Introduction to Mass Spectrometry MS Experiment Mass spectrometry (MS) is an instrument that measures the mass of charged particles. In this instrument, the structure is hit with a high energy electron which causes the loss of an electron from the original structure. e-2 e molecule molecule Molecular Ion The mass spectrometer sorts the ions produced based on their mass-to-charge ratio and a peak shows up for any positively charged ion. • What would be the largest m/z peak for this ion? o . This first ionization is called the molecular ion. Does this match the molecular weight of the original compound? Molecular Weights Most organic compounds have even molecular weights. However, If a sample has an odd number of nitrogens the molecular ion must be an odd. . What is the MW for these compounds? NH2 OH NH, NH ON ON NOZ Isotopic Abundance in Mass Spectrometry* * Spectra in this unit are from SDBS (Japan National Institute of Advanced Industrial Science and Technology). Most elements exist as mixtures of different isotopes. Since a mass spectrometer separates and detects ions of slightly different masses, it easily distinguishes different isotopes of a given element. Isotopes don't impact the mass spectra of many organic compounds. • The abundance of C is %. • The abundance of 'H is %. • The abundance of 10 is %. However, compounds containing bromine and chlorine exhibit two molecular ions as they both have two very abundant isotopes. Chlorine: 75.77% 35Cl and 24.23% CSR Bromine: 50.50% 79Br and 49.50% Br The mass spectrum of C H Br is shown below. Explain the two peaks at 156 and 158 m/z. • Predict the MF (molecular formula) for this compound. 100- 30- 90 Relative Intensity 50 TS 125 150 175 100 m/z 2 Practice • The compound that gave this mass spectrum probably contains: a) nitrogen b) bromine c) chlorine d) sulfur LOO -- 30 Relative Intensity 20- 0 10 20 30 40 50 80 30 100 60 73 m/z 130 • The compound that gave this mass spectrum probably contains: a) nitrogen b) bromine c) chlorine d) sulfur alt fn ctrl 100 30 60- Relative tensity 40 20 O TO 50 20 30 40 100 70 30 80 210 m/s • How many molecular ions (compounds with different weights) would you expect to see in a molecule with 2 bromines? Explain.
