Appendix E - An overview of analytical methods

Summary

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 developments with a large physics content about spectroscopic methods beyond the scope of this book.

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.