Interfacing Digital Microfluidics with Ambient Mass Spectrometry Using SU-8 as Dielectric Layer

Area of Science:

• Analytical Chemistry
• Microfluidics
• Mass Spectrometry

Background:

• Digital microfluidic (DMF) devices offer precise droplet manipulation for various assays.
• Ambient mass spectrometry (MS) allows direct surface analysis but interfacing with DMF is challenging.
• Optimizing surface-sensitive processes like sample precipitation and ionization is crucial for DMF-MS integration.

Purpose of the Study:

• To develop and demonstrate an integrated system combining DMF sample preparation with ambient MS analysis.
• To overcome the challenges of interfacing DMF with desorption atmospheric pressure photoionization mass spectrometry (DAPPI-MS).
• To utilize SU-8 material for both DMF dielectric layer and DAPPI-MS substrate.

Main Methods:

• Electrowetting-on-dielectric (EWOD) based digital microfluidic (DMF) devices were fabricated using SU-8.
• On-chip liquid-liquid extraction and enzymatic dealkylation reactions were performed.
• Desorption atmospheric pressure photoionization mass spectrometry (DAPPI-MS) was used for direct analysis of dried samples on the DMF device.
• SU-8's stability was leveraged for both DMF operation and MS ionization.

Main Results:

• Successful interfacing of DMF with DAPPI-MS was achieved using SU-8.
• Demonstrated feasibility for analyzing pharmaceuticals after on-chip sample preparation.
• Achieved lower limits of detection in the range of 1–10 pmol per droplet (0.25–1.0 µg/mL) for tested pharmaceuticals.

Conclusions:

• SU-8 based DMF devices are suitable for DAPPI-MS analysis.
• The integrated approach facilitates drug distribution and metabolism assays at the droplet scale.
• This work provides a robust platform for sensitive analysis of dried samples using ambient MS.