Droplet Microfluidics with MALDI-MS Detection: The Effects of Oil Phases in GABA Analysis

Microfluidic andmass spectrometry (MS) methods are widely usedto sample and probe the chemical composition of biological systemsto 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 offlineanalyses, the effects of parameters related to droplet generation,such as the type of oil phase used, have been understudied. To characterizethese effects, five different oil phases were tested in droplet microfluidicsfor producing samples for MALDI-MS analysis. Picoliter to nanoliteraqueous droplets containing 0.1 to 100 mM & gamma;-aminobutyric acid(GABA) and inorganic salts were generated inside a polydimethylsiloxanemicrofluidic chip and deposited onto a conductive glass slide. Opticalmicroscopy, Raman spectroscopy, and MALDI-mass spectrometry imaging(MSI) of the droplet samples and surrounding areas revealed patternsof solvent and oil evaporation and analyte deposition. Optical microscopydetected the presence of salt crystals in 50-100 & mu;m diameterdried droplets, and Raman and MSI were used to correlate GABA signalsto the visible droplet footprints. MALDI-MS analyses revealed thatdroplets prepared in the presence of octanol oil led to the poorestdetectability of GABA, whereas the oil phases containing FC-40 providedthe best detectability; GABA signal was localized to the footprintof 65 pL droplets with a limit of detection of 23 amol. The effectof the surfactant perfluorooctanol on analyte detection was also investigated.

Automated summary

Microfluidics and mass spectrometry are commonly used to analyze biological systems. This study focused on the effects of different oil phases in droplet microfluidics for MALDI-MS analysis, with FC-40 oil phase found to provide the best detectability. Additionally, the impact of surfactant perfluorooctanol on analyte detection was investigated.