How are microplastics measured?

In brief

• Particles picked out by eye or under a microscope are not necessarily plastic; the polymer should be confirmed with a method such as FTIR, Raman or Py-GC/MS [10].
• Of 50 studies on drinking water and freshwater, only four met all the proposed quality criteria [10].
• According to a 2025 study, Py-GC/MS is not currently a suitable method for polyethylene and PVC in biological samples; fats can produce misleading signals that resemble polyethylene [9].

Behind every “researchers found X microplastic particles” there is a long chain of lab work, and every link in that chain can change the result. This post goes through the steps in order, with a question to ask at each one when you read a study.

The structure follows a review that scored the quality of 50 studies on drinking water and freshwater [10]. It used nine criteria: sampling method, sample size, sample processing and storage, laboratory preparation, clean air conditions, negative controls (blanks), positive controls (recovery), sample treatment, and polymer identification [10].

#	Criterion	Step in this post	Stated in the text
1	Sampling method	1. Collecting
2	Sample size	1. Collecting
3	Sample processing and storage	—
4	Laboratory preparation	4. Contamination	among the lowest average scores
5	Clean air conditions	4. Contamination	among the lowest average scores
6	Negative controls (blanks)	4. Contamination
7	Positive controls (recovery)	5. Recovery	low average score
8	Sample treatment	2. Cleaning up	among the lowest average scores
9	Polymer identification	3. Polymer

1. Collecting the sample

Water, sediment, food or tissue: how much is collected, and how, determines from the outset which particles can be counted at all. The review scores sampling method and sample size separately [10].

Question to ask: Is the sample large enough for the particle size being targeted and the concentration expected?

2. Cleaning up the sample

A sample contains a lot besides plastic: organic matter, minerals, tissue. To remove these, the sample usually goes through a digestion step. In the review, this step (“sample treatment”) was among the criteria with the lowest average scores [10].

Question to ask: What treatment was used, and was it shown to remove enough of the non-plastic material?

3. Identifying the polymer

The key distinction here: is a particle called plastic because of how it looks, or because of its chemistry?

According to the review’s authors, sorting and identifying particles by hand introduces bias. It can miss transparent and small particles, and they advise against it for particles below 300 micrometres [10]. The polymer type should be confirmed, preferably with FTIR or Raman spectroscopy, pyrolysis–gas chromatography–mass spectrometry (Py-GC/MS) or TGA-GC/MS [10]. To score fully, a substantial share of the particles found must be identified this way [10].

Py-GC/MS works differently from the others. It does not see particles directly; it breaks the sample down with heat and infers the polymer from the products. According to a 2025 study on human blood, fats break down under heat into products like those of polyethylene, and PVC’s breakdown products are non-specific. The authors concluded that the technique is not currently a suitable method for polyethylene (PE) or PVC in biological samples [9]. In the same study, recovery rates ranged from 7% to 109%, and realistic detection limits in blood were up to 20 times higher than those calculated for pure water [9]. For polyethylene, the authors also proposed a quality-control check, based on the ratios of pyrolysis products, to flag when interference rather than plastic is producing the signal [9].

Question to ask: Was the polymer confirmed chemically? If Py-GC/MS was used on biological material, was the risk of false signals addressed?

4. Controlling contamination

Samples can also pick up particles from outside while in the lab. Three of the review’s nine criteria deal directly with this: laboratory preparation, clean air conditions and blank samples [10]. Laboratory preparation and clean air conditions were also among the lowest-scoring criteria on average [10].

A blank is a control that goes through every step of the procedure but contains no sample. If particles turn up in the blank too, that count has to be taken into account when interpreting the real sample.

Question to ask: Were blanks used, and are their results reported?

5. Testing recovery

A positive control means adding a known amount of plastic to a sample and measuring how much of it the method gets back. This too was among the criteria with low average scores in the review [10]. The blood study mentioned above, where recovery ranged from 7% to 109%, shows why this test matters [9].

Question to ask: Do we know how much plastic the method misses?

Why do results vary so much?

Across the 50 studies reviewed, reported concentrations ranged from roughly 0.01 to 100 million particles per cubic metre, a spread of about ten orders of magnitude [10]. Some of that reflects real environmental differences; some may come from the steps above being carried out differently. Only four studies met all the criteria [10].

So a number from one study should not be compared with a number from another without first looking at how each was obtained.

Quick checklist

• What size range was measured?
• Was the polymer confirmed chemically?
• Were blanks run, and are the results given?
• Was recovery tested?
• Are laboratory preparation and clean air conditions described?
• If Py-GC/MS was used on biological samples, is the risk of false PE and PVC signals discussed?

For a study summary where these questions come up, see: Microplastics in artery plaque: what the NEJM study found, and what it didn’t

References

1. [9]Rauert, C. et al. (2025). Assessing the Efficacy of Pyrolysis-Gas Chromatography-Mass Spectrometry for Nanoplastic and Microplastic Analysis in Human Blood. Environmental Science & Technology, 59(4), 1984–1994. https://doi.org/10.1021/acs.est.4c12599 (Open-access full text: https://pmc.ncbi.nlm.nih.gov/articles/PMC11800385/)
2. [10]Koelmans, A. A. et al. (2019). Microplastics in freshwaters and drinking water: Critical review and assessment of data quality. Water Research, 155, 410–422. https://doi.org/10.1016/j.watres.2019.02.054

How to cite this post

Mikroplastikveri. (2026, November 2). How are microplastics measured? https://mikroplastikveri.com/en/posts/how-microplastics-are-measured/

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