Fig. 6. Mass images obtained from (A) dehydrated BPA–DHB complexes and dehydrated DHB dimers detected in MALDI-MSI measurement with mass range m/z 205–550 (B) [BPA+H]⁺, [DHB+H]⁺, Na⁺, and K⁺ in MALDI-MSI measurement with mass range m/z 20–215 and the brain section used in this measurement. MALDI-MSI was performed in the half area enclosed by square. The mass images obtained at m/z 214.058, m/z 350.087, m/z 366.059 revealed the distribution in the tumor region. The mass image of [2DHB−H₂O+K]⁺ showed clear distribution in tumor region compared to the mass images of [2DHB−H₂O+Na]⁺ and [2DHB−2H₂O+H]⁺. The mass image of potassium ion clearly showed the distribution in tumor region and localization inside tumor.