Enhanced Formation of Multiply Charged Protein and Peptide Ions in SIMS Using Large Water Clusters

Area of Science:

• Analytical Chemistry
• Biophysics
• Mass Spectrometry

Background:

• Secondary Ion Mass Spectrometry (SIMS) faces challenges in protein analysis due to molecular fragmentation and low ionization efficiency.
• Traditional methods often require sample preprocessing or matrix addition, limiting direct analysis.

Purpose of the Study:

• To evaluate the efficacy of water gas cluster ion beams (GCIB) for direct protein analysis using SIMS.
• To compare the performance of water clusters with conventional (Ar/CO2)n clusters for protein ionization and fragmentation.

Main Methods:

• Application of water clusters as primary ions in SIMS for analyzing ubiquitin and angiotensin II.
• Comparison of ion yields and fragmentation patterns between water clusters and (Ar/CO2)n clusters, with and without matrix.
• Investigation of the effect of cluster energy on ion yield at equivalent energy per nucleon (E/m).

Main Results:

• Water clusters significantly enhanced ion yields for ubiquitin and angiotensin II compared to (Ar/CO2)n clusters in matrix-free conditions.
• Ion yields decreased with water clusters when a matrix was used, while (Ar/CO2)n clusters showed increased yields.
• Increased cluster energy positively correlated with ion yield at equivalent E/m.
• Water clusters successfully generated and detected multiply charged protein species (up to 4+ for ubiquitin).

Conclusions:

• Water clusters represent a significant advancement for SIMS protein analysis, overcoming fragmentation and ionization limitations.
• The ability to detect multiply charged proteins expands the accessible mass range for SIMS.
• These findings support the use of water clusters in high-resolution, multimodal mass spectrometry imaging for direct protein analysis without preprocessing.