An analytical process that sequentially chemically leaches metals from soils, sludges or sediments. Several schemes have been developed over the years, the most widely known being Tessier, Campbell and Bisson's 5 step procedure, published in Analytical Chemistry vol. 51, 1979.

The various steps of the sequential extraction are designed to dissolve the fraction of metal in the sample associated with a particular matrix component, such as carbonates, organic matter or primary and secondary minerals. Conceptually, this should merely require finding the correct chemical to selectively dissolve metals from each fraction. In practice, however, the complexity of environmental matrices coupled with the range in behaviour of the different metals makes this impossible.

As is often the case in science, the results gained from a sequential extraction procedure are operationally defined, rather than "real" values, and can only be absolutely and accurately compared against other values gained using the same procedure. Thus, the development of a standard, world-wide procedure is critical to extending the usefulness of sequential extraction schemes. The BCR 3-step procedure seems likely to fill this role in the near future, at least until more sophisticated schemes are designed and tested. Even at the present stage, the various procedures can be of great value in predicting likely behaviours of metals within environmental systems, particularly in terms of potential bioavailability.

Tessier's Sequential Extraction Procedure:

1.0 M MgCl2 pH 7.0

2:Bound to carbonates
1.0 M CH3COONa/CH3COOH ph 5.0

3:Bound to Fe and Mn oxides
0.04 M NH2OH.HCl in 25% CH3OOH pH 2.0

4:Bound to Organic Matter
85 C 30% H2O2/HNO3 ph 2.0, followed by 3.2 M Ch3COONH4/20% (v/v) HNO3

Digestion with 5:1 HF:HClO4

Disclaimer: Naturally, I don't know all of this off the top of my head. The Tessier extraction scheme is from the aforementioned paper. The rest of it is me playing at rough drafts for my PhD thesis lit review. In the unlikely event that anyone wants references, /msg me, I'm happy to oblige, but I'm not dumping them all here.