What methods exist for sample preparation of tissues and do they interfere with measuring MNPs?

If pertaining to digestion, enzymatic digestion (expensive) is the best to preserve material integrity. Oxidative, acid and alkalyne digestions all can affect plastics and alter morphology and/or surface chemistry. Use of solvent extraction methods may affect chemical signatures (for example, of sorbed contaminants or additives). If referring to tissue preparation for analysis, such as microtomy, the use of embedding media (such as parafin) may affect microplastic analysis, and, in that regard, cryosectioning would be preferable. 

Digestion of non-plastic organic matter (e.g. with H2O2, or KOH). If applied correctly, I am not aware that they interfere unduly with MNP determination.

Most sample preparation methods for biological tissues are based on acid or alkaline digestion, often combined with an oxidizing agent like H2O2. The choice of method is important as it foremost is problematic when the plastics are degraded during sample preparation. This needs to be avoided and on forehand it is needed to assess that this is not the case for the plastic(s) of interest. A second kind of interference with measuring MNPs is related to incomplete sample preparation with remainders of the biological matrix still in the sample. These remainders may directly interfere with the meaurements.

Various sample preparation methods are available for processing biological tissues, but some can interfere with the accurate detection and quantification of MNPs. Each  sample preparation  method has distinct mechanisms and different effects on the polymer's integrity and its analytical reliability. The alkaline digestion hydrolyzes proteins and lipids but it can also degrade some sensitive polymers like PET and some polyurethanes, thus affecting the spectral quality of the polycarbonate. The oxidative digestion is generally a safer approach for polymers, although an extended exposure can alter the appearance of colored particles. The  enzymatic digestion has the ability to break down proteins and fats having a minimal impact on most polymers, thus making it one of the safest options. On the other hand, the acid digestion with the aid of strong acids can destroy a great number of common plastics such as PVC, PA, PU, ABS, PMMA and it is not recommended for plastic analysis. Some mechanical treatments such as freeze–thaw cycles or homogenization could help break down the  tissue physically but could also fragment the plastic particles.

Digestion is acid or alkali is the best approach in preparing biological tissues for MNP analysis, although acid digestion has been shown to destroy some plastic types.  Availability of some labeled internal standards and inclusion of suitable surrogate standards can improve confidence in data quality.   

H202, KOH, bleach, acids, vinegar, hot water, enzymes. Yes, some do interfere especially those that are not compatible with some polymers.

Common sample preparations are digestion and separation (filtration or density separation). In some cases, small particles could loss due to absorption, and some type of particles may be destroyed by digestion as well.  

Here are some of the methods for sample preparation of tissues:
1. Enzymatic digestion
2. Alkaline Digestion (e.g., KOH)
3. Oxidative Digestion (e.g., H₂O₂, Fenton’s Reagent)
4. Solvent Extraction

Their interference with measuring MNPs depends on the polymer types and the tissue types.