Accurate analysis of N fixation in leguminous crops requires determination of N utilization within an intact plant; however, most approaches require tissue disassembly. We developed a simple and rapid technique to generate high-purity and high-yield [N-13]N-2 gas and obtained real-time images of N fixation in an intact soybean plant. The purification efficiency was similar to 81.6% after decay correction. Our method provides accurate signals of N Fixation and allows free changes to the tracer gas composition to suit different experimental designs.