Development and applications of paper-based electrospray ionization-mass spectrometry for monitoring of sequentially generated droplets

In this work, sub-microlitre droplets were generated by gravity and electrostatic attraction using a capillary tube. The parameters affecting the sizes and frequency of the droplets were investigated. The volume of droplets could be controlled in the range from 0.7 μL to 2.4 μL and the time interval from 15 s to 60 s with appropriate parameters. Combining the droplets with on-line mass spectrometry (MS) via paper-based electrospray ionization (ESI), a steady flow of solvent was delivered by the capillary tube to the base-side of the paper, which maintained the consistent state of the electrospray. With this approach, each droplet produced a peak in the ion chromatogram. Relative standard deviations (RSDs) not higher than 9% for both the intensities and the time intervals were achieved when using rhodamine 6G and L-phenylalanine as model analytes. The present method was utilized for the monitoring of the amine–aldehyde condensation reaction of butylamine and benzaldehyde. Direct analysis and distribution of molecules in fruits were also performed, which demonstrated the potential application of this approach.