alt Hacker NewsHere'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.