Assessment of quality control measures in the monitoring of microplastic: a critical review

ABSTRACT

Microplastics usually refer to solid plastic particles with particle size less than 5 mm. As a new type of pollutant unavoidable produced during human production and living activities, microplastics affect the ecological environment and human health safety. Since microplastics are widely distributed, small-sized microplastics and microfibers may be introduced from ambient air, sampling equipment and laboratory equipment during various monitoring process. Strict quality control measures were required to ensure the accuracy of monitoring results. In order to evaluate the compliance of quality control measures adopted in current researches, eight quality control parameters were statistically summarized by reviewing 30 published researches involving microplastic quality control methods. The results showed that those eight quality control measures could not be fully covered in most studies and uniform standardization of some key quality control measures have not been achieved. It was suggested to develop standardized operational protocols and perform cross-calibration among laboratories.

GRAPHICAL ABSTRACT

KEYWORDS:

• Microplastic
• environmental monitoring
• quality control
• cross-contamination

Introduction

The concept of microplastic was first introduced by Thompson in 2004. Microplastic usually referred to plastic particles or plastic fragments with their longest diameter less than 5 mm, which was a new type of pollutant inevitably produced during human production and living activities [1]. At the early stage, there were many researches on the sources and occurrence of microplastics in marine environment [2]. The microplastics were widely distributed in the coastal to oceanic circulation zone, especially in some vortex center area with high local abundance of 1000–2500 g/km² [3]. Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP) [4] and National Oceanic and Atmospheric Administration (NOAA) [5] have published experimental methods guidelines for monitoring microplastics in the marine environment, respectively. Since then, traces of microplastics have been found in freshwater, soil, sediment, atmosphere, drinking water, and diet [6–8]. Due to the property of small size and difficult degradation, microplastics can not only be transported over long distances and persist in the environment, but can also be ingested by organisms, such as zooplankton, shellfish, fish, birds, mammals and migrate along food chain [9]. Microplastics eventually accumulate into human body through the cumulative effect of biological chain, affecting the ecological environment and human health safety.

The monitoring and analysis process of microplastics mainly included three parts: sample collection, separation and extraction, and characterization, of which relevant research techniques are becoming increasingly mature. However, the color, size, morphology, composition and matrix of microplastics were highly diverse, leading to the lack of unified and standardized monitoring operational procedure used in research. The results were not comparable among laboratories, which was not conducive to the study of microplastics monitoring worldwide. In addition, unlike general pollutants, microplastics existed widely in man-made and natural environments. Samples were very susceptible to cross-contamination microplastics (especially those smaller than 500 μm) in the whole monitoring process, which can easily affect the accuracy of microplastic abundance results. For example, in the monitoring of microplastics by Klein and Fischer [10], fibers in the procedural blank accounted for 51% of the total microplastics, whereas fibers in the real samples accounted for only 5% of the total microplastics, which obviously cannot allow the application of conventional quality control methods to microplastics studies. Quality control was essential in the monitoring of microplastic to ensure the accuracy and reliability of the result data generated. Without quality control measures, the results of the monitoring may be unreliable, making it difficult to accurately assess the level of pollution in the environment. Quality control involved the implementation of a set of standard operating procedures, which included calibrating instruments, using appropriate reference materials, and adhering to established protocols for sample collection, preparation, and analysis. By following these procedures, any sources of error or bias could be identified and minimized, thereby ensuring the accuracy of the data. Furthermore, quality control measures enable the comparison of data generated over time or across different locations, facilitating the identification of trends and patterns in the levels of microplastics. Therefore, the selection of appropriate quality control (QC) methods was important for the avoidance and prevention of cross-contamination. In this paper, we compared and evaluated whether appropriate and comprehensive QC measures have been applied for microplastics monitoring in different matrices, then discussed the necessity of QC measures and specific recommendations based on potential cross-contamination risks.

