Dear isotopers

Perhaps this is a silly question but I am not able today to fine an easy answer for this theoretical problem. Any of us measuring d15N from organics in a GC-IRMS system knows that we must use a liquid nitrogen trap to remove de CO2. But if you measure d13C in the same organic you just forget about the N2 produced after the reduction reactor that is arriving to the ion source together with the CO2. By other hand measuring gases mixtures, let’s say Breath analysis, after a Plot column, give you a little but clear peak some seconds before the CO2 that I believe is N2 or perhaps a nitrogen oxide produced in the ion source and producing an isobaric interference. In any case it is something affecting the ions beams of masses 44, 45 and 46. So, my question is why this interference does not produce any effect when measuring organic carbon, for example in a sample as Caffeine with high amount of nitrogen. What I am missing in my picture? It is just a question of C/N ratios? Shoud I re-read some basic papers?

Thanks