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Fractionation as a first step towards speciation analysis


In case that it is not possible to determine the concentration of the different individual chemical species that sum up the total concentration of an element in a given matrix, that means it is impossible to determine the speciation, it is a useful practice to do fractionation instead.

Fractionation is the process of classification of an analyte or a group of analytes from a certain sample according to physical (e.g. size, solubility) or chemical (e.g. bonding, reactivity) properties.

In general, methods for fractionation are based on a separation procedure that does not have the separation power to differentiate between individual species but is able to differntiate between groups of species, such as:
  • dissolved species / particulate species
  • inorganic species / organic species
  • ionic species / uncharged species
  • free species / bound species
  • volatile / non-volatile species
  • hydrophilic / hydrophobic species

The separation procedure can be based on a whole variety of separation principles such as:
  • separation of species in the liquid phase according to size of molecule or particle (filtration, ultrafiltration, dialysis, size exclusion chromatography, field flow fractionation)
  • separation of species in the liquid phase according to their chemical interaction with a reagent or a stationary phase (liquid-liquid extraction, solid sorbent extraction, precipitation, chemical vapor generation)
  • separation of species in the solid phase according to the solubility in a special leachate (single extraction, sequential extraction, microwave-assisted extraction, ultrasonic-assisted extraction)

Since such methods are in general operationally defined and therefore lack standardization, comparabality of results is a major issue. In order to improve the situation, some of the operationally defined methods have been adopted by standardization bodies as "standard methods". For example the European Bureau Communitaire de Reference (BCR), now the Institute for Reference Materials and Measurements (IRMM) has defined a three-step sequential extraction procedure for the fractionation of trace elements from solid materials often called "BCR method".  

Very often the final goal of the fractionation is to differentiate the group of species in relation to biological effects. "Bioavailability" is the concept to differentiate between species leading to exposure from those not resulting to uptake by the biological system. In many respects leaching hehavior as reflected by the pH dependence leaching test and related characterisation leaching tests provides a better means of assessing environmental impact than analysis of total composition.

Unfortunately, the literature is spread over thousands of journals (see the EVISA Journal database) and authors do not always follow IUPAC definitions. Many authors talk about trace element speciation when they actually present methods and results for fractionation obtained by operations such as sequential leaching.

Some review articles related to sequential leaching procedures (newest first)

Amir Hass, Pinchas Fine, Sequential Selective Extraction Procedures for the Study of Heavy Metals in Soils, Sediments, and Waste Materials—a Critical Review, Crit. Rev. Environ. Sci. Technol., 40/5 (2010) 365-399. DOI: 10.1080/10643380802377992

Amanda Jo Zimmerman, David C. Weindorf, HeavyMetal and TraceMetal Analysis in Soil by Sequential Extraction: A Review of Procedures, Int. J. Anal. Chem, 2010 (2010), ID 387803. doi:10.1155/2010/387803

Janet G. Hering, Metal speciation and bioavailability: revisiting the ‘big questions’, Environ. Chem., 6 (2009) 290–293. doi:10.1071/EN09021

Jeffrey R. Bacon, Christine M. Davidson, Is there a future for sequential chemical extraction?, Analyst, 133 (2008) 25–46. DOI: 10.1039/b711896a

Patricia Smichowski, Griselda Polla, Darío Gómez, Metal fractionation of atmospheric aerosols via sequential chemical extraction: a review, Anal. Bioanal. Chem., 381 (2005) 302–316. DOI: 10.1007/s00216-004-2849-x

A. V. Filgueiras, I. Lavilla, C. Bendicho, Chemical sequential extraction for metal partitioning in environmental solid samples,  J. Environ. Monit., 4 (2002) 823-857. DOI: 10.1039/B207574C

Gemma Rauret, Extraction procedures for the determination of heavy metals in contaminated soil and sediment, Talanta, 46 (1998) 449–455. DOI: 10.1016/S0039-9140(97)00406-2
F.M. Tack, M.G. Verloo, Chemical speciation and fractionation in soil and sediment heavy metal analysis: a review. Inter. J. Environ. Anal. Chem., 59 (1995) 225-238. DOI: 10.1080/03067319508041330

