Normalization Based on m/z Shift and Ion Intensity in SALDI-TOFMS Imaging of Samples with Non-Horizontal Surface

Matrix-assisted laser desorption/ionization (MALDI), surface-assisted laser desorption/ionization (SALDI), and time-of-flight mass spectrometry (TOFMS) imaging are used for visualizing the spatial distribution of analytes. Mass spectrometry (MS) imaging of a sample with a rough surface with a uniform distribution of an analyte does not provide uniform ion intensities in the image. A shift in the m/z value of the analyte ions is also observed. To resolve these issues, we proposed a normalization method for obtaining images based on the actual localization of an analyte. A platinum (Pt) film was used to facilitate ionization in SALDI. The Pt ions obtained from the film and copper(II)-phthalocyanine from a commercial blue pigment were used to evaluate the shift in the m/z values and ion intensities originating from the sample surface placed diagonally. The relationship between the shift in the m/z value and peak area in mass spectra was determined by analyzing the relationship between the shift and irradiated height position, as well as between the peak area and irradiated height position on the sample. The SALDI image can be obtained based on the actual location of the analyte to normalize the raw image data. Thus, the normalization of the ion intensities enables SALDI-MS imaging of samples with rough surfaces using high-resolution TOFMS. The proposed normalization method for SALDI-MS imaging enhances the accuracy and resolution of analyte localization on rough surfaces, benefiting applications in material science, biomedical research, forensic analysis, and industrial quality control.