Atomic Spectroscopy for Metal Analysis - Fundamentals of Environmental Sampling and Analysis - page 225

was discussed in Chapter 8. In atomic absorption spectroscopy (FAA and
GFAA), the absorption depends on the number of ground state atoms N in the
optical path. Measurements are made by comparing the unknown to standard
solutions.

A ¼ k l C ð9 : 4Þ

where A is the absorbance, C is the concentration of the element, l is path length of
the flame, and k is a coefficient unique to each element at a given wavelength.
For flame emission spectroscopy, the emitted light intensity I of a population of
n excited atoms depends on the number of atoms dn that return to the ground state
during an interval time dt (dn/dt ¼kn). As n is proportional to the concentration of
the element, the emitted light intensity I, which varies as dn/dt, is also proportional
to the concentration:

I ¼ k l C ð9:5Þ

9.1.2 Inductively Coupled Plasma Atomic Emission

Flame emission atomic spectrometry and plasma atomic emission are the two
atomic emission techniques. However, only the latter is critically important and is
becoming a predominant tool in environmental metal analysis. The principles of
flame emission have been discussed in the previous section, because the same
processes occur with the flame atomic absorption. The plasma atomic emission
has similar processes as shown in Figure 9.1, but the principles are distinctly
different.
Inductively coupled plasma (ICP), or plasma in short, is an ionized gas at an
extremely high temperature. It is the fourth state of matter besides gas, liquid, and
solid. At a typical plasma temperature of approximately 9000 K (recall that flame
temperature is only 2000-3000 K), the commonly used argon gas (Ar) is ionized
according to the following reaction:

Ar ! Ar þ þe ð9 : 6Þ

The mean energy of argon ions (Ar þ ) thus produced is 15.76 electron volts (eV).
This energy in the plasma is transferred by collision of Ar þ with the atoms of interest
from a sample. As shown in Table 9.1, such energy is enough high to ionize many
metals with a typical ionization energy of approximately 7-8 eV. The alkali metals
are particularly vulnerable to ionization because of the low energy of ionization
(4 eV). As a result, the alkali metals emit the lowest energy or the longest wave-
length visible light. Most metals are ionizable little more readily than metalloids
followed by nonmetals. Most metals will emit in the UV range and therefore can
be measured using a UV spectrometer. For nonmetals, since their typical ionization
energies of about 12 eV are just below the ionization of Ar, nonmetals will not
readily form ions in the ICP. Because of the difficulty in ionizing nonmetals and
the high energy needed, vacuum UV (150-160 nm) is required for nonmetals such