The scientists used a special tool called an intra-cavity Raman spectrometer to detect different harmful gases in the air, like sulfur dioxide, nitric oxide, carbon monoxide, and more. They tested these gases in regular air and during burning fuel. By using this tool, they could figure out how well a small burning system was working by looking at the gases produced. They found that the tool could accurately measure sulfur dioxide, nitrogen dioxide, and carbon monoxide in normal air, and sulfur dioxide and carbon monoxide during burning. This helped them create charts to show the levels of these gases and check if the burning was efficient.