On-the-Fly Mass Spectrometry in Digital Microfluidics Enabled by a Microspray Hole: Toward Multidimensional Reaction Monitoring in Automated Synthesis Platforms

Abstract

We report an approach for the online coupling of digital microfluidics (DMF) with mass spectrometry (MS) using a chip-integrated microspray hole (μSH). The technique uses an adapted electrostatic spray ionization (ESTASI) method to spray a portion of a sample droplet through a microhole in the cover plate, allowing its chemical content to be analyzed by MS. This eliminates the need for chip disassembly or the introduction of capillary emitters for MS analysis, as required by state-of-the-art. For the first time, this allows the essential advantage of a DMF devicefree droplet movementto be retained during MS analysis. The broad applicability of the developed seamless coupling of DMF and mass spectrometry was successfully applied to the study of various on-chip organic syntheses as well as protein and peptide analysis. In the case of a Hantzsch synthesis, we were able to show that the method is very well suited for monitoring even rapid chemical reactions that are completed in a few seconds. In addition, the strength of the low resource consumption in such on-chip microsyntheses was demonstrated by the example of enzymatic brominations, for which only a minute amount of a special haloperoxidase is required in the droplet. The unique selling point of this approach is that the analyzed droplet remains completely movable after the MS measurement and is available for subsequent on-DMF chip processes. This is illustrated here for the example of MS analysis of the starting materials in the corresponding droplets before they are combined to investigate the reaction progress by DMF-MS further. This technology enables the ongoing and almost unlimited tracking of multistep chemical processes in a DMF chip and offers exciting prospects for transforming digital microfluidics into automated synthesis platforms.

Surface-Enhanced Raman Spectroscopic Probing in Digital Microfluidics through a Microspray Hole

“…To investigate the sample volume consumed during the spray process onto the SERS substrate, we observed the droplet volume shrinkage over an extended period, from which the spray volume can then be calculated . This reveals that per 60 s of spray time, only approx.…”

“…DMF Chip with Integrated μSH Fabrication. Previously, we reported a DMF chip system with an integrated microspray hole 98 fabricated using a relatively new selective laser-induced etching (SLE) technology. To generate a spray through the μSH, we utilized a 2.5 × 2.5 mm contactless actuation electrode on the bottom DMF plate.…”

“…In order to make the benefits of SERS analysis more accessible in DMF devices, we are exploring a new approach based on our recent work on the coupling of DMF and mass spectrometry. 98 There, we brought a tiny portion of a droplet contained in the DMF chip outside through a microspray hole (μSH) integrated in the DMF top plate and transferred the contained molecules ionized into an MS. The transfer of the liquid sample through a micrometer narrow hole was induced by high voltage pulses according to a DMF-compatible electrostatic spray method developed therein.…”

Toward nanofluidics‐based mass spectrometry for exploring the unknown complex and heterogeneous subcellular worlds

“…In principle, these microliter-sized droplets can be freely controlled on an array of hydrophobic and insulated electrodes (Figure 3C). 72,73 For example, Das et al presented the first approach that seamlessly combines digital microfluidics with mass spectrometry using microspray holes. 73 An electrophoresis device (also known as microchip capillary electrophoresis) is another form of continuousflow microfluidic type that is driven by an electric field instead of pressure (Figure 3D).…”

“…72,73 For example, Das et al presented the first approach that seamlessly combines digital microfluidics with mass spectrometry using microspray holes. 73 An electrophoresis device (also known as microchip capillary electrophoresis) is another form of continuousflow microfluidic type that is driven by an electric field instead of pressure (Figure 3D). 54,[74][75][76] Electrophoresis microfluidic devices are frequently coupled with electrospray ionization mass spectrometry.…”