here is my question:

I work over the field of complex numbers, and my schemes are separated and of finite type. Let $X$ be a quasi projective scheme (but not projective), let $Y$ be a regular projective scheme, and let $f: X \to Y$ be a monomorphism of schemes (but not necessary an immersion).

Is the pull-back by $f$ of an ample line bundle on $Y$ an ample line bundle on $X$? And if not, what kind of (not too strong) assumptions I should add on $f$ for this result to be true?