Atomic Spectroscopy for Metal Analysis - Fundamentals of Environmental Sampling and Analysis - page 222

Processes Occurring in Flame and Flameless Furnace

The thermal chemistry of molecules and atoms under high-temperature flame or
flameless furnace is the heart of atomic absorption techniques as well as the flame
emission techniques. When a solution of acid digestate (see Section 7.2) is
introduced into a high-temperature flame or furnace (direct introduction of solid
sample is possible only through graphite furnace), molecules containing the
elemental atoms will eventually become gaseous atom through a series of reactions.
Figure 9.1 is an illustration of these reactions when calcium (Ca) is analyzed.

Ca 0*

34

1

2

Ca 0 (g) + 2Cl 0 (g) (g)

CaCl 2 (aq) CaCl 2

56

7

CaO *
CaOH *

CaO
CaOH

8

Ca + Ca +*

1 = Desolvation 5 = Oxide & hydroxide formation

2 = Dissociation 6 = Ionization of atom

3 = Excitation 7 = Excitation of ion

4 = Emission 8 = Excitation of oxide & hydroxide

Figure 9.1 Process occurring in flame for a solution containing CaCl 2 (Christian, 2003, Reproduced
with permission from John Wiley & Sons, Ltd.)

As can be seen from Figure 9.1, Ca is initially present in its salt form such as
CaCl 2 in the aqueous sample. After the removal of water (desolvation), a gaseous
CaCl 2 is produced, which is further dissociated into gaseous atom (Ca 0 ). At an
elevated temperature, Ca can have other electronic configurations, including Ca 0*
(excited vapor Ca atom), Ca þ (ionic Ca), and Ca þ* (ionic Ca with an excited electron).
If Ca is analyzed by atomic absorption spectroscopy, we measure the radiation
absorption of gaseous atom (Ca 0 ) based on the excitation reaction in Figure 9.1. Ca
can also be analyzed by flame emission spectroscopy. In this case, we measure the
emission of Ca 0* based on the emission process shown in Figure 9.1. If M is used to
denote the vapor form of other atoms (metal elements), the general reactions are:
Mþhn !M ðfor flame atomic absorption spectrometryÞ

ð9 : 1Þ

M !Mþhnðfor flame atomic emission spectrometryÞ

ð9 : 2Þ

Note that only the reactions leading to the formation of gaseous atoms (Ca 0 ) are the
desired reactions for atomic absorption spectroscopy. All other reactions (Reactions
4-8) are undesired and will result in interferences. For flame emission spectroscopy,
reactions leading to the formation of excited gaseous atom (Ca 0* ) and the subsequent
emission to the ground state Ca 0 are the desired reactions. It is important to note that
formation of metal oxide/hydroxide (Reaction 5) and the ionization of gaseous atom
(Reaction 6) are common sources of interference that must be minimized.
Flame and flameless graphite furnaces are two common radiation sources used
in atomic spectroscopy (see Section 9.2.1 for a schematic diagram). The flame used