Surface Layer MALDI-MSI of Synthetic Materials on a Molecular Level

Importance of the topic

Synthetic polymer surfaces are integral to applications in catalysis, adhesion, coatings, printing and biomedical devices. Uniform chemical composition at the molecular level determines performance, yet common analytical tools often probe fragments or atoms rather than intact surface molecules. Surface-layer MALDI-MSI (SL-MALDI-MSI) bridges this gap by delivering intact molecular ions from the outermost 2 nm of a sample, enabling precise mapping of polymer surface chemistry.

Objectives and study overview

This work introduces SL-MALDI-MSI and applies it to characterize four types of surface defects in spin-cast poly(methyl methacrylate) (PMMA) and polystyrene (PS) thin films: contamination, masking, scratching/abrasion and solvent-induced delamination. The goal is to demonstrate solvent-free matrix application, high surface specificity and molecular-level imaging of polymer films.

Used instrumentation

• Bruker UltrafleX-III MALDI-TOF/TOF with Nd:YAG laser
• Bruker rapifleX MALDI-TOF/TOF with smartbeam 3D laser
• J.A. Woollam VASE M-200 UV-Vis-NIR spectroscopic ellipsometer
• Epson optical scanner for macroscopic imaging

Methodology

PMMA (~7 kDa) and PS (~6 kDa) films were prepared by spin-casting toluene solutions onto silicon wafers. Matrices (DHB or DCTB) and cationizing salts (NaTFA or AgTFA) were applied by sublimation at 90 °C for 3–4 h, ensuring a solvent-free deposition. All spectra were acquired in positive linear mode; data processing and image construction were performed using flexImaging software.

Main results and discussion

• Contamination defect: Molten PS stamped onto PMMA produced areas devoid of sodiated PMMA oligomers in SL-MALDI images, effectively outlining the PS region as a negative space.
• Masking defect: Scotch-tape-masked regions during spin-casting yielded clear absence of PMMA signal where no film was deposited.
• Mechanical defect: Scratched or laser-etched UA logos and mascots on PMMA were mapped by sodiated oligomer signals. The rapifleX system provided superior spatial resolution and uniform signal compared to the older UltrafleX-III.
• Solvation defect: A PS/PMMA bilayer exposed to cyclohexane showed selective dissolution of PS. Silver- and sodium-adducted ions delineated the washed versus unwashed regions with a sharp boundary in SL-MALDI images and lateral intensity profiles.

Benefits and practical applications of the method

SL-MALDI-MSI offers non-destructive, surface-specific molecular imaging without solvent interference. By detecting intact oligomer ions with nanometer depth resolution, it can be applied to quality control of coatings, monitoring of additive distributions and investigation of surface degradation in industrial and biomedical polymer systems.

Future trends and applications

• Extension to a broader range of polymers, copolymers and composites.
• Integration with higher spatial resolution optics (<10 µm) and 3D surface profiling.
• Automated image analysis and AI-driven defect recognition.
• Real-time in situ monitoring of surface treatments and manufacturing processes.

Conclusion

Surface-layer MALDI-MSI represents a powerful advance for molecular-level surface analysis of synthetic materials. By achieving intact-ion mapping within the top 2 nm, SL-MALDI-MSI successfully visualized diverse defect types in PS and PMMA films. This technology holds promise for industrial quality assurance, material development and fundamental studies of polymer surface phenomena.

Reference

• Endres KJ, Hill JA, Lu K, Foster MD, Wesdemiotis C. Anal Chem. 2018;90(22):13427-13433.
• Crecelius AC, Steinacker R, Meier A, Alexandrov T, Vitz J, Schubert US. Anal Chem. 2012;84(16):6921-6925.
• Rivas D, Zonja B, Eichhorn P, Ginebreda A, Perez S, Barcelo D. Anal Bioanal Chem. 2017;409(23):5401-5411.
• Fröhlich SM, Archodoulaki V-M, Allmaier G, Marchetti-Deschmann M. Anal Chem. 2014;86(19):9723-9732.