As increasing the distance between the spray source and the MS inlet increases the microdroplet reaction time and reduces the droplet size by its evolution (evaporation/fission) process, we tuned this distance to monitor its effect on the microdroplet reaction. Surprisingly, the increased flight time and subsequent evaporation of the microdroplets (Figure ) led to a higher fractional abundance of DA products (Figure ). This result suggests that the trace level of the DA reaction observed in water microdroplets is possibly caused by the confinement or increased concentration of reactants in a small volume during the microdroplet evolution. Indeed, when we gradually increased the reactant concentration in microdroplets, the efficacy of the DA reaction improved (Figure ). The increased nebulizing gas pressure in the spray source is known to cause faster droplet solvent evaporation (Figure ) and fission, producing smaller droplets with a high surface-to-volume ratio. When we raised the gas pressure, the tendency of the DA reaction in water microdroplets continued to increase (Figure ). This suggests that the DA reaction might predominantly occur at the air-water interface, facilitated by the accumulation (increased concentration) of hydrophobic reactants at that location. The acetonitrile droplet also exhibited a similar reaction trend up to a certain gas pressure (130 psi), after which the propensity of DA reaction decreased (Figure ). This result might be explained by the possible distribution of hydrophobic reactants throughout the acetonitrile droplet, allowing the DA reaction to occur both at the interface and within the core of the acetonitrile droplet, which is in contrast to the water droplet case as discussed above.