What are the potential interferences from biological matrices and do these depend on the tissue type and plastic chemistry?

Organic matter can impact microscopy and spectroscopic analyses; spectroscopic signatures can be affected by the presence of endogenous compounds, such as pigments. It is likely that tissue types will affect analyses. For example, fatty tissues are richer in lipids, harder to remove. Muscle tissues, on the other hand, are likely to be harsher to digest, due to higher protein content. 

Potential interferences will include lipids and other natural organic polymers that spectrally resemble MNP polymers. Such interferences will depend on the tissue type and plastic chemistry.

Potential interferences from biological matrices are significant during microplastic extraction and  depend on both tissue type and plastic chemistry. Some of the most common potential interferences from biological matrices are due to the lipids  that are found in fatty tissues that could interfere with the density separation by altering the viscosity and proteins that could  develop gels of precipitates able to retain microplastic particles. Other potential interferences that may appear are the ones due to calcium and bone fragments that are found in calcified tissues and could alter the separation density of the digestion of microplastics or either they could coprecipitate with the plastics particles. Some interferences could arise  in the case of highly vascularized tissues  due to the fact that the pigments and hemoglobin that they could bind to the plastic changing its surface properties. Interferences could also appear in the case of tendons, cartilage and skin tissues  due to their ability of resisting enzymatic and chemical digestion that could lead to an incomplete tissue breakdown and to the possibility of particles remaining  encased in the tissues. In samples that have microbial activity, they might interact with certain types of plastics and alter their structure and surface properties as well as they could form microbial biofilms on the plastic surface makin the recovery very hard.

There are two types of issues here:
1 - Degradation/removal of plastics; 2 - interference of either remainders of the biological matrix or degradation products of the biological matrix with plastics during the analysis.

1 - Degradation/removal of plastics will depend mostly on the digest used. The consequence of degradation/removal is underestimation of the actual amounts of plastics present in the sample and/or overestimation in case of fragments formed that are (partly) similar to the fragments present in the plastics of interest.
2 - Remainders of the biological matrix can also lead to underestimation of actual plastics concentrations in case of incomplete digestion and related retention of the plastics. Overestimation due to the presence of molecular fragments derived from the biological tissues and which are also present in the MNPs being quantified, is however most common.

Fatty acid esters (e.g., stearate) are potential interferences in biological tissues when pyrolysis GCMS and spectroscopy methods is used in the analysis of PE.  Yes, this will depend plastic chemistry and tissue type.  Because studies analyzing MNPs in biological samples are not many, more such interreferences are yet to be discovered.  

Many natural polymers (e.g., cellulose) are often misidentified as MNPs when spectroscopic methods are used.  

Yes, interference with the target MNP depends on tissue type: large protein structures (somewhat if not digested), polysaccharides (if not digested), fats (especially PY-GC/MS). If in sufficient amounts these substances can overwhelm the target analyte signal.

Plastic chemistry -- biological systems are water based, so polymers such as condensation polymers, which have more polar moieties,  will have more interaction with biological systems and a greater tendency to be 'comfortable' in those systems.

Lipids including fatty acids, wax, sterols, triglycerides, and phospholipids are potential interferences from biological matrices. They are dependent on the the tissue type, as certain subcategories of lipids can be rich in areas.

With spectroscopy, any particle or coating of biological matter could interfere with polymer identification of the plastic particles in the samples. With py-GCMS, biological materials can interfere with some polymers, most certainly PVC. 

Biological matrices do potentially interfere MNP analysis and the interference depends on the tissue type and the plastic chemistry.
There are different type of tissues, fatty tissues, protein-rich tissues, calcified tissues, and they have different compositions. 
On the other hand, there are different polymers, hydrophobic polymers (PE and PP), polar polymers (PLA, PET), and polymers of medium polarity (poly acrylates, polystyrene, and so on). 
The interferences also depend on the analysis methods.