Carbonate and nitrite coelute on the ICS-2100 I operate. With 27 mM KOH they can be separated at low levels but at high levels they merge completely. I've been running it at 23 mM for years and that separates them well. But now my new boss told be me to run it at 30 mM so they would merge completely in both low and high concentration standards. That would work if there was no carbonate in the samples, but some samples have quite a bit of carbonate.