ANN ARBOR—Nitrile and latex gloves that scientists wear while they are measuring microplastics may lead to a potential overestimation of the tiny pollutants, according to a University of Michigan study.
The study found that gloves may unintentionally contaminate lab equipment scientists use to measure microplastics in air, water and other samples with nonplastic particles called stearates. U-M researchers Madeline Clough and Anne McNeil suggest cleanroom gloves, which release fewer particulates, be worn instead.
Stearates are salts, or soap-like particles. Manufacturers coat disposable gloves with stearates to make them easier to peel from the molds used to form them. But stearates are also chemically very similar to some microplastics, according to the researchers, and can lead to false positives when researchers are looking for microplastic pollution.
That's not to say that there is no microplastics pollution, the U-M researchers are quick to say.
"We may be overestimating microplastics, but there should be none," said McNeil, senior author of the study and U-M professor of chemistry, macromolecular science and engineering, and the Program in the Environment. "There's still a lot out there, and that's the problem."
Clough said, "As microplastic researchers looking for microplastics in the environment, we're searching for the needle in the haystack, but there really shouldn't be a needle to begin with."
The work, led by Clough, a recent doctoral graduate, is published in the journal RSC Analytical Methods . The study was supported by a grant from the U-M College of Literature, Science, and the Arts' Meet the Moment Research Initiative.
A wild microplastics goose chase
The study began when Clough was working on a collaborative project that included graduate students and faculty in the U-M departments of Chemistry, Statistics and Climate and Space Sciences Engineering to examine microplastics in Michigan's atmosphere. To do this, Clough and McNeil turned to study collaborators U-M professor of chemistry Andy Ault and graduate students Rebecca Parham and Abbygail Ayala to assist with air sampling.
The researchers used air samplers which are fitted with a metal substrate. Air passes through the sampler, and particles from the atmosphere deposit onto the substrate. Then, using light-based spectroscopy, the researchers are able to determine what kind of particles are found on the substrate.
Clough prepared the substrates while wearing nitrile gloves, which is recommended by the guidance of literature in the microplastics field. But when she examined the substrates to estimate how many microplastics she captured, the results were many thousands of times greater than what she expected to find.
"It led to a wild goose chase of trying to figure out where this contamination could possibly have come from, because we just knew this number was far too high to be correct," Clough said. "Throughout the process of figuring it out—was it a plastic squirt bottle, was it particles in the atmosphere of the lab where I was preparing the substrates—we finally traced it down to gloves."
The researchers designed an experiment to figure out how widespread the problem is. They tested seven different kinds of gloves, including nitrile, latex and cleanroom gloves, as well as the most common techniques that microplastic researchers are using to identify microplastics.
The experiment mimicked the type of contact that would occur in a research environment between a researcher's gloved hand and a point of contact. This would include a filter or a microscope slide—any piece of technology that a researcher might use over the course of investigating microplastics.
They found that on average, the gloves imparted about 2,000 false positives per millimeter squared area.
"The type of contact we tried to mimic touches upon all varieties of microplastics research," Clough said. "If you are contacting a sample with a gloved hand, you're likely imparting these stearates that could overestimate your results."
The researchers also found that cleanroom gloves imparted the fewest particles—likely because cleanroom gloves are manufactured without the stearate coating, allowing them to be used in "ultrapure" applications.
Weeding out false positives
The researchers designed another experiment to determine whether they were able to distinguish what a true microplastic looked like versus one of the stearate salts from the gloves. Using scanning electron microscopy as well as light-based microscopy, they found that the stearate was visually impossible to distinguish from polyethylene, the plastic it resembles.
But Clough and McNeil were also able to find methods, in collaboration with graduate student Eduardo Ochoa Rivera and U-M professor of statistics Ambuj Tewari, that can differentiate between the false positives coming from the glove and microplastics in the environment. This can help researchers revisit potentially contaminated datasets.
"For microplastics researchers who have these impacted datasets, there's still hope to recover them and find a true quantity of microplastics," Clough said.
The researchers say their study highlights the importance of chemistry researchers in the field of microplastics who might be able to recognize the difference in chemical structure of plastics versus other contaminants.
"This field is very challenging to work in because there's plastic everywhere," McNeil said. "But that's why we need chemists and people who understand chemical structure to be working in this field."
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