Methods For Detecting And Representing Physical And Chemical Features Of Microplastics In Environmental Samples

Microplastic refers to small plastic pieces less than 5 millimeters in length or diameter,which mainly comes from the raw material leakage of plastic industry,breakdown of larger plastic items and micro beads in various of daily commodities.Due to its smaller size,microplastics have larger specific surface area,this characteristic enables them to absorb more pollutants,as well as gets inside of organisms much easier.That is to say,microplastics will cause more severe damage to the marine environment than larger plastic debris.One of the common difficulties in the study of microplastics is that,the sample isolated from different kind of environmental medium is a mixture containing a variety of materials with similar appearance,how can we distinguish plastic materials from these impurities?In this paper,we used stereoscopicmicroscope,micro-FTIR,micro-Ramaa,SEM/EDS and other instruments,to detect the physical characteristics,chemical composition and surface morphology of microplastics,thus allow us to explore and improve the identification and representing of microplastics.The main findings are as following:(1)For the detecting and representing of physical features,we took the accuracy and limitations of instruments into consideration,as well as the impacts of sample collection methods.Here we suggest that for the samples collected using plankton trawl,set the lower limit of size at 333 μm and the upper limit at 5 mm;for other samples,set the lower limit at 20 μm and the upper limit at 5 mm.In this way,we combine the reliability of identification instruments and operability of laboratory process to the most.For size division,it is recommended to keep the span between two dimension levels at the micron level,such as the size interval at 100,200,or 500 μm.During the identification process,the colored samples may have different detection results from uncolored ones for they contain various of pigment additives.In consideration of the probable differences caused by color,it is recommended to record the actual color ofthe sample such as transparent,translucent,white,red,yellow,blue,green,black,etc.To use the actual color instead of dark/light color,it will be more convenient to decide the identification of pigment additives in the subsequent instrument detecting process.Based on the frequency and feasibility,it is recommended to divide the shape as fiber,fragment,film/sheet,sphere/pellet.At the same time,it is suggested that the number,instead of weight,of samples should be used to interpret the abundance.And it is important to unify the way of representation among various environmental media.For sediment samples,use items/g(dry weight);and for organism samples,we should compare the size between individuals at first,if it is not applicable,use the tissue weight instead.(2)For the detecting and representing of chemical features,the feasibility of micro-FT-IR,micro-Raman and SEMIEDS in microplastic identification were investigated by using a large number of environmental samples for the first time.We have tested both LUMOS and Nicolet iN10 MX,using reflectance,transmittance and ATR mode mainly based on environmental samples.The conclusion of comparison is that Nicolet iN 10 MX provided with better results when using transmittance mode,the lower detectable limit is about 15μm.As for LUMOS,it is better to by applied under ATR mode for it integrates the ATR accessory inside its "smart objective",the lower detectable limit is about 10 μm.In addition to this,micro-Raman can also be used for routine detections of microplastics,but it is not as convenient as micro-FT-IR.The advantage of micro-Raman is that the information about the pigment in the sample can be inferred from the information below 600 cm-1 of wavelength.Furthermore,the lower limit of micro-Raman is smaller than that of micro-FT-IR due to its excitation light source.We have obtained Raman spectra of microplastics around 1 μm and successfully avoid the interference by changing the background material.At last,for the application of SEM/EDS,we have identified a lot of diatom,CaCO3 and coal ash from different environmental media,which provides the possible method to correct the basic data of microplastic abundance.We suggest that to use SEM/EDS as a supplementary tool to exclude non-plastic materials from microplastics,and improve the accuracy of data.