Angewandte Chemie International Edition 2006, 45, 7006–7009
The Smell of Iron
“Metallic” odor of iron, iron-containing water, and blood derives from the decomposition of skin oil
Contact: Dietmar Glindemann, Virginia Polytechnic Institute and State University, Blacksburg (USA)
Registered journalists may download the original article here:
The Two Odors of Iron when Touched or Pickled: (Skin) Carbonyl Compounds and Organophosphines
Where does the strange but typical “metallic” smell
come from when we touch iron objects such as tools, utensils, railings,
or coins? “The smell of iron upon contact with skin is ironically a type
of human body odor,” states Dietmar Glindemann. “That we are smelling
the metal itself is actually an illusion.” In conjunction with a team of
researchers from the Virginia Polytechnic Institute and State University in the United States, The University of Leipzig and the Leipzig Environmental Research Center, Germany, he has tracked down the responsible scent molecules.
Seven test subjects immediately recognized the
“musty” metallic odor when their hands came into contact with metallic
iron or a solution containing iron ions with a twofold positive charge.
In contrast, solutions of iron with a triple posititve charge did not
cause the odor. Analysis of gas samples from the skin of the test
subjects pointed to a bouquet of different organic compounds that seemed
to be characteristic of the metallic smell. The key component is called
1-octen-2-one, which smells fungal-metallic even when highly diluted.
The precursors to the odor molecules are lipid peroxides, which are
produced when oils on skin are oxidized by certain enzymes or other
processes (e.g. under UV light). These lipid peroxides are then
decomposed by the doubly negative iron ions, which are consequently
reduced to triply negative iron ions. When touching objects made of
iron, the required doubly negative ions are formed when perspiration on
the skin corrodes the iron.
Rubbing blood over skin results in a similar metallic
smell based on the same scent molecules. Blood also contains iron atoms.
Says Glindemann, “That humans can ‘smell’ iron can be interpreted as a
sense for the smell of blood. Early humans were thus probably able to
track down wounded prey or tribe members.”
Based on this new knowledge, medical researchers
should be able to further develop iron tests for skin, blood and tissues
in order to identify specific “fingerprints” of volatile scent molecules
as markers for individual body odor, oxidative stress, and diseases.
The researchers were also able to characterize
another iron-type smell: carbon- and phosphorus-containing cast iron and
steel develop a metallic-garlic odor when exposed to acids. Until now,
metallurgists ascribed this to the gas phosphine (PH3).
However, at breathable concentrations, pure phosphine (also known as a
pesticide) is basically odorless. The true culprits are
organophosphines, especially those champions among intensively smelly
compounds like methylphosphine and dimethylphosphine. Their structure is
like that of a phosphine molecule in which one or two of the hydrogen
atoms are replaced with methyl (CH3) groups.