ENHANCED MOLECULAR COVERAGE, RESOLUTION AND SPEED FOR IN-SITU PHARMACEUTICAL TABLET MSI ANALYSIS BY COMBINING DESI AND MALDI USING MULTI-REFLECTING Q-TOF

Importance of the Topic

In situ characterization of pharmaceutical tablets using mass spectrometry imaging (MSI) addresses the limitations of traditional dissolution assays by providing spatial distribution of active ingredients and excipients directly on the tablet surface. Combining DESI and MALDI with high-resolution Q-ToF enhances molecular coverage, mass accuracy, and speeds up analysis.

Study Objectives and Overview

The study aimed to evaluate the complementarity of desorption electrospray ionization (DESI) and matrix-assisted laser desorption ionization (MALDI) imaging on an ultra-high resolution multi-reflecting Q-ToF for homemade and commercial tablets. Key goals included mapping the distribution of small-molecule drugs, identifying polymeric species and proteins, assessing mass accuracy, and applying unsupervised segmentation to simplify complex datasets.

Methodology and Instrumentation

Sample Preparation:

• Homemade tablets: compressed lactose matrix with heterogeneously deposited aspartame, 2–3 mm thick.
• Commercial tablets: 20 mm diameter “Lysopain” lozenges containing cetylpyridinium chloride and lysozyme hydrochloride.

MALDI Protocol:

• CHCA matrix (10 mg/mL in 70/30/0.1 ACN/water/TFA) applied via HTX M5 nebulizing sprayer (8 passes).

DESI Protocol:

• Solvent: 95% MeOH/5% water with 50 pg/µL leucine enkephalin at 2 µL/min using µBSM nanoflow pump; N₂ nebulizing gas at 10 psi; High-Performance sprayer for improved focus.

Imaging Conditions:

• Pixel size: 50 µm.
• Acquisition speed: 10 Hz.
• Mass resolution: >200 000 FWHM.
• Mass accuracy: <0.5 ppm (internal lockmass).

Used Instrumentation

• MALDI/DESI™ XS SELECT SERIES™ MRT quadrupole multi-reflecting Time-of-Flight mass spectrometer (Waters).
• HTX M5 nebulizing spray device (HTX Technologies) for MALDI matrix deposition.
• µBSM nanoflow pump and N₂ nebulizer for DESI.
• High-Performance DESI sprayer (Waters) for enhanced spray focus.
• MassLynx™ and High Definition Imaging™ v1.7 for data acquisition and visualization.

Main Results and Discussion

• Both MALDI and DESI detected sodiated/potassiated lactose and protonated aspartame; spatial distributions were consistent across techniques.
• DESI spectra exhibited richer profiles, revealing adducts, multimers, and doubly/triply charged polymeric species (e.g., lactose oligomers with ∆m/z 342 Da).
• In Lysopain tablets, MALDI(+): cetylpyridinium chloride monomer (m/z 304.30) with patchy clusters; additional unidentified ubiquitous signals.
• DESI(+): multimers of cetylpyridinium chloride ([M–HCl + H]+, [M2–HCl + H]+, etc.) with sub-0.5 ppm accuracy; lysozyme protein envelope detected as +8 to +12 charge states in localized clusters.
• Unsupervised UMAP segmentation identified six clusters plus background, facilitating the visualization of chemical heterogeneity (pixel clusters) and cluster quality in reduced dimensional space.

Benefits and Practical Applications

MSI with combined DESI/MALDI delivers rapid, non-destructive, high-resolution chemical mapping of tablets, enabling:

• Direct localization of APIs and excipients without dissolution.
• High mass accuracy (<0.5 ppm) for confident identification of small molecules, adducts, and proteins.
• Automated segmentation to streamline data interpretation for QA/QC and formulation development.

Future Trends and Applications

• Integration of machine learning and advanced segmentation algorithms for real-time quality monitoring.
• Extension to multi-component and controlled-release dosage forms for detailed formulation studies.
• On-line implementation of MSI in manufacturing for continuous process analytical technology (PAT).

Conclusion

The complementary use of DESI and MALDI on a multi-reflecting Q-ToF enables comprehensive in-situ analysis of pharmaceutical tablets, capturing small molecules and proteins with high spatial resolution, mass accuracy, and acquisition speed. This approach advances non-targeted and targeted investigations in drug development and quality control.

Reference

• K. Škrášková, E. Claude, E.A. Jones, M. Towers, S.R. Ellis, R.M.A. Heeren; Methods. 2016 Jul;104:69-78