It's difficult to avoid contamination, since everything (samples, containers, equipements, etc) will have been in contact with glove at least once, and good decontamination is very hard.
You'd create a control by creating saline solution with distilled water and sodium chloride. Then you treat both the control and your sample(s) the same way in the analysis.
Surely something should tick you off when the microplastic levels aren't much lower in your control compared to your actual sample?
That's the point. If the control also gets contaminated then they should get >0 results in the control (when basically zero was expected) and that should have raised awareness of this problem pretty much immediately.