Page 42 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology en001d42 April 28, 2001 15:9

766 Atomic Spectrometry

level (10 −9 g) can be determined in samples of a few is initially volatilized and ultimately decomposed into its
micrograms (10 −6 g) by electrothermal vaporization constituent atoms (sodium and chlorine) by the heat of
(ETV) atomic absorption. Solution concentrations at the the flame. The thermal energy from the flame excites the
nanogram (10 −9 g) per milliliter level can be routinely gas-phase sodium atoms from the ground state (0) to an
measured by most of these techniques. Conversely, us- excited state (1). The efficiency of this excitation depends
ing plasma mass spectrometry, absolute mass detection on the gas-phase temperature (the wavy line in Fig. 1A
limits of a few femtograms (10 −15 g by ETV–MS) or represents the thermal excitation processes). The sodium
measuring solution concentrations of parts per quadrillion atom in the excited state then spontaneously decays back
(10 −15 g/ml) are not uncommon. All the techniques use the down to the ground state, either by collisional transfer of
phenomenon of electronic excitation. This involves transi- energy to other species or by atomic emission (loss of en-
tions among the outer electrons in the atomic orbitals, with ergy by emission of a photon). This is illustrated by the
accompanying ionization, emission, or absorption of ra- straight line from the excited state down to the ground
diation. In photon-based detection, the intensity of these state in Fig. 1A. The wavelength of this emitted photon
emissions and absorptions can be related directly to the depends on the energy difference between the excited state
concentration of an element in a sample. For ion-based and the ground state.
detection, the obtained electric current is directly propor- The amount of light that is emitted from the analyte
tional to the analyte concentration. is proportional to the number of atoms in the flame or
plasma. Hence, atomic emission spectrometry can be both
a qualitative analytical technique, in which the identity of a
I. ATOMIC SPECTROMETRIC metal is revealed by the observed color (wavelength), and
TECHNIQUES a quantitative analytical technique, in which the intensity
of the light emitted from the analyte is a function of the
There are three traditional analytical atomic spectrometric number of atoms.
techniques: atomic emission spectrometry (AES), atomic TheexperimentalarrangementinvolvedinanAESmea-
absorption spectrometry (AAS), and atomic fluorescence surement is shown in Fig. 1E. The hot analyte environ-
spectrometry (AFS). In all three cases, the analytical sig- ment, which is able to break down and excite atoms, is
nal is generated by photons. More recently, however, the called an atom cell. The atom cell can be a flame, plasma,
detection of resulting ions from an electronic excitation a heated graphite tube, or any other environment where
has led to superbly sensitive detection of elements. It is the analyte is observed in a spatially confined arrange-
possible to distinguish among these techniques by con- ment. In Fig. 1, the detector box is used to represent a
sidering the excitation mechanism and the way the an- detection system, which is able to identify the wavelength
alytical signal is detected. The general schematic rep- and measure the intensity of the emitted radiation. The ex-
resentation for various types of atomic spectrometry is perimental arrangement is the simplest of the three optical
shown in Fig. 1, where two-level electronic systems are atomic spectrometric techniques.
depicted. The ground state is labeled 0, and the excited
state is labeled 1. In general, the transition originat-
B. Atomic Absorption Spectrometry (AAS)
ing from or terminating to the ground state (or another
of the lowest lying excited states) is most commonly Figure 1B illustrates the principle of atomic absorption
used for atomic spectrometric measurements (Ingle and spectrometry. The excitation from the ground state to the
Crouch, 1981). upper state is by the absorption of light energy. An atom
cell (a flame or a heated graphite tube) is used to decom-
pose compounds, but the energy for excitation is drawn
A. Atomic Emission Spectrometry (AES)
primarily from a light source such as a white light source
The most basic measurements in analytical atomic spec- (which emits all wavelengths) or a hollow cathode lamp
trometry can be traced back to Thomas Melville, who in (which emits narrow spectral lines). In AAS, the detection
1752 reported his observations on spectra of mixtures of system looks at the light source directly (Fig. 1F) and sees
alcohol with sea salts (Laitinen and Ewing, 1981). The an intensity of the light source (I 0 ) before any atoms are
simplest example of atomic emission is the experiment of present within the atom cell. When atoms are introduced
putting table salt (sodium chloride) into a flame, which into the atom cell, they absorb some of the radiational
generates a yellow color. Although the example is a sim- energy (solid arrow in Fig. 1B) and are excited from the
ple illustration, the actual events that ultimately lead to ground state to the upper state. The detection system sees
the yellow plume are due to a complex series of chemical this absorption as a reduction in the intensity of the light
and physical processes outlined in Fig. 2. Sodium chloride source from I 0 to I (Fig. 1F). The ratio I /I 0 is called the