Visible microplastics (1 mm - 5 mm)
For larger microplastics, quality assurance and quality control are not as elaborate as when identifying microplastics < 1 mm. The hot needle test can be appropriate to use for larger visible microplastics if looking to only confirm if an item is plastic or not. However, it is important to still limit potential extraneous contamination during processing when possible and follow the same steps as if searching for small microplastics (see below).
Microscopic microplastics (1 μm - 1 mm)
During sample collection, it is important to reduce and where possible account for all sample contamination with extraneous microplastics (i.e., plastics coming from sampling gear or air). This may be harder in some environments than others (e.g., if sampling on a boat). The effectiveness of various procedures to limit contamination have recently been investigated (Jones et al. 2024). In brief, four different sources of plastic contamination in laboratory procedures were assessed, including water (e.g., Milli-Q), airflow (e.g., fume hood), dust and consumables (e.g., glassware). Findings indicate water, air flow and dust can be significant sources of contamination, and should therefore be given careful consideration during study design. While ultrapure water (Milli-Q) and reverse osmosis were found to be the least contaminated sources of water for experiments, some contamination was detected in all water types. Glass or stainless steel equivalents often used to replace plastic consumables, as well the aluminium foil used to reduce airborne contamination can also be significant sources of contamination. Given how ubiquitous contamination was, procedural blanks (more below) and the benefits/risks of alternate approaches, such as washing plastic consumables with detergent so they can potentially be reused are tested and discussed at length in Jones et al. (2024). However, as QA/QC procedures can vary depending on the aims of the study (e.g., microplastics vs nanoplastics) it is strongly recommended that researchers familiarise themselves with the specific reporting requirements and methods relating to sources of contamination, mitigation, and implementation of controls (see also recommendations in Jones et al. (2024))
At a minimum, all experimental equipment should be rinsed 2-3 times in filtered/ultrapure water (e.g., Milli-Q) before use. Avoid the use of aluminium foil to cover samples and instead use glass lids (i.e., watch glass, Petri dish lid). Sample processing should be performed in a biological safety cabinet (BSC) or similar laminar flow cabinet (LAF bench), and not in a fume hood. BSC and LAF cabinets provide a sterile and particle-free workspace by directing filtered air over the work surface, ensuring protection for sensitive materials and experiments.
It is desirable that glassware is also decontaminated using an acid wash [e.g., diluted nitric acid solution (HNO3) or diluted hydrochloric acid (HCl)], following standard operating procedures for decontamination of glassware in the laboratory.
Frequent cleaning of the lab area (not just benches) with 70% ethanol and a lint-free cloth should be undertaken to reduce the build-up and distribution of laboratory dust. Other precautions to reduce the likelihood of plastic particles being carried into the lab on operators include: the use of a sticky mat at the entrance of the lab, restricting foot traffic in the laboratory space and limiting the number of personnel working in the immediate vicinity of the workspace. There has been much discussion of researchers limiting the use of synthetic clothing while working in the lab or using brightly coloured synthetics when synthetics cannot be avoided (to help with the identification of fibres). This is a precautionary approach. At present, there are no data to confirm this is beneficial and is an area that could benefit from further investigation.
A series of blanks and controls should be used through all stages of the methods. A list is provided below, and the reader is also referred to Jones et al. (2024) for a discussion of experimental design:
Field blanks: These are taken from field items used to perform the sample collection and capture contamination from field sampling gear (i.e., ropes, nets, hoses), sampling and storage containers, and materials from the vessel (i.e., paint, decking) or operator (i.e., clothing and PPE). Field blanks help assess whether contamination has occurred during the field sampling process.
Laboratory blanks: Similar to field blanks, these are taken from laboratory items used to perform the sample processing and capture contamination from laboratory gear and materials (i.e., paint, stoppers, O-rings, labels) or operator (i.e., clothing and PPE). Lab blanks help assess whether contamination has occurred during the sample processing.
Procedural blanks: These are matrix free blanks that are treated in the same manner as environmental samples, undergoing all processing reagents and run through all laboratory sample processing steps. They are generally run at the beginning, during and at the end of a sample set. They help identify contamination introduced during laboratory processing.
Airborne contamination controls: These controls capture any contamination that may be in the air during the collection and processing of samples.
Positive controls: These involve the use of known microplastic standards of different sizes, types, and compositions. They are used to calibrate instruments, validate analytical methods, and ensure the accuracy of microplastic recovery, identification and quantification.