A new method in time-of-flight secondary ion mass spectrometry (ToF-SIMS) imaging, the droplet-enhanced method, was developed for the molecular analysis of biomaterials. To facilitate the ionization of biomolecules, a small amount of aqueous solution containing a variety of protonation agents as ionization-enhancing agents was dropped onto peptide samples before ToF-SIMS measurement. Using trifluoroacetic acid (TFA) as an enhancing agent, protonated insulin (MW 5733) ions were detected as not only [M + H](+) but also [M + H-2](2+) and [M + 3H](3+) from its film sample, using a Ga+ primary beam. TFA promoted the ionization of the large molecules much more effectively than did the other acids, and this peculiarity is related to both Na+ and Au-3(+) intensities. We also demonstrated the visualization of dot-patterned insulin drawn with our bubble jet (BJ) printing technology using insulin molecular ion signals. (c) 2006 Published by Elsevier B.V.