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The large amplification in a PM tube allows the use of narrow slit widths and low hollow-

cathode-lamp currents.

Problem If a particular photomultiplier tube contains eight dynodes and each dynode emits an

average of 4.4 electrons for each impinging electron, calculate the amplification factor of the PM
tube.

Solution:
5
d
8
Amplification = n = 4.4 = 1,40,482.23 = 1.4 x 10

Interferences in atomic absorption spectroscopy
Interferences that are encountered while performing analyses with AAS can be classified as

· Chemical

· Ionization

· Spectral or background interferences.
Chemical interferences are those that arise from chemical reactions that occur within the

cell. Because the chemical reaction removes atoms of the assayed element, the atomic population
within the cell and consequently the absorbance are decreased. Interfering chemical reactions

occur most often in flames, in which oxygen is available to form refractory oxides with the
assayed element.

In some cases chemical interferences can be eliminated by addition of a releasing agent

(spectroscopic buffer). A releasing agent is a chemical substance that prevents formation of a
chemical interference by chemically reacting with one or more of the components of the

interference to form one or more non refractory compounds. Because the analyte cannot react to
form a refractory compound, it can readily be atomized in the cell. As an example, during the

assay of calcium in the presence of phosphate, a lanthanum salt is normally added. Lanthanum
combines with phosphate and prevents formation of the refractory compound.

It is sometimes possible to compensate for chemical interference by using the graphical

standard-addition technique. When using the standard-addition technique, the analyst must
ensure that the absorbance varies linearly with the concentration of the standard additions and

that the interference affects the added standards to the same extent that it affects the analyte in

the original sample.
Ionization interference occurs when a significant proportion of atoms in the cell become

ionized. Ionization results in a decreased concentration of atoms in the cell. Because the spectral
lines of ions of an element generally do not occur at the same wavelength as the atomic lines for

the element, ionization leads to low readings of absorbance. Usually ionization is caused by a
cell temperature that is too high. The energy that is required to cause ionization is supplied by the

heat in the cell. Group IA and IIA elements in the periodic table have relatively low ionization

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