Methods

By reviewing the ISI Web of Knowledge, 30 research articles were summarized and analyzed by using ‘microplastic quality control’ as keywords for retrieval. The quality control measures in sampling, pretreatment (digestion and separation), and characterization procedures used in 30 references were compared [11–40]. At present, only a few studies have reported the ground criteria regarding quality control in microplastic research [41]. Hermsen [42] and Koelmans [43] have initially established their own analysis and evaluation systems for microplastics research methods in freshwater environments and organisms, with the aim of providing certain significance for standardizing analytical methods and improving experimental reliability. Based on the operational procedures in published studies described above, eight important quality control parameters were selected to evaluate the 30 studies by presenting assessment scores (Table 1). If the specific QC parameter was not conducted or mentioned, score 0 was assigned, otherwise score 1 was assigned. This assessment does not provide an absolute judgment of the reliability of the 30 studies, but is an indicator of cross-contaminant potential risk assessment. The eight parameters included (a) using cotton lab coats and wearing powder-free natural latex gloves or equivalent protection; (b) performing all laboratory work in clean laminar flow hoods, clean rooms with controlled access air, or other equivalent conditions; (c) covering samples with aluminum foil hats; (d) exclusive using glass or metal materials and avoiding plastic materials; (e) cleaning materials and equipment/burning filters prior to use to remove potential contaminants; (f) pre-filtrating all solution; (g) performing negative control; and (h) performing positive control. The above eight QC measures were classified into four groups: a-c for quality control of experimental environment and airborne contamination, d-f for quality control of cleaning and maintenance of experimental equipment and solution, g for negative control (blank experiment), and h for positive control (spiked recovery experiment). The 30 research studies covered various matrices, among which the sampling methods for microplastics differed somewhat, while the pretreatment and characterization methods in the laboratory were generally similar. Most of the eight quality control measures mentioned were QC methods for separation, purification, and characterization procedures in the laboratory, which were applicable to different matrices containing microplastics. Table 2 shows various sampling methods for microplastics in different matrices.

Table 1. The individual and accumulated scores of papers reporting eight parameters on microplastic quality control measures, from the best of authors knowledge and interpretation of the manuscript.

Reference	Type	Lab and air contamination			Equipment and solution cleaning and maintenance			Negative control	Positive control	Total Score
		a	b	c	d	e	f	g	h
[11]	Freshwater	1	1	1	0	1	0	1	1	6
[12]	Drinking water	1	1	1	0	1	0	1	1	6
[13]	Air	1	0	1	1	1	1	1	0	6
[14]	Freshwater	0	0	1	1	0	0	1	0	3
[15]	Drinking water	1	1	0	1	1	1	1	1	7
[16]	Seawater	0	0	1	0	1	1	1	0	4
[17]	Wastewater	1	1	0	0	1	0	1	1	5
[18]	Freshwater	1	0	1	0	1	0	1	0	4
[19]	Freshwater	1	1	1	1	1	0	1	1	7
[20]	Sediment	1	0	0	1	0	0	1	1	4
[21]	Wastewater	1	1	1	1	1	1	1	1	8
[22]	Soil	1	0	1	0	1	1	1	0	5
[23]	Sediment	1	0	1	0	1	1	1	0	5
[24]	Wastewater	1	1	0	1	1	1	1	1	7
[25]	Air	1	1	1	1	1	0	1	1	7
[26]	Organism	1	1	1	1	1	1	1	1	8
[27]	Seawater/Organism	1	1	1	1	1	1	1	0	7
[28]	Sediment	1	1	1	1	1	1	1	1	8
[29]	Sediment	1	1	1	1	1	1	1	1	8
[30]	Freshwater	1	1	1	1	1	1	1	0	7
[31]	Seawater	1	0	1	0	1	1	1	0	5
[32]	Organism	1	1	1	0	1	1	1	1	7
[33]	Sediment	1	0	1	0	1	0	1	0	4
[34]	Organism	1	0	1	1	1	0	1	0	5
[35]	Seawater	1	0	1	1	1	0	1	0	5
[36]	Seawater/Sediment	1	0	0	1	1	1	1	0	5
[37]	Organism	1	0	1	0	1	1	1	1	6
[38]	Freshwater	1	0	0	0	1	1	1	0	4
[39]	Freshwater	1	1	1	0	1	0	1	0	5
[40]	Freshwater	1	0	0	0	1	0	1	0	3
Average		0.93	0.50	0.77	0.53	0.93	0.57	1.00	0.47	5.70

Table 2. Sampling methods for microplastics in different matrices.

