Enhancing peptide molecular imaging by depositing ice in time-of-flight secondary ion mass spectrometry

• We brought out a quantitative relationship between ionization enhancement and the amounts of the adherent ice on the sample.
• Ice deposition method is suitable for mass imaging analysis because the original distribution of the sample constituents is maintained.
• The adherent ice behaves as an additional source of protons and enhances the detection intensity of the sample in TOF-SIMS measurement.

Ice deposition onto the surface of biomaterials enhances ionization in time-of-flight secondary ion mass spectrometry (TOF-SIMS). We studied this effect by depositing controlled amounts of ice onto dots containing 67 fmol of angiotensin II (MW 1046) which were printed on a silicon substrate by an inkjet. The protonated parent ion was then measured using TOF-SIMS. It was found that the protonated parent ion intensity varies with the amount of ice. A density of approximately 1 ng/mm2 of ice gave the highest intensity, which was roughly 10 times the original intensity. Comparison of optical and TOF-SIMS images of the printed dots indicated that the diffusion caused by ice deposition was negligible and that the original distribution of sample constituents was almost retained. An experiment using deuterated ice showed that the adherent ice behaved as a proton source.

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