> This is like saying "miles per gallon" when discussing weight. "nanomolar detection levels"...microplastics are individual particles identified by spectroscopy, reported as particles per mm^2. "Nanomolar" is a dissolved-species concentration unit. It has nothing to do with particle counting. (I, and other laymen, understand what you mean but you go on later in the thread to justify your unsourced and unjustified claims here via your subject-matter expertise.)
This paper used “light-based spectroscopy” [1]. Many others use methods that depend on gas chromatography or NMR. A relatively infamous recent example used pyrolysis GCMS to make low-concentration measurements (hence: nanomolar), which they credulously scaled up by some huge factor, and then made idiotic claims about plastic spoons in brains.
Relatively little quantitative science in this area depends on counting plastic particles in microscopic images, but it’s what gets headlines, because laypeople understand pictures.
[1] as an aside, the choice of terminology here is noteworthy. A simple visual light absorption spectra is also “light based spectroscopy”, but is measuring the aggregate response of a sample of a heterogeneous mixture, and is conventionally converted to molar equivalents via some sort of calibration curve (otherwise you can’t conclude anything). But there could be other approaches that are closer to microscopy, which they also discuss. “Particles per square millimeter” is also a unit of concentration (albeit a shitty one, unless your particles are of uniform mass).
Anyway, the point is that these kinds of quantitative analyses are all trying to do measurements that are fundamentally about concentration, which is why I chose the words that I did.
> ...
"1 nanomole of polyethylene" requires you to pick an arbitrary average molecular weight.
This changes the answer by orders of magnitude depending on what you pick.
Which is why nobody does it.
> Relatively little quantitative science in this area depends on counting plastic particles in microscopic images...Many others use methods that depend on gas chromatography or NMR.
So we're dismissive of some subset of papers, because they get false positives using toy methods.
Real science would use gas chromatography.
But...the paper we're dismissing tested gas chromatography. And found the same false positive. [1, in abstract]
> A relatively infamous recent example used pyrolysis GCMS to make low-concentration measurements (hence: nanomolar)
The brain study I'm guessing you are referring to, [2], measured low concentrations, yes.
But it reported them in ug/g.
Because polymers don't have a defined molecular weight.
> made idiotic claims about plastic spoons in brains
The brain study I'm guessing you are referring to, [2], does not mention spoons, or, come close.
Are we sure there's a paper that did that?
[1] Witzig et al, https://pubs.acs.org/doi/10.1021/acs.est.0c03742, "Therefore, u-Raman, u-FTIR, and pyr-GC/MS were further tested for their capability to distinguish among PE, sodium dodecyl sulfate, and stearates. It became clear that stearates and sodium dodecyl sulfates can cause substantial overestimation of PE."
[2] Campen et al, https://pubmed.ncbi.nlm.nih.gov/38765967/, "Bioaccumulation of Microplastics in Decedent Human Brains"
Doesn't take an expert to see that fatty acids and hydrocarbon chains from the degradation of polyethylene look nearly the same.
Not sure what you mean or how it’s related. If the idea is microplastics aren’t actually a problem, I’m totally open to that. But “it’s possible everyone involved is overrating it due to scientists seeing fatty acids or hydrocarbons and calling it plastic” needs a little more than anon assertion :)
PE consists of very long hydrocarbon chains. It can degrade into shorter hydrocarbon chains. Fatty acids also have long hydrocarbon chains. The detection method for microplastics commonly involves pyrolysis, which breaks down polymers into smaller molecules. It's not hard to see that they'll end up looking nearly the same.