Matrices	Sampling methods	Matrices	Sampling methods
Water body	Trawl sampling; Pump collection; Manual direct collection	Soil/Sediment	Surface sampling; Deep sampling
Organism	Biological tissues collection	Air	Wet deposition; Dry deposition

Results and discussion

Quality control of experimental environment and air cross-contamination

Among the 30 selected microplastic studies, protective measures such as lab coats and gloves were mentioned in 28 studies (93%). Microfibers, due to their small size and low density, accounted for a larger proportion of microplastic cross-contamination compared to fragments. Chemical fiber clothing was a major source of airborne microfibers. In addition to providing protection for experimenters, cotton lab coats or other clothing made from natural textile fibers can prevent the release of microfibers and other synthetic components to some extent. The usage of gloves was controversial, with most studies involving experimenters wearing nitrile or latex gloves. Witzig et al. immersed nitrile, latex, neoprene, and powder-free gloves produced by different manufacturers in water for 5 h to observe the precipitation of microplastics [44]. It was shown that polyethylene was present in the leachate of all types of gloves, while the long-chain compounds stearates and fatty acids used during the experiments may be misidentified as polyethylene, resulting in false-positive monitoring results. The study by Abel et al. [28] reached 88% recovery of microplastics with keeping hands clean and without wearing any gloves throughout the experiment. However, in the process of microplastic treatment, the acid or alkaline digestion may cause personal injury to the experimenters. When nitrile gloves were immersed in the solvents for a controlled period of time, the low concentration of microplastics precipitation was less influential. It was still recommended to wear lab coats and gloves throughout the experiment. Brightly colored style could be chosen to help identify false-positive cross-contamination of microfibers and microplastics.

Among the 30 selected microplastic studies, conducting laboratory procedures in a controlled air environment were mentioned in 15 studies (50%). Clean laminar flow hoods are designed to make air passing through a high efficiency HEPA filter at a certain air velocity to form a uniform flow layer and allow a vertical unidirectional flow of clean air, which prevent the entry of uncontrolled air and ensure the required cleanliness of the process in the working area. Many laboratories equipped without laminar flow hoods use fume hoods to control airborne cross-contamination. However, fume hoods can only extract unfiltered air from the front end of the laboratory, mostly for extracting and removing toxic and hazardous gases in the laboratory. Local exhaust air cannot guarantee the cleanliness of the entire experimental area. In addition, some studies [19,45] controlled air cross-contamination in a relatively simpler way, only requiring experimental operations to be conducted in a room with closed doors and windows. Wesch et al. [46] chose four working environments with different QC measures for air cross-contamination test, namely indoor laboratory, mobile laboratory, fume hood, and clean workbench with laminar flow hood. The results showed that the highest rate of air cross-contamination microfibers appeared in the exposed environment of the indoor laboratory (95%), followed by the mobile laboratory (89%), and significantly decreased to 50% in the fume hood experimental environment (open filtration). Only 3.7% of microfibers were found in the clean laminar flow hood environment, which demonstrated the use of fume hoods or laminar flow hoods was effective in controlling microplastic air cross-contamination in the laboratory. For the first time, Prata et al. [47] researched the false positives caused by air cross-contamination microplastic at the sampling site (outdoor). Compared with the more controllable indoor clean environment, more suspected airborne microplastics and fibers were found in outdoor. However, due to the operational difficulty of ensuring outdoor air clean, there are few publications about the subject.

