Specks of dust are as unique as snowflakes ? but no one had ever paid much attention to what individual particles are made of. Until now
THE dusty old library is a bit of a clich?. But take away all the books and you are left with something rather interesting, in its own small way.
Welcome to the dust library. With 63 individual particles catalogued, the collection is hardly dust's answer to the great library of Alexandria, but there are plans to expand. Besides, far from being dry, this is a genre of unexpected delights. If you haven't explored it much, and you probably haven't, you may be in for a pleasant surprise.
Dust means different things to different people. In the trilogy His Dark Materials, author Philip Pullman cast it as an esoteric elementary particle - one with consciousness. To you, it might be simply any fine powder, or mostly dead skin, or the state to which we shall all return. For scientists its allure is long-standing: more than a century has passed since geologist J. A. Udden's first laboratory experiments. Today, what's blowing in the wind is of interest to everyone from geologists and environmental scientists to astronomers and health experts.
Dust is hugely diverse. The biggest particles are more than 2 millimetres across; the smallest, less than 0.1 micrometres. Excitingly, some have cosmic origins: around 200,000 tonnes of extraterrestrial material enters the atmosphere each year. This is dwarfed by the 4 billion tonnes from Earth itself, though, more than 90 per cent of which comes from natural sources such as soils, volcanoes, deserts, pollen and sea salt. The human contribution is far smaller, but we are responsible for some of the nastier little particles, including those from car exhausts, industrial emissions and fertilisers.
Our knowledge of dust is also diverse, if a little haphazard. Among other things, we know that British stately homes are covered in a fine layer consisting mostly of skin, hair and fabric fibres; that indoor dust can pose more of a threat to health than the outdoor variety; and that city air is teeming with bacteria from dog faeces (New Scientist, 13 August, p 16). But there is one glaring gap in our dusty repertoire, which stems from the fact that the stuff is generally studied in bulk: no one had ever looked at what an individual dust particle is actually made of. Until now.
As with many great scientific breakthroughs, this one was serendipitous. In 2003, James Coe at Ohio State University in Columbus was using infrared light to identify the chemical composition of various substances: different molecules absorb specific frequencies of infrared according to their structure, so the emerging spectrum reveals what is there. Unfortunately, extremely small samples scatter the light, blurring the results. So Coe constructed a sensor containing a nickel mesh with 5-micrometre holes to trap individual particles and prevent scattering. The nickel absorbs infrared light, creating plasmons - electron waves with properties of photons - that help funnel the light through the trapped particles, rather than scattering it. For several years he and his team happily used the mesh to uncover the chemical make-up of all manner of things. Then, a couple of years ago, someone got sloppy, and a routine run with latex spheres embedded in the mesh turned up an unexpected spectrum: dust.
"Usually, if you see dust, you avoid it," says Katherine Cilwa, then a graduate student in Coe's lab and now at the University of Michigan, Ann Arbor. But instead of discarding the duff results, she and her colleagues embraced them, and began looking for ways to attract more dust. Soon, with the help of a miniature suction pump and some ordinary laboratory air they had captured 63 solitary dust particles in their mesh. Once these had succumbed to infrared spectroscopy, the world's first dust library was born (Journal of Physical Chemistry, vol 115, p 16910). "By studying single particles you see things that you don't see if you average all the spectra together," says Coe.
So what can this unusual library tell us? First, there is the simple parts list. The most common component was organic material, present in 40 of the 63 particles - exactly what is unclear, but it could be anything from pollen to sloughed-off bits of researcher. Quartz, found in 34 particles, came next, followed by carbonates (17 particles) and gypsum (14). "The minerals blow in," says Coe. "They come from all over the world." Other ingredients included air pollutants and fertiliser chemicals.
Anyone counting will also have noticed that there are already more components than particles. That is because most specks of dust are conglomerates, which means they may take an infinite variety of forms, much like snowflakes. The next obvious step was to find out what individual conglomerates looked like, but pinpointing exactly which speck corresponded to which spectrum wasn't going to be easy. So Coe launched a competition. The first person to capture an electron microscope image of a particle that had already been analysed with infrared light would get to name it. And, if that was not enough, there was a free dinner on offer too. What student could fail to rise to that challenge?
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