Professor Brown has left the laboratory however, you may still be able to reach her through email. Should I place a semicolon, colons, or dash?
The first part of your sentence reads "Prof Brown has left the laboratory however". This is incorrect placement as the "however" does not refer to her having left the lab, but to the fact that you can still reach her.
Professor Brown has left the laboratory. However, you may still be able to reach her by email.
Professor Brown has left the laboratory; however, you may still be able to reach her through email.
NOT A TEACHER
I most respectfully suggest the semicolon that I have placed in your sentence.