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alyte. A second UV photon is then subsequently absorbed, which raises the internal energy above the ionization energy, forming a molecular ion. The process is enhanced when the first step is in resonance with an electronic excited state. “You benefit from changing the wavelength of the laser. That provides you with a fingerprint spectrum of the compound of interest. Every molecule has its own distinct fingerprint,” explains Oudejans. “The technique is also selective in that you record the individual masses of the compounds. So you get a two-dimensional detection array that you can measure your concentrations with,” he adds. One advantage of using a real-time technique like REMPI-TOFMS is the ability to measure transient events, such as the start-up of a cold diesel engine. Because cold start-ups include a period of incomplete combustion, one would expect emissions to fluctuate during that time. And that is exactly what the researchers found. During the cold start-up of a diesel generator, they observed a sharp peak in benzene emissions lasting
