Whenever an analyst is producing data on the presence and/or concentration level of elements in a sample, the data are meant to answer one of the following questions depending on the nature of the sample:
- What is the nature of the probed entity ?
- Are there any risks or benefits related to the probed entity ?
Depending on the origin of the sample (environmental compartment, human body, food, industrial product) such questions are related to the specific activity of the chemical species, such as
- biological activity
- lifetime, fate and metabolism
- chemical and physical activity.
Since these characteristics are species related, data on the presence of elements and their total concentration do not promote relevant information.
While arsenic is synonymous to poison for most people, it has become evident that even high concentrations of arsenic in marine foodstuffs are harmless, since the main species present are the non-toxic arsenobetaine, arsenocholine and arsenosugars. The total concentration of arsenic in marine foodstuffs does not address the health risks related with its consumption and should therefore not be a criterium for its marketability.
Different elemental species might not only differ quantitatively in their characteristics (e.g. toxicity) but also qualitatively (toxic versus essential). While Cr(III) compounds do have some positive biological activity and are therefore considered to be essential, Cr(VI) compounds are carcinogenic. The value of determinations of total concentrations is therefore very questionable since not addressing the totally different characteristics of the different species. It is also very questionable, whether the meaning of such measurements can be enhanced by setting slighly different tolerance limits, such as 10 mgL-1 for total chromium in water versus 5 mgL-1 for the toxic species Cr(VI) as it is the general practice in national water monitoring legislation.
Tamas Kiss, Akira Odani, Demonstration of the importance of metal ion speciation in bioactive systems, Bull. Chem. Soc. Jpn., 80/9 (2007) 1691-1702. doi:10.1246/bcsj.80.1691
Bernhard Michalke, Element speciation definitions, analytical methodology, and some examples, Ecotoxicol. Environ. Safety , 56 (2003) 122-139. doi: 10.1016/S0147-6513(03)00056-3
Rita Cornelis, F. Borguet, J. De Kimpe, Trace elements in medicine. Speciation: the new frontier, Anal. Chim. Acta, 283/1 (1993) 183-189. doi:10.1016/0003-2670(93)85221-5
Walter Lund, Speciation analysis - why and how ?, Fresenius J. Anal. Chem., 337/5 (1990) 557-564. doi: 10.1007/BF00322862
Further chapters: About Speciation