Problems in Sequential Leaching Procedures

Francois Rapin, André Tessier, Peter G. C. Campbell, Richard Carlgnan, Potential Artifacts in the Determination of Metal Partitioning in Sediments by a Sequential Extraction Procedure, Environ. Sci. Technoi., 20/8 (1986) 836-840. DOI: 10.1021/es00150a014

J.M Martin, P. Nirel, A.J Thomas, Sequential extraction techniques: Promises and problems, Mar. Chem., 22/2–4 (1987) 313–341.   DOI: 10.1016/0304-4203(87)90017-X

Techniques for Fractionation analysis other than leaching procedures

Philip L. Verplanck, Partitioning of rare earth elements between dissolved and colloidal phases, Procedia Earth Planet. Sci., 7 ( 2013) 867–870. doi: 10.1016/j.proeps.2013.03.149

Liping Weng, Flora Alonso Vega, Willem H. Van Riemsdijk, Strategies in the application of the Donnan membrane technique, Environ. Chem., 8/5 (2011) 466-474. doi: 10.1071/EN11021

Yolanda Hedberg, Gunilla Herting, Inger Odnevall Wallinder, Risks of using membrane filtration for trace metal analysis and assessing the dissolved metal fraction of aqueous media e A study on zinc, copper and nickel, Environ. Pollut., 159 (2011) 1144-1150. doi:10.1016/j.envpol.2011.02.014

Jae Hee Hong, Susan E. Duncan, Sean F. O’Keefe, Andrea M. Dietrich, Ultrafiltration as a tool to study binding of copper to salivary proteins,  Food Chemistry 113 (2009) 180–184. doi: 10.1016/j.foodchem.2008.07.065

R.K. Singhal, J. Preetha, Rupali Karpe, K. Tirumalesh, S.C. Kumar, A.G. Hegde, The use of ultra filtration in trace metal speciation studies in sea water, Environ.  Int., 32/2 (2006)224-228.  doi:10.1016/j.envint.2005.08.015

E. Meers, G. Du Laing, V. G. Unamuno, E. Lesage, F.M.G. Tack, M.G. Verloo, Water extractability of trace metals from soils: some pitfalls, Water, Air, and Soil Pollution, 176 (2006) 21–35. DOI: 10.1007/s11270-005-9070-1

Yoshiyuki Tanizaki, Masao Yamazaki, Sumiko Nagatsuka, Physicochemical speciation of trace elements in river water by means of ultrafiltration, Bull. Chem. Soc. Japan, 58/10 (1985) 2995-3002. DOI: 10.1246/bcsj.58.2995

 Related EVISA Resources

Material Database: Certified Reference Materials for Fractionation Analysis
Brief summary: Why should elemental speciation be done ?
Brief summary: Error sources in Speciation analysis - Overview
Brief summary: Sample preservation for speciation analysis - General recommendations
Brief summary: Speciation as a discipline in Analytical Chemistry – Definitions
Brief summary: Species transformation during speciation analysis

Related Information

Leaching-Net: Standards for Leaching Procedures
Leaching-Net: Test Methods (for Leaching)

Further chapters on techniques and methodology for speciation analysis:

Chapter 1: Tools for elemental speciation
Chapter 2: ICP-MS - A versatile detection system for speciation analysis
Chapter 3: LC-ICP-MS - The most often used hyphenated system for speciation analysis
Chapter 4: GC-ICP-MS- A very sensitive hyphenated system for speciation analysis
Chapter 5: CE-ICP-MS for speciation analysis
Chapter 6: ESI-MS: The tool for the identification of species
Chapter 7: Speciation Analysis - Striving for Quality
Chapter 8: Atomic Fluorescence Spectrometry as a Detection System for Speciation Analysis
Chapter 9: Gas chromatography for the separation of elemental species
Chapter 10: Plasma source detection techniques for gas chromatography
Chapter 11: Fractionation as a first step towards speciation analysis
Chapter 12: Flow-injection inductively coupled plasma mass spectrometry for speciation analysis
Chapter 13: Gel electrophoresis combined with laser ablation inductively coupled plasma mass spectrometry for speciation analysis
Chapter 14: Non-chromatographic separation techniques for speciation analysis

last time modified: January 14, 2024


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