll need to use 2-3 times as much aliphatic organic acid eluent to achieve the same retention time. Accordingly, you will actually be operating with the background which is 5-6 times higher more than doubling baseline noise and substantially increasing temperature sensitivity problems.

We do not have experience on the PRP-X 100 column, but on a different column for non-suppressed IC we used an other eluent (3 mmol/L benzoic acid, 2% acetonitril pH 4,65 (TRIS)) for the determination of bromate. This eluent gives a low background. Bromate eluted just before chloride. You try such an eluent as well. The advantage of this eluent is the stability as corbonic acid from the air does not influence the eluent.