Atomic Spectroscopy for Metal Analysis - Fundamentals of Environmental Sampling and Analysis - page 227

9.1.3 Atomic X-ray Fluorescence

The atomic X-ray fluorescence for elemental analysis is a two-step process. The first
step is excitation of electrons at the lower shell such as K electron (i.e., n ¼ 1, the
electron that is located in the inner orbitals rather than outer shells). The energy
needed for such excitation comes from X ray or other alternative sources such as
radioactive decay. Because the inner K shell electrons are stripped off, it creates a
vacancy in this shell. The second step involves the ''jumps in'' of the electrons from
L shell (n ¼ 2) or M shell (n ¼ 3) to fill the vacancy so that the ionized atom will be
stabilized. In this second process, it emits a characteristic X ray unique to this
element and in turn, produces a vacancy in the L or M shell.
Figure 9.2 illustrates the X-ray fluorescence process of a titanium atom. The
figure on the left shows how an electron in the K shell is ejected from the atom by an
external primary excitation X ray, creating a vacancy. The figure on the right shows
how an electron from the L or M shell ''jumps in'' to fill the vacancy. In the process,
it emits a characteristic X-ray fluorescence unique to this element.
The XRF instrument measures the photon energy from the X-ray fluorescence
to identify which element is present and the intensity of the photon to measure the
amount of the element in the sample.

(b) Emission of
X-ray fluorescence

N

(a) Excitation

N

e -

L L

K K

e - e -

Nucleus Nucleus

Incoming radiation
(X-ray/radioisotope)

Figure 9.2 An example of X-ray fluorescence from a titanium atom (atomic number is 22). The
distribution of electrons in a titanium atom is: 1s 2 2s 2 2p 6 3s 2 3d 2 4s 2

9.2 INSTRUMENTS FOR ATOMIC SPECTROSCOPY

9.2.1 Flame and Flameless Atomic Absorption

Although variations exist, the basic instrument components of an atomic absorp-
tion spectrometer can be depicted in Figure 9.3 for a single-beam system (In a
double-beam system, the light from the source lamp is divided into a sample beam,
which is focused through the sample cell, and a reference beam, which is directed
around the sample cell.) The instrument components for graphite furnace atomic
absorption spectrometry are similar to the flame system except for the method of
atomization. Details of these components are described below. Practical users need