posted on 2021-10-15, 11:28authored bySara E. Bell, Insu Park, Stanislav S. Rubakhin, Rashid Bashir, Yurii Vlasov, Jonathan V. Sweedler
Microfluidic and
mass spectrometry (MS) methods are widely used
to sample and probe the chemical composition of biological systems
to elucidate chemical correlates of their healthy and disease states.
Though matrix-assisted laser desorption/ionization-mass spectrometry
(MALDI)-MS has been hyphenated to droplet microfluidics for offline
analyses, the effects of parameters related to droplet generation,
such as the type of oil phase used, have been understudied. To characterize
these effects, five different oil phases were tested in droplet microfluidics
for producing samples for MALDI-MS analysis. Picoliter to nanoliter
aqueous droplets containing 0.1 to 100 mM γ-aminobutyric acid
(GABA) and inorganic salts were generated inside a polydimethylsiloxane
microfluidic chip and deposited onto a conductive glass slide. Optical
microscopy, Raman spectroscopy, and MALDI-mass spectrometry imaging
(MSI) of the droplet samples and surrounding areas revealed patterns
of solvent and oil evaporation and analyte deposition. Optical microscopy
detected the presence of salt crystals in 50–100 μm diameter
dried droplets, and Raman and MSI were used to correlate GABA signals
to the visible droplet footprints. MALDI-MS analyses revealed that
droplets prepared in the presence of octanol oil led to the poorest
detectability of GABA, whereas the oil phases containing FC-40 provided
the best detectability; GABA signal was localized to the footprint
of 65 pL droplets with a limit of detection of 23 amol. The effect
of the surfactant perfluorooctanol on analyte detection was also investigated.