Among the 30 selected microplastics studies, the use of aluminum foil, tin foil, and glass lids to cover sample containers was mentioned in 23 studies (77%). In general, temporarily unused microplastic samples and sampling equipment need to be covered with non-plastic covers to prevent air contamination. International Council for the Exploration of the Sea (ICES) drafted a specification for monitoring microplastics in marine fish in March 2015, suggesting that the collected biota samples should be wrapped in aluminum foil and stored frozen at −20°C during transportation, or be preserved in solution such as formalin and ethanol [48].

Quality control of cleaning and maintenance of experimental equipment

Among the 30 selected microplastics studies, avoiding plastic material products and using metal or glass material products instead was mentioned in 16 studies (53%). However, some experimental procedures cannot avoid the use of plastic products. In the field sampling procedures, using plastic trawls such as manta nets and neuston nets was still the mainstream way for microplastics sampling in water. Trawl sampling method can be replaced by direct sampling method combined with metal sieves for filtering, which was the secondary preferred sampling method for most studies as more power requirement and smaller sampling volume. In addition, microplastics tended to get stuck in the mesh of both metal sieves and plastic trawls, so the equipment for water sampling should be carefully maintained and flushed [49]. In the laboratory pre-treatment procedures, many plastics components were also involved, such as ultrapure water purification system, washing bottles (perfluoroalkoxy alkane, PFA) [12], and pasteur pipettes (polyethylene, PE) [18]. After density flotation, supernatant containing microplastics was usually vacuum filtered onto filter membranes, which in many studies still were made of plastic that may cause cross-contamination. The types of plastic material membranes included polycarbonate filter (PC, 8 µm pore size) [17], polytetrafluoroethylene membrane (PTFE, 0.45 μm pore size) [25] and nylon filter membrane [33]. From the perspective of quality control, non-plastic filter membranes such as silver membranes (pore size 5 μm) [24], nitrocellulose nitrate membranes (pore size 8 μm) [11] and stainless-steel filter (pore size 20 μm) [28] were more recommended.

Among the 30 selected microplastics studies, properly cleaning materials and equipment or burning filters prior to use was mentioned in 28 studies (93%). Studies usually used washing methods to prevent cross-contamination from experimental materials, most commonly using distilled water, Milli-Q water, or pre-filtered water to thoroughly rinse containers and other materials. Some studies also reported immersing glassware in acidic solution overnight to completely remove adhering microplastics [26]. For metal utensils that would be corroded by acid, ethanol was used as cleaning solution [30,35]. Compared to washing method, the thermal treatment method cleaned utensils more thoroughly. Utensils were heated in a muffle furnace at 450°C to eliminate airborne microplastics that may adhere to the glass surface [21,35]. Prata et al. [41] conducted comparative research on thermal treatment of glass fiber filters for experimental equipment, which showed the results that 15 min of thermal treatment at 450°C was not enough and 3 h of thermal treatment was required to successfully remove contaminated microplastic fibers and fragments.

Among the 30 selected microplastics studies, pre-filtration of all solution was mentioned in 17 studies (57%). A variety of chemical solution was required in the sample pre-treatment procedures, such as H₂O₂, HCl, and HNO₃ in the digestion process, NaCl and NaI in the flotation process, and Nile Red in the dyeing process. The solution was highly likely to be contaminated by microplastics during placement, resulting in false positive. There were many studies on the effect of placement condition on microplastic contamination in different solutions. Ravanbakhsh et al. [50] studied the effect of time and light on the microplastic content in drinking water. Darena et al. [51] found that the average microplastic content in reused drinking water bottles was 118 ± 88 ind./L, while the microplastic content in disposable mineral water bottles was only 14 ± 14 ind./L. Qin et al. [52] studied the degradation of PC films in three various aqueous media: deionized water, H₂O₂ hydroxyl radical solution, and mixed acid solution. Differed from the general perception, the results showed that the concentration of degraded microplastic particles found in deionized water was the highest. Thus, it was necessary to filter all the solution using filter membrane, whose pore size was generally smaller than that of the target microplastics. The most commonly used types in references were nitrocellulose membranes, sterile acetate cellulose membranes, PTFE membranes (0.45 μm pore size) [15,26,27], and glass fiber filter membranes (0.2–1.6 μm pore size) [28,30]. Solution with different chemical properties was also filtered in different ways. For example, in the research of Tsering et al. [21], enzyme solution and SDS solution was filtered through stainless steel mesh with 20 μm pore size. Sulfuric acid solution or KOH solution which dissolved membranes was not filtered while other solution was filtered through 1.2 μm glass fiber filters prior to use.

