Page 59 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

P1: FYK/LSX Revised Pages P2: FWQ/FPW QC: FYD
Encyclopedia of Physical Science and Technology en001d42 April 28, 2001 15:9

Atomic Spectrometry 783

give a significant atomic absorption signal from the ana- V. ATOMIC FLUORESCENCE
lyte element; however, it does give a scatter signal from SPECTROMETRY
particles within the furnace and an absorption signal from
molecular species. The modulation chopper is mirrored Atomic fluorescence is an extremely sensitive technique
so that, as the chopper spins, it alternatively irradiates the for determination of elements in samples. We should reit-
furnace by using either the HCL or the deuterium lamp. erate that in atomic fluorescence an external light source
Hence, when the HCL is irradiating the furnace, a signal is used to excite the analyte atoms. An ideal light source
is obtained that is composed of the true analyte absorp- for AFS must be much more intense than a hollow cathode
tion signal plus the spurious background signal. When lamp to achieve improvements in sensitivity. As a result,
the deuterium lamp is irradiating the furnace, only the pulsed hollow cathode lamps and lasers are frequently
spurious background signal is obtained. The subtraction used in AFS measurements. Excitation with a light source
of these two signals gives the actual analyte absorption such as a hollow cathode lamp, which only emits radiation
signal. The deuterium lamp works best for elements that specific for the element of interest, makes AFS virtually
have analytical wavelengths in the ultraviolet region of the completely free from spectral interferences. In addition,
spectrum. For background correction in the visible region AFS is like AES in that a multi-element analysis can be
a tungsten–halide lamp may be used. achieved by putting several light sources around the atom
Zeeman background correction: This method of back- cell, as discussed below.
ground correction makes use of the fact that a magnetic
field is able to split the atomic energy levels of an analyte.
A. Theoretical Background
A magnet is placed around the furnace, with the lines of
force of the field perpendicular to the direction of propa- This discussion assumes that the spectral line width of the
gation of the light beam. In this configuration the atomic light source is narrow relative to the absorption profile of
energy levels of an analyte can be split from the normal the analyte atoms, as illustrated in Fig. 12A. The atoms
situation into three energy levels (σ 1 , σ 2 , and π). The π absorb light from the source, and some of the energy is
energy level absorbs only light of a particular polariza- re-emitted as fluorescence, while various collisional pro-
tion, which is a natural consequence of the splitting of the cesses in the atom cell deplete the remainder of energy.
energy levels. The light source is deliberately polarized The ratio of the amount of light absorbed to that emitted
so that no light can be absorbed by the π energy level. is called the quantum efficiency (Y) and is ideally equal to
Then two measurements are made. First, the magnetic one. In its simplest form, the equation for fluorescence ra-
field is switched off, and a measurement of signal plus diant power ( f ) resembles general expressions in atomic
background (A) is obtained. In this case, the light-source absorption (because for optically unsaturated systems the
polarization is irrelevant because atoms that are not in a fluorescence signal is a function of the initial source radi-
magnetic field absorb light of any polarization. Second, ant power, 0 ):
the magnetic field is switched on, causing the atoms to −kl
f = 0 (1 − e ) (7)
split into the various components. When the field is on,
no analyte absorption of light occurs, because the light In the above equation, k is the absorption coefficient and
source is not of the correct depolarization even though l is the optical path length for the atom cell. Therefore,
it clearly is still at the correct wavelength. The σ compo- in AFS the signal size is directly proportional to both the
nents are not involved because they are at the wrong wave- light-source intensity and the atom concentration. Calibra-
length to absorb energy from the light source. Therefore, tion curves for AFS with HCL excitation are linear with
the signal obtained (B) results only from light absorbed or a slope of 1 (on a logarithmic plot) at low concentrations
scattered by background species and particles. The simple and bend back towards the concentration axis with a lim-
subtraction (A–B) then gives the background-free analyte iting slope of –0.5 at high concentrations. The curvature
signal. This method has been proven to give reliable and at high concentration is related to self-absorption in the
accurate background correction for all elements that are atom cell.
normally analyzed by AAS. It is particularly easy to ap-
ply because only one light source is used which removes
1. Light Source
many alignment problems that causes difficulties in a two-
source system. The background measurement is made at The multi-element capability of AFS is realized by use of
exactly the same wavelength as the source and analyte several light sources. The HCLs used for AFS are special
wavelengths. In contrast, two-source correction measures high-intensity versions of the ones used for atomic absorp-
the background over the range of wavelengths that exit the tion and up to 12 of them can be arranged in a practical
monochromator. experimental arrangement. Lasers are also frequently used