Analytical Techniques (Spectroscopy & Chromatography)

Mass spectrometry

A mass spectrum plots abundance against mass/charge ($m/z$). The peak with the highest $m/z$ (ignoring small isotope peaks) is usually the molecular ion $\text{M}^{+}$, giving the $M_r$. Fragmentation produces smaller ions whose differences reveal groups (e.g. loss of 15 = $\text{CH}_3$; loss of 29 = $\text{CHO}$ or $\text{C}_2\text{H}_5$; 28 = CO).

The M+1 peak arises from the ~1.1 % $^{13}\text{C}$; its relative height indicates the number of carbon atoms. Compounds with Cl or Br show characteristic isotope patterns (e.g. a 3:1 ratio of M:M+2 for one Cl from $^{35}\text{Cl}/^{37}\text{Cl}$; a 1:1 ratio for one Br).