Introduction

The mass spectrometer provides the most powerful detector available for gas chromatography. It is sensitive, selective and offers vastly superior qualitative information over conventional detectors such as the flame ionization detector (FID) or electron-capture detector (ECD). Modern instruments for gas chromatography-mass spectrometry (GC-MS) are small, reliable and much less expensive than formerly. In many laboratories small 'bench-top' GC-MS instruments have virtually replaced 'stand-alone' gas chromatographs for even routine applications. In this short paper we shall attempt to describe some of the basic principles of mass spectrometry and how they are applied in GC-MS. In order to illustrate some of the techniques available the determination of nitrated polycyclic aromatic hydrocarbons in diesel partic-ulates will be utilized as a case study.

tion (EI) ion source. A schematic of a typical EI ion source is shown in Figure 1.

The filament, usually a simple coil of tungsten wire, is heated. On heating it produces electrons, which are then accelerated into the ion source chamber by applying a potential difference between the filament and the chamber. This potential difference is usually in the range 50-70 V, giving the electrons a kinetic energy (the electron energy) of 50-70 eV (where 1 eV = 1.602 x 10~19 J). Interaction of neutral sample molecules with the electrons causes ionization by removal of one electron:

This process creates the positively charged molecular ion of the sample molecule, i.e. a radical cation. However, the first ionization energy of most organic compounds is only of the order of 10 eV. Hence molecular ions formed in an EI ion source have excess internal energy and further fragmentation occurs in order to dissipate this energy. Fragmentation occurs via a variety of processes and leads to mass spectra containing a fingerprint of the molecule. The processes of fragmentation are shown below:

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Post a comment