Impact of different optical lenses while using the LA-REIMS imaging setup

Significance of the Topic

Laser Assisted Rapid Evaporative Ionization Mass Spectrometry (LA-REIMS) has emerged as a powerful ambient ionization technique enabling direct metabolic profiling of tissues without complex sample preparation. Refining the optical components within the LA-REIMS imaging setup is critical to improve spatial resolution, reproducibility, and sensitivity, thus expanding its potential for biomedical and veterinary diagnostics.

Aim and Overview

The study aimed to evaluate how various optical lenses—differing in material (ZnSe vs. CaF2), focal length, and diameter—affect laser spot size, energy density, and resulting mass spectral profiles when applying LA-REIMS to tissue sections. By comparing metabolite peak ratios and variance under different lens configurations, the work sought to identify optimal parameters for robust imaging.

Methodology and Instrumentation

Ultraviolet–visible–near infrared laser source:

• Optical parametric oscillator tuned to 2940 nm.

Mass spectrometer:

• Xevo G2- QTof with REIMS source for aerosol transfer and ionization.

Tested optical lenses:

• Aspheric ZnSe lenses (Ø 1 inch, focal lengths 12.7 mm and 25 mm).
• Plano-convex CaF2 lenses (Ø ½ inch, focal lengths 6 mm and 20 mm).

Data analysis:

• Principal component analysis (PCA) to detect spectral shifts.
• Peak-pair ratio evaluation for phosphatidylethanolamine lipids PE(36:2) and PE(38:4) in both [M–NH4]– and [M–H]– ion forms.

Results and Discussion

Laser spot characterization and actual energy density calculations revealed significant variation across lens types. PCA score plots of homogeneous pork liver tissue showed clear clustering by lens setup, indicating spectral differences.

• Higher energy density correlated with increased ammonia loss in PE lipids, reflected in altered [M–NH4]–/[M–H]– peak ratios.
• Shorter focal lengths and certain lens materials generated smaller spots with higher energy density, amplifying fragmentation patterns.
• Relative standard deviation (RSD) analysis demonstrated that high energy density conditions offered greater measurement robustness.

Boxplot comparisons confirmed that both focal length and energy density are the dominant factors driving lipid fragmentation variance.

Benefits and Practical Applications

The findings guide selection of optical components to tailor LA-REIMS imaging performance:

• Optimizing lens focal length and material enhances spatial resolution and reproducibility in tissue metabolic mapping.
• Controlled energy density minimizes unwanted fragmentation, improving quantitation of labile metabolites.
• Robust imaging protocols can support clinical research, veterinary diagnostics, and agro-food quality control.

Future Trends and Potential Applications

Further developments may include:

• Beam profiling for each lens to precisely map energy distribution and refine spot size calibration.
• Integration of adaptive optics to dynamically adjust focus across heterogeneous tissue surfaces.
• Expansion to multi-modal imaging by coupling LA-REIMS with optical microscopy or fluorescence guidance.

Conclusion

This work demonstrates that optical lens properties critically influence energy density and thus the mass spectral output of the LA-REIMS imaging system. By understanding how lens material, focal length, and diameter affect lipid fragmentation, researchers can standardize imaging conditions and improve data quality for metabolic tissue analysis.

Reference

[1] Waters Research Center Kft. and Eötvös Loránd University. Fully Automated Chemical Imaging with LA-REIMS poster, 2022.