I’m assuming if the syringe was wet before being placed in the microscope, the vacuum of the chamber would cause most of the water in the plasma to vaporize. The remaining salts and compounds would be much smaller than the red blood cells. The density of the red blood cells would be much larger than any remaining plasma, so the bulk of your backscattered electrons will be coming from the cells and needle, making the plasma essentially transparent. This is a fairly low magnification image for SEM, but that’s how you get such fantastic depth of field.

I plan on installing some water to feed my garden later this summer. I was considering PVC, but is this a better case for metal? How does the plastic handle the UV from the sun? I thought outdoor might be gray PVC, but I was thinking underground PVC with galvanized above.