The smell of toxic chemicals

[badhorsey]

I have read that nerve agents such as Tabun or Sarin are described as being "almost odourless" or sometimes having a faint floral odour or an odour of apples. How is the odour of highly toxic substances ascertained when the lethal dose is measured in microgrammes?

[scotman]

because, although lethal, they are not necessarily quick-acting.

[ChuckFickens]

The sarin smell tester wasn't a popular job.

[wildwood]

I was going to say "It would solve the unemployment problem" but I am not going to, someone is bound to kick up a fuss.

[boxtops]

This sounds like more homework.

[theprof]

What a great question homework or not!. Well, it is true that years ago human analysts were used to sniff toxic gases and chemicals. They came to no real harm as more often than not, the gases were combined with oxygen or air effectively diluting their toxic qualities. In addition, a good guess was often made about the likely odour from a chemical from the compounds that reacted together to make the chemical in the first place especially if the compounds themselves were relatively non-toxic but odoriferous. Having said that, fatalities have occurred with many compounds.

So how is it done nowadays? Well, there are quite a few methods available to scientists but I'll mention only one here as it's probably the simplest. The compound is analysed using Gas Chromatography and Olfactometry which effectively splits the toxic compound down to simpler compounds.

The odour evaluation is performed by a specially trained operator who in essence, smells the given compounds at special "ports" attached to the analysis equipment. Each of these evaluated compounds is less toxic than the parent compound.

The operator is then asked via a touch-screen to describe the odour, using many parameters such as "earthy", "fishy", "fruity" etc. Details of other factors such as the perception of a lingering odour are also requested. From the operator analysis (and other operators too), a profile is built up by computer of the odour of the toxic chemical as a whole.

The final data will provide an accurate description of the odour of the parent compound. I must add though, that I've greatly simplified the details used in this type of analysis and not discussed much more complex systems that are in use.

Finally, you have to remember that virtually all chemical compounds have had their odour described in chemical dictionaries such as Hawley's for many years. In effect, this reduces the need to

[theprof]

(continued)

employ the services of these odour specialists nowadays as the work has already been done. Tabun and Sarin have been described for many years.

[badhorsey]

Thanks for the answers! No, it's not homework, unless the O-Level syllabus has got a LOT more demanding since I last studied chemistry (I'm also trying to imagine the knock-on effect of setting such a question and causing a huge rise in localised searches for "Soman" and "VX"... probably result in the teacher being carted away for questioning!)

This does make a lot of sense although those early chemists who contributed to the medical dictionaries back in the '30s before advanced test equipment was available are surely some of the bravest men of their time - a good example is Joseph Barcroft and his (in)famous dog experiment: http://www.jstor.org/pss/226728