Coupling a droplet generator with conventional ESI–MS for quantitative analysis of small-volume samples

Quantitative mass spectrometric analysis of small-volume samples (e.g., < 1 μL) has been a challenge mainly due to the difficulties with sample handling and its injection into the system for analysis. Herein we report a microfluidic analytical platform coupling a droplet generator with conventional electrospray ionization–mass spectrometry (ESI–MS) that enables multiple analyses of a μL-sized sample with sensitivity and repeatability. In an analysis by droplet generator–assisted ESI–MS (DG-ESI–MS), a sample of μL volume is pulled into a sampling capillary and its equal nL-sized portions are generated by a droplet generator and analyzed by ESI–MS at time intervals of choice. The droplet generator is made of PMMA sheets by laser engraving conveniently and at a low cost. In a study to achieve effective ESI–MS detection of water-in-oil droplets, it’s found that the problem of MS signal suppression by oil can be solved by using an appropriate organic carrier with ESI-enhancing additives. The proposed DG-ESI–MS method has linear calibration curves for both adenine and phenylalanine with LODs at the sub-μM level. Application of the present analytical platform for monitoring substrate concentration changes in an enzymatic reaction solution of 3 μL is demonstrated. Graphical abstract