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Targeted leaches

Why use targeted leaches?

In the weathering environment mobile ions will bond to particle surfaces or become included in the structure of secondary minerals. An example of surface bonding is where metal ions bind to organic matter in chelation complexes. Other locations that metals are often bound is the surface of clays, amorphous Fe oxides, or Mn oxides. In environments with low precipitation, secondary salt minerals may form which can include metal ions in their structure. An example is water soluble salts that form in deserts such as the copper mineral chalcanthite.

Lixiviações direcionadas

Water soluble salts

The most water-soluble minerals are chloride, sulphate, and carbonate species. Temperature and water vapour can transform these mineral species therefore, changes can occur daily and/or seasonally. Examples of water-soluble minerals are chalcanthite, carnallite, hanksite, nahcolite, sylvite, nitre, and halite. The digestion of only the water-soluble component of a sample can be helpful to identify elements that have been mobile during the salt formation.

Method - ME-MS03

ALS offers a deionised water leach to identify the trace element concentration of all water-soluble parts of the sample. This will include the water-soluble minerals and any ions liberated from the surface of grains and maintained in solution.

Secondary minerals

The concentration of trace elements associated with secondary minerals and carbonates can be determined by weak digestion which leaves most minerals undigested. This method can be useful to identify ions that have been mobile at surface conditions in a region.

Method - ME-MS04

Ammonium acetate leaches only part of a sample; elements bonded onto exchangeable sites, water-soluble compounds, and some carbonates (Palmer et al., 2002). This digestion is slightly stronger than a water leach. Ammonia and acetic acid are formed when the neutral ammonium acetate salt is dissolved in aqueous media forming a buffered system (Kunkul et al., 2013). The pH of the solution remains near neutral due to the buffer system formed by the weak acid (acetic acid) and weak base (ammonia) (Kunkul et al., 2013).

Manganese and Iron oxides

Hydrous manganese and iron oxides act as scavenging agents in soils and sediments for many heavy metal ions (Chao, 1972). This concentration of trace elements, including target and pathfinder elements, makes them of interest for geochemical exploration.

Methods - ME-MS05 & ME-MS06

Hydroxylamine hydrochloride of variable strength and temperature is offered for the digestion of manganese oxides (ME-MS05) and amorphous iron oxides (ME-MS06).

Organic complexes

Organic matter in soils and sediment is often the location for concentration of mobile metal ions as chelation complexes form at their surfaces. The quantification of metals associated with organic matter can be helpful to identify regions where more metals have been mobile indicating a source for the metals in the vicinity.

Method - ME-MS07

Sodium pyrophosphate digestion is used to liberate organically bound metals from a sample. The digestion will not break down sulphides, silicates, or significant amounts of amorphous iron oxides.

References & further reading

Chao, T.T., 1972. Selective Dissolution of manganese oxides from soils and sediments with acidified hydroxylamine hydrochloride. Soil Society of America Journal. Volume 36. Issue 5.

Kunkul, A., Gulezgin, A., and Demirkiran, N., 2013. Investigation of the use of ammonium acetate as an alternative lixiviant in the leaching of malachite ore. Chemical Industry and Chemical Engineering Quarterly, 19(1). pp. 25-35.

Palmer, C.A., Dennen, K.O., Kolker, A., Finkelman, R.B., and Bullock, J. H., 2002. Chemical analysis and modes of occurrence of selected trace elements in a coal sample from eastern Kentucky coal bed: White Creek Mine, Martin County, Kentucky. USGS Open-file Report 02- 311.

Frequently asked questions

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