chem--partial pressure

PV=nRT

The concentration of a gas in a gaseous mixture is the same as the ratio of its partial pressure to the overall pressure. If you increase the partial pressure, you increase the overall pressure as well, but the concentration should go up.

Consider water in a closed container at room temperature and at a pressure of one atmosphere. There is a certain amount of water vapor in the gas volume but it is small. If you raise the temperature of the vessel to 100 degC, the partial pressure of the water becomes 1 atmosphere and the overall pressure rises to around 2 atmospheres. The concentration of water in the vapor phase is now 50%.