Abstract

Applications of droplets for studies in small culture volume, especially microdroplets in microfluidics, have aroused the interests from physical, chemical and biological fields. During the last 10 years, various methods for forming and manipulating small droplets have been developed. Each droplet provides a compartment in which species or reactions can be isolated and therefore is suitable for quantitative studies. Furthermore, high-throughput experiments with extremely small volumes, single molecules, or single cells can also be achieved through droplet-based systems. Ambient ionization, which aims at direct sampling of analytes in the ambient state, has emerged rapidly in recent years. Among the latest progresses in atmospheric pressure electrospray-based ionization (ESI) techniques, paper-based ESI is highly promising in consideration of its simplified protocol of sample preparation and equipment. Capability of paper-based ESI-MS for the analysis of drugs, peptides, nucleotides and phospholipids in complex biological fluid samples, such as whole blood and raw urine, has been demonstrated recently. And the transport and ionization mechanisms, the appropriate substrate and solvent for effective paper-based ESI were investigated. Therefore, coupling microfluidics and mass spectrometry can combine the advantages of these two techniques, offering a new platform for scientific research. In this work, a paper spray mass spectrometry combined with droplets generated by gravity and electrostatic attraction, microfluidic chip, and inkjet methods, was developed. The qualitative and quantitative analytical performances of this technique for single droplet were demonstrated. This manually controlled interface is straightforward, low-cost and simple to implement. Moreover, paper spray ionization mass spectrometry hold promises in direct analysis of real biological/chemical microreaction samples because of its tolerance with complex matrix. As a proof-of-concept example, the droplet-based acetylcholine hydrolysis was carried out to demonstrate the validation of our method for direct analysis of micro-chemical/biological reactions.