I missed the first week of college so I've fallen a bit behind. The main class I'm struggling with is mass spec since its relatively new to me. Whats really boggling my mind is this "nitrogen rule". Heres a quote from another site: I'm trying to get my head around this. So nitrogen has a valence of 3 and an even atomic mass. Testing this out with trimethylamine, I see it works because the 3 methyl groups will add up to 39, so adding that to nitrogens even atomic mass, you get an odd number. Does this really work in all cases? If I see a molecular ion with an odd molecular mass, is this solid evidence that the compound contains 1 or more nitrogen atoms? Does it apply to all organic compounds, or only compounds with common heteroatoms like halogens and chalcogens etc. I'm trying to get an understanding of why it works, but that seems mighty complicated.