Imaging mass spectrometry

While sucralfate can form a thick gel layer inside the gastrointestinal tract, SOS lacks this gel-forming ability²². Thus, by using IMS, we tested the hypothesis that SOS might form an alternative protective structure by directly adhering to the mucosal surface. Additionally, we evaluated the ability of SOS to accumulate at inflammation sites.

Thin sections (12-μm thick) of the oesophageal samples were prepared at −21 °C using a cryostat (CM 1850; Leica, Wetzlar, Germany), and thaw-mounted on conductive indium–tin oxide glass slides (20 Ω, SI0020N; Matsunami Glass Industry Co. Ltd.). A matrix was prepared using a solution of 10 mg of 9-AA dissolved in 1 mL of 80% ethanol²³. The 9-AA matrix solution was manually sprayed onto the samples using an airbrush (Procon Boy PS 270 WA Platinum; Mr. Hobby), until the tissue surface became uniformly light yellow in colour.

Mass spectrometry (MS)/MS-based IMS was performed using a matrix-assisted laser desorption/ionisation (MALDI) LTQ-XL instrument (Thermo Fisher Scientific)²⁴. To conduct the IMS experiment with adequate signal selectivity that could exclusively detect the SOS signal from complex tissue samples, we had to determine SOS-specific ion signals on MALDI–MS/MS. For this purpose, we measured an authentic SOS standard spotted on an oesophageal section (see Fig. 2). In the mass spectrum, a molecular ion at m/z 625 ([M-Na − 5SO₃Na + 5 H]^-) was detected, having the highest intensity (Fig. 2A). We then performed MS/MS targeting the ion at m/z 625. In the obtained product ion spectrum, a strong product ion at m/z 361 was detected with the highest sensitivity (Fig. 2B). Thus, during subsequent imaging assessments, an ion with m/z 625 was used as the parent ion, with an isolation window of m/z 1.0; the mass range from m/z 360 to 363 was recorded for each data point. The collision energy was 25% of the maximum available energy for the LTQ-XL, and the laser energy was set to 30 µJ. The scanning pitch of the laser irradiated area was 75 μm. Image reconstruction was performed using Image Quest software (Thermo Fisher Scientific, Waltham, MA).

Development of a specific SOS detection method by tandem MS. Qualitative analysis of an SOS-Na reference compound spotted on oesophageal tissues was performed using a MALDI LTQ-XL instrument. (A) On the MS spectrum (m/z 500–1600), the most intensive ion at m/z 625 was identified as ([M-Na  − 5SO₃Na + 5 H]^-); (B) Product ion spectrum (m/z 170–620) for the ion at m/z 625; (C) Structure of SOS-Na (molecular weight: 1157.628). MALDI, matrix-assisted laser desorption/ionization; MS, mass spectrometry; SOS, sucrose octasulfate.