Microdroplet fusion mass spectrometry for fast reaction kinetics

Significance Time-resolved mass spectrometry is a powerful approach for identifying reaction intermediates and measuring reaction kinetics, yet one critical limitation is temporal resolution. Here, we describe microdroplet fusion mass spectrometry on timescales as short as microseconds. In our approach, two high-speed streams of liquid microdroplets collide to make fused droplets of 13 ± 6 μm in diameter, where the reactants are mixed in a negligible time. After a short flying time of 50 μs or less during which the reaction proceeds, the fused droplets enter a mass spectrometer for chemical analysis of intermediates and reaction products. This enables observation of early events of various fast chemical reactions in the liquid phase. We investigated the fusion of high-speed liquid droplets as a way to record the kinetics of liquid-phase chemical reactions on the order of microseconds. Two streams of micrometer-size droplets collide with one another. The droplets that fused (13 μm in diameter) at the intersection of the two streams entered the heated capillary inlet of a mass spectrometer. The mass spectrum was recorded as a function of the distance x between the mass spectrometer inlet and the droplet fusion center. Fused droplet trajectories were imaged with a high-speed camera, revealing that the droplet fusion occurred approximately within a 500-μm radius from the droplet fusion center and both the size and the speed of the fused droplets remained relatively constant as they traveled from the droplet fusion center to the mass spectrometer inlet. Evidence is presented that the reaction effectively stops upon entering the heated inlet of the mass spectrometer. Thus, the reaction time was proportional to x and could be measured and manipulated by controlling the distance x. Kinetic studies were carried out in fused water droplets for acid-induced unfolding of cytochrome c and hydrogen–deuterium exchange in bradykinin. The kinetics of the former revealed the slowing of the unfolding rates at the early stage of the reaction within 50 μs. The hydrogen–deuterium exchange revealed the existence of two distinct populations with fast and slow exchange rates. These studies demonstrated the power of this technique to detect reaction intermediates in fused liquid droplets with microsecond temporal resolution.