Environmental Engineering Reference
In-Depth Information
comparisons among various elemental analysis methods. Many of these methods are
complementary to each other with regard to sensitivity, linear range, cost, sample
throughput, and skills required for operation. This chapter will be concluded with
materials on some of the practical tips helpful to beginners in this field.
9.1 INTRODUCTION TO THE PRINCIPLES OF ATOMIC
SPECTROSCOPY
To begin our discussions on atomic spectroscopy, we first need to appreciate what
differs it from molecular spectroscopy described in Chapter 8. In molecular
spectroscopy, low-energy radiation (IR, visible, UV) causes a molecule to vibrate/
rotate or outer electron (valence electron) to transit from low- to high-energy state.
In atomic spectroscopy, radiation of a much higher energy is used. As atoms are the
simplest and purest form of matter and cannot rotate or vibrate as a molecule does,
the high-energy radiation causes inner electrons to transit within the atom. This
high-energy radiation is commonly provided by (a) flame in flame atomic absorption
spectroscopy (FAA); (b) electrothermal furnace in flameless graphite furnace atomic
absorption spectroscopy (GFAA); (c) plasma in inductively coupled plasma-optical
emission spectroscopy (ICP-OES); or (d) X ray in X-ray fluorescence spectroscopy
(XRF).
The discussion in this section details the principles of FAA, GFAA, ICP-OES,
and XRF. Like molecular spectroscopy, these atomic spectroscopic techniques belong
to one of the three major types of atomic spectroscopy described in Figure 8.4,
namely absorption, emission, and fluorescene. In atomic absorption spectroscopy
(FAA and GFAA), light at a wavelength characteristic of the element of interest
radiates through the atom vapor. The atoms of that element then absorb some of the
light. The amount of light absorbed is measured. In atomic emission spectrometry
(ICP-OES), the sample is subjected to temperatures high enough to cause excitation
and/or ionization of the sample atoms. These excited and ionized atoms are then
decayed to a lower energy state through emission. The intensity of the light emitted
at a wavelength specific to the element of interest is measured. In atomic fluo-
rescence spectrometry (XRF), a short wavelength is absorbed by the sample atoms,
whereas a longer wavelength (lower energy) radiation characteristic of the element
is emitted and measured.
9.1.1 Flame and Flameless Atomic Absorption
Fundamental to atomic spectroscopy, let us now look at how elevated temperature in
flame or furnace changes the chemistry of molecules and atoms and how
temperature affects the ratio of excited and unexcited (ground state) atoms. We
will also examine how a liquid sample is aspirated to become aerosols of fine
particles (nebulization) and then vapor atoms (atomization). Finally, we will discuss
Beer's law, introduced in Chapter 8, and how it is also applicable to the quantitative
measurement of elements in atomic spectroscopy.
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