I worked with medical devices, which is different from the microplastics you are after.
For example, if we had plastics in a liquid, I would take a dropper and a watchglass and put an ml or so of liquid in the watchglass. Hopefully, I could get at least one plastic particle to transfer the microscope FTIR for analysis.
The plastic particle needs to be analyzed in several locations. Plastics can have different polymers, or be two plastics just be stuck together. By analyzing in two or three locations, you could make sure the sample was homogenous. Or if it wasnt, you knew where it had to be cut.
For a liquid, if the above went ok, we would filter through a millipore filtration apparatus to get extra particles and compare the IRs to make sure there was no problem from the filter.
By the way, this took a lab stereo microscope with about a 6" focal length. This is a reflectance microscope, not the typical slide microscope most think of. The extra length allows the use of tools like tweezers and scalpel that might be needed to slice or move the particles to the microscope FTIR. Usually, only 40x magnification was needed.
This is a quick overview, but it gives the basic idea.
Optical microscopes aren't my strong point. There are others here that can probably help you more.
But if there is anything else you need, please let me know. Regards.