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Many people notice a peculiar "metallic" smell when handling iron objects, such as tools, utensils, door handles, railings, firearms, coins, and other objects. But, iron untouched by human hand has a subtly different almost garlic like smell. Dietmar Glindemann of the University of Leipzig and his colleagues Andrea Dietrich at Virginia Polytechnic Institute and State University, and Hans-Joachim Staerk and Peter Kuschk of the Leipzig Environmental Research Center, Germany, have used a sophisticated analytical process to sniff out the reason why. It transpires that the metallic smell of iron that has been touched is a kind of body odour rather than a smelly metal. "The smell of iron upon contact with skin is ironically a type of human body odour," explains Glindemann. "That we are smelling the metal itself is actually an illusion." Glindemann and his colleagues asked seven volunteers to carry out a sniff test with iron. All of them immediately recognized the "musty" metallic odour of iron when they handled a piece of the metal or immersed a finger in a solution of iron(II) ions. In contrast a solution of iron(III) ions did not produce the characteristic odour, nor did the volunteers detect a musty smell emanating from untouched iron metal. The team has now carried out paralle solid-phase microextraction gas chromatography coupled mass spectrometry (SPME GC-MS) analysis of samples from the skin of the volunteers and have identified a veritable cocktail of organic compounds that work together to produce the characteristic metallic smell. The key component, however, is an olefinic ketone, oct-1-en-3-one. This compound has a metallic, almost fungal, odour even when it is highly diluted. But, how do these smelly compounds form? When skin touches an object made of iron, perspiration oxidises the metal to form dissolved iron(II) ions. These ions then cause natural lipid peroxides formed on the skin to decompose into the odour molecules. These lipid formed initially from enzymic action or photodegradation of skin oils ultimately oxidise the iron(II) ions to the iron(III) state. There may be an evolutionary reason for noses being tuned to this peculiar metallic scent. Rubbing blood on skin results in a similar reaction as the iron ions in blood interact with those very same lipid peroxides to produce odour molecules. "That humans can 'smell' iron can be interpreted as a sense for the smell of blood," Glindemann explains, "Early humans were thus probably able to track down wounded prey or tribe members." This seemingly esoteric research could have a practical application. The team suggests that medical researchers might be able to develop an iron-based test for skin, blood and tissues that would allow a "fingerprints" of volatile odour molecules associated with a particular individual, oxidative stress, and diseases. But, what of that "other" metallic smell associated with iron that has not been handled. The researchers identified the origin of that metallic garlic smell in carbon- and phosphorus-containing cast iron and steel as being produced by the interaction of the impure metal with acids. It is not phosphine, as was previously thought, they explain, because at breathable concentrations, pure phosphine has no smell. Instead, the team has identified the far smellier organophosphines. These are among the most odorous compounds known, they say. Related links:
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Wake up and smell the rust |
