TABLE OF CONTENTS The most fundamental property of a molecule or atom from the perspective of mass spectrometry experiments is their ability to lose or gain electrons and form ions. The energy required for a molecule or atom to gain an electron is known as its electron affinity (EA). This results in the formation of a radical-anion according to equation 2.2. The electron affinity of a molecule can be defined by equation 6.1. ? H f 0 is the heat of formation of an molecule or ion defined as the heat absorbed or released when one mole of the entity is formed at standard temperatures and pressures (298 K = 25 C, 1 atmosphere = 1.013 10 5 Pa). Values of ? H f 0 can be estimated by simple arithmetic using available thermodynamic data or by ab initio molecular orbital calculations.
Two types of experiments are employed to determine electron affinities. The first of these uses photon detachment methods. Using a crossed photon-molecular beam apparatus, a photon source such as a laser intersects at right angles with a beam of negative ions. The production of neutral molecules according to equation 6.2 is then studied.
The neutral molecules produced are detected using a particle multiplier. Ion cyclotron resonance (ICR) mass spectrometers are also used for these measurements where the photon beam runs parallel to the ion motion. Due to the long trapping times, the...
