7 —
An Introduction to Analytical Spectrometry
Spectrometric techniques from the largest and most important single group of techniques used in
analytical chemistry, and provide a wide range of quantitative and qualitative information. All
spectrometric techniques depend on the emission or absorption of electromagnetic radiation
characteristic of certain energy changes within an atomic or molecular system. The energy changes are
associated with a complex series of discrete or quantized energy levels in which atoms and molecules
are considered to exist. To understand how studies of such transitions between energy levels can yield
information requires some knowledge of the properties of electromagnetic radiation and of the nature of
atomic and molecular energy.
Electromagnetic Radiation
Electromagnetic radiation has its origins in atomic and molecular processes. Experiments demonstrating
reflection, refraction, diffraction and interference phenomena show that the radiation has wave-like
characteristics, while its emission and absorption are better explained in terms of a particulate or
quantum nature. Although its properties and behaviour can be expressed mathematically, the exact
nature of the radiation remains unknown.
The wave theory supposes that radiation emanating from a source consists of an electromagnetic field
varying periodically and in a direction perpendicular to the direction of propagation. The field may be
represented as electric and magnetic vectors oscillating in mutually perpendicular planes. If the periodic
variation is simple harmonic in nature, the radiation may also be described in terms of a sinusoidal
wave, Figure 7.1. The wave can be characterized by its frequency ν, its length λ, and its amplitude A.
Frequency is defined as the number of complete cycles or oscillations per second, wavelength as the
linear distance between any two equivalent points in successive cycles and amplitude as the maximum
value reached by the vectors in a cycle. The amplitude and frequency of the wave are determined by the
intensity of the radiation and the energy associated with it respectively.