Lab glove deposits drive false microplastic signatures in atmospheric pollution samples

Particles shed from lab gloves are being misidentified as microplastics, distorting measurements of atmospheric pollution, new research shows.

Contamination in microplastic studies typically occurs through wet contact, with particles transferred from water or reagents during liquid transfer and handling.

Here, however, dry contact from lab gloves led to results that ‘much exceeded’ expectations based on the literature, explains lead researcher Madeleine Clough at the University of Michigan, US.

‘I remember taking a gloved finger, pressing it against the substrate then looking at it with our instrument and being absolutely shocked to find the ginormous number of particles released.’

The particles come from a stearate coating that helps release gloves from moulds, which are systematically mistaken by reference spectral libraries as common microplastic polyethylene.

Because stearates and polyethylene both have long hydrocarbon chain C–H bonds, their infrared (IR) and Raman spectral signatures are difficult to tell apart.

Using photothermal IR – which enables analysis of smaller particles – Clough and colleagues found that submicron stearate deposits, many <5µm in size, inflated polyethylene estimates in their atmospheric samples.

While ‘exciting to finally track it down’ after months of searching, Clough says it was also ‘deeply devastating’ to discover much of the year’s samples seemed unusable.

To assess the extent of true microplastic pollution, Clough returned to each site and repeated sampling without gloves. The difference was vast.

Across seven types of gloves, the team found an average of 2000 particles per mm² were being misidentified as microplastics. The largest difference was around 7000 false positives for what was found to be just three microplastic particles per mm², says study co-lead Anne McNeil says.

However, McNeil and Clough are keen to emphasise that their advanced equipment allows them to see more of these smaller-sized particles. They highlight that in this evolving field, many researchers will also be ‘underestimating microplastic levels’ if analysis tools cannot capture the smaller, more dangerous particles.

‘Everyone’s trying their best with the tools and training they have and the field is in this chaos stage of figuring out how to do these studies. This is a normal part of science,’ adds McNeil.

Methods matter

Microplastics left untreated in the environment can break up into even smaller particles reaching less than 5µm in size. These particles ‘have the greatest impact to the environment and health and are most likely being undercounted,’ Clough explains.

She says more awareness of contamination risks and advanced techniques like photothermal IR will help researchers uncover the full extent of microplastic pollution in different areas.

Researchers have faced various questions over methods for identifying microplastics and Dan Biggerstaff, technical director for LGC Standards, says the field is still working to define ‘best practice’.

Photothermal IR is an ‘incredibly unique technology’ allowing researchers to assess the harder-to-detect microplastics, says Biggerstaff.

‘One of the largest unknowns in this emerging field is sample preparation – validating not only the analytical determination but also the sample preparation technique is essential,’ he adds.

Going gloveless?

Clough and Biggerstaff both point out that approaches to atmospheric pollution sample collection vary, with 80% of studies that report their methods suggesting wearing gloves.

Although cleanroom gloves reduced false positives without sacrificing safety, she says the future of atmospheric microplastic pollution may be glove-free: ‘If your process allows you to avoid contact with potential skin hazards, probably going gloveless would be best.’

Biggerstaff says he’s interested to see further investigations with lab gloves: ‘One experiment I’d have loved for them to do is to wash their hands with the gloves on with soap and water, to see if that removed the particles.’