Given the following passage "In more recent applications, hydrogen is used pure or mixed with nitrogen (sometimes called forming gas) as a tracer gas for minute leak detection. Applications can be found in the automotive, chemical, power generation, aerospace, and telecommunications industries. Hydrogen is an authorized food additive (E 949) that allows food package leak testing among other anti-oxidizing properties.", answer the following question. Note that the answer is present within the text. Question: What is a tracer gas?
A:
hydrogen is used pure or mixed with nitrogen