Laser

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Chapter

2

Laser Desorption Ionization (LDI)
The Ionization Process
A molecule naturally possesses rotational, vibrational, and electronic energy. If it is a liquid or a gas, it will also have kinetic energy. Under many everyday circumstances, if a molecule or group of molecules in a solid have their internal energy increased (e.g., by heat or radiation) over a relatively long period oftime (which may be only a few microseconds), the molecules can equilibrate the energy individually and together so that the excess energy is dissipated to the surroundings without causing any change in molecular structure. Beyond a certain threshold of too much energy in too short a time, the energy cannot be dissipated fast enough, so the substance melts and then vaporizes as internal energy ofvibration and rotation is turned into translational energy (kinetic energy or energy of motion). Simultaneous electronic excitation may be sufficient such that electrons are ejected from molecules to give ions. Thus, putting much energy into a molecular system in a very short space of time can cause melting, vaporization, possible destruction of material and, importantly for mass spectrometry,ionization (Figure 2.1). A laser is a device that can deliver a large density of energy into a small space. The actual energy released by a laser is normally relatively small, but, because the laser beam is focused into a very tiny area, the energy delivered per unit area is very large. An analogy can be drawn by considering sunlight, which will not normally cause an object to heat so that it burns.However, if the sunlight is focused into a small area by means of a lens, it becomes easy to set an object on fire or to vaporize it. Thus, a low total output of light radiation concentrated into a tiny area actually gives a high density or flux of radiation (we could even say a high light pressure). This situation is typical of a laser. As an example, a Nd-YAG laser operating at 266 nm can deliver apower output of about 10 W, somewhat like a sidelight on a motor car. However, this energy is delivered into an area of about 10–7 cm2, so that the power focused onto the small irradiated area is about 10/10–7 = 108 W/cm2 = 105 kW/cm2 — the same effect as focusing the heat energy from 100,000 1-bar electric fires onto the end of your finger! A molecular system exposed to a laser pulse (or beam) hasits internal energy greatly increased in a very short space of time, leading to melting (with increased rotational and vibrational electronic energy), vaporization (desorption; increased kinetic or translational energy), some ionization (electronic excitation energy leading to ejection of an electron), and possibly some decomposition (increase in total energy sufficient to cause bond breaking). Ifenough energy is deposited into a sample in a very short space of time, it has no time to dissipate the energy to its surroundings, and it is simply blasted away from the target area because of a large gain in kinetic energy. (The material

Copyright © 2003 by CRC Press LLC

8

Mass Spectrometry Basics
Laser pulse Neutral molecules and ions begin to desorb (a) (b)

Sample surfaceAbsorbed energy starting to be converted into kinetic energy of melted sample

Neutral molecules pumped away

Ions drawn into mass spectrometer analyzer

(c)

After a few nanoseconds, the absorbed energy has been dissipated

Figure 2.1 A laser pulse strikes the surface of a sample (a), depositing energy, which leads to melting and vaporization of neutral molecules and ions from a small,confined area (b). A few nanoseconds after the pulse, the vaporized material is either pumped away or, if it is ionic, it is drawn into the analyzer of a mass spectrometer (c).

Laser energy,
Increasing absorption

E'

(a)
Increasing absorption

Laser energy,

E'

(b)

Absorption peak

Absorption trough

Increasing wavelength

Increasing wavelength

(c) Laser beam Sample...
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