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IN 1957, I invented a device that can sniff out poisonous pesticides and ozone-eating compounds at concentrations of just one part per trillion. The , as I called it, boasted a level of sensitivity unheard of at the time and never bettered since.

I made the first one myself on a lab bench. I鈥檓 not sure it could happen that way now. For a start, I was working in a government lab in Mill Hill, north London, and I wasn鈥檛 supposed to be crafting an environmental detector at all 鈥� I was a medical researcher. But the boss allowed us to follow our noses.

The lab was a wonderful place, how all research should work. No bureaucracy. And its output was phenomenal, even though there was not much technical back-up beyond a couple of shared technicians and a titled lady who kindly washed the dishes. We invented and made our own equipment. I used to go down to Soho, the red-light district in London, to an army-surplus store, where I bought old Royal Air Force transmitters to cannibalise for electronic parts. The prostitutes always seemed to know what we were after, and ignored us. We were particularly nerdish, I expect.

I had been working on how we could freeze animals and then bring them back to life. During this work , which was good for both reviving hamsters and heating my lunch.

An important part of my work was looking at why the fatty-acid composition of cell membranes determined their resistance to damage by freezing. Cells with unsaturated fatty acids did best, and I wanted to analyse them better. So I went to see Archer Martin, a Nobel laureate, who was two doors down the corridor. He was a pioneer of gas chromatography, the only technique that would separate my membrane acids. Unfortunately, his equipment, which could find organic compounds at concentrations of less than one part per million, was still not sensitive enough for my purposes. So he challenged me to invent a better detector.

I loved a challenge, and within about three weeks, through pure serendipity and with the help of little more than a soldering iron, I came up with what became the electron capture detector. Small enough to hold in my palm, it had a chamber coated with a radioactive foil. The radiation freed electrons from a gas in the chamber 鈥� initially argon and later nitrogen. That set up an electric current. My idea was to add tiny quantities of the stuff I wanted to analyse into the chamber, and measure the effect it had on the electron flow. I figured that organic compounds would react with the free electrons and so reduce the current, and that different compounds would produce a different signal.

At first it didn鈥檛 seem to work reliably. One day, I injected a tiny amount of carbon tetrachloride, about 0.1 microlitres, into the chamber. The whole thing seized up, and it was a week before I could use it again. I thought it was broken. But as I eventually worked out, the device was so sensitive to the compound that, even after I cleaned it out, it was completely saturated. Blinded by it, you might say,

鈥淚 must have been a bit thick, because at first I didn鈥檛 realise its potential鈥�

That was my Eureka moment. The device turned out to be amazingly sensitive to other compounds too, particularly chlorinated ones. I had inadvertently made something that could measure critical human-made poisons at concentrations as low as one part per trillion 鈥� a million times better than anything before. With some refinements I made later at Yale University, it has never been bettered.

I must have been a bit thick, though, because at first I didn鈥檛 realise its potential. But word spread and shortly afterwards a researcher from the petrochemicals company Shell, which manufactured chlorinated pesticides, contacted me. He said they found it very difficult to measure low levels of pesticides in field trials, and asked if my device might be useful. I said yes, of course.

This was before the great hoo-ha created by the publication in 1962 of . She wrote that our countryside was awash with pesticides that were destroying nature, such as DDT. Some people say my device provided the data that resulted in Carson鈥檚 book. In fact, she hadn鈥檛 heard of the device, and nor had the researchers whose work she used. They could only measure down to one part per million. But research using my gadget did provide the proof that these poisons were present virtually everywhere in the environment 鈥� as Carson had claimed but couldn鈥檛 prove.

Without the electron capture detector, large chemicals companies would have been able to dismiss her findings. The reason I know that is because by then I was an adviser to Lord Victor Rothschild, a biologist in charge of science at Shell. When Carson鈥檚 book came out he was furious. He told me if what was in the book was true, it could destroy the entire pesticides industry. I had to tell him that my gadget confirmed she was right. He took it very well, considering. The pesticides industry has survived, but it is no longer based on organochlorine compounds.

The device started to be manufactured commercially soon after. I got no income from the patent, which the US government ended up owning because of the work done at Yale, but I have been giving advice to its manufacturers and users ever since. In 1961, I worked with NASA on plans for tiny gadgets that could analyse the chemistry of the atmosphere on Mars or Venus. But apart from pesticides, the biggest impact of my electron capture device was in providing the data used to predict that chlorofluorocarbons (CFCs) could harm the ozone layer.

In my work for Shell, I was looking at long-range air pollution. I knew that my detector could sniff out tiny amounts of CFCs and, because they are inert, I thought they might be accumulating in the air. When I looked, I found them even in the supposedly clean air over the Atlantic. I hitched a ride on a research boat to Antarctica and found them there too.

I subsequently did a similar trip on a German research ship. Unlike British researchers, who were not fussy about personal hygiene, the Germans used aerosol deodorants. The CFC propellants messed up my readings, so I had to get the captain to cut me adrift on a small boat before I could find suitable air to analyse. My readings led directly to warnings about CFCs eating the ozone layer. The .

The UK gave me the freedom to do my research, but it was Americans who usually spotted its importance. One exception, however, was the UK secret service. This was the height of the cold war, and they were trying to keep track of the KGB鈥檚 activities in London. The KGB had all sorts of sophisticated gear to sweep their cars for electronic bugs when they went about their clandestine business, but they never knew that we could use chemical tracers. My gadget had the scent-following powers of a bloodhound, and I was happy to put it at our guardians鈥� disposal.

Of course, a lot of people have criticised me for teaming up with Shell, the secret service and so on, but such collaboration has allowed me to be an independent scientist for the past 60 years. Like my first boss, my only interest is in doing good science and going where my curiosity and intuition takes me. That kind of approach to is all too rare these days.

As told to Fred Pearce

This article appeared in print under the headline 鈥淗ow to accidentally save the planet鈥