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Droplet microfluidics enables high-throughput manipulation of fL-μL volume samples. Methods implemented for the chemical analysis of microfluidic droplets have been limited in scope, leaving some applications of droplet microfluidics difficult to perform or out of reach entirely. Nanoelectrospray ionization-mass spectrometry (nESI-MS) is an attractive approach for droplet analysis, because it allows rapid, label-free, information-rich analysis with high mass sensitivity and resistance to matrix effects. Previous proof-of-concept systems for the nESI-MS analysis of droplets have been limited by the microfluidics used so that stable, long-term operation needed for high-throughput applications has not been demonstrated. We describe a platform for the stable analysis of microfluidic droplet samples by nESI-MS. Continuous infusion of droplets to an nESI emitter was demonstrated for as long as 2.5 h, corresponding to analysis of over 20 000 samples. Stable signal was observed for droplets as small as 65 pL and for throughputs as high as 10 droplets/s. A linear-concentration-based response and sample-to-sample carryover of <3% were also shown. The system is demonstrated for measuring products of in-droplet enzymatic reactions.
Steyer et al. (Mon,) studied this question.