In the lab with 'Mr. Clean'Pristine environment necessary for measuring microscopic materials
By Diana Steele
Munir Humayun's laboratory on the fifth floor of Hinds Geophysical Laboratory is arguably the cleanest place on campus, perhaps in the entire city of Chicago.
Visitors entering Humayun's domain tramp across a sticky mat to remove dust from their shoes. Inside an airlock -- a chamber between two sets of doors -- they don nylon booties and slip white, lint-free suits over their street clothes and place similar caps on their heads. Only then does Humayun, Assistant Professor in Geophysical Sciences, open the door to the inner sanctum -- a metal-free, dust-free, purified-air laboratory starkly free of clutter.
"If you have allergies, this place is the ultimate," said Humayun, laughing. "In order to analyze teensy amounts of materials, we need to work in an environment where all the air is filtered for dust and particulates, purify our water and eliminate all possible sources of contamination."
By measuring the tiniest fraction of some of the rarest metallic elements that make up rocks from earth and elsewhere in the solar system, Humayun hopes to answer some fairly large questions, such as how did the earth form, and is there life on other planets?
Deep in the inner sanctum -- Humayun won't reveal precisely where -- is a tiny piece of the famous Mars meteorite Alan Hills 84001, which in 1996 scientists proclaimed showed evidence of primitive life forms. Humayun, along with graduate student Nicole Foley, is examining a fragment of the potato-sized rock to see if these claims will hold up to detailed scrutiny.
If the rock indeed held primitive bacteria, Humayun said, the bacteria would have to be chemoautotrophic -- utilizing energy from chemicals to sustain them -- rather than the more familiar photosynthetic type, because the areas where bacteria might have formed are in dark cracks deep within the rock, far from the reaches of sunlight. The chemical processes that created this bacteria would have left characteristic chemical signatures in the form of the buildup and transport of certain elements, such as iron and sulfur, that are essential for life. It is these signatures that Humayun and Foley are looking for. It is safe to say they will leave no stone unturned.
Humayun, who joined the Chicago faculty last year, was recently awarded the prestigious F.W. Clarke Medal from the Geochemical Society for a significant contribution to the field of geochemistry before age 35. Humayun, who received his Ph.D. from Chicago in 1994 -- studying under Robert Clayton, Enrico Fermi Distinguished Service Professor in Chemistry and Geophysical Sciences -- won the medal for contributing to our understanding of the formation of the solar system. The award will be presented during the society's annual meeting in August.
Specifically, the prize has been awarded for the technique Humayun developed with Clayton to precisely measure the isotopic composition of potassium. Their measurements of potassium isotopes in meteoritic samples helped resolve a long-standing debate about the temperature of the solar system during its early formation.
Humayun's primary analytic tool is an inductively coupled plasma mass spectrometer. This newly acquired, highly sophisticated machine can analyze chemical elements in a sample to a detection limit of one femtogram per gram. That's one part per quadrillion, or 10-15 grams per gram, of a sample. It is one of the most sensitive instruments of its kind in the world; perhaps only five other earth sciences departments have this degree of sensitivity.
"We can see trace elements in very small amounts of material," said Humayun, "so we can attack problems other people can't because of analytical constraints." The high sensitivity means Humayun can track trace elements in concentrations as low as a few parts per trillion in a one-gram sample of material, or a few parts per billion in smaller samples, such as a precious milligram of a meteorite or moon rock.
This extreme sensitivity is the main reason for the scrupulously clean lab. All of the hinges, taps and screws are made of nylon or other inert polymeric materials, because the acids Humayun uses to dissolve the samples would corrode anything metal, which would contaminate the samples. The floor, ceiling and walls are manufactured out of large sheets of polyvinyl chloride -- even the cabinets are plastic. The room air is multiply filtered and maintained at positive pressure so that no dust-laden air enters from the outside. The water is doubly-deionized, carbon-filtered and passed through another filter that removes organic resins.
Asked what would happen if the power went off -- shutting down the air pumps that keep the dusty world at bay -- Humayun exclaimed, "That is my worst nightmare!" It actually happened once, and the entire lab -- three rooms -- had to be washed from floor to ceiling and flushed out. It was weeks before work resumed.
All this caution is well-justified. Headlines in the 1960s trumpeted, "Life in meteorites!" after evidence of life was found in a meteorite. Through careful detective work and scrupulous analysis, Humayun's intellectual predecessor at Chicago, Edward Anders, the Horace B. Horton Professor Emeritus in Geophysical Sciences, showed that the sample had been contaminated by ragweed pollen.
In addition to studies of the solar system and life on other planets, Humayun's chemical investigations carry him deep into Earth's mantle, studying the role of meteoritic bombardment in the chemical composition of the Earth. "The elements that we depend on for life -- carbon, nitrogen, hydrogen -- none of them were present when the earth formed," said Humayun. "About one tenth of one percent of Earth's mass fell to the surface in the form of meteorites, mostly during the first half billion years after the earth formed. They brought these elements and many others, including the ones that we study here: platinum, iridium and palladium."
Because of Earth's active geologic processes -- as evidenced by earthquakes, volcanoes and plate tectonics -- these meteorites have been subsumed into the earth's mantle and mixed with the rest of the material.
Humayun is analyzing chunky fragments of the earth's solid mantle that get thrown to the surface during violent volcanic eruptions. One source of mantle material is Peridot Mesa, in Arizona -- peridot is the gem-quality version of the mantle's most abundant component, the sea-green olivine. The granular olivine contains chemical clues to the earth's early history.
If the platinum-group elements had been present at the earth's formation, Humayun said, because of their chemical properties they would have migrated to the metallic core. It was Anders at Chicago in the 1970s who first proposed that meteorite bombardment explains the presence of these elements at the earth's surface. But precisely where these meteorites came from has never been determined. Humayun's deciphering of the chemical clues in olivine may provide some long-sought answers.