Chemicals are ubiquitous – finding a simple and cheap technique to identify them is important.
If you scour the Internet, you’ll find companies offering to certify produce as containing no detectable pesticide residues.
For the health-conscious consumer that’s a powerful claim. But if you read the fine print, you’ll likely find it’s subject to limitations. That’s because there are a lot of pesticides — more than 1,000 different chemicals according to the World Health Organization, and more than 17,000 ‘pesticide products’ according to US environmental group Pesticide Action Network.
Testing for these is not as simple as on TV or in the movies. There is no Star Trek tricorder-like device that can simply be waved over a radish or stalk of celery to see what’s in it, nor do existing tests work the way they do on TV, says Michelangelo Anastassiades, an analytical chemist who coordinates the European Union’s official laboratories on pesticide residues.
True chemical analysis involves several steps. First, that radish or celery stalk has to be prepared so that chemicals of interest can be removed. Then they have to be separated from each other and measured, often at very low levels.
Pesticides aren’t a new problem. They’ve been around for a long time—a lot longer than most people realise. The first pest-killer, Anastassiades suggests, may not even have been a chemical, but the mousing abilities of your favorite tabby’s distant ancestor. Even today, he says, the Indian Council of Agricultural Research reports that rodents account for 8% of crop loss.
Chemical pesticides also go back to antiquity. In a 2019 law review article, Minnesota attorney Kate Graham notes that records of sulfur being used as a fumigant and insect repellant go all the way back to Homer’s Odyssey. But until the 19th Century, she says, most were derived from botanical preparations, sulfur, oil soaps, kerosene emulsions, lime, and salt.
Then French grape growers discovered arsenic and copper sulfate. Others combined lead and arsenic: cheap and effective, but wickedly frightening by today’s standards. By the 20th Century the hunt was on for regulation, and with it testing.
Many of the early tests would look familiar to anyone who has used a chemistry set. “[They] were primitive and maybe also complex,” Anastassiades says. He notes they worked by chemically reacting the sample extract with reagents designed to produce colour changes or other easily detected changes if the target pesticide is present. Unfortunately, if you wanted to test for multiple pesticides, you probably needed a separate test for each: a daunting task.
There was at least one early test that could detect residues of a large number of pesticides, but it wasn’t chemical. It used fruit flies. That’s right, those tiny, irritating insects that appear as if by magic if you leave a banana peel or apple core too long in the trash.
It worked, because fruit flies are sensitive to an entire class of insecticides known as organophosphates (the US EPA lists nearly 100 varieties). All that was needed to detect them was to let the flies feed on the product being tested, then wait and see what happens. Organophosphates work by inhibiting the enzyme cholinesterase, which is important for proper neurological functioning, both in flies and in people. If the residue was too high, the flies would quickly show the effects. “They started to fly in a very strange way,” Anastassiades says. That made it a very simple and effective test in a canary-in-the-coal-mine sort of way.
The problem with these early tests was that none were all that sensitive. When he was young, analytical chemist Ronald Hites, now retired from the University of Indiana, says initially conducting analyses at the “milli-” level (one part in 1,000) was considered good. But as time passed, tests started working at the micro-, pico-, and even femto- level (one part in a trillion): a rough counterpoint to the expansion of computer memory from kilobytes to megabytes, gigabytes, and terabytes. In pesticide detection, as in computers, technology marches in prefixes.
The revolution, Anastassiades says, came in two parts. One involved better methods of separating chemicals from each other. An example is thin-layer chromatography, in which a tiny dollop is put on a strip of a paper-like substance that is then wet at one end with a solvent. As the solvent wicks through the paper, it picks up materials from the sample and moves them across the paper at different paces. If you want to see this in action, he says, put an ink mark on a strip of paper, and wet one end of the strip. As the moisture moves through the paper, it will break the soluble portions of the ink into a rainbow of shades, a great way to fingerprint what the ink is made of.
To make the various bands stand out in a true lab test, Anastassiades adds, one approach is to illuminate them with ultraviolet light and see how they fluoresce. “So, we have here the blue light,” he says.
The process can also be done by putting the test material at one end of a column filled with a porous solid and watching how its components move under the influence of solvent. Another approach does the same, with a long, thin tube filled with gas. “It’s like 30 metres [long],” Anastassiades says.
There has also been a revolution in the type of instrumentation used for identifying the chemicals that come out of the separation process. No more fruit flies or color changes: far better is a mass spectrometer, which ionises these chemicals, accelerates them, and spews them at a detector that can measure incredibly tiny differences in the speed at which they move. “This was an epic advancement,” Anastassiades says.
Drawing on this, Anastassiades recently won a lifetime achievement award for his work on an analytical approach called QuEChERS (pronounced ‘kechers’.) The name stands for Quick, Easy, Cheap, Effective, Rugged, and Safe. “Maybe 90% of labs use it now,” he says.
The heart of QuEChERS is an extraction method in which the sample is mixed with water and acetonitrile. Solids are removed with a centrifuge, then chemicals are added to induce the water and acetonitrile to separate, with dissolved organics (like pesticide residues) going with the acetonitrile. That layer is then cleaned up as needed to remove non-pesticides, fed into liquid or gas chromatography, then to a mass spectrometer.
“Quick” barely begins to describe it—the whole process can be done in as little as an hour. “Safe” is also important, because acetonitrile is fairly benign and easy to dispose of. “Past methods used large amounts of solvents including dichloromethane, which was banned by the Montreal Protocol [due to damaging effects on the ozone layer],” Anastassiades says.
The simplicity of the extraction processes also allows a single test to be able to detect an entire smorgasbord of pesticides—probably 75 to 80% of all the ones currently in existence, Anastassiades says.
It’s also a blueprint for detecting new pesticides, because if there’s one thing history has shown, it’s that farmers are always looking for new ways to protect their crops, and even natural methods can leave residues. If you don’t believe that, you’ve never had to clean a cat litter box.
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Originally published by Cosmos as Detecting pesticide residues will soon be quick and safe
Richard A Lovett
Richard A Lovett is a Portland, Oregon-based science writer and science fiction author. He is a frequent contributor to Cosmos.