For a long time gold prospectors and metallurgists have been trying to get accurate determinations of trace levels of gold in samples obtained in field or from any stream of the recovery process. Then, the need for reliable analysis of gold in ores, and solutions is important due to the value of the gold and its impact on the prospective gold project and metallurgical process efficiency. Basically, in the analytical route of ores and solutions, a rapid routine quality control method is the key factor. For example the distribution of gold in many geological samples is not homogeneous. In this way, it is necessary to be sure that a representative sample was selected for gold assay. Sample preparation of geological samples usually comprises drying, crushing, splitting, and pulverizing, and some problems such contamination or loss of samples is possible to detect in labs without an appropriate procedure.
The common procedure to determine gold has been fire assay and the theoretical understanding of the process, temperature and process control are important to get reliable results. The process uses lead as the gold collector and it is removed by cupellation where the lead is oxidized and removed leaving behind a bead or prill of gold, which is dissolved in nitric acid to separate silver and gold. In some labs the common analytical the final stage in the gold determination usually comprises an analytical procedure such atomic absorption spectrometry, inductively coupled plasma atomic emission spectrometry (ICP), chromatography, X-Rays or neutron activation.
By atomic adsorption is possible to detect low levels of gold in solutions and is relatively cheap. In this method, solutions are aspirated into an atomization source and the radiation absorbed at the gold resonance wavelength is proportional to the gold concentration in solution. This procedure may be supported by solvent extraction. Basically, the use of solvent extraction makes possible to remove gold from its interferents and improve the sensitivity of the analytical procedure. Typical solvents are methyl isobutyl ketone (MIBK) and di- isobutyl ketone (DIBK). The use of solvent extraction with atomic absorption enhances the sensitivity of gold assay and the typical limit for gold is 0.3 ng/g.
The ICP technique employs argon gas and is a source of ions that may be divided and measured by a mass spectrometer. This analytical procedure has few interference, is fast and give information on other elements. Other advantage is related to the fact that solutions have a low dissolved solid concentration and some dilution types must be diluted. The detection limit for gold is 0.07 µg/L. The chromatography is useful for the separation of metal cyano complexes and may be used to determine gold in solutions from leaching. It has been recognized that the main advantage is the capacity to detect one specific gold specimen and the detection limit for gold is 9 µg/L. X-rays generates electronically excited ions and some excited species emit characteristic radiation when they return to the ground state. Neutron activation comprises the irradiation neutron particles from a nuclear source and the procedure is limited by the capacity of an irradiation facility, analysis of several days and the separation of gold from other elements to avoid any interference. The detection limit for gold is 0.1 ng/g.