1. Created with MindGenius Business 2005® Isotopes (1) Isotopes (1) It is important to use the correct method to calculate molecular weight for MS. Average atomic masses take into account the different abundances of isotopes. Need to use exact mass for MS. CARBON 1.1% of naturally occurring carbon is 13 C: gives 1% abundance m/z = 17 signal (M+H) + in spectrum of Methane. HALOGENS Significant amounts of different isotopes, visible patterns within spectrum.

2. Created with MindGenius Business 2005® Isotopes (2) Isotopes (2) Chlorine: Average Atomic weight = 35.453 Isotope 35 Cl 37 Cl Atomic weight 3537 Abundance75.824.2 Ratio ~31 Relative abundance M M+2

3. Created with MindGenius Business 2005® Isotopes (3) Isotopes (3) Bromine: Average atomic weight = 79.904 Isotope 79 Br 81 Br Atomic weight 7981 Abundance50.649.4 Ratio ~11 Relative abundance MM+2 m/z Isotopic pattern for one bromine atom Relative abundance M M+2 M+4 eg CH 3 Br Mass: 50% CH 3 79 Br = 94 50% CH 3 81 Br = 96 CH 2 Br 2 Mass: 25 % CH 2 79 Br 79 Br = 172 25 % CH 2 79 Br 81 Br = 174 25 % CH 2 81 Br 79 Br = 174 25 % CH 2 81 Br 81 Br = 176 Isotopic pattern for one bromine atom