Matrix Removal and Purification of Microplastic Samples
For a correct identification of the microplastics, it is necessary to eliminate all the
organic and inorganic compounds adhered to the surface thereof. In addition,
particularly matrix removal, it is necessary to remove microplastic from biological
samples. The softest method to clean samples is washing with fresh water
(McDermid and McMullen 2004). Other purification techniques have also been
used in the literature, for example, ultrasonic cleaning, treatments with hydrogen
peroxide, and treatments with mineral acids (Andrady 2011; Cooper and Corcoran
2010; Liebezeit and Dubaish 2012).
Other authors have used mainly 37% of hydrochloric acid (HCl), various concentrations of sodium hydroxide (NaOH), and 30% of hydrogen peroxide (H 2 O 2 ) or
a specific mixture of them, for the tissue digestion of biotic samples (Claessens et al.
2013; Davidson and Dudas 2016; Dehaut et al. 2016; Löder and Gunnar 2015;
Lusher 2015; Zhao et al. 2017). However, special care must be taken in the use of
these techniques because some plastics can react especially to strong acid or alkaline
solutions (Liebezeit and Dubaish 2012; Claessens et al. 2013). It notably restricts the
applicability of these reagents. In this sense, the most promising technique is the use
of enzymatic digestion which has shown good preliminary results (Cole et al. 2014;
Catarino et al. 2017).
1.3.3 Quantification and Identification of Microplastics
Manual Counting by Visual Identification
The use of microscopes is widely extended to identify microplastics (Hanvey et al.
2017). The main drawbacks of this technique are the limitation in the identification
of particles below a certain size and an excessive slowness. Another major drawback
is that the quality of the data produced depends to a large extent on the microscope
used, the characteristics of the person performing the study, and the sample matrix
(i.e., sediment or intestinal content). Finally, mistakes by counting nonplastic particles as plastic can be made. According to all the limitations mentioned, the error rate
of the visual classification increases with decreasing particle size and can fluctuate
from 20% to 70% (Eriksen et al. 2013; Hidalgo-Ruz et al. 2012). This is the reason
why it is important to analyze then the particles by other methods for a correct
identification of plastics (Dekiff et al. 2014; Hidalgo-Ruz et al. 2012).
Norén (2007) suggests the following criteria for the visual identification of larger
microplastics: (1) in the plastic particle, no structure of biological origin should be
distinguished, (2) the plastic fibers must have a folded three-dimensional shape and a
similar thickness to assure that there is no biological origin, (3) the particles should
be of homogeneous color, and (4) those transparent or whitish particles should be
inspected with the support of fluorescence at high magnification to exclude an
organic origin.
1 Microplastic Pollution in Water
9
For a correct identification of the microplastics, it is necessary to eliminate all the
organic and inorganic compounds adhered to the surface thereof. In addition,
particularly matrix removal, it is necessary to remove microplastic from biological
samples. The softest method to clean samples is washing with fresh water
(McDermid and McMullen 2004). Other purification techniques have also been
used in the literature, for example, ultrasonic cleaning, treatments with hydrogen
peroxide, and treatments with mineral acids (Andrady 2011; Cooper and Corcoran
2010; Liebezeit and Dubaish 2012).
Other authors have used mainly 37% of hydrochloric acid (HCl), various concentrations of sodium hydroxide (NaOH), and 30% of hydrogen peroxide (H 2 O 2 ) or
a specific mixture of them, for the tissue digestion of biotic samples (Claessens et al.
2013; Davidson and Dudas 2016; Dehaut et al. 2016; Löder and Gunnar 2015;
Lusher 2015; Zhao et al. 2017). However, special care must be taken in the use of
these techniques because some plastics can react especially to strong acid or alkaline
solutions (Liebezeit and Dubaish 2012; Claessens et al. 2013). It notably restricts the
applicability of these reagents. In this sense, the most promising technique is the use
of enzymatic digestion which has shown good preliminary results (Cole et al. 2014;
Catarino et al. 2017).
1.3.3 Quantification and Identification of Microplastics
Manual Counting by Visual Identification
The use of microscopes is widely extended to identify microplastics (Hanvey et al.
2017). The main drawbacks of this technique are the limitation in the identification
of particles below a certain size and an excessive slowness. Another major drawback
is that the quality of the data produced depends to a large extent on the microscope
used, the characteristics of the person performing the study, and the sample matrix
(i.e., sediment or intestinal content). Finally, mistakes by counting nonplastic particles as plastic can be made. According to all the limitations mentioned, the error rate
of the visual classification increases with decreasing particle size and can fluctuate
from 20% to 70% (Eriksen et al. 2013; Hidalgo-Ruz et al. 2012). This is the reason
why it is important to analyze then the particles by other methods for a correct
identification of plastics (Dekiff et al. 2014; Hidalgo-Ruz et al. 2012).
Norén (2007) suggests the following criteria for the visual identification of larger
microplastics: (1) in the plastic particle, no structure of biological origin should be
distinguished, (2) the plastic fibers must have a folded three-dimensional shape and a
similar thickness to assure that there is no biological origin, (3) the particles should
be of homogeneous color, and (4) those transparent or whitish particles should be
inspected with the support of fluorescence at high magnification to exclude an
organic origin.
1 Microplastic Pollution in Water
9