The Molecular Ion Peak

The molecular ion peak, M⁺, is produced by molecules that have lost one electron but have not broken apart:M(g) → M⁺(g) + e⁻Because the molecular ion has the same mass as the whole molecule, it is the peak with the highest m/z value in the spectrum, ignoring the very small M+1 peak one unit above it. Its m/z value gives the relative molecular mass, M_r, of the compound.The molecular ion peak is not always the tallest peak. Many molecular ions break into fragments, so the M⁺ peak can be small. Always look at the far right of the spectrum, not for the tallest line.Once you know M_r, you can test possible molecular formulae, or use it with an empirical formula to find the molecular formula.Worked example: an alkane has its molecular ion peak at m/z = 72. Find its molecular formula.
Alkanes are C_nH_2n+2, so M_r = 12n + (2n + 2) = 14n + 2.14n + 2 = 72, so n = 5The alkane is C₅H₁₂ (for example pentane).Second example: an empirical formula of C₂H₄O has mass 44. If M⁺ is at m/z = 88, the molecular formula is C₄H₈O₂.