The M+1 Peak

Look closely at a mass spectrum and you will usually see a very small peak one unit to the right of the molecular ion peak. This is the M+1 peak.It is caused by molecules that contain one atom of carbon-13, ¹³C, instead of carbon-12. About 1.1% of all carbon atoms are carbon-13, so a small fraction of molecules are one mass unit heavier than the rest. For example, most ethane molecules have M_r = 30, but a few contain one ¹³C atom and have a mass of 31.The more carbon atoms a molecule has, the greater the chance that one of them is ¹³C, so the M+1 peak becomes larger compared with the M⁺ peak.The M+1 peak matters when you read a spectrum: the molecular ion is the tall line just before the tiny M+1 line, not the tiny line itself. For a compound with M_r = 58, the M+1 peak is at m/z = 59.Extension, estimating the number of carbon atoms: dividing the percentage ratio of the M+1 and M⁺ peak heights by 1.1 gives the approximate number of carbon atoms.number of C atoms ≈ (height of M+1 ÷ height of M⁺) × 100 ÷ 1.1Worked example: M⁺ has relative abundance 40% and M+1 has 1.8%.
1.8 ÷ 40 × 100 = 4.5%, and 4.5 ÷ 1.1 = 4.1, so the molecule contains 4 carbon atoms.