Negative control

Among the 30 selected microplastics studies, using blank as negative control during the procedures was mentioned in 30 studies (100%). Although the negative control measure was highly used in the studies, the specific experimental methods varied greatly. Only five out of the 30 studies provided clear information on the quantities of blank samples, spiked samples and actual samples, and the proportions of these three types of samples varied among studies [12,20,25,32,37]. Some studies reported that no microplastic contamination was found in blank experiments [34,35], presumably due to the high LOD of microplastics detection size, while some studies did not specify the concentration of microplastics in the blank control group despite conducting blank experiments [16,17]. There were also obvious differences in values and units among studies that reported the content of microplastics in the negative control, such as 100 ind./m³ [14], 10.33 ind./L [15], and 0.9 ind./sample [23], which was mainly caused by the ambiguous definition of negative control and unclear description of actual experimental operation. For example, the microplastic sampling methods in the studies of Talbot [53] and Kaliszewicz [54] both collected samples by in situ trawling, while the field blank methods were different. In research of Talbot et al., a field control jar containing deionized water opened at the sampling point during active sample collection was considered as blank control background. In research of Kaliszewicz et al., 14.4 dm³ deionized water was poured into the plankton net and then transferred to the sampling bottle as blank control. Some negative control measures were less reliable but still provide some useful information on the level of contamination, such as placing clean paper filter/clean film next to the samples to correct the environmental background [20,33]. Both high-viscosity transparent film and clean filter paper could be used to capture microplastics that may cause cross-contamination in the air of the work area. The captured microplastics are then observed under a microscope to obtain the blank value.

Procedural blank generally included field blank and laboratory blank. Due to the particularity of microplastics sampling, many studies tended to omit field blank and only perform laboratory blank. Usually, laboratory blank was conducted by replacing the real water sample with filtered water and subjected to the same experimental steps of placement, filtration, and pretreatment (digestion, peroxidation, extraction) as the real sample [13]. For the study on microplastics in sediments, Xue et al. [55] used quartz sand as blank control, which was treated with the same laboratory experimental procedure used for the real sediment samples. Field blank, on the other hand, was usually ignored by most researches and varied among sampling methods. There were 16 chosen papers related to microplastics in freshwater and seawater, respectively, classified by sampling methods [14,53,54,56–84].

The methods of sampling microplastics in water can be divided into volume-reduced sampling and bulk sampling. Different sampling methods caused great diversity in sample representativeness. Trawl sampling was a typical volume-reduced sampling method (sample concentrated) that mainly used vessels towing neuston net, manta net, or other types of plankton net to sample at the surface of water, with flowmeters to determine the amount of water passing through the net. The main advantages of the trawl collection were large sampling volume and sampling area, which was especially suitable for low microplastic concentration water area, while the disadvantages were expensive equipment and vessel requirement. Trawls were mainly composed by nylon plastic (polyamide fiber), which could be the source of microplastic cross-contamination. The pore sizes of most manta trawls varied from 112 to 500 μm, which may result in the missing of small-sized microplastics. The pore sizes of plankton trawls were smaller, ranging from 20 to 300 μm, while the surface resistance was relatively large during the sampling procedure, which was difficult to drag and easily to be broken. Bulk sampling included pump collection and manual direct collection methods. By in situ pump-powered filtration, larger volume of water sample could be collected with less manpower consumption, theoretically allowing the collection of smaller particle size and lower visibility microplastics present. In addition, the in situ pump collection method reduced the possibility of sample exposure to the surrounding environment and mitigated cross-contamination. The main disadvantage was the requirement of energy power to operate electric pump equipment. In the work of Li et al. [84], an in situ filtration plankton pump (KC Denmark A/S, Denmark) equipped with an external battery and pressure compensation was used to sample. As to manual collection method, the sampler/bucket collected water samples were sent to the cabin or laboratory for sieving and filtering, which was easy to operate and the volume of collected water was also easy to calculate. As no energy equipment was required, the method has little environmental pollution problem. However, smaller sampling volume and sampling area may result in the lack of sample representative.

