The first breakthrough in the detection of arsenic poisoning was in 1775 when Carl Wilhelm Scheele discovered a way to change arsenic trioxide to garlic-smelling arsine gas (AsH), by treating it with nitric acid (HNO) and combining it with zinc.

Who devised the first test for detection of arsenic in 1775? Let's have some stream of consciousness first.
To answer the question, consider the following: The first breakthrough in the detection of arsenic poisoning was in 1775 when Carl Wilhelm Scheele discovered a way to change arsenic trioxide to garlic-smelling arsine gas (AsH), by treating it with nitric acid (HNO) and combining it with zinc. So, the answer is Carl Wilhelm Scheele.