The limitations of the study are those characteristics of design or methodology that impacted or influenced the application or interpretation of the results of your study. They are the constraints on generalizability and utility of findings that are the result of the ways in which you chose to design the study and/or the method used to establish internal and external validity.
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What is the focus of your study?
Where are you going to conduct the experiment part of your study and how much time it will consume?
What are the organism/chemical/machine/etc that your study will mainly use?
What are the things that are out of the scopes of your study?

That's the basics and you can add more. It's like telling people that your study is about this and that, and not about those others. Answer that in paragraph form.

Good luck on your study :D
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