Among 16 studies on freshwater microplastics collection, 10 used trawl collection method, one used pump collection method, and four used manual direct collection, while one used trawl collection method combined with manual direct collection. Among 16 studies on seawater microplastics collection, eight were collected by trawl, five by pump, and three by manual sampler. Overall, trawling was still the dominant sampling method in both freshwater and seawater. For the remaining two bulk sampling methods, pump sampling was mostly used in seawater while manual direct sampling was mostly used in freshwater. The field blank in manual direct collection method was simple, which was usually performed by filling glass or metal containers with deionized water under the environmental conditions at sampling sites. In contrast, the field blank in trawling method was performed more complicated. According to the requirements for the collection of field blank samples of seawater in the technical regulation for marine microplastics monitoring of China National Marine Environmental Monitoring Center [85], when using neuston net, manta net or catamaran net for sampling, the outside of mesh should be washed with seawater from top to bottom. The blank water sample obtained was transferred from the net bottom tube to glass container/jar. In Hung’s study [86], manta mesh was rinsed by deionized water instead of seawater to obtain field blank water sample. The different selection of the types of blank water would inevitably result in the differences of microplastic abundance results.

Figure 1 shows the distribution of pore size of plankton trawl, manta trawl (concentrated sampling) and filter membrane (bulk sampling) used in freshwater and seawater sampling. In the 32 selected papers, the pore sizes of trawls used in freshwater sampling were 60–120 μm for plankton trawl, 300–500 μm for manta trawl and 20–63 μm for filter membrane, while the pore sizes of trawls used in seawater sampling were 20–200 μm for plankton trawl, 250–500 μm for manta trawl and 1–250 μm for filter membrane. As can be seen from Figure 1, in addition to the glass fiber membrane used in the study of Cincinelli [72] with the diameter of only 1 μm, the minimum filter membrane pore size in other studies was 10 μm. Compared with the pore size on trawl mesh, the aperture size of filter membrane using in the bulk sampling method was finer, resulting in higher microplastic abundance. Generally, the collected microplastics were slightly smaller than the mesh size of the trawl net. Microplastic particles larger than the mesh size of the trawl net could be quantitatively analyzed, while microparticles fraction below the size could also be characterized for the qualitative analysis study. However, the operation was more time-consuming and prone to mesh clogging. The manta trawl with larger pore size was generally more suitable for the study of larger particle size microplastics, while the plankton trawl with moderate pore size was applicable for targeted microplastics of medium particle size. Scientific standardization of trawl and filter membrane pore size was of great significance for the study of microplastic abundance.

Figure 1. The distribution of pore size of plankton trawl, manta trawl and filter membrane used in freshwater and seawater sampling.

