Anion exchange column regeneration

[Edde]

I use IC-Pak anion HC 4.6 x 150mm, isocratic with 70mM KH2PO4 10% ACN, pH 5.5, flow 1mL/min to analyze nitrite and nitrate in urine.
Urine was filtered, then passed through C18 SPE, the flow-through was diluted x2 or x10 for analysis.
However, I can't get proportional increase in urine spiked with increasing amount of standards. Sometimes, the response in blank urine is even higher than that of spiked same urine matrix.
I suspect the various ions in urine is affecting unpredictably the column ion exchange mechanism.
I regenerate the column with butanol with acetonitrile in water. There seems some improvement but the condition worsened after some injections.
I intend to install a guard column and regenerate the column more frequently but I don't know if this is the cause for non-linearity.

Please comment.

[Edde]

Or is UV 214nm appropriate for urine nitrate, nitrite?
I tried in serum and the method was validated in serum.

[Markus Laeubli, Metrohm]

Are you following a known application?
I do not have experience with that column. But I assume that the eluent is quite high in concentration.
UV should work as well.

[Edde]

Thanks for your comment.
Yes, I follow an published article but I increased the conc. of KH2PO4 after experiments. In this conc., the interference peak can be separated from nitrate peak.
Is that conc. of salt detrimental to the HPLC system?

[Markus Laeubli, Metrohm]

Its not the bare concentration. that should be OK.
But I doubt whether the eluent is not already too strong.
What is the composition of the standard eluent (from the test chromatogram) of that column?

[Edde]

The Waters IC-Pak anion column insert recommends the following eluent for efficiency testing:
2% borate gluconate (3.4% boric acid, 1.2% d-gluconic acid, 0.86% lithium hydroxide monohydrate), 2% n-butanol, 12% ACN in water.

I'm not sure if this eluent is used for conductivity detection rather than UV detection or for both.

Thanks.