Introduction to Soil Chemistry

Wavelength
selector
Gas flow
control
atomic spectroscopy 153
Window to
monochromator
Flame
Hollow cathode
lamp
Figure 8.4. An atomic absorption spectrophotometer capable of functioning in either the EM or
AA mode.
this instrument, the composition of many samples can be determined very
quickly.
The analysis of a well-characterized sample by atomic spectroscopic analysis
is straightforward. The instrument is adjusted to isolate and direct the analytical
wavelength of light to the photomultiplier. It can be adjusted for
maximum sensitivity by introducing a known sample of the element of interest
into the flame and adjusting the instrument, including the flame, to maximize
the reading obtained. The sample is then introduced into the flame, and
the amount of light in the analytical wavelength emitted is measured and
recorded. A standard curve (see Section 8.8) is prepared and used to determine
the amount of element in the samples.
Even when soils are from the same area, significant variations in concentration
and content are found. This leads to the possibility of diagnostic wavelength
overlap. Even though each element has a unique spectrum when it is
taken alone, two elements may produce wavelengths of light that are the same
or very close together. This can lead to the overdetermination of the quantity
of an element because the total amount of light measured is partially from the
desired element and partially from another element.
Matrix interferences can be observed in a number of different forms. Components
in the extract may interfere with excitation of electrons. Absorbence
of light by unexpected, metals or organic compounds, either generally or
specifically, would be another example. Another would be complexation with