Positive control

Among the 30 selected microplastics studies, using microplastics of known polymer identity and targeted sizes as positive control was mentioned in 14 studies (47%), to quantify the false low value results during microplastics processing. To facilitate observation, standard microplastics with obvious characteristics, clear identification, and homogeneous particle sizes were generally chosen. Commercial standard microplastic products had higher recoveries compared to laboratory manually cut fragments/manually ground particles [11]. Standard microplastic samples were added to water or sediment samples then pre-treated, after which the recovery of standard microplastic was calculated. In study by Exposito et al. [32], standard commercial PE microplastics of different particle sizes were added in real samples, which showed the recovery of 60% for 53–63 μm and 125–150 μm , 74% for 250–300 μm and 99% for 425–500 μm. The pore size of microplastic particles was positively correlated with their recovery. The study of Tsering et al. [21] was also consistent with the above result, with the average recovery of microplastic fragments (77–80%) being much higher than that of microfibers (20–33%). Moreover, the recovery of microplastics decreased with increasing sample processing steps. For example, more complicated methods were generally used for the extraction of microplastics from sediment and soil matrices, such as density flotation, whereas the extraction of microplastics from water samples was easier. The recovery of microplastics from sediment and sludge was generally lower than that of microplastics from aqueous matrices [21]. For example, in the study by Horton et al. [24], the recovery of PA particles in sludge was 52.4% while in wastewater was 101%. Similar to the condition of negative control, most of the current studies on spiked positive recovery involved only the pre-treatment procedure within the laboratory and omitted sampling procedure [87]. Borton et al. [88] added microplastic particles at a concentration of 100 MP/m³ in a continuous stirred stainless-steel tank (containing 2400 m³ filtered tap water). The experiments collected data on recoveries by modeling different turbulence conditions and sampling depth. The maximum recovery of microplastics sampled from the water surface was 31.4%, which indicated that the current measured data of recovery in most researches including only the laboratory pre-treatment might be inaccurate. Microplastics reported in environmental monitoring studies might be less than the real concentration.

General discussion of quality control measures

The cumulative total score of studies involving the parameters of quality control measures was evaluated (Figure 2a). In descending order of frequency, eight QC measures mentioned in the studies were g (100%) > a (93%) = e (93%) > c (77%) > f (57%) > d (53%)> b (50%) > h (47%). Compared to the previous review of microplastic quality control, most recent studies have more comprehensive QC measures, consciously increasing the emphasis on quality control and taking corresponding measures. Measures such as a (wearing lab coats and gloves) and measure e (cleaning materials and equipment) were relatively easy to operated, resulting in more frequency presence in studies. Some measures were limited by experimental condition, such as measure b requiring laminar flow hoods, while some measures were time consuming, such as measure f requiring to pre-filter all solution and h requiring to perform the recovery experiment, which naturally made these QC measures less frequently adopted in research. Moreover, although all the 30 researches mentioned blank control measures, it was difficult to achieve standardization in the specific experimental operation as discussed above. The average cumulative total score of the 30 researches was 5.70, among which four researches got the highest score of 8 (accounting for 13%), and 19 researches got 5–7 scores (accounting for 64%), and seven researches got 3–4 scores (accounting for 23%). In Figure 2b, the number of references for each microplastic matrix was displayed alongside its corresponding average cumulative total score, which ranged from 5 to 7 for different matrices. Most of the studies described the necessary quality control measures avoiding cross-contamination, but the detailed descriptions of the sampling and pretreatment steps still need to be further optimized and supplemented.

Figure 2. Cumulative total score and percentage of individual quality control parameters (a); the number of references on different microplastic matrices and their corresponding average cumulative total score (b).

Conclusions

It was important to take appropriate quality control measures to prevent environmental cross-contamination during the process of microplastic monitoring, which affected the reliability and accuracy of results. In this paper, 30 published researches involving microplastics quality control measures were selected to statistically summarize eight important quality control parameters that might occur frequently in each process. The quality control measures of experimental environment and air cross-contamination, cleaning and maintenance of experimental equipment, negative control and positive control were of great significance for the standardization of microplastics analysis methods and the improvement of the reliability of results. Especially for the sampling methods and negative blank control, it was suggested to describe the corresponding experimental methods and procedures more clearly in the manuscript. However, it should be noted that the number of reviewed references in this study was still limited. The discussions on quality control methods during on-site sampling mainly focused on waterbody matrices, while discussions on other microplastic matrices were relatively few. The study of quality control methods for microplastic still needed to be more comprehensive and accurate so as to establish the evaluation system and methodological standards of microplastics monitoring methods in the future.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported the Projects of State Environmental Protection Key Laboratory of Quality Control in Environmental Monitoring [KF202205].