You can try something with RP, but I am not sure how well it will work. Most of the RP columns have some residual silanols. You need to inonize these silanols by going to higher pH, something like 6-8. You will be exploring ion-exclusion mechanism. Your flow rate needs to be 25% of you usual rate, but you are expanding this region 4 times because of the lower flow rate (6.5 min for 150 mm column, 10 min for 250 mm column). Nitrate/nitrite will elute BEFORE void, but might be separated. I don’t know how much organic will give you better results, try to explore 20-70%. Use 10-20 mM buffer.
Alternatively you can use one of our methods:
https://helixchrom.com/compounds/nitrate-ion/Contact me through our website “Contact Us” if you have any question or would like to discuss other options.