The combination of acoustically levitated droplets, mid-IR
laser
evaporation, and subsequent post-ionization by secondary electrospray
ionization was applied for monitoring the enzymatic digestion of various
proteins. Acoustically levitated droplets are an ideal, wall-free
model reactor, readily allowing compartmentalized microfluidic trypsin
digestions. Time-resolved interrogation of the droplets yielded real-time
information on the progress of the reaction and thus provided insights
into reaction kinetics. After 30 min of digestion in the acoustic
levitator, the obtained protein sequence coverages were identical
to the reference overnight digestions. Importantly, our results clearly
demonstrate that the applied experimental setup can be used for the
real-time investigation of chemical reactions. Furthermore, the described
methodology only uses a fraction of the typically applied amounts
of solvent, analyte, and trypsin. Thus, the results exemplify the
use of acoustic levitation as a green analytical chemistry alternative
to the currently used batch reactions.