I am using an extract in my experiment that has been prepared by another researcher, "as a gift" how can I write that in my paper "materials and methods"?
You will write in the material and results briefly , if they published give the paper as Reference , if it is Thesis give it as Ref, if neither write the Ref, In press,
I would recommend, though as gracious as the gift was, that you use your own extract. It may be ok to rely on the extract of another, and of course properly cited, but you need to think about defensibility when you complete your work. If at all possible, control as many of the inputs as possible. If that isn't possible, you have to be able to justify the quality of what you were given or your work may have a structural weakness.