Geoscience Reference
In-Depth Information
E
Appendix E
An overview of analytical methods
The analytical methods of geochemistry are many and varied, but they can be grouped by
family depending on what is to be analyzed. Setting aside the high-temperature and high-
pressure experiments that involve methods often borrowed from mineralogy and petrology,
these methods fall roughly into three groups:
•
concentrations;
•
isotopic ratios;
•
speciation of elements in solutions, mineral phases, or organic matter.
We will omit the last item here, as the variety of methods would involve substantial devel-
opments with a large physics content about spectroscopic methods beyond the scope of
this topic.
Measurement of concentrations
Two general principles are commonly used. The first one uses comparison with a reference
material by means of calibration curves and only requires off-the-shelf reagents, while
the second one, isotope dilution, requires artificially altered nuclide mixtures. In the first
case, the operator compares the response to physical stimulation (radiation, ionization)
by means of a suitable detector upon the passage of a solution containing the dissolved
sample and a set of reference solutions. The first step of most procedures is the dissolution
of the powdered sample in hydrofluoric acid (HF), the only acid to dissolve silicates. Often
this attack phase is replaced by melting of the sample powder in a lithium meta-borate
“flux," the addition of which lowers the melting point of the sample-flux mixture for all
the minerals, even the most refractory ones (zircon, oxides). The resulting glass can be
dissolved in hydrochloric acid, which is far less dangerous than HF. The attack solution
is then diluted so as to minimize problems of interference between the elements and the
solution (matrix effects). Let us review four types of methods, each being associated with
a different detector:
1.
Atomic emission spectroscopy
(AES) operates by light excitation of rays specific to
an element. The method involves spraying the solution in a flame and is excellent for
measuring the alkaline elements. As for all optical methods, the different wavelengths
of the light beam are separated by an optical grating, a device which sorts the wave-
lengths by their interference patterns. Inductive plasma excitation, which we will
