Firstly brass is not copper. If your pipes are extruded copper tubing used for domestic drinking water, then I am confident you can get the material specification from the manufacturer, because that is most probably controlled by the EPA and Government Law. If your pipe is cast and the casting plant buys it from some back yard non ferrous metal supplier, then there is a good probablitiy it contains lead, used to enhance machining. Finally if its not too complicated or expensive, you get the supplier to give you a material analysis down to parts per million, spectral analysis.