A SHORT PRIMER ON MASS SPECTROSCOPY

1. A SHORT PRIMER ONMASS SPECTROSCOPY Tim Hubin and William Kelly Southwestern Oklahoma State University

2. WHAT IS MASS SPECTROSCOPY • In mass spectrometry, a substance is bombarded with an electron beam having sufficient energy to ionizeand fragment the molecule. The mass spectrum of a compound is a function of the behavior of the molecular ion thus formed when a high energy electron strikes a neutral molecule, knocking an electron from the neutral molecule, ionizing it. The molecule now has an odd number of electrons, it is a radical as well as a cation. • The electron bombardment transfers such a large amount of energy (~ 550 kJ/mole) to the molecule that it usually begins to break apart or fragment soon after the initial ionization. Some of the fragments retain the positive charge, some are neutral. Thus within a short period of the initial ionization process there is not one ion but many different ion structures. • Shown below is the output of the mass spectrometer, or mass spectrum, for ethyl acetate which plots the intensity (or relative number) of the cationic fragments of ethyl acetate versus the mass-to-charge ratio, m/e (or molecular weight) of the same fragments. Mass spectra are bothfingerprints of compounds, and, if interpreted in terms of the stability and ease of formation of forming different cations, sets of clues as to the structure of the compound.

3. THE MASS SPECTROMETER

4. Electron beam knocks electron out of neutral molecule forms Molecular Ion During time of flight a fractionof Molecular Ions break into smaller fragment ions Molecular Ion accelerates toward cathode Neutral molecules Enter ionization chamber

5. MASS ANALYZER A heavier particle will have a greater radius A lighter particle will have a smaller radius When H, r and V are known m can be easily determined!

6. MASS SPECTRUM FRAGMENTATION PATTERNS Lowest energy fragments are most favored