Here's a very naïve example to help illustrate how you can do a "blank" (a control).

Say you're testing a sample of water in a test tube. Repeat all steps in exactly the same way, but use distilled water. You can even do all the steps and use no water! (Including having an empty container and pouring nothing from the empty container into the test tube).

By doing things like this you create samples that allow you to look for contamination. How do you know that the thing you're testing has microplastics? (Or whatever) because it has more than the blanks/controls. That's it. Congrats, you've isolated a variable in your experiment.

Btw, this is pretty common practice. In fact! Here's a video of someone doing exactly that "nothing" control looking for microplastics. Those steps are done at 10:20.

https://youtube.com/watch?v=oDDQjEpuFfQ

I don't believe the point is to construct a blank without them. Rather, the point is to capture what is already there (contaminants) so you can calibrate during the real sample.

The point isn’t to have 0 microplastics according to the test.

The point of the blank is to identify the base level given the current testing environment. Then you test again with the variable.

If the majority of the microplastics contaminants were introduced in the blank, the variable would show minimal, if any, bump.

Exactly.

If you run a blank and it has no fewer microplastics than the thing you are studying, then that tells you something.

That's EXACTLY what the blanks for! It lets you see what to expect with "All things being the same - just not with the sample in place."

You sure you're a coder?