White powder paranoia world problem

By Associate Professor Allan Blackman

This article was orignally published in the Otago Daily Times on Monday 5 May 2003.

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On Friday October 5, 2001, Robert Stevens died in a hospital in Palm Beach, Florida. While the name may not be familiar to you, his death had worldwide ramifications. For Robert Stevens died of anthrax, spores of which were later found at the offices of the newspaper where he worked. And thus the world became paranoid about white powers.

Thankfully, confirmed anthrax attacks have been few in number, but in recent times, "cyanide" has been sent to foreign embassies and newspapers in New Zealand, and last week saw the conviction of a Tauranga man for filling a packet of tobacco with flour and tissue paper. Even Dunedin hasn't been immune — only last week emergency services were called out to investigate a white powder in Taieri Rd, only to find it was sand. So let's suppose that you're opening the mail and some white powder spills out of an envelope. Is it sodium cyanide or sodium chloride (table salt)? Strychnine or sucrose (sugar)? One white powder looks pretty much like another to the naked eye, so how do you tell what it is?

One obvious method by which you could distinguish between the above four compounds (but one that I certainly wouldn't recommend) might be to taste the powder — if it doesn't taste sweet or salty, you know it's poisonous! In days gone by, chemists did indeed taste materials in the lab — for example, the German word for "sour" also means "acidic" because of the characteristic taste of acids. However, such practices were not without their dangers. The highly toxic element beryllium was originally named glucinium (from the Greek meaning "sweet") because its compounds tasted sweet. You really have to wonder how many intrepid but unfortunate researchers died before they figured out that these compounds were not only sweet, but also deadly.

Nowadays, chemists do not have to resort to such a potentially hazardous technique in order to identify unknown compounds. Instead, they can use a variety of methods ranging from simple physical tests to experiments utilising equipment worth hundreds of thousands of dollars. An example of the former is the measurement of the melting point of a solid. Most pure solids have a well-defined and hence diagnostic melting point; for example, strychnine melts at 286°C while sucrose melts at 185°C, and use of a simple laboratory melting point device would allow these two white powders to be easily distinguished. However, this technique is routinely used only for solids that melt below about 300°C and more sophisticated apparatus is required for high-melting solids such as sodium chloride (801°C) and sodium cyanide (564°C). In addition, if the solid is impure, then measurement of its melting point is of little use.

Chemical tests are often used to aid the identification of unknown materials. For example, sodium cyanide can be unambiguously detected through its reaction with iron salts to give a blue solid called "Prussian Blue", sucrose will react with a copper salt following treatment with acid to give the brick-red solid copper oxide, sodium chloride will give a white solid on reaction with silver nitrate and strychnine gives a blue colouration on treatment with potassium dichromate in sulfuric acid. While they are often useful, many chemical tests are not absolutely specific for one particular compound and can give positive results with more than one species.

The most powerful techniques for identifying unknown substances inevitably turn out to be the most expensive. Nuclear magnetic resonance (NMR) is today the method of choice for the identification of materials that contain either carbon or hydrogen. All university chemistry departments of any note will possess at least one NMR spectrometer, and such machines sell for close to $1 million. This technique relies on the fact that the nuclei of some atoms behave as tiny magnets and will align with or against a large magnetic field (the machines in our department utilise a field about 100,000 times as strong as the magnetic field of the Earth).

Some of you may have in fact had first-hand experience of NMR — if you've ever had an MRI (magnetic resonance imaging) scan, you've been inside an NMR spectrometer. I suspect that NMR was renamed MRI for medical procedures because of the public's fear of the word "nuclear".

X-ray crystallography is probably the ultimate technique for the unambiguous identification of solids. X-rays are fired at a crystal of the substance and are scattered in different directions. Computer analysis of the pattern generated by scattered X-rays allows a 3-dimensional picture of the solid to be obtained to an accuracy of billionths of a millimetre. This technique was perfected in the early 20th century by William and Lawrence Bragg, an Australian father and son, and sodium chloride was one of the first solids whose structure was determined using this method.

With any luck, the white powder scares will soon be a distant memory. And I fervently hope that whoever was behind the murder of Robert Stevens and four others (using anthrax manufactured in a US Army laboratory) will be brought to justice.