Page 78 - Geochemistry of Oil Field Waters

66 ANALYSIS OF OILFIELD WATERS

results from lack of absorption of atoms bound in molecular combination in
the flame and can occur when the flame is not hot enough to dissociate the
molecule. It also occurs when a dissociated atom immediately oxidizes to a
compound that cannot dissociate further at the temperature of the flame.

Interferences

Ionization

When a significant number of the atoms of the element being determined
are ionized in the flame, an error in the analysis can result. This ionization is
because of excessive flame temperature, which, however, can be changed to
control this interference. Another type of interference can be caused by the
presence in the sample of other, more easily ionizable elements than the one
sought. The resulting increase can be controlled by the addition of a suffi-
cient amount of the interfering element to both sample and standards to
produce a “plateau” in the absorbance above which no further increase
occurs.

Che m ica 1

A chemical interference is caused by the formation, in the flame, of salts
of the element sought which are difficult to decompose, thus reducing the
amount of the element available for absorption. The formation of such
compounds may often be precluded by the addition of another element,
such as lanthanum, which forms a less-soluble salt with the interfering anion
than does the element desired. The interfering anion is thus removed from
the flame, and the interference is eliminated.
Phosphate combines with calcium and magnesium and produces an inter-
ference; however, the addition of lanthanum largely overcomes this inter-
ference. Addition of an excess of a cation having a similar or lower ioniza-
tion potential usually reduces interference problems.

Matrix
Matrix interference is caused by unequal amounts of dissolved solids in
the standards and samples. This can cause error because of differences in
aspiration rates through the atomizer. Often this can be controlled by
matching the specific gravities of the standards and samples or by adding
salts to the standards.

Burners and solvents
Various types of burners are used with atomic absorption spectrophoto-
meters. For example, a Boling burner usually is used for aqueous solutions,