Is the taxidermist the murderer?

Hint 1 of 8:

Well, you can't be sure now.

Hint 2 of 8:

But there is some evidence to suggest that he is.

Hint 3 of 8:

He has some chemicals that will help prove your case.

Hint 4 of 8:

Did you analyze the powder on Sarah's body?

Hint 5 of 8:

Use the lab table at Baker St.

Hint 6 of 8:

You'll find arsenic in the powder and at the taxidermist.

Hint 7 of 8:

And that strange blade will help prove your case.